Дисертації з теми "Fault rheology"

In seismically active regions, faults nucleate, propagate, and form networks that evolve over time. Progressive strain localization and periodic fault pattern re-configuration induce the accumulation and healing of fault zone damage. The damage zones are characterized by distributed fractures, veins, and secondary faults, and may act as barriers for propagating earthquake ruptures, or as nucleation sites for earthquakes. They interact with seismic waves, promoting strong surface motions during earthquakes, and can focus fluid flow and enhance mineralization. In spite of their great scientific, social, and economic significance, interactions between these evolving damage zones and crustal deformation remain unresolved. Indeed, geodynamic models generally treat active faults as surfaces embedded in a medium with non-evolving material properties. For my dissertation projects, I have simulated fault system evolution over thousands of years, applying a rheological model which incorporates concepts of damage mechanics. This model accounts for crack nucleation, growth and concentration (i.e., material degradation), macroscopic failure, and material healing. My Simulations show that strike-slip faults form as segmented structures before evolving into contiguous, simpler structures. Flower structures rapidly form along fault segments (before a total offset of 0.05 km), and stepovers display extensive, permanent damage and ongoing seismicity throughout the seismogenic crust. My models also indicate that the “effectiveness” of material healing strongly affects the spatial extent of damage zones and long-term fault complexity. Effective healing promotes rapid evolution of segmented faults to a simpler through-going fault, and ineffective healing preserves fault complexities, resulting in long-lasting, distributed deformation. I also find that lateral contrasts in lithosphere viscosity structure (or effective plate thickness) attract evolving faults and cause damage and strain to concentrate on the “weaker” side. Realistic contrasts in crustal rigidity, however, have only a minor effect on the symmetry of damage, deformation, or the propagation of faults. In addition, lower crust and mantle viscosity contrasts induce the formation of broad shear zones with relatively high strain-rate in the “weaker” side of the interface. These results demonstrate that reasonable, lateral contrasts in viscosity (rather than extreme, unrealistic contrasts in elasticity) can explain GPS observations of highly asymmetric, interseismic deformation around strike-slip faults.

This dissertation presents the collection and processing of dense high-precision geode- tic data across major faults throughout Southern California. The results are used to inform numerical models of the long-term slip rate and interseismic behavior of these faults, as well as their frictional and rheological properties at shallow depths. The data include campaign surveys of dense networks of GPS monuments crossing the faults, and Interferometric Synthetic Aperture Radar (InSAR) observations from ENVISAT. Using a Bayesian framework, we first assess to what extent these data constrain relative fault slip rates on the San Andreas and San Jacinto faults, and show that the inferred parameters depend critically on the assumed fault geometry. We next look in detail at near-field observations of strain across the San Jacinto fault, and show that the source of this strain may be either deep anomalous creep or a new form of shallow, distributed yielding in the top few kilometers of the crust. On the San Andreas fault, we show that this type of shallow yielding does occur, and its presence or absence is controlled by variations in the local normal stress that result from subtle bends in the fault. Finally, we investigate shallow creep on the Imperial fault, and show that thanks to observations from all parts of the earthquake cycle it is now possible to obtain a strong constraint on the shallow frictional rheology and depth of the material responsible for creep. The results also suggest activity on a hidden fault to the West, whose existence has been previously suggested but never confirmed.

The main objective of this work is to test the effect of two conical-shaped positive topographic obstacleson propagation of a discrete basement dextral strike-slip or transcurrent fault. A set of sandbox analogue (physical) models was constructed, in which two consecutive sand cones were placed progressivelycloser to each other. Key structural and strain parameters, such axial strain ratios and angular strain, aswell as the width and direction of the basins which formed during deformation were measured and analyzed. This procedure was then repeated with a basal decoupling layer of PDMS beneath each cone,to test the influence of this layer on the deformation.The results show that, for models without a basal decoupling layer, the distance between the two cones governs the end-stage deformation patterns of the topographic obstacles. The proximity of the topographic obstacles causes an increase of their deformation, i.e., results in higher axial strain ratios and angular strain. This effect is particularly noticeable in the first obstacle, which is affected by a strong clockwise rotation. The basal ductile which partly decouples the basement fault from the cover units nullifies the previous effect (the increase in deformation caused by proximity) and, when present, localizes the deformation by not only producing narrower pull-apart basins within the obstacles but alsoby increasing their rotation.
O objectivo deste trabalho foi o de estabelecer os efeitos de uma única falha de desligamento direito emdois obstáculos cónicos consecutivos, de relevo positivo. Adicionalmente, procura-se estabelecer o efeito que uma camada basal dúctil poderá ter na deformação dos obstáculos.Como tal, uma série de modelos análogos foram efetuados onde dois cones de areia consecutivosforam colocados sistematicamente mais próximos um do outro. Durante estas experiências, parâmetros chave de natureza estrutural e de strain foram medidos, tais como os rácios de strain axial e angular,bem como a direção e largura das bacias formadas. Este procedimento foi repetido com uma camadabasal de silicone (PDMS) colocada por baixo dos obstáculos. Os resultados mostram que, para modelos sem a camada de silicone basal, a distância de separação dos cones tem uma influência muito forte no produto final da deformação nos cones. A proximidade dos obstáculos causa um aumento da deformação (ex. valores mais elevados de strain angular e strain axial) em ambos os obstáculos. Este efeito é particularmente visível no primeiro obstáculo, sendo este afetado por uma rotação no sentido dos ponteiros do relógio mais elevada que o segundo.Por fim, verifica-se que a presença da camada basal dúctil nulifica o efeito anterior e, quando presente, focaliza a deformação, não só criando bacias de pull-apart mais estreitas mas tambémcausando uma maior rotação nos obstáculos.

The Alpine Fault is the major structure of the Pacific-Australian plate boundary through New Zealand�s South Island. During dextral reverse fault slip, a <5 million year old, ~1 km thick mylonite zone has been exhumed in the hanging-wall, providing unique exposure of material deformed to very high strains at deep crustal levels under boundary conditions constrained by present-day plate motions. The purpose of this study was to investigate the fault zone rheology and mechanisms of strain localisation, to obtain further information about how the structural development of this shear zone relates to the kinematic and thermal boundary constraints, and to investigate the mechanisms by which the viscously deforming mylonite zone is linked to the brittle structure, that fails episodically causing large earthquakes. This study has focussed on the central section of the fault from Harihari to Fox Glacier. In this area, mylonites derived from a quartzofeldspathic Alpine Schist protolith are most common, but slivers of Western Province-derived footwall material, which can be differentiated using mineralogy and bulk rock geochemistry, were also incorporated into the fault zone. These footwall-derived mylonites are increasingly common towards the north. At amphibolite-facies conditions mylonitic deformation was localised to the mylonite and ultramylonite subzones of the schist-derived mylonites. Most deformation was accommodated by dislocation creep of quartz, which developed strong Y-maximum crystallographic preferred orientation (CPO) patterns by prism (a) dominant slip. Formation of this highly-oriented fabric would have led to significant geometric softening and enhanced strain localisation. During this high strain deformation, pre-existing Alpine Schist fabrics in polyphase rocks were reconstituted to relatively well-mixed, finer-grained aggregates. As a result of this fabric homogenisation, strong syn-mylonitic object lineations were not formed. Strain models show that weak lineations trending towards ~090� and kinematic directions indicated by asymmetric fabrics and CPO pattern symmetry could have formed during pure shear stretches up-dip of the fault of ~3.5, coupled with simple shear strains [greater than or equal to]30. The preferred estimate of simple:pure shear strain gives a kinematc vorticity number, W[k] [greater than or equal to]̲ 0.9997. Rapid exhumation due to fault slip resulted in advection of crustal isotherms. New thermobarometric and fluid inclusion analyses from fault zone materials allow the thermal gradient along an uplift path in the fault rocks to be more precisely defined than previously. Fluid inclusion data indicate temperatures of 325+̲15�C were experienced at depths of ~45 km, so that a high thermal gradient of ~75�C km⁻� is indicated in the near-surface. This gradient must fall off to [ less than approximately]l0�C km⁻� below the brittle-viscous transition since feldspar thermobarometry, Ti-inbiotite thermometry and the absence of prism(c)-slip quartz CPO fabrics indicate deformation temperatures did not exceed ~ 650�C at [greater than or equal to] 7.0-8.5�1.5 kbar, ie. 26-33 km depth. During exhumation, the strongly oriented quartzite fabrics were not favourably oriented for activation of the lower temperature basal(a) slip system, which should have dominated at depths [less than approximately]20 km. Quartz continued to deform by crystal-plastic mechanisms to shallow levels. However, pure dislocation creep of quartz was replaced by a frictional-viscous deformation mechanism of sliding on weak mica basal planes coupled with dislocation creep of quartz. Such frictional-viscous flow is particularly favoured during high-strain rate events as might be expected during rupture of the overlying brittle fault zone. Maximum flow stresses supported by this mechanism are ~65 Mpa, similar to those indicated by recrystallised grain size paleopiezometry of quartz (D>25[mu]m, indicating [Delta][sigma][max] ~55 MPa for most mylonites). It is likely that the preferentially oriented prism (a) slip system was activated during these events, so the Y-maximum CPO fabrics were preserved. Simple numerical models show that activation of this slip system is favoured over the basal (a) system, which has a lower critical resolved shear stress (CRSS) at low temperatures, for aggregates with strong Y-maximum orientations. Absence of pervasive crystal-plastic deformation of micas and feldspars during activation of this mechanism also resulted in preservation of mineral chemistries from the highest grades of mylonitic deformation (ie. amphibolite-facies). Retrograde, epidote-amphibolite to greenschist-facies mineral assemblages were pervasively developed in ultramylonites and cataclasites immediately adjacent to the fault core and in footwall-derived mylonites, perhaps during episodic transfer of this material into and subsequently out of the cooler footwall block. In the more distal protomylonites, retrograde assemblages were locally developed along shear bands that also accommodated most of the mylonitic deformation in these rocks. Ti-in-biotite thermometry suggests biotite in these shear bands equilibrated down to ~500+̲50�C, suggesting crystal-plastic deformation of this mineral continued to these temperatures. Crossed-girdle quartz CPO fabrics were formed in these protomylonites by basal (a) dominant slip, indicating a strongly oriented fabric had not previously formed at depth due to the relatively small strains, and that dislocation creep of quartz continued at depths [less than or equal to]20 km. Lineation orientations, CPO fabric symmetry and shear-band fabrics in these protomylonites are consistent with a smaller simple:pure shear strain ratio than that observed closer to the fault core (W[k] [greater than approximately] 0.98), but require a similar total pure shear component. Furthermore, they indicate an increase in the simple shear component with time, consistent with incorporation of new hanging-wall material into the fault zone. Pre-existing lineations were only slowly rotated into coincidence with the mylonitic simple shear direction in the shear bands since they lay close to the simple shear plane, and inherited orientations were not destroyed until large finite strains (<100) were achieved. As the fault rocks were exhumed through the brittle-viscous transition, they experienced localised brittle shear failures. These small-scale seismic events formed friction melts (ie. pseudotachylytes). The volume of pseudotachylyte produced is related to host rock mineralogy (more melt in host rocks containing hydrated minerals), and fabric (more melt in isotropic host rocks). Frictional melting also occurred within cataclastic hosts, indicating the cataclasites around the principal slip surface of the Alpine Fault were produced by multiple episodes of discrete shear rather than distributed cataclastic flow. Pseudotachylytes were also formed in the presence of fluids, suggesting relatively high fault gouge permeabilities were transiently attained, probably during large earthquakes. Frictional melting contributed to formation of phyllosilicate-rich fault gouges, weakening the brittle structure and promoting slip localisation. The location of faulting and pseudotachylyte formation, and the strength of the fault in the brittle regime were strongly influenced by cyclic hydrothermal cementation processes. A thermomechanical model of the central Alpine Fault zone has been defined using the results of this study. The mylonites represent a localised zone of high simple shear strain, embedded in a crustal block that underwent bulk pure shear. The boundaries of the simple shear zone moved into the surrounding material with time. This means that the exhumed sequence does not represent a simple 'time slice' illustrating progressive fault rock development during increasing simple shear strains. The deformation history of the mylonites at deep crustal P-T conditions had a profound influence on subsequent deformation mechanisms and fabric development during exhumation.

La rugosité multi-échelle de l'interface d'une faille est à l'origine de multiples aspérités qui établissent un ensemble complexe et discret de contacts réels. Puisque les aspérités contrôlent l'initiation et l'évolution du glissement de la faille, il est important d'explorer les relations intrinsèques entre le comportement collectif des aspérités locales et la stabilité frictionnelle de la faille globale. Nous proposons ici une nouvelle approche expérimentale analogique, qui nous permet de capturer l'évolution temporelle du glissement de chaque aspérité sur une interface de faille. Nous constatons que de nombreux événements déstabilisants à l'échelle de l'aspérité locale se sont produits dans la phase de renforcement du glissement, qui est conventionnellement considérée comme le régime stable d'une faille. Nous calculons le couplage intersismique pour évaluer les comportements de glissement des aspérités pendant la phase de renforcement du glissement. Nous montrons que le couplage intersismique peut être affecté par les interactions élastiques entre les aspérités par l'intermédiaire de la matrice molle encastrée. Les lois d'échelle des événements naturels de glissement lent sont reproduites par notre configuration, en particulier l'échelle moment-durée
The multi-scale roughness of a fault interface is responsible for multiple asperities that establish a complex and discrete set of real contacts. Since asperities control the initiation and evolution of the fault slip, it is important to explore the intrinsic relationships between the collective behavior of local asperities and the frictional stability of the global fault. Here we propose a novel analog experimental approach, which allows us to capture the temporal evolution of the slip of each asperity on a faulting interface. We find that many destabilizing events at the local asperity scale occurred in the slip-strengthening stage which is conventionally considered as the stable regime of a fault. We compute the interseismic coupling to evaluate the slipping behaviors of asperities during the slip-strengthening stage. We evidence that the interseismic coupling can be affected by the elastic interactions between asperities through the embedding soft matrix. Scaling laws of natural slow slip events are reproduced by our setup in particular the moment-duration scaling

On modelise une source sismique par un glissement se produisant sur un plan de faille. On montre l'influence des proprietes mecaniques des materiaux et l'on etudie l'endommagement qui se repentit de facon asymetrique par rapport au plan de faille. La distribution du tenseur du moment sismique montre l'importance des composantes mxx et myy dans les zones les plus eloignees du plan de faille, correspondant a un mode d'ouverture

Les objectifs de cette these sont de decrire les mecanismes de deformations de la lithosphere en regime compressif, et le controle impose par les parametres mecaniques sur la maniere dont le raccourcissement horizontal est accommode (par la formation de plis, de chevauchements, ou encore par epaississement homogene). Nous avons etudie la nature des instabilites susceptibles de se developper en utilisant des calculs analytiques bases sur la resolution des equations de navier-stokes ainsi que leur evolution pour des taux de deformation importants a partir de modeles analogiques et de calculs numeriques par la methode des elements finis. Les calculs analytiques ont permis de determiner l'influence des differents parametres mecaniques de la lithosphere sur le developpement d'instabilites. En domaine oceanique, le raccourcissement est essentiellement accommode par la formation de plis affectant l'ensemble de la lithosphere. Les parties fragiles de la lithosphere et les contrastes de densite controlent la croissance des instabilites. Deux series d'experiences analogiques ont ensuite permis de confirmer les resultats precedents et d'etudier l'evolution tridimensionnelle d'instabilites lithospheriques apres l'apparition de la fracturation. En domaine continental, le passe tectonique et les heterogeneites mecaniques qui en resultent joue un role essentiel pour l'initiation des plis. Les heterogeneites initiales peuvent favoriser l'apparition de failles aux depends des plis de grandes longueurs d'onde puis la subsidence des portions de lithosphere ainsi delimitees. Les structures ainsi formees s'apparentent a des bassins compressifs. Leur longueur d'onde reste cependant controlee en partie par celle des plis lithospheriques. Ces resultats ont ete completes par des calculs numeriques bases sur la methode des elements finis. Les plis ne se developpent qu'apres plastification complete des parties fragiles de la lithosphere oceanique ou continentale.

The Copper Creek thrust fault in the southern Appalachians places Cambrian over Ordovician sedimentary strata. The fault accommodated displacement of 15-20 km at 100-180 °C. Along the hanging wall-footwall contact, microstructures within a ~2 cm thick calcite and shale shear zone suggest that calcite, not shale, controlled the rheology of the shear zone rocks. While shale deformed brittley, plasticity-induced fracturing in calcite resulted in ultrafine-grained (<1.0 μm) fault rocks that deformed by grain boundary sliding (GBS) accommodated primarily by diffusion creep, suggesting low flow stresses. Optical and electron microscopy of samples from a transect across the footwall shale into the shear zone, shows the evolution of rheology within the shear zone. Sedimentary laminations 1 cm below the shear zone are cut by minor faults, stylolites, and fault-parallel and perpendicular calcite veins. At vein intersections, calcite grain size is reduced (to ~0.3 μm), and microstructures include inter-and-intragranular fractures, four-grain junctions, and interpenetrating boundaries. Porosity rises to 6 percent from <1 percent in coarse (25 μm) areas of calcite veins. In coarse-grained calcite, trails of voids follow twin boundaries, and voids occur at twin-twin and twin-grain boundary intersections. At the shear zone-footwall contact, a 350 μm thick calcite band contains coarseand ultrafine-grained layers. Ultrafine-grained (~0.34 μm) layers contain microstructures similar to those at vein intersections in the footwall and display no lattice-preferred orientation (LPO). Coarse-grained layers cross-cut grain-boundary alignments in the ultrafine-grained layers; coarse grains display twins and a strong LPO. Within the shear zone, ultrafine-grained calcite-aggregate clasts and shale clasts (5-350 μm) lie within an ultrafine-grained calcite (<0.31 μm) and shale matrix. Ultrafinegrained calcite (<0.31 μm) forms an interconnected network around the matrix shale. Calcite vein microstructures suggest veins continued to form during deformation. Fractures at twin-twin and twin-grain boundary intersections suggest grain size reduction by plasticity-induced fracturing, resulting in <1 μm grains. Interpenetrating boundaries, four-grain junctions, and no LPO indicate the ultrafine-grained calcite deformed by viscous grain boundary sliding. The evolution of the ultrafine-grain shear zone rocks by a combination of plastic and brittle processes and the deformation of the interconnected network of ultrafine-grained calcite by viscous GBS enabled a large displacement along a narrow fault zone.

Ample experimental and observational evidence suggests that friction properties on natural faults vary spatially. In the lab, rock friction depends on temperature and confining pressure and it can be either velocity weakening or velocity strengthening, leading to either unstable or stable slip. Such variations in friction rheology can explain patterns of seismic and aseismic fault slip inferred from field observations.

This thesis studies earthquake source processes using models with relatively simple but conceptually important patterns of velocity-weakening and velocity-strengthening friction that can arise on natural faults. Based on numerical and analytical modeling, we explore the consequences of such patterns for earthquake sequences, interseismic coupling, earthquake nucleation processes, aftershock occurrence, peak ground motion in the vicinity of active faults, and seismic slip budget at shallow depths. The velocity-dependence of friction is embedded into the framework of logarithmic rate and state friction laws.

In addition to using existing boundary integral methods, which are accurate and efficient in simulating slip on planar faults embedded in homogeneous elastic media, the thesis develops spectral element methods to consider single dynamic ruptures and long-term histories of seismic and aseismic slip in models with layered bulk properties.

The results of this thesis help to understand a number of observed fault slip phenomena, such as variability in earthquake patterns and its relation to interseismic coupling, seismic quiescence following decay of aftershocks at inferred rheological transitions, instances of poor correlation between static stress changes and aftershock occurrence, the lack of universally observed supershear rupture near the free surface, and coseismic slip deficit of large strike-slip earthquakes at shallow depths. The models, approaches, and numerical methods developed in the thesis motivate and enable consideration of many other earthquake source problems, such as the combined effect of two or more triggering mechanisms on aftershock rates, inferring friction properties on natural faults based on seismic and geodetic measurements, seismic hazard assessment based on observed interseismic coupling, and the effect of heterogeneous and/or nonelastic bulk properties on earthquake sequences.

碩士
國立中正大學
地震研究所暨應用地球物理研究所
97
We investigate the rheology of the Chelungpu fault and the stress perturbations following the Chi-Chi earthquake to account for the postseismic deformation recorded at 4 permanent GPS reference stations and repeatedly measured at 78 temporary monuments. Kinematic analysis shows that the Chelungpu decollement embedded in midcrust took up most of the afterslip following the Chi-Chi earthquake. We conduct a numerical modeling that involves a mechanical system with multiple spring-sliders to demonstrate that the dynamics of the afterslip may most likely result from stress transferred from the mainshock on the decollement that follows a velocity-strengthening friction law at the brittle creep segment and power-law flow at the ductile shear zone. We find the rate parameter . This value is similar to what found at Hole B in TCDP. It also agrees with the result inferred from the single spring-slider model. However, our model also implies that the distribution of the afterslip is not homogeneous in time or in space; it started at the east of the focus of the Chi-Chi earthquake, and gradually propagated toward northeast and northsouth.

Lo scopo fondamentale di questo lavoro è l’applicazione delle tecniche di modellazione numerica per lo studio di sistemi di faglie per verificarne il loro potenziale sismogenetico. Determinare quale faglia merita più attenzione, dal punto di vista del rischio sismico, è una questione attualmente ancora dibattuta. Lo confermano, ad esempio, i terremoti di l’Aquila nel 2009 e di Sumatra nel 2004. Inoltre, secondo uno studio di Wyss et al. (2012), il numero di morti causati dai recenti terremoti è da 100 a 1000 volte più elevato rispetto ai valori predetti dalla mappa mondiale di hazard. Le problematiche riguardanti le mappe di hazard dipendono principalmente dal fatto che sono calcolate mediante cataloghi sismici e dati di tipo geologico. Questo comporta un problema dal punto di vista temporale, in quanto i cataloghi sismici registrano eventi che non coprono un intero ciclo sismico, mentre i dati geologici contengono più eventi registrati, ad esempio, dal rigetto superficiale delle faglie. La questione temporale può essere risolta mediante la modellazione numerica che permette di raccordare i dati a lungo e corto periodo. Infatti, tramite la modellazione numerica, è possibile stimare l’evoluzione di una faglia (in superficie e in profondità) nel periodo intersismico e simulare il caso cosismico. Inoltre la modellazione numerica permette di distinguere le faglie bloccate da quelle sbloccate. Questa distinzione fornisce un elemento utile per valutare la possibilità di un’eventuale rottura. Inoltre è possibile stimare lo stress, la deformazione e la velocità di ricarica di un terremoto. Ho applicato la modellazione numerica a tre aree rappresentative del territorio italiano. Partendo dal centro Italia, ho studiato la faglia a basso angolo dell’Altotiberina e la sua relazione con le faglie di Colfiorito e della Valle Umbra. Ho approfondito lo studio delle faglie a basso angolo, analizzando il caso della faglia di Messina (Sud Italia). Infine, ho studiato l’area esterna del sud Alpino (nord Italia), caratterizzata da un sistema compressivo, che comprende il thrust del Montello ed il thrust di Bassano. Ho modellato numericamente ognuna di queste faglie o sistemi di faglie utilizzando diverse condizioni al contorno e parametri reologici in accordo con l’area di studio. I risultati sono stati confrontati con dati di tipo geodetico, geologico e geofisico. E’ stato possibile verificare che, la modellazione numerica fornisce un ottimo sostegno per la modellazione analogica, contribuendo a dare maggiore completezza al risultato e a simulare alcune proprietà dei materiali con grande precisione. Il risultato di un modello numerico varia principalmente al variare delle condizioni al contorno imposte, quindi dalla geometria, dai parametri reologici, e dal tipo di meccanismo utilizzato per riprodurre la deformazione di un’area. I risultati ottenuti in questo lavoro mostrano che la faglia Altotiberina è completamente bloccata al contrario della faglia di Colfiorito e la faglia della Valle Umbra che si muovono in parte come delle faglie sbloccate. Il campo deformativo dell’area sembra essere guidato da una trazione posta alla base della litosfera. Per quanto riguarda il sistema di thrust del Montello, ho potuto verificare che la porzione bloccata del thrust di Bassano ha un grande potenziale sismogenetico rispetto al thrust del Montello e al thrust antitetico al Montello, che risultano sbloccate. Assumendo che l’ampiezza delle faglie bloccate sia proporzionale all’ampiezza del terremoto, è stato possibile stimare la magnitudo massima attesa per ogni porzione di faglia bloccata, calcolata mediante la modellazione numerica. In particolare, la faglia di Bassano e la faglia Altotiberina sembrano avere un forte potenziale sismogenetico, in quanto potrebbe avere una magnitudo massima attesa di circa 7.
Università degli studi di Urbino
Unpublished
open

During the last decades, the study of seismic anisotropy has provided useful information for the interpretation and evaluation of the stress field and active crustal deformation. Seismic anisotropy can yield valuable information on upper crustal structure, fracture field, and presence of fluid-saturated rocks crossed by shear waves. Several studies worldwide demonstrate that seismic anisotropy is related to stress-aligned, filled-fluid micro-cracks (EDA model, Crampin et al., 1984b; Crampin, 1993). The seismic anisotropy is an almost ubiquitous property of the Earth and the Shear Wave Splitting is the most unambiguous indicator of anisotropy, but the automatic estimation of the splitting parameters is difficult because the effect of the anisotropy on a seismogram is a second order, not easily detectable effect. Different researchers developed automated techniques aimed to study the Shear Wave Splitting: in this study, the results of different codes are compared in order to evaluate the best method for automatic anisotropy evaluation. In the last three years, an automatic analysis code, “Anisomat+”, was developed, tested and improved to calculate the anisotropic parameters: fast polarization direction () and delay time (∂t). “Anisomat+” consists of a set of MatLab scripts able to retrieve automatically crustal anisotropy parameters from three-component seismic recordings of local earthquakes. It needs waveforms and hypocentral parameters in the format routinely archived by the Istituto Nazionale di Geofisica e Vulcanologia (INGV). The code uses horizontal component cross-correlation method: a mathematical algorithm aimed to measure the similarity of the pulse shape between two shear waves. Anisomat+ has been compared to other two automatic analysis codes (SPY and SHEBA) and tested on three zones of the Apennines (Val d’Agri, Tiber Valley and L’Aquila surroundings). It was observed that, if the number of measures is large enough, at each station the average values of the parameters (fast direction and delay time) are comparable. The main goal in developing of an automatic code was to have tool able to work on a big amount of data, in a short time, by reducing the errors due to the subjectivity. These two acquirements are very useful and are the basis to develop a quasi real-time monitoring of the anisotropic parameters. The anisotropic parameters, resulting from the automatic computation, have been interpreted to determine the fracture field geometries; for each area, I defined the dominant fast direction and the intensity of the anisotropy, interpreting these results in the light of the geological and structural setting and of two anisotropic interpretative models, proposed in the literature. In the first one, proposed by Zinke and Zoback (2000), the local stress field and cracks are aligned by tectonics phases and are not necessarily related to the presently active stress field. Therefore the anisotropic parameters variations are only space-dependent. In the second, EDA model (Crampin, 1993), and its development in the APE model (Zatsepin and Crampin, 1995) fluid-filled micro-cracks are aligned or ‘opened’ by the active stress field and the variation of the stress field might be related to the evolution of the pore pressure in time; therefore in this case the variation of the anisotropic parameters are both space- and time- dependent. I recognized that the average of fast directions, in the three selected areas, are oriented NW-SE, in agreement with the orientation of the active stress field, as suggested by the EDA model, proposed by Crampin (1993), but also, by the proposed by Zinke and Zoback model; in fact, NW-SE direction corresponds also to the strike of the main fault structures in the three study regions. The mean values of the magnitude of the normalized delay time range from 0.005 s/km to 0.007 s/km and to 0.009 s/km, respectively for the L'Aquila (AQU) area, the High Tiber Valley (ATF) and the Val d'Agri (VA), suggesting a 3-4% of crustal anisotropy (Piccinini et al., 2006). In each area are also examined the spatial and temporal distribution of anisotropic parameters, which lead to some innovative observations, listed below. o The higher values of normalized delay times have been observed in those zones where most of the seismic events occur. This aspect was further investigated, by evaluating the average seismic rate, in a time period, between years 2005 and 2010, longer than the lapse of time, analyzed in the anisotropic studies. This comparison has highlighted that the value of the normalised delay time is larger where the seismicity rate is higher. o In the Alto Tiberina Fault area the higher values of normalised delay time are not only related to the presence of a high seismicity rate but also to the presence of a tectonically doubled carbonate succession. Therefore, also the lithology, plays a important role in hosting and preserving the micro-fracture network responsible for the anisotropic field. o The observed temporal variations of anisotropic parameters, have been observed and related to the fluctuation of pore fluid pressure at depth possibly induced by different mechanisms in the different regions, for instance, changes in the water table level in Val D’Agri (Valoroso et al., GJI submitted), occurrence of the April 6th Mw=6.1 earthquake in L’Aquila (Lucente et al., 2010). Since these variations have been recognized, it is possible to affirm that the models that better fit my results, both in term of fast directions and of delay times, seems to be those proposed by Crampin (1993) and Zatsepin & Crampin (1995), respectively EDA and APE models.
Università degli studi di Perugia
Published
1.11. TTC - Osservazioni e monitoraggio macrosismico del territorio nazionale
3.1. Fisica dei terremoti
3.2. Tettonica attiva
3.8. Geofisica per l'ambiente
open

The strain-stress state generated by faulting or cracking and influenced by the strong heterogeneity of the internal earth structure precedes and accompanies volcanic and seismic activity. Particularly, volcanic eruptions are the culmination of long and complex geophysical processes and physical processes which involve the generation of magmas in the mantle or in the lower crust, its ascent to shallower levels, its storage and differentiation in shallow crustal chambers, and, finally, its eruption at the Earth’s surface. Instead, earthquakes are a frictional stick-slip instability arising along pre-existing faults within the brittle crust of the Earth. Long-term tectonic plate motion causes stress to accumulate around faults until the frictional strength of the fault is exceeded. The study of these processes has been traditionally carried out through different geological disciplines, such as petrology, structural geology, geochemistry or sedimentology. Nevertheless, during the last two decades, the development of physical of earth as well as the introduction of new powerful numerical techniques has progressively converted geophysics into a multidisciplinary science. Nowadays, scientists with very different background and expertises such as geologist, physicists, chemists, mathematicians and engineers work on geophysics. As any multidisciplinary field, it has been largely benefited from these collaborations. The different ways and procedures to face the study of volcanic and seismic phenomena do not exclude each other and should be regarded as complementary. Nowadays, numerical modeling in volcanology covers different pre-eruptive, eruptive and post-eruptive aspects of the general volcanic phenomena. Among these aspects, the pre-eruptive process, linked to the continuous monitoring, is of special interest because it contributes to evaluate the volcanic risk and it is crucial for hazard assessment, eruption prediction and risk mitigation at volcanic unrest. large faults. The knowledge of the actual activity state of these sites is not only an academic topic but it has crucial importance in terms of public security and eruption and earthquake forecast. However, numerical simulation of volcanic and seismic processes have been traditionally developed introducing several simplifications: homogeneous half-space, flat topography and elastic rheology. These simplified assumptions disregards effects caused by topography, presence of medium heterogeneity and anelastic rheology, while they could play an important role in Moreover, frictional sliding of a earthquake generates seismic waves that travel through the earth, causing major damage in places nearby to the modeling procedure This thesis presents mathematical modeling and numerical simulations of volcanic and seismic processes. The subject of major interest has been concerned on the developing of mathematical formulations to describe seismic and volcanic process. The interpretation of geophysical parameters requires numerical models and algorithms to define the optimal source parameters which justify observed variations. In this work we use the finite element method that allows the definition of real topography into the computational domain, medium heterogeneity inferred from seismic tomography study and the use of complex rheologies. Numerical forward method have been applied to obtain solutions of ground deformation expected during volcanic unrest and post-seismic phases, and an automated procedure for geodetic data inversion was proposed for evaluating slip distribution along surface rupture.
Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania
Unpublished
3.6. Fisica del vulcanismo
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