Dissertations / Theses on the topic 'Seismic source inversion'

To obtain the rupture history of the Parkfield, California, earthquake, we perform 12 kinematic inversions using elliptical sub-faults. The preferred model has a seismic moment of 1.21 x 10^18 Nm, distributed on two distinct ellipses. The average rupture speed is ~2.7 km/s. The good spatial agreement with previous large earthquakes and aftershocks in the region, suggests the presence of permanent asperities that break during large earthquakes. We investigate our inversion method with several tests. We demonstrate its capability to retrieve the rupture process. We show that the convergence of the inversion is controlled by the space-time location of the rupture front. Additional inversions show that our procedure is not highly influenced by high-frequency signal, while we observe high sensitivity to the waveforms duration. After considering kinematic inversion, we present a full dynamic inversion for the Parkfield earthquake using elliptical sub-faults. The best fitting model has a seismic moment of 1.18 x 10^18 Nm, distributed on one ellipse. The rupture speed is ~2.8 km/s. Inside the parameter-space, the models are distributed according the rupture speed and final seismic moment, defining a optimal region where models fit correctly the data. Furthermore, to make the preferred kinematic model both dynamically correct while fitting the data, we show it is necessary to connect the two ellipses. This is done by adopting a new approach that uses b-spline curves. Finally, we relocate earthquakes in the vicinity of the Darfield, New-Zealand earthquake. 40 years prior to the earthquake, where there is the possibility of earthquake migration towards its epicentral region. Once it triggers the 2010-2011 earthquake sequence, we observe earthquakes migrating inside regions of stress increase. We also observe a stress increase on a large seismic gap of the Alpine Fault, as well as on some portions of the Canterbury Plains that remain today seismically quiet.

Seismic traveltime inversion, traveltime tomography and seismic reflection techniques have been applied for two dimensional (2D) and three dimensional (3D) data acquired in conjunction with site characterization and monitoring aspects at a carbon dioxide (CO₂) geological storage site at Ketzin, Germany (the CO₂SINK project). Conventional seismic methods that focused on investigating the CO₂ storage and caprock formations showed a poor or no image of the upper 150 m. In order to fill this information gap, an effort on imaging the shallow subsurface at a potentially risky area at the site is the principal goal of this thesis.

Beside this objective, a seismic source comparison from a 2D pilot study for acquisition parameter testing at the site found a weight drop source suitable with respect to the signal penetration, frequency content of the data and minimizing time and cost for 3D data acquisition.

For the Ketzin seismic data, the ability to obtain high-quality images is limited by the acquisition geometry, source-generated noise and time shifts due to near-surface effects producing severe distortions in the data. Moreover, these time shifts are comparable to the dominant periods of the reflections and to the size of structures to be imaged. Therefore, a combination of seismic refraction and state-of-the-art processing techniques, including careful static corrections and more accurate velocity analysis, resulted in key improvements of the images and allowed new information to be extracted. The results from these studies together with borehole information, hydrogeologic models and seismic modeling have been combined into an integrated interpretation. The boundary between the Quaternary and Tertiary unit has been mapped. The internal structure of the Quaternary sediments is likely to be complicated due to the shallow aquifer/aquitard complex, whereas the heterogeneity in the Tertiary unit is due to rock alteration associated with fault zones. Some of the major faults appear to project into the Tertiary unit. These findings are important for understanding the potentially risky anticline crest and can be used as a database for the future monitoring program at the site.

2010/2011
ABSTRACT One of the principal goals of seismology is to infer the nature of an earthquake source from observations of seismic ground motion. This work shall discuss the seismic source both in the 2D finite-fault and in the point-source approximation. By inverting 3-component accelerograms the rupture history and the slip distribution for the Mw 6.3 earthquake occurred in central Italy on April 6, 2009 are determined. The method of linear programming is used for the inversion and the simplex method is applied to solve the linear programming problem (Das and Kostrov, 1994). All known parameters, such as crustal structure and station distribution are kept fixed and a large-enough fault area is considered. Physical constraints such as the positivity of the slip rates on the fault and a pre-assigned seismic moment are used to stabilize the solution. Using synthetic data with a checkerboard slip distribution shows that the obtainable spatial resolution is around 2 km. Observed records acquired from local stations of the national strong-motion network are inverted. Only data from rock stations distributed uniformly around the fault at epicentral distances less than 80 km are used. The accelerograms are filtered at 1 Hz and about 15 seconds of the signals are modelled. The obtained slip distribution shows a single major asperity and is in agreement with other similar studies of the L’Aquila earthquake. The main event of L’Aquila is used to validate a stable and automatic procedure implemented by SeiSRaM group (Dep. of Mathematics and Geosciences, University of Trieste) for the SE Alps transfrontier network to estimate in real time the seismic moment, moment magnitude and corner frequency of events recorded by broad-band velocimeters and accelerometers. The procedure has two steps: the first one consists in an interface with the Antelope system (a software that manages the network) from which pre-processed waveforms are retrieved. The second step consists in estimating the seismic moment and the corner frequency by spectral analysis. The S-wave train is identified through an automatic picking procedure of Antelope software or, if that procedure fails, through the estimates arrival times based on the travel-time. The transversal component of motion is used to minimize conversion effects. The analyzed frequency window is selected on the basis of the signal-to-noise ratio (SNR). The source spectrum is obtained by correcting the signals for geometrical spreading and intrinsic attenuation. For the latter, different relationships are tested for frequency-dependent Q value in order to characterize the anelastic proprieties of the seismic region. Source spectra for both velocity and displacement are computed and, following Andrews (1986), the seismic moment and the corner frequency are estimated. The procedure is successfully validated using the recordings of some recent strong earthquakes like Carnia 2002 (Mw=4.9), Bovec 2004 (Mw =5.1), Parma 2008 (Mw =5.4) and Aquila 2009 (Mw =6.3) and the recording of some minor events in the SE Alps area for which independent seismic moment and Mw estimates are available. Since one year the procedure is applied to events recorded by the National Accelerometric Network (RAN). The agreement between moment magnitudes estimated by the SeiSRaM procedure and the INGV local magnitudes is very good.
RIASSUNTO I terremoti sono fenomeni fisici molto complessi a partire dai processi di sorgente alla determinazione della magnitudo, argomenti fondamentali nelle indagini sismologiche. Questa tesi si propone di indagare i processi fisici degli eventi sismici. L’approccio è studiare la sorgente sismica del terremoto a partire dai dati delle registrazioni, 'decifrando' le informazioni contenute in esse con l’uso sia delle teorie fisiche che con modelli matematici. In questo lavoro si discute la sorgente sismica sia nel suo modello più semplice, il caso della sorgente puntiforme, sia nella sua descrizione realistica con dimensioni finite. Una descrizione teorica delle caratteristiche e delle rappresentazioni della sorgente estesa sono rappresentate nel Cap. 1. Sono descritti i fondamenti teorici che, sulla base di numerosi studi sperimentali, sembrano meglio descrivere gli eventi sismici, gli strumenti matematici che governano i processi di rottura, i modelli che rappresentano meglio la situazione fisica che sta alla base dei terremoti, quali il modello di Haskel ed il modello di Brune (1970) e l’approssimazione della sorgente estesa come somma di sorgenti puntiformi. Il tema centrale di questo studio riguarda la comprensione e la modellazione cinematica del processo di rottura di un terremoto su una faglia finita, attraverso l'inversione dei dati accelerometrici. L’inversione di dati simici permette di ottenere gran parte delle informazioni sul comportamento spazio-temporale del processo di rottura. L'approccio cinematico consente di interpretare le forme d’onda che si irradiano dalla sorgente in termini di spostamento relativo lungo il piano di faglia in funzione dello spazio e del tempo (la storia dello scorrimento). Usando il teorema di rappresentazione, lo spostamento registrato da una stazione durante un terremoto può essere espresso in termini della distribuzione di scorrimento sulla superficie di faglia. Assumendo che la faglia sia piana e la direzione di scorrimento costante, il problema può essere discretizzato, vincolato e ricondotto ad un sistema di equazioni lineare Ax=b (Cap. 2). La soluzione a questo problema è tutt’altro che banale. E’ ben noto che il problema è instabile e dal punto di vista computazionale questa instabilità è equivalente alla non unicità della soluzione. Quindi, per ottenere una soluzione definita vi è la necessità di inserire alcuni vincoli fisici nel processo di sorgente in aggiunta alla semplice richiesta di riprodurre i dati osservati (Das e Kostrov, 1990, Das e Suhadolc, 1996). Strumento fondamentale nella procedura di calcolo e cuore della procedura di inversione adottata in questa tesi, il metodo del simplesso viene introdotto nell’ambito dello studio della programmazione lineare e applicato ad un piccolo esempio esplicativo (Cap. 3). Seguendo la formulazione sviluppata da Das e Kostrov (1990,1994) si è applicata la procedura di inversione all’evento principale dell’Aquila avvenuto il 6 aprile 2009. Dopo una breve descrizione geologica dell’Aquila, della struttura utilizzata e del modello di sorgente adottato (Cap. 4) vengono presentati i risultati sia in termini di distribuzione del momento sismico sulla faglia sia in termini di confronto tra le forma d’onda reali e sintetiche (Cap. 5). E’ la prima volta che si utilizzano dati reali con il tempo assoluto. Questo ha portato non pochi problemi principalmente nella scelta del modello di velocità e nella scelta delle stazioni. Sono state considerate solo stazioni della Rete Accelerometrica Nazionale (RAN) su roccia con distanze epicentrali tra 20 km ed 80 km. Attraverso test sintetici e confrontando con quanto riportato in letteratura, è stato scelto il modello di sorgente che meglio si adatta ai dati disponibili. Tutte le inversioni sono state fatte imponendo vincoli fisici quali la casualità, la positività e il momento prefissato totale. Questi vincoli sono stati necessari per avere una soluzione più stabile. Sono stati investigati differenti modelli di faglia, differenti distribuzioni di stazioni e due modelli di velocità. I risultati migliori sono stati ottenuti considerando una faglia lunga 28 km a larga 12 km discretizzata in celle 2km per 2 km, e considerando solo le quattro stazioni situate sul tetto di faglia (Saraò et al.,1996). Il modello di velocità è quello proposto da Costa et al. (1992). La distribuzione del momento mostra somiglianze con i risultati ottenuti dell’inversione di dati sismici proposta da altri autori, confermando che la massima energia è nella parte SE della faglia. Nella seconda parte della tesi l’attenzione si è focalizzata sulla determinazione dei parametri di sorgente. Si è utilizzata la procedura implementata dal gruppo SeisRaM del Dipartimento di Matematica e Geoscienze, che stima in real-time il momento sismico, la magnitude da momento e la frequenza d’angolo. La determinazione della grandezza di un terremoto è un problema aperto. Esistono differenti scale di magnitudo e differenti metodi di calcolo, tanto da ottenere diversi valori per lo stesso evento, da parte dei diversi enti che li determinano. Nel Cap. 6 sono trattate le scale di magnitudo in uso ed in particolare la magnitudo da momento. Infine, sono descritti due metodi utilizzati per il calcolo in real-time della magnitudo da momento, tra cui il metodo di Andrews (1986) utilizzato nella procedura. Nel Cap. 7 dopo una descrizione della procedura automatica, si riportano la validazione ed i risultati. Questo metodo automatico stima in real-time i parametri di sorgente degli eventi delle Alpi sud orientali registrati dalla rete Transfrontaliera e da circa un anno anche degli eventi registrati dalla RAN. La procedura è stata validata sugli eventi recenti avvenuti in Italia e Slovenia: L’Aquila 2009, Parma 2008, Bovec 2004 e Carnia 2002. Il confronto della magnitudo da momento stimata della procedura in studio e quella calcolata con metodi di inversione da altri istituzioni è molto buono, dimostrando l’affidabilità e la robustezza di questo metodo. Questo è stato confermato dalla stima della magnitudo degli ultimi eventi avvenuti in Italia durante la scrittura finale di questa tesi: Verona, 24 gennaio 2012 e Reggio Emilia, 25 gennaio 2012. La magnitudo stimata in real-time dalla procedura è in ottimo accordo con quella stimata dall’INGV. Inoltre sia l’Agenzia sismologica della Slovenia, l’ARSO, che quella romena hanno richiesto di poter utilizzare questa procedura real-time. Questo lavoro spera di essere una fonte di utili suggerimenti nello studio dei processi di sorgente.
XXIV Ciclo
1979

La caractérisation des tremblements de terre est un domaine de recherche primordial en sismologie, où l'objectif final est de fournir des estimations précises d'attributs de la source sismique. Dans ce domaine, certaines questions émergent, par exemple : quand un tremblement de terre s’est-il produit? quelle était sa taille? ou quelle était son évolution dans le temps et l'espace? On pourrait se poser d'autres questions plus complexes comme: pourquoi le tremblement s'est produit? quand sera le prochain dans une certaine région? Afin de répondre aux premières questions, une représentation physique du phénomène est nécessaire. La construction de ce modèle est l'objectif scientifique de ce travail doctoral qui est réalisé dans le cadre de la modélisation cinématique. Pour effectuer cette caractérisation, les modèles cinématiques de la source sismique sont un des outils utilisés par les sismologues. Il s’agit de comprendre la source sismique comme une dislocation en propagation sur la géométrie d’une faille active. Les modèles de sources cinématiques sont une représentation physique de l’histoire temporelle et spatiale d’une telle rupture en propagation. Cette modélisation est dite approche cinématique car les histoires de la rupture inférées par ce type de technique sont obtenues sans tenir compte des forces qui causent l'origine du séisme.Dans cette thèse, je présente une nouvelle méthode d'inversion cinématique capable d'assimiler, hiérarchiquement en temps, les traces de données à travers des fenêtres de temps évolutives. Cette formulation relie la fonction de taux de glissement et les sismogrammes observés, en préservant la positivité de cette fonction et la causalité quand on parcourt l'espace de modèles. Cette approche, profite de la structure creuse de l’histoire spatio-temporelle de la rupture sismique ainsi que de la causalité entre la rupture et chaque enregistrement différé par l'opérateur. Cet opérateur de propagation des ondes connu, est différent pour chaque station. Cette formulation progressive, à la fois sur l’espace de données et sur l’espace de modèle, requiert des hypothèses modérées sur les fonctions de taux de glissement attendues, ainsi que des stratégies de préconditionnement sur le gradient local estimé pour chaque paramètre du taux de glissement. Ces hypothèses sont basées sur de simples modèles physiques de rupture attendus. Les applications réussies de cette méthode aux cas synthétiques (Source Inversion Validation Exercise project) et aux données réelles du séisme de Kumamoto 2016 (Mw=7.0), ont permis d’illustrer les avantages de cette approche alternative d’une inversion cinématique linéaire de la source sismique.L’objectif sous-jacent de cette nouvelle formulation sera la quantification des incertitudes d’un tel modèle. Afin de mettre en évidence les propriétés clés prises en compte dans cette approche linéaire, dans ce travail, j'explore l'application de la stratégie bayésienne connue comme Hamiltonian Monte Carlo (HMC). Cette méthode semble être l’une des possibles stratégies qui peut être appliquée à ce problème linéaire sur-paramétré. Les résultats montrent qu’elle est compatible avec la stratégie linéaire dans le domaine temporel présentée ici. Grâce à une estimation efficace du gradient local de la fonction coût, on peut explorer rapidement l'espace de grande dimension des solutions possibles, tandis que la linéarité est préservée. Dans ce travail, j'explore la performance de la stratégie HMC traitant des cas synthétiques simples, afin de permettre une meilleure compréhension de tous les concepts et ajustements nécessaires pour une exploration correcte de l'espace de modèles probables. Les résultats de cette investigation préliminaire sont encourageants et ouvrent une nouvelle façon d'aborder le problème de la modélisation de la reconstruction cinématique de la source sismique, ainsi, que de l’évaluation des incertitudes associées
The earthquake characterization is a fundamental research field in seismology, which final goal is to provide accurate estimations of earthquake attributes. In this study field, various questions may rise such as the following ones: when and where did an earthquake happen? How large was it? What is its evolution in space and time? In addition, more challenging questions can be addressed such as the following ones: why did it occur? What is the next one in a given area? In order to progress in the first list of questions, a physical description, or model, of the event is necessary. The investigation of such model (or image) is the scientific topic I investigate during my PhD in the framework of kinematic source models. Understanding the seismic source as a propagating dislocation that occurs across a given geometry of an active fault, the kinematic source models are the physical representations of the time and space history of such rupture propagation. Such physical representation is said to be a kinematic approach because the inferred rupture histories are obtained without taking into account the forces that might cause the origin of the dislocation.In this PhD dissertation, I present a new hierarchical time kinematic source inversion method able to assimilate data traces through evolutive time windows. A linear time-domain formulation relates the slip-rate function and seismograms, preserving the positivity of this function and the causality when spanning the model space: taking benefit of the time-space sparsity of the rupture model evolution is as essential as considering the causality between rupture and each record delayed by the known propagator operator different for each station. This progressive approach, both on the data space and on the model space, does require mild assumptions on prior slip-rate functions or preconditioning strategies on the slip-rate local gradient estimations. These assumptions are based on simple physical expected rupture models. Successful applications of this method to a well-known benchmark (Source Inversion Validation Exercise 1) and to the recorded data of the 2016 Kumamoto mainshock (Mw=7.0) illustrate the advantages of this alternative approach of a linear kinematic source inversion.The underlying target of this new formulation will be the future uncertainty quantification of such model reconstruction. In order to achieve this goal, as well as to highlight key properties considered in this linear time-domain approach, I explore the Hamiltonian Monte Carlo (HMC) stochastic Bayesian framework, which appears to be one of the possible and very promising strategies that can be applied to this stabilized over-parametrized optimization of a linear forward problem to assess the uncertainties on kinematic source inversions. The HMC technique shows to be compatible with the linear time-domain strategy here presented. This technique, thanks to an efficient estimation of the local gradient of the misfit function, appears to be able to rapidly explore the high-dimensional space of probable solutions, while the linearity between unknowns and observables is preserved. In this work, I investigate the performance of the HMC strategy dealing with simple synthetic cases with almost perfect illumination, in order to provide a better understanding of all the concepts and required tunning to achieve a correct exploration of the model space. The results from this preliminary investigation are promising and open a new way of tackling the kinematic source reconstruction problem and the assessment of the associated uncertainties

Nesta dissertação foi desenvolvida uma metodologia baseada no cruzamento de diferentes técnicas e de dados geodésicos e sísmicos, para estudar o processo de rotura de sismos. A metodologia consiste, numa primeira fase, em determinar o campo de deslocamento cossísmico produzido por um evento, através da técnica InSAR. Numa fase seguinte o modelo de deslocamentos cossísmicos é determinado através das equações de Okada utilizando o modelo de distribuição de deslizamentos obtido pela inversão das formas de onda registadas em estações de banda larga a distâncias telessísmicas. Para comparar o modelo de deslocamentos cossísmicos com o interferograma é aplicado um algoritmo que projeta os deslocamentos do modelo na direção do satélite e de seguida procura a região homóloga entre esse modelo e o interferograma, pelo cálculo da máxima correlação entre ambas as regiões, que resulta também na relocalização da fonte. O processo de inversão/modelação da deformação/comparação é repetido iterativamente até se conseguirem bons ajustes simultaneamente nos dados sísmicos e geodésicos. Esta metodologia foi aplicada no estudo dos sismos ocorridos a 12 de janeiro de 2010 no Haiti; a 22 de fevereiro de 2006 em Moçambique; e a 21 de maio de 2003 na Argélia. No estudo do sismo ocorrido no Haiti foi utilizado um par interferométrico do sensor ALOSPALSAR, relativo à órbita descendente 447, onde foi medido o deslocamento máximo de ~70 cm na direção do satélite. O conjunto de 32 registos das formas de onda permitiu obter o modelo de rotura e respetivos deslocamentos superficiais, para quatro soluções de parâmetros da geometria da falha. Após a comparação entre os modelos de deslocamentos e o interferograma é concluído que os parâmetros que melhor justificam a deformação observada no interferograma são: a falha orientada segundo um azimute de 262º, com uma inclinação de 42º para norte e um rake médio de 42º. No estudo do sismo de Moçambique foi usado um par interferométrico do satélite ENVISAT e um conjunto de 36 registos telessísmicos. Desta forma foi possível concluir que a rotura ocorreu na direção 165ºN numa falha com inclinação de 76º para oeste e os deslizamentos ocorreram com um rake médio de 90º, sobre uma falha com um comprimento de 40.6 km por 29 km de largura. Neste modelo de rotura foi obtido o momento sísmico de 3.9x1019Nm, com um deslizamento máximo de 4.1 m próximo do hipocentro. A modelação dos deslocamentos cossísmicos representa bem os deslocamentos observados no terreno e medidos no interferograma. Para o estudo do sismo de Zemmouri-Boumedès foram utilizados alguns pares interferométricos do satélite ENVISAT, as medições realizadas ao longo da costa da Argélia e um conjunto de 28 registos das formas de onda. Os interferogramas revelaram uma fraca coerência, mas mesmo assim foi possível observar 19 franjas (~53 cm) a oeste de Boumerdès. Os parâmetros que justificam os deslocamentos cossísmicos são: strike=64º; dip=50º; rake=97º. Este modelo permite gerar a sobre-elevação observada ao longo da costa, como a configuração das franjas interferométricas. O plano desta solução localiza-se no mar, a 9 km da linha de costa e o respetivo epicentro está localizado no mar; Modelling of active internal processes through observation data of surface deformation. ### Abstract: In this dissertation a methodology that consists of the cross of different techniques and geodetic and seismic data, to study the earthquake rupture process was developed. The methodology consists initially in determining the field of co-seismic displacements caused by an event using the InSAR technique. In a next step the co-seismic displacements model is determined by the equations of Okada using the model of rupture obtained from the inversion of waveforms recorded in the broadband stations at teleseismic distances. To compare the co-seismic displacement model with the interferogram is applied an algorithm that project the model of the displacements toward the satellite and is then applied to search the homologous region between the two region, which also results the re-location of the source. The process of inversion/modeling of the deformation/comparison is repeated iteratively until achieving good adjustments in both seismic and geodetic data. This methodology was applied in the study of the earthquakes that occurred on January 12, 2010 in Haiti, on February 22, 2006 in Mozambique, and on May 21, 2003 in Algeria. In the Haiti earthquake study an interferometric pair of the ALOS-PALSAR sensor of the descending orbit 447 was used, where it was measured the maximum co-seismic displacement of ~70 cm in the direction of the satellite. The set of 32 registers of the waveforms allows obtaining the model of the rupture and the displacements on the earth surface, for four solutions with different geometries parameters. After comparing the models of the displacements with the interferogram is concluded that the parameters that better explain the deformation observed in the interferogram is the fault azimuth of 262° with an inclination of 42º north and the rupture occurred with an rake of 42º. In the Mozambique earthquake study was used an interferometric pair of the ENVISAT satellite and a set of 36 teleseismic registration. Thus it was concluded that the rupture occurred with an azimuth of 165º North with an inclination of 76º westward, the slip occurred with a rake of 90°, on a fault with a length of 40.6 km to 29 km wide. The seismic moment obtained was 3.9x1019 Nm, the maximum slip was 4.1m near the hypocenter and the model of the displacements is well fit to the co-seismic displacements observed on the coastline and in the measurements in the interferogram. To study the earthquake Zemmouri-Boumedès were used some interferometric pairs of the ENVISAT satellite, the measurements along the coastline of Algeria and a set of 28 records of waveforms. The interferograms revealed a low coherence, but it was still possible to observe 19 fringes (~53 cm) west of Boumerdès. The parameters that better justify the coseismic displacements are strike=64°, dip=50°, rake=97º. This model allows us to cause the uplift observed along the coastline, such as the configuration of the interferometric fringes. The plan of this solution is located at the sea, 9 km of coastline and also its epicenter is located at the see.

Les séismes sont le résultat de glissements rapides le long de failles actives chargées en contraintes par le mouvement des plaques tectoniques. Il est aujourd'hui établi, au moins pour les grands séismes, que la distribution de ce glissement rapide le long des failles pendant les séismes est hétérogène. Imager la complexité de ces distributions de glissement constitue un enjeu majeur de la sismologie en raison des implications potentielles dans la compréhension de la genèse des séismes et la possibilité associée de mieux anticiper le risque sismique et les tsunamis. Pour améliorer l'imagerie de ces distributions de glissement co-sismique, trois axes peuvent être suivis: augmenter les contraintes sur les modèles en incluant plus d'observations dans les inversions, améliorer la modélisation physique du problème direct et progresser dans le formalisme de résolution du problème inverse. Dans ce travail de thèse, nous explorons ces trois axes à travers l'étude de deux séismes majeurs: les séisme de Tohoku-Oki (Mw 9.0) et de Sumatra-Andaman (Mw 9.1-9.3) survenus en 2011 et 2004, respectivement
Earthquakes are the results of rapid slip on active faults loaded in stress by the tectonic plates motion. It is now establish - at least for large earthquakes - that the distribution of this rapid slip along the rupturing faults is heterogeneous. Imaging the complexity of such slip distributions is one the main challenges in seismology because of the potential implications on understanding earthquake genesis and the associated possibility to better anticipate devastating shaking and tsunami. To improve the imaging of such co-seismic slip distributions, three axes may be followed: increase the constraints on the source models by including more observations into the inversions, improve the physical modeling of the forward problem and improve the formalism to solve the inverse problem. In this PhD thesis, we explore these three axes by studying two recent major earthquakes: the Tohoku-Oki (Mw 9.0) and Sumatra-Andaman (Mw 9.1-9.3) earthquakes, which occured in 2011 and 2004 respectively

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.

There are many examples of earthquakes whose surface expressions are much more complicated than the seismologically derived faulting models. Seismologists also have found seismic source complexity and improved seismicity data have shown that rupture may occur on irregular or multiple shear surfaces. To simultaneously map both geometrical and temporal variation of the seismic sources for a complex rupture history from observed seismograms, it is possible to use a time dependent moment tensor (TDMT) inversion. The TDMT inversion algorithm has been tested with three synthetic data examples with varying degrees of complexity. The first example demonstrates that a multiple source with no focal depth change can be recovered, and the source parameters of each of the subevents can be accurately determined. In the second case we allowed the depth to vary for subevents (9-km and 13-km source depth, respectively). The two subevents can be identified on the basis of simultaneous shape-change of the moment tensor elements along with non-causality and the size of the CLVD component. The third example introduced source complexity by having two subevents which overlapped in time. The overlapped period could be seen in the moment tensor elements as unusually abrupt changes in the time function shape. The TDMT inversion was also performed on long-period body waves for three earthquakes: the 1982 Yemen earthquake, the 1971 San Fernando earthquake, and the 1952 Kern County earthquake. The Yemen earthquake was mapped as two simple, normal-slip subevents (with onset timing of the second subevent 5 seconds after the first) without a significant component of left- or right-lateral displacement or source depth change. The San Fernando earthquake is interpreted as two shear dislocation sources with changing source depths, possibly indicating upward rupture propagation (from 13-km to 7-km). The interpretation of the TDMT inversion for the Kern County earthquake was also a double point source which propagates upward from 20-km to 5-km. The resultant moment tensor functions from inversion of the synthetic waveforms, a combination of isotropic and tectonic release, demonstrated that the tectonic release associated with underground nuclear explosion can be separated and identified if the source depth between the explosions and tectonic release are different.

2006/2007
The Scotia Sea region is found between the south American and Antarctic plates and constitutes a complex area tectonics, characterized from numerous active processes and changes in the movement and in the configuration of the plates. The main tectonics characteristics of the Scotia Sea were object of different studies, nevertheless some details of the interactions tectonics, of the margin of plates and of their relative movement remain still uncertain. In this sense, the determination of the features of the lithosphere and the study of the focal mechanisms develop an important role to understand the geodynamic evolution of the area. This study proposes to use the present technologies for the inversion of wave- forms to the end of to obtain the source mechanisms for a series of earthquakes recorded in the proximity of the Antarctic Base Argentina Orcadas. The appli- cation of the method is possible thanks to the digital seismograms recorded from a regional network installed to leave from 1992 and that includes the Antarctic Seismographic Argentinean Italian Network (ASAIN) and other three seismo- graphic stations of the Global Seismographic Network (GSN) operating in the antarctic Peninsula, Tierra del Fuego and in the islands of the Scotia arc. Were analyzed and reproduced seven events that followed the earthquake of magnitude 7,6 Mw 4 August 2003 known in literature like Centenary Earthquake. The main unit of the study (Chapter 3) is preceded of two chapters dedicated respectively to a detailed description of the situation tectonics and the seismicity of the Scotia Sea region (Chapter 1) and the Italian-Argentinean seismographic network ASAIN with special attention for the response in frequency of the seismograph and the characterization of the seismic noise levels (Chapter 2). Like complement to the analysis of the focal mechanisms, applying the Omori’s law, it has determined the curve of temporal decay of the seismic se- quence recorded from the Orcadas station in sixty days following to the Cente- nary Earthquake.
XIX Ciclo
1960

This dissertation is a comprehensive summary of four papers on the development and application of new strategies for locating tremor and relocating events in earthquake catalogs. In the first paper, two new strategies for relocating events in a catalog are introduced. The seismicity pattern of an earthquake catalog is often used to delineate seismically active faults. However, the delineation is often hindered by the diffuseness of earthquake locations in the catalog. To reduce the diffuseness and simplify the seismicity pattern, a relocation and a collapsing method are developed and applied. The relocation method uses the catalog event density as an a priori constraint for relocations in a Bayesian inversion. The catalog event density is expressed in terms of the combined probability distribution of all events in the catalog. The collapsing method uses the same catalog density as an attractor for focusing the seismicity in an iterative scheme. These two strategies are applied to an aftershock sequence after a pair of earthquakes which occurred in southwest Iceland, 2008. The seismicity pattern is simplified by application of the methods and the faults of the mainshocks are delineated by the reworked catalog. In the second paper, the spatial distribution of seismicity of the Hengill region, southwest Iceland is analyzed. The relocation and collapsing methods developed in the first paper and a non-linear relocation strategy using empirical traveltime tables are used to process a catalog collected by the Icelandic Meteorological Office. The reworked catalog reproduces details of the spatial distribution of seismicity that independently emerges from relative relocations of a small subset of the catalog events. The processed catalog is then used to estimate the depth to the brittle-ductile transition. The estimates show that in general the northern part of the area, dominated by volcanic processes, has a shallower depth than the southern part, where tectonic deformation predominates. In the third and the fourth papers, two back-projection methods using inter-station cross correlations are proposed for locating tremor sources. For the first method, double correlations, defined as the cross correlations of correlations from two station pairs sharing a common reference station, are back projected. For the second method, the products of correlation envelopes from a group of stations sharing a common reference station are back projected. Back projecting these combinations of correlations, instead of single correlations, suppresses random noise and reduces the strong geometrical signature caused by the station configuration. These two methods are tested with volcanic tremor at Katla volcano, Iceland. The inferred source locations agree with surface observations related to volcanic events which occurred during the tremor period.

Sedimentary rock-hosted deposits are a major source of copper and cobalt, with the Neoproterozoic central African Copperbelt being among the largest Cu-Co provinces in the world, accounting for around 15% of its copper resource. The deposits occur primarily in the carbonates and siliciclastic sediments overlying the basement, and formed during early diagenesis (around 820 Ma) and late diagenesis/metamorphism during the Pan-African Orogeny (580-520 Ma). The northwest province of Zambia hosts three major copper deposits, amongst which is Kansanshi: the focus of this study. The deposit, which lies north of the Solwezi dome, is hosted within the Katangan Supergroup, particularly within the carbonaceous phyllites and porphyroblastic schists of the Mshwaya subgroup and lower Nguba Group and extends along the strike length of the North-West trending Kansanshi antiform. In this study, tomographic inversion is applied to first arrival refraction data collected at the Kansanshi Copper Mine with the aim of locating potential copper-bearing structures.  The survey was carried out using both dynamite and VIBSIST sources along 3 profiles; 2 trending North-East across the Kansanshi anticline and 1 trending north-west parallel to it. Seismic refraction tomography is an excellent tool for investigating the shallow subsurface, providing a velocity distribution. Unlike conventional refraction seismics, it allows for the velocity calculation of each cell in a non-homogeneous earth model, rather than just the average velocity of individual layers - allowing us to map structure and infer geological units and weathering profiles. The data highlights abundant faulting and varying depth to fresh bedrock. The various lithologies have also been interpreted.

Mise en place d'un modèle de propagation sismique moyen à travers le lagon de l'atoll Mururoa et calcul de sismogrammes synthétiques, en champ intermédiaire (5 à 20km) par la technique de sommation des nombres d'ondes discrets. Utilisation du modèle de terrain et d'une méthode d'inversion linéaire de moments sismiques pour estimer les termes de source élastique équivalents à la source nucléaire. Seule la partie isotrope est considérée ici. La stabilité de la solution est accrue par utilisation d'un lissage spectral implicite et d'une hypothèse de phase minimale

Ce travail de these a eu pour objectif d'utiliser des enregistrements effectues a distance regionale pour determiner la nature d'une source sismique et etudier sa complexite eventuelle, a savoir l'existence de plusieurs sous-evenements. A l'issu d'une synthese bibliographique, nous avons choisi d'utiliser une methode d'inversion lineaire qui permet d'estimer le tenseur des moments sismiques representant la source. Dans un premier temps, nous avons teste le programme d'inversion sur des donnees numeriques, en s'affranchissant des problemes lies a la modelisation de la propagation. L'influence du type d'ondes inversees et de la profondeur de la source sur la qualite du resultat de l'inversion a ete evaluee, ainsi que l'impact de la geometrie du reseau de stations et du type de source. Dans un deuxieme temps, nous avons traite trois cas d'evenements reels. Nous nous sommes tout d'abord interesses a un effondrement de mine, de magnitude 4. 8, dont les caracteristiques de la source etaient a priori connues, ce qui nous a permis de mettre au point une methodologie d'inversion. Nous avons egalement mis en evidence l'existence d'une composante isotrope de la source de l'ordre de 35%. Nous avons ensuite etudie le seisme de roermond (avril 1992) comme un evenement tectonique de reference. Le mecanisme obtenu par l'inversion des donnees basses frequences est coherent avec les etudes precedemment publiees. Des inversions a plus hautes frequences ont permis d'evaluer la limite de validite de la methode. Enfin, nous avons traite un evenement tectonique de plus petite taille, pour lequel un enregistrement en champ proche etait disponible. Nous avons montre que les mecanismes issus de l'inversion de donnees regionales d'une part, et de champ proche d'autre part, sont compatibles entre eux, ainsi qu'avec le mecanisme issu de l'etude du sens des premiers mouvements.

Presented in this thesis are some basic concepts and applications of seismic hazard analysis and the elements that influence the amplitude and geometric attenuation of earthquake ground motion. This thesis centers on the identification of the styles of failure, focal mechanisms, and the state of regional stress in the study area. Seismic hazard is a complex problem often involving considerable uncertainties. Therefore it is reasonable to consider different seismic hazard analysis approaches in order to as robustly as possible define zones of different levels of hazard. With the aim of characterizing and quantifying hazard in the east African region of Eritrea and its surroundings, a study is included in the thesis presenting hazard maps constructed using two non-parametric probabilistic seismic hazard analysis (PSHA) approaches. Peak ground acceleration (PGA) values for 10% probability of exceedence in 50 years are computed at given grid points for the whole selected area and results from both methods are compared. Other aspects addressed in the thesis include the determination of source parameters of selected earthquakes that occur in the Afar region. The styles of faulting, the mechanisms involved during the rupture process and the states of stress along the major tectonic features are also highlighted. Source parameters for selected events in the region were re-evaluated and improved solutions obtained. An aftershock sequence in the Hengill volcanic area in SW Iceland, following the major event that occurred on June 4, 1998, was used to investigate improved methodologies for moment tensor using a relative approach. The sensitive and spatially dense seismic network in this area reveals large sets of clustered events allowing the power of the new methodology to be demonstrated and providing greater insight into the tectonic implications of the activity in the area.

Cette these presente une methode d'etude de la source sismique composee de deux etapes. La premiere etape consiste a deconvoluer, a chaque station, le sismogramme du seisme etudie par la fonction de green pour retrouver la fonction source temporelle apparente. Afin d'eliminer le plus efficacement possible les effets de la propagation des ondes, les effets du site et de la reponse instrumentale, on utilise un seisme petit par rapport a celui que l'on etudie et localise au meme endroit comme fonction de green empirique. L'operation de deconvolution etant souvent instable par les methodes classiques, nous avons developpe une nouvelle technique basee sur une inversion non lineaire par recuit simule afin d'obtenir une fonction source stable et positive. Un calcul base sur la theorie des validations croisees appliquee aux sismogrammes a trois composantes permet d'estimer les incertitudes sur les fonctions source obtenues. Dans la deuxieme etape, l'information obtenue sur les fonctions source temporelles de plusieurs stations est retro-propagee au niveau du plan de faille. En utilisant une inversion par recuit simule, on reconstitue la distribution du glissement sur le plan de faille pendant le seisme. Trois applications de la methode sont presentees pour des seismes d'italie du sud (naples), d'italie du nord (ligure) et de la region de patras en grece

Nous avons developpe une methode de sommation de faisceaux gaussiens (gbs) afin de modeliser des tremblements de terre de magnitudes 5-6 aux distances epicentrales comprises entre 17\ et 30\. La modelisation depend du choix d'un modele de manteau superieur approprie. Des etudes synthetiques de structures du manteau comme la zone a faible vitesse ou des discontinuites de vitesse ont montre que la sommation de faisceaux gaussiens peut etre employee pour determiner un modele correct. En utilisant l'evenement bien connu de kosani-grevena qui a eu lieu le 13 mai 1995 en grece, nous avons choisis une colonne de 3smac comme modele moyen de manteau superieur pour modeliser des seismes mediterraneens enregistres en europe de l'ouest. Nous avons etudie quatre evenements ayant des localisations, des magnitudes et des contextes tectoniques differents : le seisme du 20 juillet 1996 pres des iles dodecanese, l'evenement turc du 1 e r octobre 1995, le seisme de chypre du 23 fevrier 1995 et celui du 27 juin 1998 au niveau de la terminaison sud de la faille est-anatolienne. Pour tous les seismes, nous avons determine la profondeur, la fonction source et le moment sismique. Pour le tremblement de terre du 1 e r octobre 1995, une localisation et un mecanisme en accord avec la tectonique regionale ont ete egalement determines. L'etude du seisme du 27 juin 1998 n'est que preliminaire. Neanmoins, nous avons pu determiner une solution de source ponctuelle avec deux sous-evenements bien que ce tremblement de terre semble avoir une histoire complexe de rupture. Les phases du manteau superieur pour tous ces evenements sont bien identifiees et modelisees. Nous concluons que la sommation des faisceaux gaussiens peut etre employee pour modeliser des sismogrammes aux distances intermediaires, permettant d'obtenir les parametres de la source et, dans l'avenir, d'ameliorer notre connaissance de la structure du manteau superieur.

Micro-seismology is attracting more and more attention in the exploration seismology community. The main goal in micro-seismic imaging is to find the source location and the ignition time in order to track the fracture expansion, which will help engineers monitor the reservoirs. Conventional imaging methods work fine in this field but there are many limitations such as manual picking, incorrect migration velocity and low signal to noise ratio (S/N). In traditional surface survey imaging, full waveform inversion (FWI) is widely used. The FWI method updates the velocity model by minimizing the misfit between the observed data and the predicted data. Using FWI to locate and image microseismic events allows for an automatic process (free of picking) that utilizes the full wavefield. Use the FWI technique, and overcomes the difficulties of manual pickings and incorrect velocity model for migration. However, the technique of waveform inversion of micro-seismic events faces its own problems. There is significant nonlinearity due to the unknown source location (space) and function (time). We have developed a source independent FWI of micro-seismic events to simultaneously invert for the source image, source function and velocity model. It is based on convolving reference traces with the observed and modeled data to mitigate the effect of an unknown source ignition time. The adjoint-state method is used to derive the gradient for the source image, source function and velocity updates. To examine the accuracy of the inverted source image and velocity model the extended image for source wavelet in z-axis is extracted. Also the angle gather is calculated to check the applicability of the migration velocity. By inverting for the source image, source wavelet and the velocity model simultaneously, the proposed method produces good estimates of the source location, ignition time and the background velocity in the synthetic experiments with both parts of the Marmousi and the SEG Overthrust model. On the other hand, a new imaging condition of natural Green’s function has been implemented to mitigate the effect of the unknown velocity model. It is based on putting receivers in a horizontal well close to the micro-seismic events so that only a small part of the velocity model is required for the imaging. In order to focus the multi scattering energy to the source location, as well as to suppress the influence of the noise in the data, we introduced a new method to compensate the energy in the receiver wavefield. It is based on reflection waveform inversion (RWI) theory. We simply migrate for the scatters (reflectors) in the medium, and set the image as a secondary source to compensate for the multi scattering energy in the receiver wavefield. By applying the same imaging condition, the energy of those scattering events can be traced to the source location. Thus the source point has higher energy in the source image. A simple two-layer medium test demonstrates the features.

Reflection seismologists illuminate the subsurface by introducing energy into the ground. These propagating waves encounter heterogeneities in the subsurface material and are partly reflected back up to the surface where they are recorded as seismograms. The seismic energy source in most cases cannot be reliably measured in a laboratory but must be accurately estimated to allow one to extract the physical parameters which characterize the subsurface (such as velocity and density). The source and multiple earth parameters may be simultaneously successfully estimated by inversion. When the seismogram model is the plane-wave convolutional model derived from the constant density, variable sound velocity acoustic wave equation, perturbations in the seismic data stably determine perturbations in the source and reflectivity (the high-frequency relative fluctuation in the velocity). The stability of this determination improves as the angular range over which the data is defined increases. A more realistic model for wave propagation in the earth is the plane-wave convolutional model derived from the viscoelastic wave equation. Waveform inversion applied to field data from the Gulf of Mexico successfully estimates the long-wavelength compressional velocity, three elastic parameter reflectivities, and the anisotropic seismic source. The resulting reflectivities match measured well log data and agree with commonly-accepted lithological relationships. These inversion results predict 70% of the total seismic data and 90% of the data in an interval around the gas sand target. The resolution matrix measures how close inversion-estimated reflectivities are to the true parameters which generated the data and is useful when independent information such as well logs is unavailable. However, computing the resolution matrix from the singular value decomposition of the forward map (the usual technique) is prohibitive for real seismic inverse problems. Instead we approximate the resolution matrix from Lanczos estimates of the eigenvectors of the normal matrix. The resolution matrix indicates that our inversion-estimated source provides well resolved reflectivities in the depth interval of interest.

Seismic waves are used to study the Earth, exploit its hydrocarbon resources, and understand its hazards. Extracting information from seismic waves about the Earth’s subsurface, however, is becoming more challenging as our questions become more complex and our demands for higher resolution increase. This dissertation introduces two new methods that use scattered waves for improving the resolution of subsurface images: natural migration of passive seismic data and convergent full-waveform inversion. In the first part of this dissertation, I describe a method where the recorded seismic data are used to image subsurface heterogeneities like fault planes. This method, denoted as natural migration of backscattered surface waves, provides higher resolution images for near-surface faults that is complementary to surface-wave tomography images. Our proposed method differ from contemporary methods in that it does not (1) require a velocity model of the earth, (2) assumes weak scattering, or (3) have a high computational cost. This method is applied to ambient noise recorded by the US-Array to map regional faults across the American continent. Natural migration can be formulated as a least-squares inversion to furtherer enhance the resolution and the quality of the fault images. This inversion is applied to ambient noise recorded in Long Beach, California to reveal a matrix of shallow subsurface faults. The second part of this dissertation describes a convergent full waveform inversion method for controlled source data. A controlled source excites waves that scatter from subsurface reflectors. The scattered waves are recorded by a large array of geophones. These recorded waves can be inverted for a high-resolution image of the subsurface by FWI, which is typically convergent for transmitted arrivals but often does not converge for deep reflected events. I propose a preconditioning approach that extends the ability of FWI to image deep parts of the velocity model, which significantly improves the chances for finding hydrocarbon deposits.

One of the most important aspects of seismology is explaining the generation of seismic waves during earthquakes. The first mathematical models of earthquakes involved shear faulting, where deformation of rocks surrounding the fault increases the stress level, causes rock fracturing and results in radiation of elastic waves. Over the years, a large number of earthquakes that cannot be explained only with shear faulting have been observed. Hence, the mathematical model of seismic sources evolved into a seismic moment tensor (MT), which also includes isotropic and compensated linear vector dipole components. Although uncertainties in MT inversions are important for estimating solution robustness, they are rarely available. Furthermore, noise in the data can alter the waveform and cause spurious non-double-couple components. In this thesis, I address these issues using Bayesian hierarchical inversion, a relatively novel technique in seismology. Its probabilistic approach gives an ensemble of solutions instead of just one best-fit solution, thus, it can be used to estimate MT uncertainties. The algorithm developed as a part of this thesis uses waveform data of regional earthquakes and explosions with moderate magnitudes to compute the centroid location and the seismic moment tensor. The algorithm includes a sophisticated treatment of data noise utilising an empirical noise covariance matrix, and including the level of noise as an unknown in the inversion. As a result, the model complexity is determined by the data themselves. There are two major groups of events for which the Bayesian approach can be of great importance, and to which the algorithm has been applied. The first one is seismic events in complex geological environments, such as volcanic and geothermal areas. A significant number of these events are expected to have source processes that require the full MT. The second group is explosions, where the algorithm can be valuable for nuclear proliferation. The feasibility of the approach is initially demonstrated on synthetic data contaminated with noise. It is shown that the empirical covariance matrix improves the location estimate. This is followed by application to a well-studied earthquake from Long Valley caldera, a volcanic environment in California, where a statistically significant isotropic component of the source is confirmed. The method was further improved to include multiple noise parameters that determine the fit on each record, and in turn weight the stations' contribution in the inversion. Subsequently, I have analysed several earthquakes from a geothermal field in California, The Geysers. The double-couple components of the sources agree well with the regional stress field, but the non-double-couple components show a variety of values. Finally, the method is applied to the 2013 Democratic People's Republic of Korea nuclear explosion. Since the paths to the recording stations in the region traverse significantly different crustal structures, a linear inversion was initially conducted to create a composite structural model that better explained the oceanic raypaths. The Bayesian inversion shows exceptionally low uncertainties in the moment tensor solution for this event, characterising it as a crack mechanism, which explains the non-isotropic radiation as a result of material damage.

Ricostruzione del processo di rottura cosismico su faglia finita attraverso l’inversione congiunta di dati sismologici e geodetici. Implementazione e validazione di una nuova tecnica di inversione non lineare, di tipo global search, per l’inversione congiunta di dati GPS e dati strong motion. Analisi statistica dell’ensemble dei modelli di rottura esplorati dall’algoritmo di inversione. Analisi sulla consistenza dinamica dei modelli cinematici di rottura. Applicazioni: (1) Test Sintetici atti a validare la capacità di risoluzione e robustezza della tecnica sviluppata; (2) Analisi del terremoto di Tottori (2000); (3) Analisi del terremoto di Niigata (2007); (4) Determinazione di scenari di scuotimento in aree di interesse prioritario e strategico (terremoto dell’Irpinia, 1980).
Istituto Nazionale di Geofisica e Vulcanologia
Unpublished
3.1. Fisica dei terremoti
open

碩士
國立臺灣大學
海洋研究所
104
We apply ambient noise analysis to continuous seismic waveform data recorded by ocean-bottom seismometers (OBS) deployed in the Huatung Basin and adjacent regions off the east coast of Taiwan. Taiwan is a young and active orogenic belt resulting from the oblique subduction and collision between the Eurasian Plate and the Philippine Sea Plate. Sitting on the westernmost edge of the Philippine Sea Plate, the Huatung Basin is directly involved in the subduction-collision processes. The structural characteristics of the basin provide important constraints not only on its own history but the tectonic evolution in this complex region. We first analyze the noise signals by stacking cross-correlation functions from station pairs between OBS stations and between land and OBS stations. We find that, unsurprisingly, the noise power of the offshore OBS stations is larger than that of onshore stations. Further, we integrate the OBS data with selected land station data along the east coast, deriving Rayleigh wave Green’s functions from cross-correlation between all available station pairs. We measure phase velocity dispersion at periods from 4 to 20 sec, and invert for 2-D anisotropic phase velocity maps based on a wavelet-based multi-scale inversion scheme. Our results show that, at periods 12-20 s, significant seismic anisotropy is present in the Huatung Basin close to Taiwan’s southeast coast, with fast direction sub-parallel to the direction of convergence. On the other hand, to the north across the Ryukyu trench, anisotropy becomes weaker and the fast direction appears to rotate clockwise toward NW-SE to N-S directions. These characteristics represent the property of the mantle lithosphere of the Huatung Basin, implying for current influence of asthenospheric flow due to currently the PSP plate motion.

A genetic algorithm has been developed for the inversion of seismic traveltime data and applied to a tomographic investigation of central-eastern Tasmania. The investigation used four shots and twelve stations across a 100 km east-west section of the post Devonian cover of the Tasmania Basin. This area lies across the inferred but poorly understood contact between the Eastern and Western Tasmanian Terranes, which has proven difficult to image in previous geophysical studies. The genetic algorithm developed in this project is portable across all parallel and non-parallel Unix-based computing platforms, and interfaces with an existing, ad- vanced, fast marching forward model code. The algorithm was applied both at low resolution with subsequent model refinement by a subspace inversion method in a two- step approach, and at higher resolution to directly invert the data using a one-step approach. The one-step implementation yielded superior exploration of the model space, and sufficient exploitation of the possible solutions when applied to the sparse noisy data acquired during the controlled source investigation. This demonstrates the viability of a one-step Monte-Carlo approach to seismic traveltime tomography in cases of sparse data coverage. The results of the inversions show a high velocity anomaly at 6 km depth and 147.4 degrees longitude, coincident with a long-wavelength magnetic anomaly, and is interpereted as an ultramafic unit of possible oceanic crustal affinity. This supports both thick and thin-skinned tectonic models with oceanic crust beneath the Eastern Tasmanian Terrane, though the thin-skinned scenario is preferred on the basis of ex- isting gravity data. A model is suggested in which this oceanic unit is part of the allochthonous boninite-tholeiite stack overlying the Western Tasmanian Terrane.

Nel presente lavoro la dinamica dei processi sismogenetici è stata studiata attraverso un metodo innovativo basato su una soluzione dell’equazione dell’elastodinamica che esprime lo sforzo di taglio agente sul piano di faglia come funzione della velocità di dislocazione e della sua evoluzione temporale. Il dato di ingresso della procedura numerica è quindi l’evoluzione nel tempo della velocità di dislocazione in ciascun punto del piano di faglia. Questo metodo permette di vincolare l’evoluzione della trazione in funzione del tempo e della posizione sulla faglia e consente quindi la stima dei principali parametri dinamici per terremoti reali. Il vantaggio di tale modello è che non viene imposta una legge costitutiva a priori. La procedura numerica è stata applicata a forti terremoti reali, per i quali sono disponibili i modelli cinematici che descrivono la propagazione della rottura cosismica, allo scopo di studiare il comportamento meccanico delle strutture sismogenetiche ed i meccanismi responsabili del rilascio di energia. L’applicazione del metodo ha prodotto risultati originali ed interessanti: gli andamenti della trazione in ciascun punto del piano di faglia, sia in funzione del tempo sia in funzione della dislocazione, mostrano l’andamento atteso in base all’interpretazione teorica del processo di propagazione della rottura cosismica, ovvero un chiaro andamento di tipo ‘dynamic weakening’. I parametri dinamici risultano ben vincolati, sebbene dipendenti dalla risoluzione dei modelli cinematici, e mostrano una distribuzione eterogenea sul piano di faglia. Un parametro molto importante ottenuto dagli andamenti della trazione dinamica è il breakdown work che, come definito da Tinti et al. (2005), fornisce una stima dell’energia spesa per far propagare il fronte di rottura. I valori ottenuti per i diversi terremoti sono in accordo con quelli pubblicati recentemente in letteratura (Rice et al., 2005; Tinti et al., 2005) e dimostrano che il breakdown work costituisce un contributo commensurabile alle stime dell’energia irradiata.
Università degli studi di Roma La Sapienza, Istituto Nazionale di Geofisica e Vulcanologia
Unpublished
4T. Fisica dei terremoti e scenari cosismici
open

The basic idea of this thesis is to exploit the capabilities of neural networks in a very new framework: the quantitative modelling of the seismic source and the interferogram inversion for retrieving its geometric parameters. The problem can be sum up as follows. When a moderateto- strong earthquake occurs we can apply SAR Interferometry (InSAR) technique to compute a differential interferogram. The latter is used to detect and measure the surface displacement field. The earthquake has been generated by an active, seismogenic, fault having its own specific geometry. Therefore each differential interferogram contains the information concerning the geometry of the seismic source the earthquake comes from; its shape and size, the number of fringes, the lobe orientation and number are the main features of the surface effects field. Two problems have been analysed in this work. The first is the identification of the seismic source mechanism. The second is a typical inversion exercise concerning the fault plane parameter. To perform both exercises of the seismic fault a huge number of synthetic interferograms has been computed. Each of them is generated by a different combination of such geometric parameters. As far as the retrieval of the geometric parameters is concerned an artificial neural network has been properly generated and trained to provide an inversion procedure to single out the geometric parameters of the fault. Five among these latter, Length, Width, Dip, Strike, Depth, have been simultaneously inverted. The result is in agreement with those results based on different approaches. Furthermore the method seems very promising and leads to improve the studies concerning the combined use of neural networks and InSAR technique.
Tor Vergata University
Unpublished
open

The primary focus of this thesis is to examine the detailed seismic structure of the northern Cascadia margin, including the Cascadia basin, the deformation front and the continental shelf. The results of this study are contributing towards understanding sediment deformation and tectonics on this margin. They also have important implications for exploration of hydrocarbons (oil and gas) and natural hazards (submarine landslides, earthquakes, tsunamis, and climate change). The first part of this thesis focuses on the role of gas hydrate in slope failure observed from multibeam bathymetry data on a frontal ridge near the deformation front off Vancouver Island margin using active-source ocean bottom seismometer (OBS) data collected in 2010. Volume estimates (∼ 0.33 km^3) of the slides observed on this margin indicate that these are capable of generating large (∼ 1 − 2 m) tsunamis. Velocity models from travel time inversion of wide angle reflections and refractions recorded on OBSs and vertical incidence single channel seismic (SCS) data were used to estimate gas hydrate concentrations using effective medium modeling. Results indicate a shallow high velocity hydrate layer with a velocity of 2.0 − 2.1 km/s that corresponds to a hydrate concentration of 40% at a depth of 100 m, and a bottom simulating reflector (BSR) at a depth of 265 − 275 m beneath the seafloor (mbsf). These are comparable to drilling results on an adjacent frontal ridge. Margin perpendicular normal faults that extend down to BSR depth were also observed on SCS and bathymetric data, two of which coincide with the sidewalls of the slump indicating that the lateral extent of the slump is controlled by these faults. Analysis of bathymetric data indicates, for the first time, that the glide plane occurs at the same depth as the shallow high velocity layer (100±10 mbsf). In contrast, the glide plane coincides with the depth of the BSR on an adjacent frontal ridge. In either case, our results suggest that the contrast in sediments strengthened by hydrates and overlying or underlying sediments where there is no hydrate is what causing the slope failure on this margin. The second part of this dissertation focuses on obtaining the detailed structure of the Cascadia basin and frontal ridge region using mirror imaging of few widely spaced OBS data. Using only a small airgun source (120 cu. in.), our results indicate structures that were previously not observed on the northern Cascadia margin. Specifically, OBS migration results show dual-vergence structure, which could be related to horizontal compression associated with subduction and low basal shear stress resulting from over-pressure. Understanding the physical and mechanical properties of the basal layer has important implications for understanding earthquakes on this margin. The OBS migrated image also clearly shows the continuity of reflectors which enabled the identification of thrust faults, and also shows the top of the igneous oceanic crust at 5−6 km beneath the seafloor, which were not possible to identify in single-channel and low-fold multi-channel seismic (MCS) data. The last part of this thesis focuses on obtaining detailed seismic structure of the Vancouver Island continental shelf from MCS data using frequency domain viscoacoustic full waveform inversion, which is first of its kind on this margin. Anelastic velocity and attenuation models, derived in this study to subseafloor depths of ∼ 2 km, are useful in understanding the deformation within the Tofino basin sediments, the nature of basement structures and their relationship with underlying accreted terranes such as the Crescent and the Pacific Rim terranes. Specifically, our results indicate a low-velocity zone (LVZ) with a contrast of 200 m/s within the Tofino basin sediment section at a depth 600 − 1000 mbsf over a lateral distance of 10 km. This LVZ is associated with high attenuation values (0.015 − 0.02) and could be a result of over pressured sediments or lithology changes associated with a high porosity layer in this potential hydrocarbon environment. Shallow high velocities of 4 − 5 km/s are observed in the mid-shelf region at depths > 1.5 km, which is interpreted as the shallowest occurrence of the Eocene volcanic Crescent terrane. The sediment velocities sharply increase about 10 km west of Vancouver Island, which probably corresponds to the underlying transition to the Mesozoic marine sedimentary Pacific Rim terrane. High attenuation values of 0.03 − 0.06 are observed at depths > 1 km, which probably corresponds to increased clay content and the presence of mineralized fluids.
Graduate
0373
0372
0605
subbarao@uvic.ca

This thesis presents a concise introduction to the spectral-element method and its applications to the simulation of seismic wave propagation in 3-D earth models. The spectral-element method is implemented in the regional scale for a 3-D integrated southern California velocity model. Significantly better waveform fits are achieved for the 3-D synthetics calculated compare to the 1-D synthetics generated from the 1-D standard southern California model, especially for many basin stations where strong amplifications are observed due to the very low wave-speed sediments. A hypothetical earthquake rupturing from northeast to southwest at the southern end of the San Andreas fault is simulated to improve our understanding of the seismic hazards in the Salton Trough region.

With the improved 3-D Green's function, we perform source inversions for both the source mechanisms and event depths of $M_w geq 3.5$ earthquakes in southern California. The inversion results generally agree well with the results obtained by other traditional methods, but with significantly more stations used in the inversions. Time shifts are generally required to align the data and the synthetics, which provides a great dataset for the improvement of the 3-D velocity models in southern California.

We use the adjoint method to formulate the tomographic inverse problem based upon a 3-D initial model. We calculate the sensitivity kernels, a key component of the tomographic inversion, that relate the perturbations of observations to the perturbations of the model parameters. These kernels are constructed by the interaction of the regular forward wavefield and the adjoint wavefield generated by putting the time-reversed signals at the receivers as simultaneous adjoint sources. We compute the travel-time sensitivity kernels for typical phases in both regional and global problems for educational purposes, and outline the procedures of applying the conjugate-gradient method to solve both source and structural inversion problems.