Dissertations / Theses on the topic 'Crustal tomography'

In this thesis, a finite-frequency theory is developed to calculate Born sensitivity kernels for Rayleigh-wave phase and amplitude measurements that are valid in regions near seismic stations. Calculations of sensitivity kernels for inter-station measurements show that exact travelling-wave representation of Green tensor is necessary when station spacing is close to or smaller than the seismic wavelength. This finite-frequency theory will allow us to take advantage of dense seismic arrays to obtain high-resolution surface-wave tomography using inter-station measurements. The non-linear dependence of surface wave phase upon large perturbations in crustal thickness as well as finite-frequency effects in global surface-wave tomography are investigated using wave propagation simulations. Calculations show that non-linearity as well as finite-frequency effects can be accounted for by using 2D phase-velocity kernels for boundary perturbations. A 3D-reference tomographic approach is developed for iterative inversions of global crustal structure where Frechet kernels are calculated in 3D reference models. A global dataset of minor-arc and major-arc Rayleigh wave dispersion measurements at periods between 25 seconds and 100 seconds are built and global phase velocity maps based on the dataset are obtained using diffractional tomography. The phase velocity model confirms many general features associated with surface tectonics including the ocean-continent dichotomy and the signature of lithospheric cooling in oceanic plates. There are significant differences between the phase velocity model and calculations based on a current global model CRUST2.0+S20RTS in oceanic regions, Archean and Proterozoic cratons as well as orogenic belts. In addition, the high resolution phase velocity maps reveal a major change in the distribution of small scale anomalies in the Pacific at different wave periods.
Ph. D.

Passive tectonic margins, like the eastern North American margin (ENAM), represent the meeting of oceanic and continental material where no active deformation is occurring. The recent ENAM Community Seismic Experiment provides an opportunity to examine the crustal structure across the ENAM owing to the simultaneous deployment of offshore and onshore seismic instrumentation. Using Rayleigh wave phase and group velocities derived from ambient noise data, we invert for shear velocity across the ENAM. We observe a region of transitional crustal thicknesses that connects the oceanic and continental crusts. Associated with the transitional crust is a localized positive gravitational anomaly. Farther east, the East Coast magnetic anomaly (ECMA) is located at the intersection of the transitional and oceanic crusts. We propose that underplating of dense magmatic material along the bottom of the transitional crust is responsible for the gravitational anomaly and that the ECMA demarks the location of initial oceanic crustal formation.

xv, 151 p. : ill. (some col.) A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number.
To explain the origin of several distinct aspects of the Galápagos volcanic hotspot, such as the broad geographical extent of recent volcanism and the unusual pattern of geochemical anomalies, we conducted seismic tomography studies of the upper mantle and crust beneath the Galápagos Archipelago. The studies combine measurements of group and phase velocities of surface waves and delay times of body waves. We find that upper mantle seismic velocities are lower than those beneath other regions of comparable age in the Pacific and consistent with an excess temperature of 30 to 150°C and ∼0.5% melt. We attribute the excess temperature and presence of melt to an upwelling thermal mantle plume. Crustal seismic velocity is up to 25% lower than that of very young crust at the East Pacific Rise (EPR) and is comparable to that of Hawaii, which we attribute to heating by increased intrusive activity above the Galápagos plume and the construction of a highly porous volcanic platform. In addition, we find that the Galápagos hotspot is underlain by a high-velocity region whose thickness varies from 40 to 100 km. The tomographic images reveal that the upwelling mantle plume tilts northward (towards the nearby Galápagos Spreading Center) as it rises and then spreads laterally when it reaches the bottom the lid. The lid, which we attribute to residuum from melting, is thickest where it is farthest from the spreading center, suggesting that ridge processes may affect the generation and amount of thinning of the residuum layer. In addition, the thickness of the lid correlates well with the geographical pattern of geochemical anomalies of erupted lavas, suggesting that the lid may control the final depth of decompression melting. We conclude that many of the distinct characteristics of the Galápagos can be attributed to the interaction of the upwelling plume with the lid and the nearby ridge. We further suggest that the ridge affects the geometry of plume upwelling in the upper mantle and also the pattern of lateral spreading of the plume due to its effect on the thickness of the residuum layer. This dissertation includes previously published co-authored material.
Committee in charge: Dr. Douglas R. Toomey, Chairperson; Dr. Eugene Humphreys, Member; Dr. Emilie Hooft Toomey, Member; Dr. Paul Wallace, Member; Dr. John Conery, Outside Member

Over recent years geological, geochemical and geophysical surveys of mid-ocean ridges have revealed a significant degree of along-axis variability not only in seabed morphology, but also in crustal structure, particularly Numerous geophysical surveys of the Valu Fa Ridge, southwest Pacific, have mapped the extent of an axial mid-crustal reflector. This reflector has been interpreted as representing the top of a sill-like melt lens, comprising a high percentage of partial melt, lying at the top of a crustal magma chamber. In 1995, a controlled-source, wide-angle seismic dataset was acquired at the Valu Fa Ridge during RN Maurice Ewing cruise EW9512, to investigate the mid-deep crustal structure at this ridge, and particularly the crustal magma chamber associated with the melt lens beneath the ridge axis. The EW9512 acquisition geometry was primarily two-dimensional in design, and modelling of these 2-D profiles revealed the presence of an axial low velocity zone beneath the melt lens. This low velocity zone is thought to represent a region of crystal mush comprising a much lower percentage of partial melt than is present in the overlying melt lens. Similar structures have been modelled beneath a number of other mid-ocean ridges. The primary aim of this study was to build on this 2-D interpretation by taking advantage of three-dimensional ray coverage in the axial region in order to assess the along-axis continuity of the magmatic system, correlate this to any ridge segmentation apparent in the seabed morphology, and determine if ridge segmentation is related to the magma supply. The 3-D data were analysed using a tomographic inversion technique. The inversion results suggest that the axial low velocity zone may be segmented on a scale of 5-10 km, which correlates with the morphological segmentation of the ridge crest and is believed to reflect episodic magma supply with different ridge segments at different stages of a cycle of magmatic and amagmatic extension. However, three- dimensional ray coverage is not ideal owing to the dominantly 2-D acquisition geometry. Therefore a detailed assessment of data uncertainty and resolution was undertaken to enable a meaningful interpretation of the inversion results in terms of which features have a geological origin and which are artefacts of the inversion process. P-S mode converted arrivals arising from mid-crustal interfaces were also modelled in order to obtain improved geological constraints on the crustal structure than is possible from P-wave studies alone. This modelling indicates that the uppermost crust is pervaded by thin cracks. In addition, techniques were developed for modeling the polarisation of 5-wave arrivals with low signal strength. Application of these methods suggests that the thin cracks have a preferred orientation parallel to the ridge crest on-axis, and oblique to the ridge crest off-axis which is thought to reflect the pattern of southward propagation of the ridge system inferred from regional tectonic and bathymetric studies. Modelling of P-S mode converted arrivals arising from conversion at the top of the melt lens provided additional constraints on the properties of the melt lens. In conjunction with the 3-D tomographic results, this work suggests that the southernmost ridge segment in the study area has recently become magmatically active following a period of amagmatic extension suggested by its morphology, thus providing evidence for episodic melt supply at this ridge. As part of the suggestions for further work, a theoretical investigation of survey resolution was undertaken to test commonly adopted acquisition geometries with a view to optimising the design and cost-effectiveness of future 3-D controlled-source tomographic experiments.

xii, 98 p. : ill. (some col.)
The upper-crustal seismic-velocity structure of Newberry volcano, central Oregon, is imaged using P-wave travel time tomography. The inversion combines a densely-spaced seismic line collected in 2008 with two USGS seismic experiments from the 1980s. A high-velocity ring (7 km EW by 5 km NS) beneath the inner caldera faults suggests an intrusive ring complex 200 to 500 m thick. Within this ring shallow low velocities (<2 km depth) are interpreted as caldera fill and a subsided block. High velocities below 2 km depth could be intrusive complexes. There appears to be a low-velocity body at 3-6 km depth beneath the center of the volcano. This region is poorly resolved in the inversion because the ray paths bend around the low-velocity body. The 2008 data also recorded a secondary arrival that may be a delayed P-wave interacting with the low-velocity body.
Committee in charge: Emilie E.E. Hooft, Chairperson; Douglas R. Toomey, Member; Katharine V. Cashman, Member

The formation of oceanic lithosphere along ocean ridges, and the role that crustal magma chambers play in the accretionary process, continue to be fundamental issues in plate tectonics. To address these issues, a multi-receiver airgun/ocean bottom seismograph refraction line, designed to allow definition of lateral velocity and attenuation variations within the shallow crust, was shot across the Endeavour segment of the Juan de Fuca Ridge near 48° N, 129° W. A tomographic inversion procedure has been developed to invert the first arrival travel times and amplitudes from this profile for 2[sup D] velocity and attenuation structure. The inversion method is suited to multi-source, multi-receiver refraction profiles where source/receiver spacings are denser than for conventional profiles. The travel time-velocity inversion scheme is based on an iterative solution of the linearized problem and allows for determination of continuous velocity variations as well as geometry of subhorizontal interfaces. The iterative procedure requires a good initial estimate of the velocity model. In each iteration, two-point ray tracing is performed to construct a linear system relating travel time residuals to velocity perturbations. A damped least-squares algorithm is used to solve this system for a velocity perturbation which is used to update the current velocity estimate. Once the final velocity structure of the model has been determined, amplitudes can be inverted directly for attenuation. Tests to ascertain resolution of the method reveal horizontal smearing of the solution due to ray geometry, drop-off in resolution with depth, and the effects of source-receiver geometry and velocity structure on resolution. Parameter weighting is important in removing streaking effects (caused by inhomogeneous ray coverage) from the solution. For the purposes of ray tracing, the model is parameterized in terms of constant gradient (velocity and attenuation) cells, which allow use of analytic expressions for kinematic and dynamic ray properties, attenuation and inversion quantities. This parameterization causes scatter in the amplitudes calculated using zero-order asymptotic ray theory, a problem which is remedied by smoothing the velocity models before amplitude calculation. Application of this 2[sup D] tomographic inversion scheme to first arrival travel times and amplitudes for the cross-ridge refraction line produced a 4-layer model for the shallow crust. Layer 1 is 250 — 650 m thick, with v₁ = 2.5 km/s and [Nabla, sub z]v₁ = 0.5 s⁻¹. Layer 2 is ~800 m thick, v₂ = 4.8 km/s and [Nabla, sub z]v₂ — 1.0 s⁻¹. Layer 1 and layer 2 likely represent the sequence of extrusives whereas layer 3 (~800 m thick, v₃=5.8 km/s, [Nabla, sub z]v₃=0.5 s-1) and layer 4 (v₄=6.3 km/s, [Nabla, sub z]v₄=0.3 s⁻¹) are associated with the dike complex and massive gabbro sequence, respectively. An abrupt velocity transition between layer 1 and layer 2 may be a metamorphic front within the pillow basalts. A low velocity-high attenuation anomaly (velocities decreased by < 0.4 km/s and Q ~20-100), which is interpreted as a zone of increased fracture porosity and/or permeability associated with axial hydrothermal circulation, exists beneath the ridge in layer 2 and upper layer 3. Smaller low velocity-attenuative zones in layer 2, located 8 km to either side of the ridge may be loci of off-axis hydrothermal circulation. No evidence is found for the existence of a crustal magma chamber in the depth range of 1.5 — 3.0 km below the seafloor. Tests indicate that a 1 X 1 km zone of partial melt represents the minimum dimension of such a feature that would be clearly detected by this refraction experiment. These results suggest that Endeavour Ridge may be experiencing a period of diminished magma supply with the magma chamber reduced or eliminated by hydrothermal circulation. Asymmetry of the velocity anomalies observed in layer 3 and layer 4 suggest that crustal temperatures are elevated by 125 — 200° C beneath the ridge and to the east relative to temperatures west of the ridge, indicating that a deep crustal or upper mantle melting anomaly may exist east of the ridge.
Science, Faculty of
Earth, Ocean and Atmospheric Sciences, Department of
Graduate

The neotectonic evolution of the eastern Mediterranean is intimately tied to interactions between the underthrusting/subducting slab along the southern margin of Anatolia and the overriding plate. The lateral variations in the subduction zone can be viewed as a temporal analogue of the transition between continuous subduction and subduction termination by continent-continent collision. By investigating the lateral variations along this subduction zone in the overriding plate, we can gain insight into the processes that precede continent collision. This dissertation summarizes the results of three studies that focus on different parts of the subduction margin: 1) In the west, where the development of a slab tear represents the transition between continuous and enigmatic subduction, 2) In the east, where continent-continent collision between the Arabian and Eurasian Plate is leading to the development of the third largest orogenic plateau on earth after complete slab detachment, and 3) In central Anatolia, where the subducting slab is thought to be in the processes of breaking up, which is affecting the flow of mantle material leading to volcanism and uplift along the margin. In the first study, we interpret that variations in the composition of material in the downgoing plate (i.e. a change from the subduction of oceanic material to continental material) may have led to the development of a slab tear in the eastern Aegean. This underthrusting, buoyant continental fragment is controlling overriding plate deformation, separating the highly extensional strains of western Anatolia from the much lower extensional strains of central Anatolia. Based on intermediate depth seismicity, it appears that the oceanic portion of the slab is still attached to this underthrusting continental fragment. In the second study, we interpret that the introduction of continental lithosphere into the north-dipping subduction zone at the Arabian-Eurasian margin led to the rollback and eventual detachment of the downgoing oceanic lithosphere attached to the Arabian Plate. After detachment, high rates of exhumation in the overriding plate are recorded due to the removal of the oceanic lithosphere and accompanying rebound of the Arabian continental lithosphere. In the third study, we image a transitional stage between the complete slab breakoff of the second study and the continuous subduction slab of the first study. We interpret that trench-perpendicular volcanism and ~2 km of uplift of flat-lying carbonate rocks along the southern margin of Turkey can be attributed to the rollback and ongoing segmentation of the downgoing slab as attenuated continental material is introduced into the subduction zone. Combining these three studies allows us to understand the terminal processes of a long-lived subduction zone as continental material is introduced.

Controlled source seismic methods were employed in this study to investigate the reflectivity and velocity structure of two Hercynian orogens – the Uralides and Variscides. Conventional common depth point (CDP) sections from five reflection seismic campaigns and a velocity model obtained from tomographic inversion of wide-angle observations were the main datasets studied from the Middle Urals. These were complemented with the near-vertical seismic sections and velocity models from the Southern Urals. In the Variscides, conventional CDP processing, along with non-standard processing and synthetic data modeling, were used to obtain and interpret reflection seismic images of the Southwestern Iberian crust. Although, the Uralian and Variscan belts were formed in Late Paleozoic time in apparently similar plate collisional settings, a comparison of the seismic results show that the crust of these two orogens looks quite different at depth. In the Urals, collision of Baltica with Asian terranes (Siberia and Kazakhstan) resulted in a highly diversely reflective crust of 40-45 km thickness. The axial zone of the orogen is characterized by a high velocity crustal root of diffuse reflectivity and an imbricated Moho, with a crustal thickness reaching 55-60 km. The Moho discontinuity is marked by a sharp decrease in reflectivity and is well imaged in most locations except in the crustal root zone. The Southwestern Iberian Variscan crust is 30-35 km thick and is characterized by a highly reflective two-layered structure that resulted from collision of Luarussia and Gondwana, including terranes in-between them. This type of crustal structure is very similar to those imaged in other regions of the Variscan belt in the Europe. The Moho discontinuity is flat and appears to be the deepest reflection. This thesis compares the deep structure of the two orogens and interprets mountain building processes related to late Paleozoic plate movements.

For the past three decades, deep crustal studies of the British Isles have been restricted to the interpretation of 2-D seismic reflection and refraction profiles, mostly acquired offshore. During this period, the British Geological Survey (BGS) seismic monitoring network has developed to an unrivalled density for a region of low intraplate seismicity. In an average year, the modern network records approximately 40 earthquakes in the crust beneath the British Isles with local magnitudes > 2. Statistical tests show the modern and historical pattern is not random. Understanding of the tectonic processes behind the pattern are hindered by the sparseness of onshore deep crustal studies where the majority of earthquakes are concentrated. For the first time local earthquake tomography, a method more commonly applied to tectonically active regions, is used to produce high resolution 3-D images of seismic P-wave velocity (Vp) and the P- to S-wave velocity ratio (Vp/Vs) in the crust beneath England, Wales and the Irish Sea. To account for low seismicity, over 1,000 earthquakes are utilised from the past 25 years of monitoring. The existing BGS digital catalogue is enhanced by a two-fold increase in seismic arrival time picks, significantly reducing earthquake location errors in the input dataset. The tomographic models establish a strong and previously undemonstrated link between Palaeocene magmatism and more widespread earlier phases of Caledonian magmatism. A regional Vp anomaly (> 7.2 km/s) in the lower crust centred on the East Irish Sea Basin is inferred as Palaeocene magmatic underplate with seismicity concentrated around its eastern and southern margins. In the mid- and lower-crust earthquake clusters are evident around the edges of local Vp/Vs anomalies (> 1.80), most significantly beneath the Ordovician volcanic centre in Snowdonia. The models are supplemented by the inversion of 185 independently determined focal mechanisms to consider the influence of local variations in far-field intraplate stresses alongside lithostatic stress from overburden pressure.

2012/2013
I terremoti costituiscono un disastro naturale ricorrente su tutto il territorio italiano e per questo sono estremamente importanti interventi mirati e rapidi di protezione civile. La rapidità di questi interventi dipende dalla produzione di localizzazioni veloci e possibilmente in tempo reale degli eventi sismici. La precisione delle localizzazioni, inoltre, è necessaria per identificare le faglie sismogenetiche. Per questi due aspetti, è necessario un miglioramento dei sistemi di monitoraggio esistenti in modo da poter accrescere la qualità delle localizzazioni automatiche in tempo reale. Lo scopo di questo studio è la scrittura di una procedura che localizza accuratamente eventi sismici in tempo reale. La qualità delle localizzazioni è fortemente dipendente dalla corretta determinazione delle fasi P ed S. A volte è difficile riconoscere il corretto arrivo di una fase, poiché il segnale sismico può essere di difficile lettura per differenti motivi, come, ad esempio, la complessità del meccanismo della faglia generatrice e la presenza di rumore sia naturale che artificiale. Per questo motivo abbiamo studiato, analizzato e comparato differenti metodi per la rilevazione delle fasi e per la localizzazione degli eventi sismici. Gli algoritmi di rilevazione delle fasi che sono stati valutati sono lo Short Time Average su Long Time Average ratio (STA/LTA) e la funzione di Akaike Information Criterion (AIC). Il primo di questi è una tecnica comune usata per distinguere il segnale sismico dal rumore. E’ basato sul calcolo continuo di due valori medi dell’ampiezza assoluta di un segnale sismico in due finestre di tempo di differente lunghezza: media sull’intervallo breve (STA) e media sull’intervallo lungo (LTA). Il rapporto di queste due medie (STA/LTA) viene comparato ad un valore di soglia. Quando questo rapporto è maggiore della soglia, viene rilevata una fase nel segnale sismico analizzato. Il settaggio di questo sistema dipende dalla scelta dei parametri, questo prouce instabilità. La funzione di AIC è una metodologia sofisticata e precisa [Akaike and Hirotugu, 1974], basata sul classico metodo della massima verosimiglianza. La sua applicazione più comune consiste nella selezione tra pi` modelli: la stima della massima verosimiglianza dei parametri del modello da il minimo della funzione AIC. Questo metodo è strettamente correlato alla scelta della finestra di tempo nella quale applicare la funzione. Per questo motivo è necessaria una combinazione di più tecniche in modo da poter scegliere automaticamente la finestra corretta. In un segnale sismico il minimo della funzione AIC identifica l’arrivo delle onde P o delle onde S. Questa funzione è utilizzata nella procedura dell’AutoPicker [Turino et al., 2010]. Una volta identificate le fasi, è necessario elaborarle in modo da poter localizzare eventi sismici. In Antelope la procedura di localizzazione è chiamata orbassoc. Questa metodologia legge le fasi rilevate tramite il metodo STA/LTA e cerca di produrre una localizzazione dell’evento sulle tre possibili griglie: telesismica, regionale e locale. La soluzione, che produce tempi teorici di percorrenza per ogni stazione, che si accordano maggiormente con le osservazioni, viene considerata la migliore. Nell’AutoPicker l’algoritmo di localizzazione è Hypoellipse [Lahr, 1979], nel quale i tempi di percorrenza sono stimati utilizzando una struttura a strati piani paralleli e gli ipocentri sono calcolati utilizzando il metodo di Geiger [Geiger, 1912]. In questo lavoro abbiamo utilizzato metodologie per la localizzazione diverse da quelle assolute come Hypoellipse. L’HypoDD [Waldhauser and Ellsworth, 2000] è un algoritmo relativo, ovvero le localizzazioni vengono calcolate in riferimento alla localizzazione di un evento principale o dal sito di una stazione. Questo metodo può essere applicato solo nel caso in cui la distanza ipocentrale tra i due terremoti è piccola comparata alla distanza evento-stazione e alle eterogeneità laterali del campo delle velocità. In questi casi il percorso del raggio tra le due sorgenti e una stazione comune sono simili per gran parte del percorso del raggio. Per testare le prestazioni dell’AutoPicker, lo abbiamo applicato ad un database di 250 eventi registrati nell’area di contatto tra le Alpi e le Dinaridi nell’anno 2011 dalla rete C3ERN - the Central Eastern European Earthquake Reasearch Network [Dipartimento di Matematica e Geoscienze (DMG), Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Agencija RS za okolje (ARSO) e Zentralanstalt fr Meteorologie und Geodynamik (ZAMG)]. L’algoritmo automatico proposto è risultato essere un utile strumento per l’assegnazione automatica degli arrivi delle onde P ed S. Questo risultato incoraggiante ci ha permesso di procedere nel confronto tra questa nuova metodologia e Antelope, utilizzato da noi quotidianamente in tempo reale per rilevare fasi e localizzare eventi. La complessità del contesto tettonico influenza il percorso dei raggi e conseguentemente la localizzazione degli eventi. In regioni dove sono presenti molte strutture sismogenetiche, una localizzazione precisa della sequenza sismica è essenziale, in modo da capire quale è la faglia generatrice. In questi casi l’uso di modelli 1-D potrebbe non essere sufficiente, mentre un modello 3-D potrebbe descrivere al meglio l’area interessata. La tomografia dei primi arrivi è una tecnica comune per ottenere un modello tridimensionale dalla localizzazione degli eventi. In questo studio abbiamo utilizzato una tomografia di eventi locali (Local Earthquake Tomography, LET) [Aki, 1982]. La tomografia dei primi arrivi e la localizzazione 3-D degli eventi sono state eseguite, rispettivamente, utilizzando il Computer Aided Tomography per modelli 3D (Cat3D) [Cat3D user manual, 2008] e il Non Linear Location (NonLinLoc) [Lomax et al., 2000] attraverso una procedura iterativa. Il Cat3D viene utilizzato solitamente in sismica attiva, mentre in questo studio è stato applicato ad un caso sismologico. La principale differenza tra la sismica attiva e la sismologia sono le incertezze nel sistema tomografico. Nella sismica attiva la localizzazione della sorgente è ben definita mentre nella sismologia è una variabile con incertezza elevata che si propaga nella stima del percorso del raggio e dei tempi di percorrenza. Per risolvere questo problema, abbiamo utilizzato una procedura iterativa composta dalla tomografia dei primi arrivi e dalla rilocalizzazione degli eventi con il modello 3-D risultante. Dopo il verificarsi della sequenza sismica emiliana nel Maggio-Giugno 2012, abbiamo deciso di analizzarla come interessante caso di studio. La sequenza sismica è iniziata il 20 Maggio (02:03:53 UTC), con un terremoto di Ml 5.9 [Scognamiglio et al., 2012]. Questa sequenza è composta da migliaia di eventi, sei dei quali con Ml maggiore di 5.0, tra cui un evento di magnitudo locale 5.8, il 29 Maggio (07:00:03 UTC). Su questi eventi abbiamo testato le prestazioni dell’AutoPicker e di Antelope. Per fare ciò abbiamo rilevato manualmente le fasi e localizzato alcuni degli eventi maggiori della sequenza sismica. Questi eventi sono caratterizzati da fasi P, ma in particolar modo fasi S, difficili da rilevare, probabilmente a causa del complesso meccanismo di faglia. Inoltre la complessità del sistema tettonico assieme all’incertezza della profondità focale rendono problematiche le localizzazioni degli eventi. La sequenza sismica emiliana ha interessato un’area di 50 km con andamento E-W localizzata nell’angolo sud della Pianura Padana, interessando il settore centrale dell’arco di Ferrara appartenente al sistema esterno della cintura degli Appennini Settentrionali. L’arco di Ferrara è composto da due sistemi: le pieghe di Ferrara nel nordest e la piega di Mirandola localizzata nella parte più interna a sudovest [Govoni et al., 2014]. Abbiamo elaborato gli arrivi P ed S in modo da poter localizzare la sequenza sismica utilizzando differenti modelli di velocità trovati in letteratura: Bragato et al. [2011], Ciaccio and Chiarabba [2002],Costa et al. [1992], Iside, Zollo et al. [1995], Malagnini et al. [2012], Massa [2012] e quattro modelli geologici proposti da Lavecchia et al. [in prep.] L’idea è di produrre un insieme di localizzazioni di eventi clusterizzati con residui minimi, in modo da poter capire quale è la faglia generatrice. Questo lavoro è stato svolto in collaborazione con l'Università di Chieti e il Dipartimento di Protezione Civile (DPC). Dalla distribuzione ipocentrale delle soluzioni, sembra che l'arco di Mirandola non sia coinvolto nella sequenza sismica, mentre i segmenti della parte interna e centrale del sistema di sovrascorrimento di Ferrara sembrano essere stati attivati dalle sequenze sismiche del 29 e del 20 Maggio, rispettivamente. La complessità dell'area interessata dalla sequenza sismica dell'Emilia, richiede il calcolo di modelli tridimensionali di velocità in modo da poter localizzare più precisamente gli eventi. Come già detto, abbiamo elaborato una procedura iterativa: tomografia dei primi arrivi e localizzazioni 3-D degli eventi, attraverso l'uso rispettivamente del Cat3D e del NonLinLoc, in collaborazione con l'OGS. La sequenza sismica copre solo una piccola parte della regione (30x30 km^2 di larghezza e 20 km di profondità), per questo l'area investigata si limiterà alla porzione superiore della crosta. Come modelli iniziali di velocità abbiamo scelto: Costa et al, 1992; Massa et al. 2013 e NewModel1 (LaVecchia et al., in prep., i quali avevano errori verticali inferiori al chilometro nello studio precedente. Il miglior modello iniziale sembra essere quello di Massa et al. (2013), il quale mostra valori di rms bassi rispetti alle altre soluzioni. I tre modelli tridimensionali di velocità per le onde P risultanti mostrano caratteristiche comuni: uno strato superficiale a bassa velocità e uno strato spesso (5-20 km in profondità) a 5.5km/s. I risultati tomografici per i modelli Vs presentano un comune strato superficiale a bassa velocità e uno strato caratterizzato da valori di velocità per le onde S di 3.0 km/s. Le tre serie di soluzioni, dei differenti modelli di velocità, sono comparabili all'interno dell'intervallo di errore, anche in termini di qualità. Le localizzazioni per la scossa principale del 20 maggio 2012 sono sparpagliate rispetto a quelle della seconda scossa principale del 29 maggio. Una possibile causa potrebbe essere l'installazione delle stazioni temporanee nel campo vicino della sequenza sismica dopo il 20 maggio 2012. Per l'evento del 29 maggio, infatti, si hanno molte più registrazioni che per il primo evento del 20 e tutte in campo vicino. Le localizzazioni degli eventi ottenute da modelli tomografici tridimensionali sono meno disperse di quelle ottenute con modelli unidimensionali, anche se le localizzazioni dei due eventi principali sono simili. In profondità le due serie di soluzioni non differiscono in modo significativo. Per migliorare la qualità della procedura di localizzazione nel nostro centro di raccolta dati, vorremo installare una procedura automatica sia rapida sia precisa. Per raggiungere questo risultato abbiamo comparato l'AutoPicker con Antelope sulla sequenza sismica dell'Emilia. Questo confronto è di fondamentale importanza per comprendere quale dei due algoritmi rileva fasi e/o localizza eventi in modo più preciso. Il nostro scopo, infatti, è quello di unire ed implementare queste due tecniche in modo da ottenere un miglior rilevatore di fasi e localizzatore. I risultati di questo confronto ci hanno portato a concludere che l'AutoPicker trova più fasi e con maggior precisione rispetto ad Antelope, sia per le fasi P che per le fasi S. Nonostante ciò il processo di associazione delle fasi in Antelope è in grado di correggere gli errori delle fasi e trovare la corretta localizzazione dell'evento. Questo ci ha suggerito di implementare l'algoritmo dell'AutoPicker nella procedura di Anteope, in modo tale che l' AutoPicker definisca gli arrivi P ed S e Antelope li associ e localizzi gli eventi. Con il miglioramento delle reti sismiche e la possibilità di raccogliere enormi quantitativi di dati, è necessario produrre enormi database, in modo da poter avere un rapido accesso ad essi e di poterli rielaborare in tempo reale o quasi reale. Per questi enormi database la rilevazione manuale delle fasi è un lavoro oneroso, che richiede tanto tempo. La possibilità di avere uno strumento che rilevi automaticamente fasi di ottima qualità, che producano risultati similari a quelli ottenuti dall'inversione tomografica utilizzando le fasi rilevate manualmente, è sicuramente conveniente ed utile. Per questa ragione abbiamo confrontato due differenti tomografie dei primi arrivi, prodotte con la stessa tecnica dell'analisi precedente, che differiscono solo per i dati di partenza: la prima è stata ottenuta dalle fasi rilevate manualmente, la seconda dalle fasi rilevate automaticamente con l'AutoPicker per la sequenza sismica dell'Emilia. I risultati ottenuti indicano un incremento del valore medio dell' rms sia nelle localizzazioni sia nella tomografia per le fasi automatiche. Nonostante questo i modelli tridimensionali ottenuti ( Vp, Vs and Vp/Vs) sono comparabili. Pertanto sembra che gli errori nelle localizzazioni non influenzino i risultati tomografici ma inficino la precisione del sistema tomografico stesso. Quindi per database contenenti enormi quantità di dati è possibile utilizzare le fasi automatiche come dati di partenza, ottenendo risultati comparabili a quelli ottenuti con le fasi manuali.
Earthquakes constitute a recurring natural disaster all over the Italian territory, and therefore civil defence focused interventions are extremely important. The rapidity of such interventions strongly depend on the production of fast and possibly real-time locations of the seismic events. The precise location of events is also needed to identify seismogenic faults. For these two aspects, an upgrade of the existing monitoring systems is fundamental to improve the automatic locations quality in a quasi real-time mode. The main purpose of this study is the production of a routine that will accurately locate seismic event in real-time. The quality of the locations strongly depends on the correct determination of the P- and S- phases. Sometimes it is hard to recognize the correct onset of a phase, since the signal can be blurred by various causes, such as, e.g., the complexity of the generating fault mechanism and the presence of natural or man-made noise. For this reason we have studied, analyzed and compared different phase picking and location methods. The picking algorithms that were evaluated are the Short Time Average over Long Time Average ratio (STA/LTA) and the Akaike Information Criterion (AIC) function. The first one is a common technique used to distinguish the seismic signal from noise. It is based on the continuous calculation of the average values of the absolute amplitude of a seismic signal in two moving-time windows with different lengths: the short-time average and the long-time average. The STA/LTA ratio is compared with a threshold value. When the ratio is larger than this threshold, the onset of a seismic signal is detected. The main disadvantage of this method is its instability, due to the parameters choice: a too long STA window could cause the non-detection of local events, whereas a too short STA window could cause the detection of man-made seismic noise. A high STA/LTA threshold records less events than the ones those have occurred, but false triggers are eliminated. If this value is chosen to be lower, more events will be detected, but more frequent false triggers could be recorded. This algorithm is part of the Antelope (BRRT, Boulder) detection procedure, used in this study. The AIC function is a precise and sophisticated methodology, being a revision of the classical maximum likelihood estimation procedure (Akaike, 1974). The AIC function is designed for statistical identification of model characteristics. Its most classical application consists in the selection of the best among several competing models; the maximum likelihood estimate of the model parameters gives the minimum of AIC function. It is strictly correlated to the correct choice of the time window in which apply the function, so it is necessary combined with other techniques, in order to automatically choose a correct window. This dependence on other methods, makes the application of the AIC function to detect phases, a complex methodology, which can be affected by errors in the parameter choices. The AIC function is used in the AutoPicker procedure (Turino et al., 2012). In a seismic signal the minimum of the AIC function identifies the P- or S- onset. In this automatic phase picker the time window in which to apply the function, in the case of P phases, is chosen by a combination of a band-pass filter and an envelope time function, used as “energy” detector to select the event in the waveform; for the S phases, the selection of the window is guided by a preliminary location of the P- phases. Once the P- and S- phases are identified, it is necessary to elaborate them in order to locate the seismic event. In Antelope the location procedure is called orbassoc. This methodology reads the pickings, determined through the use of the STA/LTA technique, and tries to produce an event location over three possible grids: teleseismic, regional and local. The solution that produces the minimum travel time residuals set (differences between synthetic travel times and observed travel times) is considered as the best one. In the AutoPicker the location algorithm is Hypoellipse (Lahr, 1979), in which the travel-times are estimated from a horizontally-layered velocity-structure and the hypocenter is calculated using Geiger's method (Geiger, 1912) to minimize the root mean square (rms) of the travel time residuals. In order to improve the location quality we have used in this work various location methodologies with respect to the absolute ones, such as Hypoellipse. The HypoDD (Waldhauser et al., 2000) is a relative algorithm, the locations depend either on the location of a master event or on a station site. This method can be applied only in the case when the hypocentral separation between two earthquakes is small compared to the event-station distance and the scale length of the velocity heterogeneities. In such cases the ray paths between the source region and a common station are similar along almost the entire ray path. In order to test the performances of the AutoPicker, we have applied it to a database of 250 events recorded in the year 2011 by the C3ERN - the Central Eastern European Earthquake Reasearch Network [Department of Mathematics and Geosciences (DMG), Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Agencija RS za okolje (ARSO) and Zentralanstalt für Meteorologie und Geodynamik (ZAMG)] – at the Alps-Dinarides contact. The proposed automatic picker appears to be a useful tool for assigning automatically onset P and S times to detected seismic signals for the purpose of rapid hypocenter calculations. These encouraging results have allowed us to proceed comparing this new picking methodology to another one, tested and used daily and in real-time by us to detect and locate events, the Antelope software. The complexity of the tectonic environment influences ray tracing and consequently the event locations. In regions where many seismogenic structures are present, a precise location of a seismic sequence is essential, in order to understand which fault is the generating one. In such cases the use of a 1-D velocity model might not be sufficient, so a 3-D velocity model is a better solution to describe the studied area. The travel-time tomography is a common technique to obtain a 3-D velocity model, from event locations. In this study we have chosen a local earthquake tomography (LET) (Aki, 1982). The travel time tomography and the 3-D event location are performed, respectively, using the Computer Aided Tomography for 3D models (Cat3D) software (Cat3D manual, 2008) and the Non Linear Location (NonLinLoc) software (Lomax et al., 2000) through an iterative procedure. The Cat3D is basically used in active seismics, but in this study it is applied to a seismological case. The main difference between active seismics and seismology are the unknowns in the tomographic system. In seismology the source location is an unknown parameter with a high uncertainty, while in active seismics the source locations are well defined. In this study, the introduction of the source location in the tomographic system, introduces uncertainties in obth the ray tracing and travel-times estimation. In order to solve this uncertainty, we used an iterative procedure composed by the application of tomography and the event location in resulting 3-D velocity model. After the occurrence of the Emilia seismic sequence in May-June 2012, we have decided to investigate it as an interesting study case. The sequence started on May 20 (02:03:53 UTC), with a ML 5.9 earthquake, preceded by a M_L 4.1 foreshock, three hours earlier (Scognamiglio et al., 2012). Theaftershock sequence comprised thousands of earthquakes, six of them with M_L ≥ 5.0. Among these, a M_L 5.8 earthquake, on May 29 (07:00:03 UTC), caused probably more damages than the first shock. Through the study of this seismic sequence we have tested the performances of the automatic picking algorithms. In order to do that, we have manually picked and located some of the major events of this seismic sequence. These events are characterized by P- and especially S-phases, which are really difficult to detect, probably because the fault system of the Emilia earthquake area is complex. Moreover, the complexity of the tectonic environment along with the focal depth uncertainty make the event locations problematic, because it is not always easy to assess which fault has moved. The Emilia sequence occurred in the central, roughly E-W trending, sector of the Ferrara arc belonging to the external fold-and-thrust system of the Northern Apennines belt. The Ferrara arc is structured into two major fold-and-thrust systems: the Ferrara system in the northeast and the Mirandola system located in a more internal position to the southwest (Govoni et al., 2014). We have processed the P- and S- onsets in order to locate the seismic sequence using different velocity models found in literature: Bragato et al. (2011), Ciaccio et al. (2002), Costa et al. (1992), “Iside”, Zollo et al. (1995), Malagnini et al. (2012), Massa (Rapporto DPC-INGV S1-2013) and four geological models proposed by Lavecchia et al. (in prep). The idea is to produce a set of clustered event locations with the lowest residuals, in order to understand which is the generating fault in the complex system of faults. This work is being performed in collaboration with Università di Chieti and Department of Civil Defence (DPC). From the hypocentral distribution, it seems that the Mirandola thrust was not involved during the Emilia sequence, whereas the internal and middle segments of the Ferrara thrust systems were activated by 29 and 20 May seismic sequences, respectively. The complexity of the seismic sequence area in Emilia requires the calculation of a tridimensional velocity model in order to locate more precisely the events. As already said, we elaborated an iterative procedure: travel-time tomography and 3-D event locations, through the use of the Cat3D and NonLinLoc softwares, in collaboration with OGS. This is done to minimize the uncertainties introduced in the tomographic system by the unknown source locations. Since the seismic sequence covers only a small part of this region (about 30x30km^2 wide and 0-20 km deep), the investigated area will be limited to its upper crustal part. As initial velocity models, we have chosen those ones: Costa et al, 1992; Massa et al. 2013 and NewModel1 (LaVecchia et al., in prep.) that have vertical errors lower than one km. The best velocity model is the one, obtained using as initial model the Massa et al. (2013), which shows rms values lower than the others. The three resulting 3-D Vp velocity models shows similar characteristcs: a surface layer (0 – 5 km) of low Vp velocity, about 1,8 km/s, and a thick layer (5 – 20 km) of 5.5 km/s. The tomographic results for Vs velocity model present a common shallow layer (0 - 3 km) of low velocity (about 1 km/s) and a thick layer (3 - 13 km) characterized by a Vs velocity value of about 3.0 km/s. The three set of solutions, from the different velocity models, are comparable in the errors range. The locations for the main-shock of the 20th of May, 2012 are more scattered respect the solutions for the 29th's. A possible reason could be the installations of temporary stations in the near field of the sequence after the 20th of May, 2012. For the 29th event, in fact, we have more waveforms than for the previous main-shock, and all of them in the near field. We calculated the rms for each event in order to discriminate a velocity model with respect to another from the quality of the locations. We obtained three similar rms values trends, so we were not able to choose a best velocity model. The events locations from 3-D tomographic models are less scattered than those one computed from the 1-D ones; otherwise the locations of the two main-shock events seem to be quite similar. In depth the two set of solutions do not differ in a significative way. To improve the quality of the location procedure in our datacenter, we would like to install a precise and rapid automatic procedure. Therefore, we have compared the AutoPicker method with a more tested and solid one, the Antelope picking method, on the Emilia seismic sequence of data, using as reference pickings and locations the manual ones. This comparison is of fundamental importance which one of the two algorithms better detects phases and/or locates events. Our aim is, in fact, to merge and implement these two techniques to obtain a better detector and locator. AutoPicker finds more and preciser phases than Antelope both P- and mainly S-phases. Despite that the associator process in Antelope, is able to correctly associate the detections and to find the correct location. The obtained results suggest us to implement the AutoPicker algorithm in the Antelope procedure in order to use the AutoPicker to define P- and S-onset and Antelope to associate them and locate the events. With the improvement of seismic networks and the possibility to store huge amounts of data, it is necessary to produce big databases, in order to have a rapid access to the data and to re-elaborate them in real time o quasi real time mode. For big databases, the manual picking is an onerous work, requiring a lot of time. The possibility to have a good-quality automatic tool for phase recognition and picking, which produces similar results to those obtained from the tomographic inversion by using manual phases picking, is certainly convenient and useful. For this reason, we have compared two different travel time tomographic inversions made with the same technique of the previous analysis, differing only in the input phase files: the first one obtained from manual pickings, the second one from the automatic AutoPicker pickings of the Emilia sequence. The obtained results indicate an increase of the average rms both on the locations and on the tomography. Despite that, the tridimensional velocity models (Vp, Vs and Vp/Vs) are comparable, therefore, it seems that the location errors do not influence the tomographic results but the precision of the tomographic system. So for a large database it is possible to use automatic phases as input in a travel-time tomography, obtaining similar results as those obtained using manually picked phases.
XXVI Ciclo
1985

Continental rifting ultimately creates a deep accommodation space for sediment. When a major river flows into a late-stage rift, thick deltaic sediment can change the thermal regime and alter the mechanisms of extension and continental breakup. The Salton Trough, the northernmost rift segment of the Gulf of California plate boundary, has experienced the same extension as the rest of the Gulf, but is filled to sea level by sediment from the Colorado River. Unlike the southern Gulf, seafloor spreading has not initiated. Instead, seismicity, high heat flow, and minor volcanoes attest to ongoing rifting of thin, transitional crust. Recently acquired controlled-source seismic refraction and wide-angle reflection data in the Salton Trough provide constraints upon crustal architecture and active rift processes. The crust in the central Salton Trough is only 17-18 km thick, with a strongly layered but relatively one-dimensional structure for ~100 km in the direction of plate motion. The upper crust includes 2-3 km of Colorado River sediment. The basement below the sediment is interpreted to be similar sediment metamorphosed by the high heat flow and geothermal activity. Meta-sedimentary rock extends to at least 7-8 km depth. A 4-5 km thick layer in the middle crust is either additional meta-sedimentary rock or stretched pre-existing continental crust. The lowermost 4-5 km of the crust is rift-related mafic magmatic material underplated from partial melting in the hot upper mantle. North American lithosphere in the Salton Trough has been almost or completely rifted apart. The gap has been filled by ~100 km of new transitional crust created by magmatism from below and sedimentation from above. These processes create strong lithologic, thermal, and rheologic layering. Brittle extension occurs within new meta-sedimentary rock. The lower crust, in comparison, stretches by ductile flow and magmatism is not localized. This seismic interpretation is also supported by 1D thermal and rheological modeling. In this passive rift driven by far-field extensional stresses, rapid sedimentation keeps the crust thick and ductile, which delays final breakup of the crust and the initiation of seafloor spreading.
Ph. D.

Cette thèse a été conduite dans le cadre du programme de coopération de recherche Algéro-française SPIRAL (Sismique Profonde et Investigations Régionales du Nord de l’Algérie). Ce projet vise à étudier la structure profonde de la marge algérienne par une approche combinée des techniques sismiques ; grand-angle et multi-canal. Le domaine couvert par la présente étude se concentre dans la région de Jijel dans la marge algérienne orientale. L’objectif principal de notre thèse est d'améliorer en profondeur l'imagerie de la marge algérienne en utilisant une combinaison de données sismiques grand-angle (OBS, sismomètres de fond de l'océan) et multi-canal (MCS). Le but de cette thèse est d'apporter de nouvelles connaissances pour répondre à quelques questions sur la nature de la croûte terrestre, la zone de transition continentale-océanique, la présence du sel messénien, sa distribution et sa relation entre les formations sédimentaires superficielles et les structures crustales. Dans cette étude, notre approche est une inversion jointe des enregistrements grand-angle et des données sismiques multi-canal. Nous avons conduit une série de tomographie des premières arrivées, une inversion jointe des arrivées réfractées et réfléchies ainsi qu’une modélisation gravimétrique. Etant donné que la solution du problème inverse n’est pas unique, deux programmes de tomographie ont été utilisés sur les mêmes données pour la même région d’étude à savoir : FAST (First Arrival Seismic Tomography) et Tomo2D. La tomographie a été suivie par une inversion jointe des arrivées réfractées et réfléchies suivant une approche basée sur la combinaison de la migration en profondeur « Kirchhoff » avant sommation (PSDM) des données de sismique réflexion multi-canal (MCS) et la modélisation directe des enregistrements grand-angle sur le fonds marin (OBS). Afin de vérifier la consistance du modèle de la vitesse avec les données gravimétriques, l’anomalie à l'air libre a été modélisée. Les résultats de l’imagerie conduite dans ce travail montrent la structure de la marge, la croûte continentale, la zone de transition continent-océan et la croûte océanique de la Méditerranée. La structure du modèle confirme les études antérieures basées sur des données bathymétriques, gravimétriques et magnétiques. Cette structure montre essentiellement : - un plateau continental étroit et pente continentale une très raide.- l’Expulsion du sel vers le nord impliquant la formation de diapirs au-dessus du flanc nord du bassin (plaine abyssale).- L’approfondissement et l’épaississement des séquences sédimentaires (bassin sédimentaire) près de la marge algérienne. Le modèle de vitesses obtenu et l’épaisseur des différentes unités structurales formant ce modèle apportent des arguments quantitatifs pour enrichir la connaissance de cette partie de la Méditerranée occidentale. Les couches sédimentaires dans le bassin montrent des vitesses sismiques allant de 1,9 km / s à 3,8 km / s. Les formations messéniennes ont été modélisées en utilisant une vitesse située entre 3,7 km / s à 3,8 km / s. La croûte continentale s’amincit sur une bande étroite de la marge dont la distance est d'environ 15 km. La vitesse de la croûte océanique dans cette région présente deux couches distinctes : l’une caractérisée par des vitesses variant de 4,7 km / s à 6.1 et l’autre de 6.2 à 7.1 km / s. La vitesse du manteau supérieur quant à elle a été modélisée par 7,9 km / s
This thesis has been conducted within the framework of the Algerian-French research cooperation program SPIRAL (Sismique Profonde et Investigations Régionales du Nord de l’Algérie). This project aims to study the deep structure of the Algerian margin. The area covered by this study focuses in the region of Jijel in eastern Algerian margin.The main objective of our thesis is to improve depth imaging of the Algerian margin using a combined approach of seismic techniques; wide-angle and multi- channel seismic data. The purpose of this thesis is to bring new knowledge to answer some questions about the nature of the crust, the area of continental -oceanic transition, the presence of Messinian salt, its distribution and relationship between surface sedimentary formations and crustal structures.This study presents the results of a deep seismic survey across the north Algerian margin, based on the combination of 2D multi-channel and wide-angle seismic data simultaneously recorded by 41 ocean bottom seismometers deployed along a North-South line extending 180 km off Jijel into the Algerian offshore basin, and 25 land stations deployed along a 100 km-long line, cutting through the Lesser Kabylia and the Tellian thrust-belt.In this study, our approach is a joint inversion of wide-angle seismic recordings (OBS, ocean bottom seismometers) and multi- channel seismic data (MCS). We conducted a series of first arrivals tomography, a joint inversion of reflected and refracted arrivals and gravity modelling. Since the solution of the inverse problem is not unique, two tomography programs were applied using the same data for the same study area; FAST (First Arrival Seismic Tomography) and Tomo2D. Tomography was followed by a joint inversion of reflected and refracted arrivals following an approach based on the combination of Kirchhoff prestack depth migration (PSDM) for MCS data and forward modelling of OBS. To check the consistency of the velocity model with gravity data, the free air anomaly was modeled.The final model obtained using forward modelling of the wide-angle data and pre-stack depth migration of the seismic reflection data provides an unprecedented view of the sedimentary and crustal structure of the margin. The sedimentary layers in the Algerian basin are 3.75 km thick to the north and up to 4.5 to 5 km thick at the foot of the margin. They are characterised by seismic velocities from 1.9 km/s to 3.8 km/s. Messinian salt formations are about 1 km thick in the study area, and are modelled and imaged using a velocity between 3.7 km/s to 3.8 km/s. The crust in the deep sea basin is about 4.5 km thick and of oceanic origin, presenting two distinct layers with a high gradient upper crust (4.7 km/s - 6.1 km) and a low gradient lower crust (6.2 km/s - 7.1 km/s). The upper mantle velocity is constrained to 7.9 km/s. The ocean-continent transition zone is very narrow between 15 km to 20 km wide. The continental crust reaches 25 km thickness as imaged from the most landward station and thins to 5 km over a less than 70 km distance. The continental crust presents steep and asymmetric upper and lower crustal geometry, possibly due to either asymmetric rifting of the margin, an underplated body, or flow of lower crustal material towards the ocean basin. Present-time deformation, as imaged from 3 additional seismic profiles, is characterized by an interplay of gravity-driven mobile-salt creep and active thrusting at the foot of the tectonically inverted Algerian margin

Bien que de nombreuses études géodynamiques et tectoniques aient été effectuées à partir l'activité sismique en Equateur, il n'existait pas à ce jour une tomographie complète utilisant l'ensemble des données du réseau sismologique Equatorien (RENSIG), mise à part une étude prélimaire sur la partie centrale de l'Equateur menée en 1994 par Prévot et coll. et de plusieurs profils sismiques déterminés à la suite des campagnes marines SALIERI et SISTEUR. Inverser les centaines de millier de temps d'arrivées d'ondes P et S, de qualité inégale, formant le catalogue RENSIG était le défi qu'a constitué le sujet de cette thèse.Nous décrivons comment nous avons complété le catalogue RENSIG par des données provenant du Nord du Pérou et comment nous avons homogénéisé et filtré l'ensemble de données résultant, comportant plus de 800 000 temps d'arrivée correspondant à plus de 50 000 séismes. Pour inverser ces données nous avons adopté une approche Bayésienne. Nous montrons comment le problème peut être reformulé dans un contexte Gaussien par un changement de variables, tout en imposant une statistique robuste aux données, qui conduit à un problème de moindre carrés non linéaire. Nous détaillons particulièrement la régularisation du problème au travers des noyaux de covariance qui conduit à définir des paramètres de contrôle fort utils pour l'inversion. Nous montrons également qu'inverser des différences de données revient à introduire des termes spécifiques de corrélation dans la matrice de covariance des données, tout en conservant les données brutes. Nous indiquons finalement comment le calcul de l'indice de restitution permet de définir une zone de confiance du modèle résultant de l'inversion.L'inversion a été menée pratiquement en utilisant les codes informatiques (en Fortran 2003 par B. Potin, B. Valette, V. Monteiller): LOCIN (localisation) et INSIGHT (tomographie). La région finale d'étude est constituée par une boite parallélipipédique de dimension 590$times$770 km$^2$ de base et de 244 km de hauteur qui contient la topographie de la surface. Le modèle est constitué d'une part des valeurs de $v_P$ et $v_P/v_S$ sur une grille ayant 5 km de pas horizontal et 2 km de pas vertical et, d'autre part, des paramètres d'identification spatiale et temporelle des séismes. Un ensemble de tests nous a permis de déterminer des valeurs raisonables de ces paramètres au travers d'un analyse de type courbe en L.Nous avons obtenu une amélioration de la localisation de la sismicité, qui nous a permis de mieux décrire les essaims superficiels comme ceux de Pisayambo, Macas et du Reventador et d'identifier des linéaments en relation avec la Tectonique. Nous avons également obtenu une image de la sismicité à profondeur intermédiaire qui est dominée par la présence de 4 nids sismiques, ceux de Madonaldo, La Man'a et de Guayaquil à des profondeurs entre 75 et 115 km et celui de Puyo à de plus grandes profondeurs. La zone de Wadati-Benioff nous a permis de définir la profondeur du slab jusqu'à des profondeurs de 100-150 km en fonction de la latitude et d'observer la décroissance du pendage de 25° environ au nord et au centre de l'Equateur jusqu'à environ 10° au sud puis au nord du Pérou. Par ailleurs, l'analyse du champ de vitesse des ondes P suggère fortement que le slab est coupé en deux morceaux, le morceau sud passant sous le morceau nord au niveau du nid sismique de Puyo. Le modèle $v_P/v_S$ présente une forte anomalie positive de ce rapport le long de la cordillère occidentale à des profondeurs entre 30 et 50 km qui caractérise des matériaux partiellement fondus et correspond au réservoir d'alimentation profond de l'arc volcanique. Enfin, nous avons déduit de notre modèle un modèle de profondeur de Moho en prenant la profondeur de maximum de la norme du gradient de vitesse entre les vitesses de 7.2 et 7.4 km/s et en incorporant l'information sur la profondeur de Moho provenant des campagnes SALIERI et SISTEUR dans la marge active
Although there have been numerous studies on the geodynamics and the tectonics in Ecuador based on the seismic activity, there has not been to date a comprehensive tomography study using the entire database of the National Seismic Network (RENSIG). Only a preliminary limited study was performed by Prevot et al. to infer a simple P velocity model in central Ecuador, and several profiles in the South-Colombian-Ecuador margin were also investigated by using travel time inversion of wide-angle seismic data obtained during the two marine experiments SISTEUR and SALIERI. Inverting the hundreds of thousands of arrival times of P and S waves of uneven quality that constitutes the RENSIG catalogue is the challenging subject of this thesis.We describe how we complemented the RENSIG catalogue with data from the Northern Peru network and how we homogenized and filtered the resulting dataset of more than 800 000 first arrival times of P and S waves corresponding to more than 50 000 earthquakes. To invert these data for both the velocity models and the event locations we adopted a Bayesian approach. We show how the problem can be recast in the Gaussian framework by changes of variable while imposing a robust statistics to the data, and how it leads to a generalized nonlinear least squares problem. We detail in particular the regularization of the models through the smoothing and damping properties of the covariance kernels. We also show that inverting differences in data instead of the raw data amounts to the introduction of specific correlation terms in the data covariance matrix, while keeping the same set of data. We finally indicate how the computation of the averaging index allows the delimitation of a confidence region for the resulting model.The practical inversion has been carried out by using the two Fortran 2003 codes (B. Potin, B. Valette, V. Monteiller): LOCIN (prior localization) and INSIGHT (tomography). The final study region is a parallelepipedic box of 590$times$770 km$^2$ area and 244 km height that contains the topography of the surface. The models consist of the $v_P$ and $v_P/v_S$ fields discretized over a grid, the spacing of which is 5 km in the horizontal directions and 2 km in the vertical one, and of the spatial and temporal parameters of the seismic events. A battery of tests allowed us to set reasonable values for these tuning parameters through an L-curve analysis.We obtained the spatial distribution of the seismicity with an improved accuracy which allows us to describe with more details the shallow seismic clusters, as those of Pisayambo, Macas, Reventador, and to identify lineaments in the seismicity in relation with tectonics. We obtained also a clear image of the intermediate depth seismicity wich is dominated by 4 nests, namely the Maldonado, La Man'a, and Guayaquil nests, at depths ranging between 75 km and 115 km, and the Puyo nest at much deeper depths. The Wadati-Benioff zone allowed us to clearly defined the topography of the slab only to a depth to about 110-150 km, depending on the latitude, and to observe the decrease of the dip angle from about 25° in northern and central Ecuador down to about 10° in southern Ecuador and northern Peru. On the other hand, the analysis of the P velocity clearly suggests that the slab is broken in two pieces, the southern one passing under the northern at the level of the Puyo nest. The $v_P/v_S$ model presents a high anomaly of the ratio along the western cordillera at a depth ranging between 30 km and 50 km that characterized partially melted rocks and corresponds to the feeding reservoir of the volcanic arc. Finally, we deduced the Moho depth from our model by taking the depth for which the norm of the velocity gradient is maximum between 7.2 and 7.4 km/s and by incorporating information on the Moho depth provided by the SISTEUR and SALIERI experiments in the convergent margin

Durant cette thèse, nous appliquons la méthode de la corrélation de bruit ambiant dans la région particulièrement hétérogène des Pyrénées et des ses alentours (socle rocheux affleurant et bassin sédimentaires épais). Le jeu de données utilisé est une combinaison de deux réseaux temporaires large-bande français et espagnol (PYROPE et IBERARRAY) et de stations des réseaux permanents large-bande français et catalan. Le bruit sismique, enregistré pendant un an par les 158 stations est utilisé pour calculer les corrélations dans la gamme de période 5-55 s. Les vitesses de groupe de l'onde de Rayleigh et de l'onde de Love entre paires de stations sont inversées de manière linéarisée et nous obtenons, pour chaque période, des cartes de vitesses de groupe avec une résolution latérale d'environ 40 km. La comparaison entre les deux types d'ondes montre qu'il existe une anisotropie radiale à courtes périodes, alors que peu ou pas d'anisotropie radiale n'est visible aux périodes plus longues. Nous avons développé une nouvelle stratégie d'inversion des courbes de dispersion en modèle de vitesse d'onde S que nous avons appliqué sur les vitesses de groupe de l'onde de Rayleigh. Cette approche d'inversion est basée sur une exploration complète de l'espace des modèles et une inversion linéarisée. Le modèle obtenu, validé par la comparaison avec des résultats provenant d'autres méthodes, est le premier modèle 3-D crustale en vitesse d'onde S de la région et il permet d'apporter des contraintes sur la géodynamique des Pyrénées et de ses alentours. Deux points importants sont soulevés : (1) Des profils de vitesses atypiques sous l'Est du Massif Central, avec une croûte amincie et des vitesses anormalement faibles dans le manteau supérieur. (2) Deux anomalies de vitesses rapides sous la zone du Labourd-Mauléon et dans le prolongement du bassin de Parentis. Ces anomalies, situées à 25 km de profondeur, sont interprétées comme les traces de l'hyper-extension qui aurait précédée la phase de collision amenant à la formation des Pyrénées. La forte hétérogénéité de la zone permet également de faire l'analyse de l'influence de la non prise en compte des déviations de rais lors de l'inversion. Les premiers résultats montrent que le modèle obtenu en utilisant la théorie des rais droits ne permet pas d'expliquer les déviations calculées par la méthode de formation de voie. Ces déviations observées peuvent donc apporter une amélioration du modèle en considérant l'utilisation d'une inversion combinée
In this thesis, we applied the ambient noise correlation method in the very heterogeneous region of the Pyrenees and the surrounding areas (mountain belt and thick sedimentary basins). The dataset used is a combination of two temporary broadband arrays from France and Spain (PYROPE and IBERARRAY) and stations of the French and Catalan permanent broadband arrays. Seismic noise recorded over years by the 158 stations was used to calculate correlations in a period range of 5-55 s. Observed Rayleigh and Love wave group velocities between pairs of stations were used as input to a linearized inversion scheme, where we obtained for each period group velocities maps, with a lateral resolution of approximately 40 km. The comparison between the two type of waves demonstrates radial anisotropy at short periods, while little or no radial anisotropy is present at long periods. We developed a new strategy for the inversion of dispersion curves to shear wave velocity models and applied it to the Rayleigh waves group velocity. This approach is based on the combination of a full exploration of the model space and a linearized inversion. The obtained model, validated by the comparison of our results with the results of other methods, is the first complete 3-D crustal Vs model of the region. We in particular note : (1) Atypical S-waves profiles in the Eastern part of the Massif Central which indicate a thinned crust and low velocities in the uppermost mantle. (2) High-velocity anomalies at 25 km depth beneath the Labourd-Mauléon area and the on-land continuation of the Parentis basin. We suggest that they are the traces of the hyper-extension which might have preceded the collision phase which lead to the formation of the Pyrenees. The strong heterogeneity of our study region is also well-adapted to analyze the influence of the ray deviations on the reconstruction of the model. First results show that the reconstructed model, using great-circle paths, does not explain the ray deviations as observed by beamforming. Observed deviations therefore carry the potential of improving the model in a combined inversion scheme

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2014.
Cataloged from PDF version of thesis.
Includes bibliographical references.
The primary objective of this thesis is to improve our understanding of the crustal structure and deformation in the southeastern Tibetan Plateau and adjacent regions using surface wave tomography. Green's functions for Rayleigh and Love waves are extracted from ambient noise interferometry. Using the Green's functions, we first conduct traditional traveltime tomography for the two shear wavespeeds Vsv and Vsh Their differences are measured as radial anisotropy. We then conduct Eikonal tomography to study azimuthal anisotropy in the crust. Our tomography results are well consistent with geology in the study region. In the Sichuan Basin, low wavespeed and positive radial anisotropy (Vsh> Vsv) in the upper crust reflect thick sedimentary layers at surface; high wavespeed and small radial anisotropy in the middle and lower crust reflect a cold and rigid basin root. Little azimuthal anisotropy is observed in the Basin, indicating small internal deformation. In the Tibetan Plateau, we observe widespread low wavespeed zones with positive anisotropy in the middle and lower crust, which may reflect combined effects of weakened rock mechanism and horizontal flow in the deep crust of southeastern Tibet. The northern part of the Central Yunnan block, which geographically coincides with the inner zone of the Emeishan flood basalt, reveals relatively higher wavespeeds than the surrounding regions and little radial anisotropy throughout the entire crust. We speculate that the high wavespeeds and small radial anisotropy are due to combined effects of the remnants of intruded material from mantle with sub-vertical structures and channel flow with sub-horizontal structures. In general, the azimuthal anisotropy in our study region is consistent with a clockwise rotation around the Eastern Himalayan Syntaxis. Careful examination reveals large angular differences between the azimuthal anisotropy in the upper and lower crust, suggesting different deformation patterns at the surface and in depth. Therefore, our tomography results support models with ductile flow in the deep crust of the southeastern Tibetan Plateau; however, the large lateral variation of both wavespeeds and anisotropy indicates that the flow also varies greatly in intensity and pattern in different geological units.
by Hui Huang.
Ph. D.

This thesis is directed at exploiting information in the coda of seismic phases and the ambient noise field to provide new constraints on the structure of the Australian Continent. ¶ The exploitation of the immediate coda following the onset of P waves from a distant earthquake using radial receiver functions is now a well established method. The 40 sec interval following P contains reverberations and conversions, by deconvolving the radial component trace with the vertical components, the conversions are emphasized by canceling the part of the response that are common to both components. A member of different styles of such deconvolution, are investigated and a variant of the multitaper method is adopted for subsequent applications. The TASMAL experiment 2003-2005 spans the expected location of the transition between Precambrian and Phanerozoic Australia. The 20 portable broadband stations were exploited in receiver function studies to extract S wave crustal structure through the inversion of stacked receiver functions using the Neighbourhood Algorithm. There is no clear crustal transition associated with the presence of Tasman Line. The Precambrian Cratons tend to exhibit crustal thicknesses close to 40 km but such values are also found in some Phanerozoic sites. ¶ The second part of the thesis is directed at the exploitation of ambient noise or seismic coda to gain information on the Green's function between seismic stations. The TASMAL experiment covered a significant fraction of the Australian continent with a simultaneous deployment of portable broadband stations. From these continuous records, it has proved possible to extract very clear Rayleigh wave signals for station separations up to 2000 km, and to demonstrate the frequency dependent variations in group velocity behaviour. The combination of the paths between the 20 stations localize such behaviour, but detailed images needed more data. The entire archive of portable broadband data recorded by RSES was mined, and combined with data from permanent stations to provide more than 1100 estimates of interstation Green's functions within Australia. Group velocity analysis as function of frequency was followed by nonlinear tomography with the Fast Marching Method. The resulting images of group velocity patterns as a function frequency show pronounced regions of lowered group velocities, most of which match regions of thick sediment. The frequency dependence is not consistent with just sedimentary structure and low midcrustal velocities, most likely due to elevated temperatures, are also needed. ¶ The surface wave portion of the interstation Green's function is the most energetic, and is normally all that seen in ambient noise studies. However, in the coda of events record at the broadband Warramunga seismic array in the Northern Territory, the P and S body wave components also emerge. The characteristics of these arrivals match those observed from nearby small earthquakes. The stacked cross-correlation is the normal approach to enhance Green's function information from ambient noise, but a broader spectral band width with the same phase response can be found by spectral division. It appears advantageous to compare both approaches and select the best result, since very little modifications to procedures are needed. ¶ The properties of the ambient noise at a single station have been investigated in the logarithmic spectral domain and a station dependent signal can be extracted by stacking. The signal appears to be related to the local structure beneath the station, and when fully characterized may provide a new means of investigating structure.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2005.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Includes bibliographical references.
This thesis involves inverting the seismic structure of the crust and uppermost mantle in China from the P- and S-wave travel-time tomography. The main contributions of this research are: 1) introducing the adaptive moving window method to obtain 2338 1D P and S models in China; 2) introducing a tomographic method to perform the 3D body wave travel-time tomography with the Moho discontinuity included. Both horizontal and vertical resolutions are highly controlled and smooth transitions among adjacent locations are guaranteed in the final models. To achieve these objectives, the Monte-Carlo (random search) method and the Gauss-Newton method are applied iteratively to find the nonlinear least square solutions and to optimize the models in the crust and uppermost mantle. The models we obtained provide accurate travel-time calculation, ground-truth event relocation and seismogram fittings. These models can therefore be applied to reliable earthquake location. Geological, geodynamic, and volcanic implications of our models are discussed in this thesis. Our tomographic models provide new insights into the geological structure and tectonics of the region, such as lithological variations and large fault zones across the major geological terranes.
(cont.) Compared with previous tomographic studies, we have used a larger, higher quality data set and applied an updated tomographic method to take into account the effects of the complex Moho geometry in this region. Our results cast a new light over the complex structure and seismotectonics of China and surrounding areas.
by Youshun Sun.
Ph.D.

Depuis 10 ans, l'imagerie de la croûte par corrélations de bruit a été utilisée dans différentes régions. Ces études sélectionnent les corrélations sur la base du rapport signal sur bruit des signaux utilisés. Selon la théorie, les corrélations convergent vers les fonctions de Green en temps positif et négatif si la distribution des sources est homogène. Seule la vérification de la parité des corrélations permet d'évaluer la qualité de la convergence. Un écart à la parité fournit des informations sur la synchronisation des horloges des stations et la distribution des sources de bruit. Ainsi, l'analyse des variations des temps de propagation des parties causale et acausale nous a permis d'identifier des erreurs d'horloge pour des stations du jeu de données SIMBAAD. Une analyse de polarisation et l'utilisation de la formation de voie sur les corrélations ont permis de localiser des sources de bruit d'ondes P en océan profond. Dans la région Egée-Anatolie, à 15 s de période la distribution des sources de bruit est homogène et elle devient hétérogène à plus longues périodes. Nous utilisons la propriété de symétrie pour sélectionner les temps de propagation des ondes de surface (Love et Rayleigh) les plus représentatifs des fonctions de Green. L'inversion de ces temps de propagation permet de reconstruire les variations latérales des vitesses de groupe à toutes les fréquences. Les courbes de dispersions locales obtenues sont inversées pour construire un modèle en 3D de la vitesse des ondes S de la croûte pour la région Egée-Anatolie. Les variations latérales de la vitesse et de l'épaisseur de la croûte sont cohérentes avec la structure géologique connue, avec les résultats d'autres études géophysiques et avec la déformation récente.

Advances in seismology allow us to obtain "high-resolution" images of the Earth's subsurface. This dissertation summarizes the results of three seismic studies on three different continents, with the aim of better understanding the crust and upper mantle structure of seemingly disparate yet ultimately related regions. The seismic techniques of Pn tomography and P-wave receiver function (RF) analysis are applied to central Turkey (Pn tomography), western Argentina and southwestern Wyoming, USA (RF analysis). These studies look at both a present-day convergent margin (Andean subduction zone, Argentina) and two ancient ones (Bitlis-Zagros collision zone of Arabia-Africa with Eurasia, Turkey; Farallon subduction zone, Wyoming).Using Pn tomography, we were able to detect the limit of the slab rupture edge along the Central Anatolian Fault Zone, Turkey. Slab break-off is an important process that modifies the mantle in tectonically active regions, and the limit of the oceanic Arabian slab break-off along the Bitlis-Zagros Suture Zone, thought to have begun at 11 Ma, was previously undetermined.Using RF analysis, we obtained high-resolution images of the subducting slab beneath the Sierras Pampeanas, Argentina. Continental Moho contours roughly follow terrane boundaries, suggesting that ancient terranes continue to exert control over present-day continental deformation. Overthickened oceanic crust is often cited as a cause of flat slab subduction; our RF results indicate that the crust is moderately overthickened, around 11-16 km. Further, we image offsets in the RF arrivals that indicate the subducted slab is broken or offset in along trench-subparallel fractures.The crustal structure beneath southwestern Wyoming, the location of ancient Farallon flat slab subduction, was studied using RF analysis. Looking at regional crustal structure, results include a new depth to Moho map. Coherency of the seismic signal across the dense LaBarge array (55 stations, ~250 m spacing) was investigated, with results showing that complicated shallow structure can greatly impact the resulting RF signal. Modeling of RFs using synthetics helped to separate the complex signal containing multiple primary conversions and their reverberations, which interact constructively and destructively. The dense spacing of the LaBarge array allowed unique opportunities to investigate coherency of waveforms across very short distances.

Teleseismic data recorded by broad-band seismic stations in the Swedish National Seismic Network (SNSN) have been used in a suite of studies of seismic wave conversion in order to assess the structure of the crust and upper mantle beneath the Baltic Shield. Signals of seismic waves converted between P and S at seismic discontinuities within the Earth carry information on the velocity contrast at the converting interface, on the depth of conversion and on P and S velocities above this depth.

The conversion from P to S at the crust-mantle boundary (the Moho) provides a robust tool to constrain crustal thicknesses. Results of such analysis for the Baltic Shield show considerable variation of Moho depths and significantly improve the Moho depth map. Analysis of waves converted from S to P in the upper mantle reveals a layered lithosphere with alternating high and low velocity bodies. It also detects clear signals of a sharp velocity contrast at the lithosphere-asthenosphere boundary at depths around 200 km.

Delay times of P410s, the conversion from P to S at the upper mantle discontinuity at 410 km depth, were used in a tomographic inversion to simultaneously determine P and S velocities in the upper mantle. The polarisation of P410s was also used to study anisotropy of the upper mantle. Results of these analyses are found to be in close agreement with independently derived results from arrival time tomography and shear-wave splitting analysis of SKS.

The results presented in this thesis demonstrate the ability of converted wave analysis as a tool to detect and image geological boundaries that involve sharp contrasts in seismic properties. The results also show that this analysis can provide means of studying aspects of Earth’s structure that are conventionally studied using other types of seismic data.

Le Massif Armoricain est situé dans la syntaxe Ibéro-Armoricaine, à l'extrémité Ouest de la chaîne hercynienne d'Europe. Cette structure arquée se referme pendant toute l'histoire de la collision continentale jusqu'au cours du Carbonifère, et de grands décrochements se forment à travers ces domaines. Deux de ces grands décrochements, le Cisaillement Nord Armoricain et le Cisaillement Sud Armoricain, délimitent un domaine supra-crustal peu épaissi au cours de l'orogenèse hercynienne : la Bretagne Centrale. La partie orientale de ce domaine présente tous les caractères d'une zone de cisaillement simple dextre. L'étude présentée dans cette thèse aboutit à une modélisation nouvelle du cisaillement simple. La modélisation est basée sur une analyse géostatistique des données structurales compilées à travers toute la zone d'étude. Cette analyse permet en outre de filtrer les variations locales du champ de déformation par rapport à celles régionalement corrélées et de quantifier ainsi les paramètres cinématiques du cisaillement simple en Bretagne Centrale. La restauration de ce cisaillement régional conduit à réinterpréter les déformations subies aux limites du domaine et dans les zones adjacentes ; par exemple, le bassin Carbonifère de Châteaulin, jusqu'à présent interprété comme extensif s'avère être vraisemblablement transpressif. Le domaine situé au Sud de la Bretagne Centrale a aussi pu être restauré à l'aide de l'interprétation des structures d'échelle crustale imagées par les données géophysiques récemment acquises dans le cadre du projet de recherche Armor2-Géofrance3D (données sismiques, gravimétriques et magnétiques). On montre que dans cette zone, le cisaillement régional est largement accommodé par de grandes structures chevauchantes en ciseaux. Enfin, la confrontation des données de tomographie sismique au modèle de déformation obtenu à partir des données de terrain suggère que le cisaillement simple de Bretagne Centrale a affecté l'ensemble de la lithosphère sous-jacente.

Sous le Chili central et l’ouest de l'Argentine (29°-35°S), la plaque océanique Nazca, en subduction sous la plaque continentale Amérique du Sud, change radicalement de géométrie : inclinée à 30°, puis horizontale, engendrée par la subduction de la chaine de volcans de Juan Fernandez. Le but de mon étude est d'évaluer, la variation de nature et de propriétés physiques de la lithosphère chevauchante entre ces deux régions afin de mieux comprendre (1) sa structure profonde et (2) les liens entre les déformations observées en surface et en profondeur. Pour répondre à cette thématique, j’utilise une approche originale couplant la sismologie, la thermométrie, et la pétrologie. Je montre ainsi des images 3-D de tomographie sismique les plus complètes de cette région par rapport aux études précédentes, qui intègrent (1) de nombreuses données sismiques provenant de plusieurs catalogues, (2) un réseau de stations sismiques plus dense permettant de mieux imager la zone de subduction. J’apporte la preuve que la plaque en subduction se déshydrate dans deux régions distinctes : (1) le coin mantellique, et (2) le long de la ride subduite avant que celle-ci ne replonge plus profondément dans le manteau. La croûte continentale au-dessus du flat slab possède des propriétés sismiques très hétérogènes en relation avec des structures de déformation profondes et des domaines géologiques spécifiques. La croûte chevauchante d’avant-arc, au-dessus du flat slab, est décrite par des propriétés sismiques inhabituelles, liées à la géométrie particulière du slab en profondeur, et/ou liées aux effets du séisme de 1997 de Punitaqui (Mw 7.1). Mes résultats, confirmant les études antérieures, montrent que : - le bloc Cuyania situé plus à l’est, dans la zone d’arrière-arc est plus mafique et contient une croûte inférieure éclogitisée ; quant à, la croûte continentale inférieure sous l’arc Andin, est épaisse et non-éclogitisée, décrivant surement le bloc felsique de Chilenia
Beneath central Chile and western Argentina, the oceanic Nazca slab drastically changes geometry from horizontal to dipping at an angle of 30°, and correlates with the subduction of the Juan Fernandez seamount ridge. The aim of our study is to assess, using a thermo-petrological-seismological approach, the differences of the overriding lithosphere between these two regions, in order to better understand the deep structure of the continental lithosphere above the flat slab, and the links between the deformations at the surface and at depth. We show the most complete regional 3-D seismic tomography images of this region, whereby, in comparison to previous studies, we use (1) a much larger seismic dataset compiled from several short-term seismic catalogs, (2) a much denser seismic station network which enables us to resolve better the subduction zone. We show significant seismic differences between the flat and normal subduction zones. As expected, the flat slab region is impacted by colder temperatures, and therefore by faster seismic velocities and more intense seismic activity, compared to the normal slab region. We show evidence that the flat slab dehydrates within the mantle wedge, but also along the subducting ridge prior to re-subducting. The forearc crust above the flat slab is described by unusual seismic properties, correlated to the slab geometry at depth, and/or, to the aftershock effects of the 1997 Mw 7.1 Punitaqui earthquake which occurred two years before the recording of our events. The continental crust above the flat slab has very heterogeneous seismic properties which correlate with important deformation structures and geological terranes at the surface. We confirm previous studies that have shown that the thick lower crust of the present day Andean arc is non-eclogitized and maybe representing the felsic Chilenia terrane, whereas to the east, the Cuyania terrane in the backarc is more mafic and contains an eclogitized lower crust

Abstract The crustal and upper mantle structures of the Shield on the regional scale were investigated using the data of the POLENET/LAPNET passive seismic array and the previously published models of active and passive seismic experiments in the study area. This area is centred in northern Finland and it extends to surrounding areas in Sweden, Norway and northwestern Russia. The bedrock there is mostly of the Archaean origin and the lithosphere of the region was reworked by two orogenies during Palaeoproterozoic. One of the results of the thesis was a new map of the Moho depth of the study area, for which new estimates of the crustal thickness were obtained using receiver function method and complemented by published results of receiver function studies and controlled source seismic profiles. The map differs from the previously published maps in two locations, where we found significant deepening of the Moho. The 3D structure of the upper mantle was studied using teleseismic traveltime tomography method. The resulting model shows high seismic velocities below three cratonic units of the study area, which may correspond to non-reworked fragments of cratonic lithosphere and a low velocity anomaly separating these cratonic units from each other. The regional scale studies were complemented by two smaller scale studies in upper crust level using combined interpretation of seismic profiling and gravity data. These studies were centred on Archaean Kuhmo Greenstone Belt in eastern Finland and central Lapland in northern Finland located in the crust reworked during Palaeoproterozoic. Both areas are considered as prospective ones for mineral exploration. Both studies demonstrate the advantage of gravity data inversion in studying 3D density structure of geologically interesting formations, when the Bouguer anomaly data is combined with a priori information from petrophysical and seismic datasets.

La lithosphère continentale est physiquement et chimiquement segmentée. La cartographie des isotopes radiogéniques de roches plutoniques acides, représentatives de la croûte continentale, et de laves basiques, représentatives du manteau, possède des similarités avec la cartographie sismique de la lithosphère sous-jacente. Ces similitudes permettent d'interpréter les observations sismiques en étudiant leurs caractéristiques chimiques et leur âge. Les isotopes du plomb permettent de dater et d'identifier l'empilement de segments crustaux qui forment la croûte. L'écart des âges modèles du plomb avec d'autres systèmes identifie le recyclage crustal et le réchauffement de la croûte au dessus de la température du système plomb-plomb. Le système plomb-plomb donne également accès au sous-étudié rapport Th/U qui contraint la profondeur de la source des roches continentales. Certains échantillons de l'ouest U.S.A. proviennent de la croute inférieure, et se sont formés par l'extension crustale ou par un flux de matériel au sein de la croûte. Les isotopes du néodyme et de l'hafnium marquent la fusion du manteau lithosphérique enrichis sous le Colorado Plateau, une région où est observée le détachement du manteau lithosphérique sub-continental. Ce manteau fond par décompression adiabatique, par extension localisée ou remontée asthénosphérique engendrée par la convection locale. Au final, l'association des systèmes isotopiques du plomb, du néodyme, et de l'hafnium avec la sismologie est une approche puissante pour étudier la formation et la déformation de la lithosphère continentale.

Differentes methodes d'inversion de la vitesse de ces ondes sont appliquees a plusieurs regions du globe: ocean pacifique, afrique, ocean indien. La regionalisation suivant l'age du fond oceanique montre que des heterogeneites laterales de vitesse existent jusqu'a au moins 400 km de profondeur et que la correlation entre la tectonique de surface et la structure profonde est meilleure sous l'ocean pacifique que sous l'ocean indien

L'étude des données Lithoscope permet d'étudier les structures profondes de la lithosphère. Trois méthodes ont été développées. La première porte sur l'analyse des ondes converties à l'interface croûte-manteau afin de contraindre les variations d'épaisseur de croûte. La seconde méthode caractérise l'orientation des structures dans le manteau, par la rnesure de l'anisotropie des ondes S. La troisième méthode mesure l'atténuation des milieux échantillonnés à l'aide de deux méthodes d'analyse fréquentielle et temporelle. Ces trois méthodes ont été appliquées aux enregistrements provenant du Nord Tibet, du Massif Central et de l'Abitibi, L'étude des données du plateau tibétain a permis de mettre en évidence des variations latérales très importantes de vitesse et de structure dans la lithosphère. Un saut de Moho de 20km a été observé sans qu'aucune variation de topographie ne soit observable. Ces travaux montrent que les phénomènes de l'équilibrage dans la lithosphère sont lents comparés aux mouvements horizontaux et verticaux. L'étude de l'anisotropie a permis de révéler l'importance de l'extrusion latérale le long de la faille du Kunlun. Les valeurs d'anisotropie obtenues impliquent que les mouvements horizontaux des blocs lithosphériques orientent les minéraux sur des épaisseurs mantelliques supérieures à 260 km. L'étude de l'atténuation sous le Massif Central a permis de constater qu'il existait une bonne corrélation entre les zones atténuantes et ies édifices volcaniques majeurs de cette région. Les valeurs du facteur de qualité obtenues montrent que les perturbations occasionnées par le volcanisme intéressent l'ensemble de la lithosphère mais sur de zones très étroites. Les modé!isations de perturbation thermique dans la croûte montrent de très bonnes corrélations avec les âges des différentes crises volcaniques. L'application systématique de ces trois analyses sur les enregistrements télésismiques du programme Lithoscope a permis de contraindre et de compléter les images de la lithosphère obtenues par la tomographie en vitesse des ondes P.

This dissertation work applies seismic tomographic inversion methods to two different datasets - one to address the earthquake hazard within the Strait of Georgia and the other to estimate hydrate concentration and distribution in the continental slope off Vancouver Island. In the first part of the study, seismic refraction/wide-angle reflection data from onshore-offshore experiments in 1998 and 2002 were inverted for a smooth three-dimensional (3D) velocity structure down to depths of 6-7 km beneath the Strait of Georgia, a seismically active region where an earthquake swarm (with magnitude up to 5) occurred in 1995-1997. The objectives were to map structures that contribute to seismic hazard evaluation in the Georgia Basin. The main structural features obtained from the inversion are: a northeast-southwest trending hinge line at the location of the earthquake swarm, where the basin deepens rapidly to the southeast; a northwest-southeast trending velocity discontinuity that correlates well with the surface expression of the shallow Outer Island fault; sediment thickening from north to south; and basement uplift at the San Juan Islands, possibly caused by a thrust fault. In the second part of the dissertation, seismic single channel and wide-angle reflection data collected in September 2005 were analyzed for a 2D profile of ocean bottom seis¬mometers (OBSs) on the continental slope region off Vancouver Island, near ODP Site 889 and IODP Site 1327. The objectives were to determine the shallow sediment velocity structure associated with marine gas hydrates and to estimate the hydrate concentration in the sediment pore space. Combined inversion of single channel and OBS data produced a P-wave velocity model down to the depth of the BSR at 230 m below seafloor. Strong attenuation of P-waves below the BSR indicates the presence of free gas. To investigate structures below the BSR, forward modelling of S-waves was carried out using the data. from the OBS horizontal components. Both the P- and S-wave models match very well with the sonic log data from ODP Site 889 and IODP Site 1327. The increase in P-wave velocity of the hydrate bearing sediments relative to the background no-hydrate velocity was utilized to estimate the hydrate concentration by using a simple porosity-reduction equation. An average concentration of 15% was estimated from the P-wave velocity model. Prestack depth migration was applied to the OBS data to image the structure along the 2D profile containing the OBSs. The primary and multiple arrivals were migrated separately. Conventional migration of the primary arrivals produced an image with a very narrow illumination and the shallow subsurface layers including the seabottom were not imaged. However, migration of the OBS multiples, using a mirror imaging technique, pro¬duced a continuous structural image of the subsurface including the shallowest layers. The lateral illumination is much wider with a quality comparable to that of vertical incidence reflection data.

碩士
國立中央大學
地球科學學系
105
1906 Ｍ7.1 Meishan earthquake occurred near Chiayi city in southwestern Taiwan and resulted in more than three thousands casualties and six thousands of buildings collapsed. Based on the geological survey, the Meishan fault zone a right-lateral strike-slip fault with a length of 25 kilometers, was the main contributor for this event. In previous studies, many researchers have done lot of investigation about the Meishan Fault zone, including geologic researches at surface and seismic explorations at shallow crust. However, there is still limited information about 3-D shallow crustal structure of this study area. Therefore, we deployed 100 Texan instruments (~2 km interval) between Aug. and Nov. 2015, covered around the Meishan Fault zone. We obtained a 3-D shear wave shallow crustal velocity structure using ambient noise tomography. The reliable periods of phase velocity from Rayleigh wave are 0.6 to 5.8 seconds, which correspond to around 0-5 km at depths. The results are consistent with surface geology and seismic explorations study. Furthermore, Meishan fault, Chiayi blind thrust, and Xiaomei anticline near the fault zone are observed from this study.

In the BATHOLITHSonland seismic project, a refraction - wide-angle reflection survey was shot in 2009 across the Coast Mountains and Interior Plateau of central British Columbia. Part of the seismic profile crossed the Nechako Basin, a Jurassic-Cretaceous basin with potential for hydrocarbons within sedimentary rocks that underlie widespread volcanics. Along this 205-km-long line segment, eight explosive shots averaging 750 kg were fired and recorded on 980 seismometers. Forward and inverse modelling of the traveltime data were conducted with two independent methods: ray-tracing based modelling of first and secondary arrivals, and a higher resolution wavefront-based first-arrival seismic tomography. Gravity modelling was utilized as a means of evaluating the density structure corresponding to the final velocity model. Material with velocities less than 5.0 km/s is interpreted as sedimentary rocks of the Nechako Basin, while velocities from 5.0-6.0 km/s may correspond to interlayered sediments and volcanics. The greatest thickness of sedimentary rocks in the basin is found in the central 110 km of the profile. Two sub-basins were identified in this region, with widths of 20-50 km and maximum sedimentary depths of 2.5 km and 3.3 km. Such features are well-defined in the velocity model, since resolution tests indicate that features with widths greater than ~13 km are reliable. Beneath the sedimentary rocks, seismic velocities increase more slowly with depth – from 6.0 km/s just below the basin to 6.3 km/s at ~17 km depth, and then to 6.8-7.0 km/s at the base of the crust. The Moho is interpreted at a depth of 33.5-35 km along the profile, and mantle velocities are high at 8.05-8.10 km/s.

碩士
國立臺灣大學
地質科學研究所
101
Surface wave travel-time tomography has been widely used as a powerful strategy to image shear wave velocity structure of the Earth’s crust and upper mantle. Traditionally with either ray theoretical great-circle approximations or 2-D phase kernels, phase velocity maps are first obtained at multiple frequencies. They are then combined to invert for shear wave velocity structure using 1-D depth-varying Frechet derivatives of phase velocity with respect to shear wave speed. Such approach runs short on considering the directional- and depth-dependence of scattering while surface wave propagating through laterally heterogeneous Earth. We here present a fully 3-D finite-frequency method based on the Born scattering theory in conjunction with surface-wave mode summation and apply it to regional fundamental Rayleigh wave tomography in central Tibet. Our data were collected from Hi-CLIMB array in the central Tibet during 2004-2005. Following a standard procedure to obtain empirical Green’s functions of Rayleigh waves from ambient noise cross correlation functions (CCFs) between station pairs, the phase differences between the CCFs and corresponding synthetics are measured by a multi-taper cross-spectral method. We apply the 3-D sensitivity kernels at individual frequencies convolved with the same eigentapers used in the phase measurement to conduct a 3D tomography of shear wave velocity perturbations with respect to a spherically-symmetric earth model suitable for central Tibet. A wavelet-based, multi-scale parameterization is invoked in the tomographic inversion to deal with the intrinsic problem of unevenly distributed data and resolve the structure with data-adaptive spectral and spatial resolutions. The result shows that the crust is generally slower to the north of the Bangong-Nujiang Suture (BNS) in marked contrast to the south with higher speeds. The absence of pervasive low velocity anomalies in the mid-to-lower crust indicates that the ductile channel flow of the lower crust may be inactive beneath southern Tibet. The model resolution in the lithospheric mantle can be improved by integrating longer-period surface data from distant earthquakes, which will yield better constrains on the geodynamic process of the Himalayan-Tibetan orogeny.

Glacial isostatic adjustment (GIA) is the process by which the solid Earth responds to past and present-day changes in glaciers, ice caps, and ice sheets. This thesis focuses on vertical crustal motion of the Earth caused by GIA, which is influenced by several factors including lithosphere thickness, mantle viscosity profile, and changes to the thickness and extent of surface ice. The viscosity of the mantle beneath Antarctica is a poorly constrained quantity due to the rarity of relative sea-level and heat flow observations. Other methods for obtaining a better-constrained mantle viscosity model must be investigated to obtain more accurate GIA model predictions. The first section of this study uses seismic wave tomography to determine mantle viscosity. By calculating the deviation of the P- and S-wave velocities relative to a reference Earth model (PREM), the viscosity can be determined. For Antarctica mantle viscosities obtained from S20A (Ekstrom and Dziewonski, 1998) seismic tomography in the asthenosphere range from 1016 Pa∙s to 1023 Pa∙s, with smaller viscosities beneath West Antarctica and higher viscosities beneath East Antarctica. This agrees with viscosity expectations based on findings from the Basin and Range area of North America, which is an analogue to the West Antarctic Rift System. Section two compares bedrock elevations in Antarctica to crustal thicknesses, to infer mantle temperatures and draw conclusions about mantle viscosity. Data from CRUST 2.0 (Bassin et al., 2000), BEDMAP (Lythe and Vaughan, 2001) and specific studies of crustal thickness in Antarctica were examined. It was found that the regions of Antarctica that are expected to have low viscosities agree with the hot mantle trend found by Hyndman (2010) while the regions expected to have high viscosity are in better agreement with the trend for cold mantle. Bevis et al. (2009) described new GPS observations of crustal uplift in Antarctica and compared the results to GIA model predictions, including IJ05 (Ivins and James, 2005). Here, we have generated IJ05 predictions for a three layered mantle (viscosities ranging over more than four orders of magnitude) and compared them to the GPS observations using a χ2 measure of goodness-of-fit. The IJ05 predictions that agree best with the Bevis et al. observations have a χ2 of 16, less than the null hypothesis value of 42. These large values for the best-fit model indicate the need for model revisions and/or that uncertainties are too optimistic. Equally important, the mantle viscosities of the best-fit models are much higher than expected for West Antarctica. The smallest χ2 values are found for an asthenosphere viscosity of 1021 Pa•s, transition zone viscosity of 1023 Pa∙s and lower mantle viscosity of 2 x 1023 Pa∙s, whereas the expected viscosity of the asthenosphere beneath West Antarctica is probably less than 1020 Pa∙s. This suggests that revisions to the IJ05 ice sheet history are required. Simulated annealing was performed on the ice sheet history and it was found that changes to the recent ice load history have the strongest effect on GIA predictions.
Graduate

This thesis is directed at exploiting information in the coda of seismic phases and the ambient noise field to provide new constraints on the structure of the Australian Continent. ¶ The exploitation of the immediate coda following the onset of P waves from a distant earthquake using radial receiver functions is now a well established method. The 40 sec interval following P contains reverberations and conversions, by deconvolving the radial component trace with the vertical components, the conversions are emphasized by canceling the part of the response that are common to both components. A member of different styles of such deconvolution, are investigated and a variant of the multitaper method is adopted for subsequent applications. ... ¶ The second part of the thesis is directed at the exploitation of ambient noise or seismic coda to gain information on the Green's function between seismic stations. ¶ ...

碩士
國立中央大學
地球科學學系
103
The Ilan Plain (IP) in NE Taiwan locates on the western end of the Okinawa trough and exhibits high geothermal gradients with abundant hot springs, likely resulting from magmatism associated with the back-arc spreading as manifested by the offshore volcanic island (Kueishantao)(Tong et al., 2008). North and south sides of IP are divided by Lan-Yang River with distinctive characteristics. Comparing to the northern part, the southern part exhibits, relatively, thin unconsolidated Quaternary alluvium layer with depths ranging from 0 to 1 km (Chiang, 1976), high on-land seismicity and significant SE movements relative to Penghu island. Purposes of this study are two folds. By obtaining a high-resolution 3-D shear wave upper-crust structures, we aim at (1) assessing the extent of underground geothermal sources as revealed by low velocity anomalies, (2) mapping 3-D sedimentary structures as revealed by the structures of very low velocity zones at surface. To fulfill this goal, we deployed 89 Texan instruments (~2 km station interval) between Aug. 2014 and Jan. 2015, covering most of the IP and its vicinity. We conduct methods of ambient noise tomography for inversion of high-resolution 3-D shear wave upper-crust velocity structures. Firstly, we estimate empirical Green’s functions (EGF) of Rayleigh wave between station pairs by ambient noise cross-correlation. Secondly, dispersion curves of group and phase velocities are measured at the frequency range between 0.25 and 1.67 Hz from each EGFs. Multiple filter Technique and Image transformation technique are used to measured group and phase velocities at each period, respectively. Finally, we apply a fast marching method for inhomogeneous-medium ray tracing and for calculations of velocities between station pairs. We also adopt a wavelet-based sparsity-constrained tomography method for the direct inversion of 3-D shear wave velocity structures. Results show that the lowest shear wave velocity can be as low as 0.4 km/s. mostly at depths shallower than 500 meters. Having examined the vertical cross-sections of each profiles, the spatial distributions of low velocity zones well match to those of sedimentary structures as shown by seismic reflection survey (Chiang, 1976). Results in west IP show that local low velocity anomalies with depth shallower than 1 km display in regions of known geothermal wells.

High-resolution seismic images of lithospheric structures allow us to infer the tectonics that modified the lithosphere. We apply such methods to understand Cenozoic modification of the lithosphere by tectonic and magmatic processes in the tectonically active western United States. Using USArray Transportable and Flexible Array data, we present high-resolution images for three regions in this thesis. (1) In the Mendocino triple junction, we use a joint inversion of Rayleigh-wave dispersion data and receiver functions to obtain a new crust and upper Vs model to ~150km depth. The model shows four distinct, young lithosphere-asthenosphere boundary systems. A low-Vs anomaly beneath the Great Valley-Sierra Nevada reconciles existing slab window models with the mantle-wedge geochemical signatures in Coast Range volcanics, and explains the ~3 Myr delay of the onset of volcanism after slab removal. Uppermost mantle low velocities provide evidence for forearc mantle serpentinization extending along the Cascadia margin. (2) In the Colorado Plateau, a Rayleigh wave tomography model sheds light on the volcanism along the margins and plateau uplift. Strong upper mantle heterogeneity across the plateau edge results from the combined effect of a ~200-400 K temperature difference and ~1% partial melt. A ring of low velocities under the plateau periphery suggests that the rehydrated Proterozoic lithosphere is progressively removed by convective processes. Particularly, a high-Vs anomaly imaged beneath the western plateau adds evidence for a downwelling/delamination hypothesis [Levander et al., 2011]. Thermo-chemical edge-driven convection causing localized lithospheric downwelling provides uplift along the margins and magmatic encroachment into the plateau center. (3) In the final study, we developed a 3-D teleseismic scattering wave imaging technique based on the Kirchhoff approximation and 3-D inverse Generalized Radon Transform. Synthetic tests demonstrate higher resolution imaging for continuous, irregular interfaces or localized scatterers, in comparison to conventional methods. Applied to the High Lava Plains dataset, the transmission coefficient structure shows a deepening Moho near 117.6°W and three negative events that correlate well with the Rayleigh wave low-Vs zones. Images made with the Mendocino data clearly show rapidly decreasing lithosphere-asthenosphere boundary depths from the subduction to transform regime.

Seismic tomography is arguably the most powerful geophysical method for imaging the Earth's interior. From its origins in the early 1970s, rapid growth in computing power, increased availability of high quality digital data, and ongoing technique development have all conspired to propel it to a stage where images of unprecedented detail are now produced on a regular basis. In this thesis, cutting edge seismic tomography methods will be used to image 3-D crustal structure in southeast Australia, with the aim of better understanding the tectonic evolution of the former east Gondwana margin. A key objective of the first strand of this thesis is to use data from an array of 24 seismometers to image the crust beneath the Flinders Ranges by using Local Earthquake Tomography. A subset of P- and S-wave traveltimes is inverted to jointly recover earthquake hypocenters, P-wave velocity structure and Vp/Vs anomalies to improve our understanding of crustal structure, rheology, and the mechanism responsible for the localised intraplate deformation that characterises this area. Clusters of seismicity are observed within distinct low velocity regions (i.e., between the Archean-Mesoproterozoic Gawler Craton and the Palaeo-Mesoproterozoic Curnamona Province and along a major sequence of N-S trending Ross-Delamerian thrust faults). I postulate that pre-existing mechanical weaknesses in the lithosphere, principally due to structure and composition, exert first-order control on the distribution of seismicity in the Flinders Ranges. In the second strand of my research, ambient noise data from an array of 24 seismometers is used to produce a 3-D crustal shear wave velocity model of Bass Strait, the key to understanding the missing link between Tasmania and mainland Australia. I apply a transdimensional, hierarchical Bayesian inversion approach to construct a 3-D shear wave velocity model of the area. This allows the entire crust beneath Bass Strait to be imaged in high detail and elucidates the geometry and position of key crustal features. A key feature of the 3-D tomography model is a distinct mid-lower crustal NW-SE high velocity zone that extends from northwestern Tasmania to south-central Victoria, confirming a Proterozoic geological connection that has not previously been established in the deep crust. A recent study that uses 3-D geodynamic modelling has controversially suggested that the entrainment of exotic continental fragments may exert first-order control on continental growth via subduction accretion. Evidence from kinematic modelling suggests that the pre-Carboniferous Tasmanides in southeastern Australia may have been subjected to this process. In the final strand of my research, I present the first high-resolution crustal 3-D shear velocity model of southeastern Australia by applying the same inversion approach that was used for the Bass Strait datasets to seismic noise recorded by the WOMBAT seismic array. The main elements of the proposed continental accretionary model are fully imaged here for the first time, including the remains of the ingested continental fragment and the tectonic escape of the back arc region. To date, no other geophysical study has revealed structures in the deep crust related to the entrainment of continental fragments at former convergent accretionary margins.

La chaîne du Zagros située sur la marge septentrionale de la plaque Arabie, est l'une des plus jeunes chaînes de collision continentale. Elle a été structurée par la collision de la plaque Arabie avec le microcontinent d'Iran central. Une expérience sismologique, appelée « Zagros 2000-2001 », a été réalisée dans le cadre d'une collaboration entre le LGIT et l'IIEES pour étudier la structure lithosphérique sous cette chaîne de collision et une partie du bloc d'Iran Central. Le jeu de données de cette expérience nous a permis de caractériser la structure de la croûte et du manteau lithosphérique sous le réseau de stations. Les variations de l'épaisseur de croûte ont été mises en évidence par analyse en fonctions récepteur. Elles sont caractérisées par un sur-épaississement maximum de 20 km sur une largeur d'environ 100 km immédiatement au nord-est du MZT (« Main Zagros Thrust »). Une épaisseur moyenne de croûte de 45 km a été trouvée sous le Zagros et de 40 km sous l'Iran Central. Nous avons ensuite proposé un modèle de croûte, contraint par la géométrie du Moho tirée de l'analyse en fonctions récepteur, qui est aussi compatible avec les données gravimétriques. Le sur-épaississement est interprété comme lié à un redoublement crustal avec chevauchement de la croûte d'Iran central sur celle du Zagros le long du MZT. L'inversion de plus de 5000 temps d'arrivée P télésismiques nous a permis de caractériser la structure du manteau supérieur jusqu'à 350 km de profondeur. Les résultats de cette inversion montrent un manteau supérieur rapide sous le Zagros et lent sous l'Iran central. Ceci peut être lié à une délamination du manteau lithosphérique sous l'Iran central. La présence d'un manteau lent et léger sous l'Iran central peut expliquer la haute altitude moyenne du plateau iranien. L'analyse de la biréfringence des ondes S télésismiques montre une différence majeure entre la lithosphère du Zagros et celle de l'Iran Central en terme d'anisotropie sismique. Cette analyse met en évidence l'absence de biréfringence des ondes S télésismiques sous le Zagros par opposition à certaines régions d'Iran Central. D'autre part, aucun lien n'est observé entre la direction de l'axe rapide de la biréfringence observée en Iran central et le déplacement actuel relatif ou absolu des plaques. La biréfringence observée doit donc avoir son origine dans une anisotropie gelée dans la lithosphère du bloc d'Iran central liée à un épisode tectonique plus ancien que la collision continentale entre les deux plaques.

Passive seismic experiments, MOSAIC, BOHEMA I-III, EgerRift, or, PASSEQ, carried out in the region of the Bohemian Massif (BM), allowed a detailed study of velocity structure of the upper mantle. We present results of tomography studies of the upper mantle beneath the north-eastern and southern parts of the BM based on the data from the BOHEMA II and BOHEMA III experiments (2004-2006). Despite the fact that regions with the highest resolution of velocity perturbations differ in the models, tomography images are similar in overlapping parts. Models of the upper mantle show mostly low- velocity perturbations relatively to radially symmetric velocity model of the Earth beneath the BM. Limited high-velocity heterogeneity beneath the Moldanubian unit, extended in the NE-SW direction, reflects thickening of the lithosphere due to a collision of the BM with the Brunovistulian micro-plate during the Variscan orogeny. The tomography based on the data from the BOHEMA III experiment revealed significant high-velocity heterogeneity in the southern margin of the model with a subduction of the lithosphere beneath the Eastern Alps. Tomographic tests showed that effects of uncorrected velocity heterogeneities within the crust can appear as deep as 100 km and, therefore, they could lead to erroneous interpretation of...

title: Surface Wave Analysis and Inversion Application to the Bohemian Massif author: Mgr. Petr Kolínský, DiS. author's e-mail address: kolinsky@irsm.cas.cz departments: Department of Geophysics Faculty of Mathematics and Physics Charles University Prague V Holešovičkách 2, Praha 8 - 180 00, Czech Republic and Department of Seismology Institute of Rock Structure and Mechanics, v.v.i. Academy of Sciences of the Czech Republic V Holešovičkách 41, Praha 8 - 182 09, Czech Republic supervisor: RNDr. Johana Brokešová, CSc. supervisor's e-mail address: johana.brokesova@mff.cuni.cz consultant: RNDr. Jiří Málek, PhD. consultant's e-mail address: malek@irsm.cas.cz keywords: surface waves, group velocity, phase velocity, frequency-time analysis, multiple filtering, tomography, inversion problems, Earth crust structure, Bohemian Massif An overview of surface wave analysis methods as well as of inversion techniques is given. Special attention is paid to the multiple filtering method for dispersion curve estimation, which is described by two different ways in detail. The isometric method is used for dispersion curve inversion and its description and tests are presented. Described methods are further used in applications. The applications show examples of surface wave analysis and inversion for 1D and 2D...