The Pacific-North American plate boundary is typified by transpression and convergence, yet the relationship between interplate deformation and long-term crustal shortening is not fully understood. The continuous belt of rugged topography that extends along the entire plate boundary is generally associated with oblique tectonic plate motion, strong interplate coupling, and terrane accretion, but relating plate boundary orogenesis to variations in plate geometry and behavior requires detailed case studies. The northern San Gabriel Mountains along the San Andreas fault and the Chugach-Kenai Mountains above the Aleutian subduction zone are located along highly tectonically active sections of the Pacific-North American plate boundary and have not been studied from the context of long-term landscape development. To determine whether mountain building along these sections of the plate boundary reflects recent, rapid exhumation as observed in bordering mountain belts, low-temperature thermochronometry and topographic analyses were applied to each area. In the northern San Gabriel Mountains, apatite (U-Th)/He ages are >10 Ma along narrow crystalline ridges topped by low-slope erosional surfaces located within ~5 km of the San Andreas fault zone. In the Chugach-Kenai Mountains, the youngest apatite (U-Th)/He ages (~5 Ma) are an order of magnitude older than those from the Yakutat collision zone to the east, despite the presence of a continuous swath of glaciated, rugged topography between the two areas. Exhumation rates inferred from these ages are <1 mm/yr, suggesting that there has been minimal recent denudation in the northern San Gabriel and Chugach-Kenai Mountains. The lack of evidence for recent mountain building in both of these case studies implies that interplate deformation is heterogeneous and that other factors (secondary structures, climate) besides plate kinematics and topographic character must be considered for understanding landscape development. / Ph. D.

Liquefaction is one of the major causes of ground failures during an earthquake. Recent evidence shows that the existing variants of the "simplified" liquefaction evaluation procedure lead to inaccurate results for megathrust earthquakes in subduction interfaces. To overcome this drawback and to achieve better prediction of liquefaction cases in subduction zones, this research intends to develop new empirical models that could be used for the prediction of liquefaction triggering in subduction zones. Towards this goal, new models for number of equivalent cycles (n_eq) and stress-reduction factor (r_d) have been proposed. The models are developed by regressing site response data obtained from 254 pairs of subduction ground motions and 77 representative soil profiles. To account for tectonic differences and magnitude scaling, separate models are developed for interface and intraslab earthquakes. The uncertainties involved in the proposed models are quantified through standard deviations of regression coefficients, event, site, and residual terms. The resulting models differ from other published models, especially the model for number of equivalent cycles. It was found that n_eq is greatly influenced by the fundamental site period. The model for r_d predicts higher values at shallow depths and lower values at deeper layers than other published models. Comparing the factors of safety against liquefaction with those from other existing models revealed that the use of models proposed in this research is more likely to reduce the "false positives" in liquefaction predictions, especially when design ground motion acceleration is high. / Master of Science / During earthquake shaking, loose saturated sands may lose strength and behave more like a liquid than a solid. This phenomenon is referred to liquefaction. Liquefaction has been responsible for infrastructure failure during past earthquakes, thus leading to major economic losses. This prompts the prediction and mitigation of potential liquefaction effects in a building site. However, the current state-of-the-practice for predicting liquefaction is inaccurate for large magnitude earthquakes in subduction zones. This provided the impetus for this research which focusses on developing new liquefaction evaluation models for large magnitude earthquakes. New models for number of equivalent cycles and stress reduction factor are developed by analyzing the representative ground motions and soil strata. These empirical parameters are central to the prediction of liquefaction triggering. Comparing the new models with the existing models revealed that the factor of safety against liquefaction estimated using new models are greater than those obtained using existing models for large magnitude earthquake scenario when the ground acceleration is high. This implies that using the existing models for predicting liquefaction in a site subjected to high values of ground acceleration from a subduction earthquake will lead to "false positives." Developed using a comprehensive dataset and robust regression techniques, the models developed in this research will lead to better predictions of liquefaction due to large subduction events.

<p dir="ltr">The Cascadia subduction zone features many along-strike variations in geophysical signatures that appear independent of properties in the subducting Juan de Fuca plate. Past studies have hypothesized that controls on these variations, namely subcretion, seem linked to overriding plate characteristics but may be influenced by characteristics of the downgoing slab as well. Nowhere is this more apparent than in southern Cascadia, which features the highest seismogenesis, broadest forearc topography, and lowest Bouguer gravity along the Cascadia margin. Additionally, the northward migration of deformation related to the San Andreas fault’s evolution and potential subslab buoyancies introduce further complexities making it difficult to parse contributions of tectonic processes to individual geophysical observations. To better understand contributions from Cascadia subduction and San Andreas evolution on tectonic processes, 60 Magseis Fairview nodal seismometers were deployed in southern Cascadia (Klamath Mountains) between April and May of 2020. We perform ambient noise tomography using Rayleigh and Love waves to constrain radial anisotropy and reveal seismic characteristics in the forearc. We find low VSV (<3.4 km/s) in the lower crust of the forearc consistent with previous studies. This is paired with high (>10%) positive radial anisotropy suggesting these materials are dominated by (sub)horizontal fabrics. We also observe relatively high VSV and VSH and negative radial anisotropy (~ -10%) in the upper crust of the forearc to ~10 km depth. These results suggest that the upper crust, which is dominated by the Klamath terrane, is characterized by (sub-vertical) deformational fabrics, likely related to brittle deformation superimposed on the accretionary history of the Klamath terrane, while the lower crust shows fabrics consistent with what would be expected due to basal accretion of oceanic crust (e.g, sedimentary rocks with or without basaltic slivers). The correlation of positive radial anisotropy with low shear-wave velocities (~3.4 km/s), low Bouguer gravity, high conductivity, and high rates of seismogenic activity (LFEs, tremor distribution, and episodic slow slip events) suggest that this basally accreted material may be infiltrated by fluids derived from the downgoing oceanic lithosphere.</p>

Tsunami hazards were greatly underestimated along the coasts of countries bordering the northeastern Indian Ocean until the occurrence of the 26 December 2004, Mw 9.2 earthquake and its ensuing tsunami. Sourced off the coast of northern Sumatra, on the plate boundary between the Indo-Australian and Eurasian plates, the rupture of the 2004 earthquake propagated ~1300 km northward. The magnitude of this earthquake and the reach of its tsunami exceeded all known precedents, based on instrumental and historic records. The coseismic deformational and post-tsunami depositional features facilitated opportunities to conduct tsunami geology studies along the coasts of countries bordering the Indian Ocean. Several questions are being posed, the answers of which have implications for tsunami hazard assessment. How did this plate boundary behave prior to and after the great earthquake? Was the 2004 earthquake the first of its kind on the Sumatra-Andaman plate boundary? If it had a predecessor, when did it occur and was it a true predecessor in terms of its rupture dimensions and tsunamigenic potential? What types of depositional evidence are preserved and how can we use them to develop the history of past tsunamigenic earthquakes? Researchers are exploring the affected regions and using the imprints left by the 2004 event, to address these questions. There are two components to this study: one, a seismotectonic analysis of the region from the perspective of plate driving forces and their relative roles in the interseismic and post-seismic phases. This study uses global data catalogs like the NEIC PDE (National Earthquake Information Centre Preliminary Determination of Epicenters) and the Global Centroid Moment Tensor (CMT) solutions for earthquake source parameters to understand the along-strike variations in seismicity patterns before and after the 2004 earthquake. The 2004 experience was unprecedented in South Asia. Unaffected by tsunami hazards in the past, tsunami geology is a nascent field for most South Asian researchers. Very little background field data is available on the deformational features of great earthquakes along this plate boundary and the depositional characteristics of extreme coastal surges, such as tsunamis and storms. Where do we begin our search for evidence of past tsunamigenic earthquakes? How best can we use the 2004 tsunami and its deposits as a proxy? What problems are encountered in the interpretations? This thesis addresses these questions in part and presents observations from the Andaman Islands (the ~400 km, northern segment of the Sumatra-Andaman subduction zone) and the southeast coast of India, towards developing a reliable database of tsunami geology for 2004-type events. The premise is that regions affected by the 2004 earthquake are more likely to conserve signatures from older events. Based on the stratigraphic context of the proxy and quality of age estimates, this work presents evidence for past earthquake related deformation and tsunami deposition. In this work we use deformational and depositional features from the Andaman Islands, falling within the 2004 rupture zone and from one location on the Tamil Nadu coast of India (Kaveripattinam). From a perceptive understanding of the features related to tectonic deformation of the Sumatra-Andaman subduction zone, we have selected the Andaman segment that demonstrates explicit evidence for deformation and tsunami deposition through geomorphological and stratigraphic features, which are key to our exploration. A gist of each chapter is given below. The introduction (chapter 1) presents the background, motivation and scope of this work and the organization of this thesis, also summarizing the contents of each chapter. Chapter 2 provides a review of literature on subduction zone earthquakes and updates on tsunami geology, to place this study in the global context. The next two chapters discuss the seismotectonics of the Sumatra-Andaman plate boundary, the important earthquakes and their source processes. In chapter 3 we discuss the Andaman segment (from 10–15° N), characterized by relatively lower level seismicity, but distinctive, as it falls within the northern limit of the 2004 rupture. The deformational and depositional features here are better exposed due to availability of land straddling the hinge line separating the areas of 2004 uplift and subsidence. Here, the pre-2004 earthquakes used to occur along a gently dipping subducting slab, up to a depth of about 40 km. Post-2004, the earthquakes moved up-dip, extending also to the outer-rise and outer-ridge regions, expressing post-earthquake relaxation [Andrade and Rajendran, 2011]. The southern Nicobar segment (5–10° N) differs from the Andaman segment in its style of deformation and seismic productivity. The decreasing obliquity of convergence, the likely influence of a subducting ocean ridge on the subducting plate and the character of the subducting oceanic plate make this segment distinctly different. In chapter 4 we present an analysis of its seismotectonic environment based on the well-constrained focal mechanisms of historic and recent earthquakes. We report that left-lateral strike-slip faulting on near N-S oriented faults control the deformation and the style of faulting is consistent to ~80 km within the subducting slab [Rajendran, K. et al., 2011]. The 11 April 2012 sequence of earthquakes on the subducting oceanic plate, between the Sumatra Trench and the Ninety East Ridge are the more recent among the oceanic intraplate earthquakes that demonstrate the reactivation of N-S oriented fossil fractures. The limited availability of land and the 2004 coseismic deformation dominated by subsidence, followed by prolonged waterlogging makes exploration difficult in the Nicobar segment. Thus, we focus on the Andaman Islands for deformational and depositional evidence, using observations that can be corroborated through multiple proxies and depositional environments that are not prone to other coastal surges, such as cyclones and storms. The criteria for selection of sites, evaluation of deposits and determination of limiting ages are discussed in chapters 5 through 9. In chapter 5 we discuss different types of coastal environments and their response to high-energy sea surges. We also give a brief review of the comparative analyses of storm and tsunami deposits, a highly debated issue and then discuss important characteristics of these two deposits, using examples from the 2004 tsunami and the 2011 Thane cyclone that affected parts of the Tamil Nadu coast. An important component of tsunami geology is the ability to identify and select datable material from tsunami deposits and chose an appropriate method for dating (chapter 6). The types of material used vary from peat layers, peat-rich soil, gastropod shells, wood, charcoal, organic remains such as bones, coral fragments, pottery sherds and buried soil. Techniques such as AMS Carbon-14 and Thermoluminescence are commonly used with appropriate calibrations and corrections. In addition to the dates generated in this study (based on wood and shell dates) we use some previous dates from the entire stretch of the rupture within the Indian Territory and assign a relative grading to these ages, based on the quality criterion evolved in this study. We believe that this is the first attempt to segregate age data obtained from coastal deposits, and assign them a specific quality grading based on their environment of deposition and the type of material dated. Chapter 7 presents results of our investigations in the Andaman Islands, which cover ~30% of the rupture area. A coseismically subsided mangrove from Rangachanga (Port Blair, east coast of South Andaman) led us to a former subsidence during AD 770–1040, which we believe is the most convincing evidence for a previous tectonic event. Data based on inland deposits of coral and organic debris yielded a younger age in the range of AD 1480–1660. Both these dates fall in the age brackets reported from other regions of this plate boundary (mainly Sumatra) as well as distant shores of Sri Lanka, Thailand and mainland India. To understand the nature of distant deposits, we present observations from Kaveripattinam, an ancient port city on the east coast of India, where a high-energy sea surge deposit, found 1 km inland is attributed to a paleotsunami. The inland location of this archeological site at an elevation of 2 m and characteristics of the deposit that help discriminate it from typical storm deposition provide clinching evidence in favor of a 1000-year old regional tsunami (chapter 8). In chapter 9 we discuss the results of our study. We evaluate the nature of deformation/deposition and the calibrated age data in the context of their environments. Ages based on the organic material associated with coral debris (at Hut Bay and Interview Island) and the remains of mangrove roots, 1 m below the present ground level (at Port Blair) are considered as reliable estimates, due to their sheltered inland location and the in situ root horizon used for dating. Age data from Kaveripattinam is also considered reliable, based on its inland location beyond the reach of storm surges, sediment characteristics typical of tsunami deposition and ages based on multiple methods and samples. The age data based on the sites presented in this thesis are more conclusive about the 800 to 1100 AD and 1250 to 1450 AD tsunamis, and the former is represented from regions closer to the 2004 source as well as distant shores reached by its tsunami. Chapter 10 presents our conclusions and the scope for future studies. We present this as the first study of its kind in the northeastern Bay of Bengal, wherein the coseismic vertical coastal deformation features along an interplate subduction boundary and a variety of tsunami deposits are used to categorize depositional environments and ages of paleoearthquakes and tsunamis. To our knowledge, this is the first study of its kind where the effects of a recent tsunami have been used to evaluate paleodeposits based on their respective environments of occurrence. Our results have implications for tsunami geology studies in coastal regions prone to tsunami hazard.

La partie superficielle des zones de subduction (0-10 km de profondeur) a longtemps été considérée comme asismique. Cependant la découverte de séismes très basses fréquences dans cette zone, ainsi que la propagation très superficielle de la rupture cosismique lors du séisme de Tohoku-Oki (Japon) remettent en question cette hypothèse jusqu’alors largement admise. L’une des raisons pour lesquelles le potentiel sismogénique de cette zone est mal contraint réside dans le fait que les processus qui y règnent sont complexes, mêlant déformation et diagenèse (principalement la transformation des argiles de type transition smectite vers illite), et ainsi difficilement reproductibles en laboratoire. Au cours de cette thèse, des expérimentations en presse triaxiale sur échantillons smectitiques (représentatifs des matériaux accrétés dans les prismes d’accrétion) et illitiques ont été réalisées sous différentes conditions de pression de confinement (de 50 à 200 MPa) et de température (20°C et 300°C). Ces expériences ont été menées afin d’identifier les modes de déformation de ces échantillons, et de déterminer les effets couplés de cette déformation et de la diagenèse sur la rhéologie de ces roches et notamment leur potentiel à générer des instabilités de glissement. Dans ces expériences, le style de déformation est à chaque fois similaire, avec, tout d’abord, une localisation progressive de la déformation le long d’une zone de cisaillement, puis formation d’une fracture. Malgré cette constance dans le style de déformation, le comportement rhéologique des échantillons, lui, est drastiquement opposé entre les expériences à 20 et à 300°C, avec un comportement exclusivement durcissant à 20°C (i.e. stable) quel que soit la minéralogie, et un comportement qui évolue systématiquement vers du stick-slip (i.e. instable) à 300°C (pour les échantillons smectitiques). Ces résultats montrent que la réactivité chimique des smectites au cours de la diagenèse (activée dans les expériences à 300°C) conditionne la formation d’instabilités de glissement. Nous proposons que la réactivité chimique des smectites dans les zones de subduction pourrait promouvoir la propagation de la rupture cosismique vers la surface. / The shallow portion of subduction zones (0-10 km depth) has long been considered as unable to store and release seismic energy. However, the detection of very-low frequency earthquakes in this zone, as well as the propagation of the coseismic rupture to the trench during the Tohoku-Oki earthquake, question this hypothesis. The difficulty to assess the seismogenic potential of this shallow portion lies principally in the complexity of the processes that occur in this zone, combining deformation and diagenesis (especially the smectite-to-illite transition), and hence not easily reproducible in laboratory. In order to analyse the mechanical properties of the shallow portion of subduction zones, triaxial tests have been performed with smectitic and illitic samples, under confining pressure between 50 and 200 MPa and at temperature of 20 and 300°C. The aim of these experiments was to identify the deformation modes of such sedimentary material and to determine the effects of deformation and diagenesis on rheology of these materials and on the rock potential to exhibit instable failure. In the experiments, deformation operates under the same pattern with a progressive localisation from shear band to fracturation. Even if the deformation style does not differ much between experiments, the rheology of the samples tested at 20°C and at 300°C contrasts drastically. Indeed, while the samples (smectitic and illitic) tested at 20°C show exclusively a strengthening behaviour (i.e. stable), the smectitic samples tested at 300°C exhibit a rheology that systematically evolves from strengthening to stick-slip behaviour (i.e. unstable). These results indicate that the chemical reactivity of smectite under diagenetic conditions (diagenesis is activated in the experiments conducted at 300°C) constitutes a weakening mechanism promoting unstable sliding. Finally, we propose that, at the subduction zone scale, the chemical metastability of smectite could promote the propagation of the coseismic rupture to the very shallow portion of accretionary prisms.

Ophiolites - thrust sheets of oceanic lithosphere that have been emplaced onto the continental margin - provide the opportunity to explore the structure and genesis of oceanic crust. As many ophiolites formed above subduction zones, they also allow for the investigation of mantle wedge and subduction interface processes. This the- sis examines the Oman-United Arab Emirates (UAE) ophiolite, which is the largest and most intensely studied ophiolite on Earth. Three distinct problems are addressed. (1) Recent research has proposed that the architecture and tectonic evolution of the ophiolite in the UAE differs from in Oman. In Chapter 2, I test this hypothesis by integrating new geological mapping and field observations with previously published maps of the ophiolite in the UAE. My results indicate that the ophiolite is gently folded, but otherwise largely intact. I demonstrate that the architecture of the ophi- olite in the UAE is not significantly different from in Oman. Thus, there is no basis for a different tectonic evolution as recently proposed. (2) Observations from exper- iments and small-scale natural shear zones indicate that volumetrically-minor phases can control strain localization. In Chapter 3, I test the hypothesis that minor phases control strain-localisation at plate boundaries. To do so, I analyzed peridotites from the base of the ophiolite, a palaeosubduction interface. My results demonstrate that minor phases limited olivine grain growth, which led to rheological weakening. (3) The mechanisms by which metamorphic soles detached from the downgoing slab and accreted to the hanging-wall mantle is unclear. In Chapter 4, I examine a transect across the metamorphic sole in the UAE. My results reveal that granulite formation was more extensive than is typically considered. I propose that granulite formation resulted in rheological strengthening, which caused the subduction interface to migrate into the downgoing slab and accrete the metamorphic sole.

abstract: The movement between tectonic plates is accommodated through brittle (elastic) displacement on the plate boundary faults and ductile permanent deformation on the fault borderland. The elastic displacement along the fault can occur in the form of either large seismic events or aseismic slip, known as fault creep. Fault creep mainly occurs at the deep ductile portion of the crust, where the temperature is high. Nonetheless, aseismic creep can also occur on the shallow brittle portion of the fault segments that are characterized by frictionally weak material, elevated pore fluid pressure, or geometrical complexity. Creeping segments are assumed to safely release the accumulated strain(Kodaira et al., 2004; Rice, 1992)(Kodaira et al., 2004; Rice, 1992)(Kodaira et al., 2004; Rice, 1992)(Kodaira et al., 2004; Rice, 1992)(Kodaira et al., 2004; Rice, 1992) on the fault and also impede propagation of the seismic rupture. The rate of aseismic slip on creeping faults, however, might not be steady in time and instead consist of successive periods of acceleration and deceleration, known as slow slip events (SSEs). SSEs, which aseismically release the strain energy over a period of days to months, rather than the seconds to minutes characteristic of a typical earthquake, have been interpreted as earthquake precursors and as possible triggering factor for major earthquakes. Therefore, understanding the partitioning of seismic and aseismic fault slip and evolution of creep is fundamental to constraining the fault earthquake potential and improving operational seismic hazard models. Thanks to advances in tectonic geodesy, it is now possible to detect the fault movement in high spatiotemporal resolution and develop kinematic models of the creep evolution on the fault to determine the budget of seismic and aseismic slip. In this dissertation, I measure the decades-long time evolution of fault-related crustal deformation along the San Andrea Fault in California and the northeast Japan subduction zone using space-borne geodetic techniques, such as Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR). The surface observation of deformation combined with seismic data set allow constraining the time series of creep distribution on the fault surface at seismogenic depth. The obtained time-dependent kinematic models reveal that creep in both study areas evolves through a series of SSEs, each lasting for several months. Using physics-based models informed by laboratory experiments, I show that the transient elevation of pore fluid pressure is the driving mechanism of SSEs. I further investigate the link between SSEs and evolution of seismicity on neighboring locked segments, which has implications for seismic hazard models and also provides insights into the pattern of microstructure on the fault surface. I conclude that while creeping segments act as seismic rupture barriers, SSEs on these zones might promote seismicity on adjacent seismogenic segments, thus change the short-term earthquake forecast. / Dissertation/Thesis / Doctoral Dissertation Geological Sciences 2018

Arc magmatism is sustained by the complex interactions between the subducting slab, the overriding plate, and the mantle wedge. Partial melting of mantle peridotite is achieved by fluid-induced flux melting and decompression melting due to upward flow. The distribution of melting is sensitive to temperature, the pattern of flow, and the pressure in the mantle wedge. The arc front is the surface manifestation of partial melting in the mantle wedge and is characterized by a narrow chain of active volcanoes that migrate in time. The conventional interpretation is that changes in slab dip angle lead to changes in the arc front position relative to the trench. We explore an alternative hypothesis: evolution of the overlying plate, specifically thickening of the arc root, causes arc front migration. We investigate the effects of varying crustal morphology and viscous decoupling of the shallow slab-mantle interface on melt production using 2D numerical models involving a stationary overriding plate, a subducting plate with prescribed motion, and a dynamic mantle wedge. Melt production is quantified using a hydrous melting parameterization. We conclude: 1) Localized lithospheric thickening shifts the locus of melt production trenchward while thinning shifts melting landward. 2) Inclined LAB topography modulates the asthenospheric flow field, producing a narrow, well-defined arc front. 3) Thickening of the overriding plate exerts increased torque on the slab, favoring shallowing of the dip angle. 4) Viscous decoupling produces a cold, stagnant forearc mantle but promotes arc front melting due to reduction in the radius of corner flow, leading to higher temperatures at the coupling/decoupling transition.

The Nicoya Peninsula of Costa Rica is unusually close to the Middle America Trench (MAT), such that interface locking along the megathrust is observable under land. Here, rapid convergence between the downgoing Cocos and the over-riding Caribbean plates at ~85mm/yr allows for observable high strain rates, frequent large earthquakes and ongoing micro-seismicity. By taking advantage of this ideal location, a network of 20 on-land broadband seismometers was established in cooperation between UC Santa Cruz, Georgia Tech, and OVSICORI, with most stations operating since 2008. To evaluate what seismicity tells us about the ongoing state of coupling along the interface, we must consistently evaluate the location and magnitude of ongoing micro- seismicity. Because of large levels of anthropogenic, biologic, and coastal noise, automatic detection of earthquakes remains problematic in this region. Thus, we resorted to detailed manual investigation of earthquake phases. So far, we have detected nearly 7,000 earthquakes below or near Nicoya between February and August 2009. From these events we evaluate the fine-scale frequency-magnitude distribution (FMD) along the subduction megathrust. The results from this b-value mapping‟ are compared with an earlier study of the seismicity 9 years prior. In addition, we evaluate them relative to the latest geodetically derived locking. Preliminary comparisons of spatial and temporal variations of the b-values will be reported here. Because ongoing manual detection of earthquakes is extremely laborious and some events might be easily neglected, we are implementing a match-filter detection algorithm to search for new events from the continuous seismic data. This new approach has been previously successful in identifying aftershocks of the 2004 Parkfield earthquake. To do so, we use the waveforms of 858 analyst-detected events as templates to search for similarly repeating events during the same periods that have been manually detected. Preliminary results on the effectiveness of this technique are reported. The overall goal of this research is to evaluate the evolution of stress along the megathrust that may indicate the location and magnitude of potentially large future earthquakes. To do so, I make the comparison between the FMD and the interface locking. Only positive correlations are observed in the Nicoya region. The result is different from the one derived from the seismic data set that was recorded 9 years before our data. Therefore, to substantiate the causes for the different relationships between the b-value and the coupling degree, we need additional data with more reliable magnitudes.

This dissertation consists of two parts and investigates the crustal deformation associated with great subduction zone earthquake at two different spatial scales. At the small scale, I investigate the stress transfer along the megathrust during great earthquakes and its effects on the forearc wedge. At the large scale, I investigate the viscoelastic crustal deformation of the forearc and the back arc associated with great earthquakes. Part I: In a subduction zone, the frontal region of the forearc can be morphologically divided into the outer wedge and the inner wedge. The outer wedge which features much active plastic deformation has a surface slope angle generally larger than that of the inner wedge which hosts stable geological formations. The megathrust can be represented by a three-segment model, the updip zone (velocity-strengthening), seismogenic zone (velocity-weakening), and downdip zone (velocity-strengthening). Our dynamic Coulomb wedge theory postulates that the outer wedge overlies the updip zone, and the inner wedge overlies the seismogenic zone. During an earthquake, strengthening of the updip zone may result in compressive failure in the outer wedge. The inner wedge undergoes elastic deformation. I have examined the geometry and mechanical processes of outer wedges of twenty-three subduction zones. The surface slope of these wedges is generally too high to be explained by the classical critical taper theory but can be explained by the dynamic Coulomb wedge theory. Part II: A giant earthquake produces coseismic seaward motion of the upper plate and induces shear stresses in the upper mantle. After the earthquake, the fault is re-locked, causing the upper plate to move slowly landward. However, parts of the fault will undergo continuous aseismic afterslip for a short duration, causing areas surrounding the rupture zone to move seaward. At the same time, the viscoelastic relaxation of the earthquake-induced stresses in the upper mantle causes prolonged seaward motion of areas farther landward including the forearc and the back arc. The postseismic and interseismic crustal deformation depends on the interplay of these three primary processes. I have used three-dimensional viscoelastic finite element models to study the contemporary crustal deformation of three margins, Sumatra, Chile, and Cascadia, that are presently at different stages of their great earthquake cycles. Model results indicate that the earthquake cycle deformation of different margins is governed by a common physical process. The afterslip of the fault must be at work immediately after the earthquake. The model of the 2004 Sumatra earthquake constrains the characteristic time of the afterslip to be 1.25 yr. With the incorporation of the transient rheology, the model well explains the near-field and far-field postseismic deformation within a few years after the 2004 Sumatra event. The steady-state viscosity of the continental upper mantle is determined to be 10^19 Pa S, two orders of magnitude smaller than that of the global value obtained through global postglacial rebound models. / Graduate