Part I of this thesis investigates current deformation in western British Columbia from northern Vancouver Island in the south to Haida Gwaii in the north. The area is characterized by transition from the Cascadia subduction zone to the Queen Charlotte transform fault. The tectonic setting involves interactions between the Pacific, North America, Juan de Fuca, and Explorer plates, and the Winona block, involving a number of plate boundaries: the mainly strike-slip Queen Charlotte, Revere-Dellwood-Wilson and Nootka faults, the Explorer ridge, and the Cascadia subduction zone. Using GPS campaign data from 1993 to 2008 I derive a new crustal velocity field for Northern Vancouver Island and the adjacent mainland, and integrate it with previous velocity fields developed for Haida Gwaii, southern Vancouver Island and the adjacent mainland. The northern limit of the subduction zone is confirmed to be at Brooks Peninsula, where the direction of the crustal motion changes abruptly from ENE to NNE. I use viscoelastic models to explore what percentage of the observed deformation is transient, related to the earthquake cycle, and how much is permanent ongoing deformation, distributed off the continental margin. Previous authors have developed two competing end-member models that can each explain how the Pacific/North America plate convergence is accommodated off Haida Gwaii. These models assume either internal crustal shortening or underthrusting of the Pacific plate. These new GPS data allow me to conclude that underthrusting does occur, and that a small component (<15%) of the observed data reflects long-term deformation. South of Haida Gwaii the distinction between transient and long-term deformation is not as clear; however, I conclude that transient deformation alone cannot fully explain the observed velocities, and so long-term deformation likely must also occur. Part II of the thesis investigates the updip and downdip limits of the seismogenic zone of the Sumatra megathrust fault. Temperature and downdip changes in formation composition are controls proposed for these limits. To examine the thermal control I developed 2-D finite element models of the Sumatra subduction zone with smoothly varying subduction dip, variable thermal properties of the rock units, frictional heating along the rupture plane, and an appropriate thermal state for the incoming plate. The common updip thermal limit for seismic behaviour of 100-150°C occurs close to or at the trench in agreement with the rupture limit of the 2004 earthquake. Off central Sumatra the common downdip thermal limit range of 350-450°C occurs at 30-60 km depth. The 350°C isotherm location is in agreement with the earthquake limits but 450°C is deeper. North of Sumatra, 350°C occurs ~14 km deeper than the earthquake rupture limit. The proposed composition control for the downdip limit, the intersection of the subduction thrust with the forearc mantle, is at a depth of ~30 km, 140-200 km from the trench, in good agreement with the earthquake limits. These results support the conclusion that the Sumatra updip seismogenic limit is thermally controlled, but the downdip limit is governed by the intersection of the downgoing plate with the forearc Moho. / Graduate

In this thesis I use Interferometric Synthetic Aperture Radar (InSAR) and GPS geodetic observations, along with numerical models, to examine the distribution of strain, assess seismic hazard, and study the dynamics of deformation across Turkey and Iran. I measure interseismic strain accumulation across the Ashkabad fault using InSAR, and find that atmospheric corrections using MERIS (Medium Resolution Imaging Spectrometer) data are necessary in order to retrieve the tectonic signal in the presence of large atmospheric delays. I estimate a slip rate of 5-12 mm/yr for the Ashkabad fault which is faster than previous geodetic estimates. I also attempt to validate atmospheric corrections derived from the ERA-Interim numerical weather model and find that they do not work satisfactorily for this region. I produce InSAR-derived velocity maps for five overlapping tracks in Eastern Turkey, covering both the North Anatolian Fault (NAF) and East Anatolian Fault (EAF), and measure slip rates for the NAF and EAF of 20+/-3 mm/yr and 10+/-2 mm/yr respectively. I calculate a velocity field for Eastern Turkey from these InSAR data and a compilation of GPS data, and find that strain is mainly localised across the NAF and EAF and that there is negligible differential vertical motion across the Anatolian Plateau. I construct a thin viscous sheet model for Iran and find that the GPS velocity field is well described by deformation of a ductile lithosphere. Contrary to previous suggestions, a rigid central Iran is not required to match the kinematics of Iranian deformation, but buoyancy forces acting in the lithosphere are found to play an important role. I develop a new method to assess slip rates and therefore seismic hazard on major faults in Iran from this continuum model. In this thesis I have measured slip rates across three major strike-slip faults using InSAR; the first time this has been achieved for the Ashkabad fault and the EAF. I have demonstrated the importance of atmospheric correction for these results, and have shown that Iran deforms as a continuous medium.

L’interprétation structurale de sections de Sismique Réflexion Multitrace-2D acquises pendant la campagne SISTEUR sur la marge de l’Équateur Central et migrées en profondeur avant sommation (PSDM) a été combinée avec la bathymétrie multifaisceaux, des modèles tomographiques de sismique grand-angle OBS, un modèle d’inversion GPS, et 13 années de sismicité relocalisée, afin de déchiffrer les causes de la variabilité de la sismicité et du Couplage Inter Sismique (CIS) le long de la subduction. La partie marine de cette marge est étroite et érosive. Elle chevauche vers l’Ouest, à 4.7 cm/an, la Ride de Carnégie. Le segment nord de la zone d’étude est bloqué, et aucun chenal de subduction n’est identifié. Ce segment révèle la présence d’un important (50 X 40+ km) massif océanique (MO) subduit, haut de ~2.5 km, et dont le flanc arrière plonge vers le continent de 2-4°, et coïncide avec la zone de CIS bloquée, et avec le socle océanique résistant (Vp= 5 km/s) de la marge. Le flanc avant du MO déduit de notre étude coïncide avec une zone de CIS partiel et des essaims de séismes chevauchant déformant le socle de la marge. A l’inverse, le segment sud est découplé, et affiche une pente sous-marine très perturbée avec des escarpements abrupts. Le contact interplaque plonge de 6-7° sous le continent et porte des monts sous-marins isolés séparés par un chenal de subduction de ~1km d’épaisseur qui agit comme lubrifiant. Un scénario en 3 étapes est proposé pour la subduction d’un MO de forme émoussée sous la marge résistante de l’Ile La Plata. Un modèle cinématique est proposé pour rendre compte de la surrection de l’île de La Plata en réponse au MO au cours des derniers 1.3-1.4 Ma. / The structural interpretation of 2D-Pre-stack Depth Migrated Multichannel Seismic Reflection sections collected during the SISTEUR cruise across the Ecuadorian margin was combined with multibeam bathymetry, OBS wide-angle tomographic models, a GPS inversion model, and 13 years of relocated seismicity to decipher the causes of the along-trench variability of the seismicity and Inter-Seismic Coupling (ISC). The margin submarine part is narrow and dominated by subduction erosion. It is underthrust eastward at 4.7 cm/yr by Carnegie Ridge, and figures a decoupled subduction centered over La Plata Island region. Our study shows that the Central Ecuador margin divides in two contrasting segments with dissimilar long-lived physical properties that may account for their specific ISC and seismicity patterns. The locked northern segment shows a smooth outer-wedge slope scalloped by a gentle re-entrant. No subduction channel is detected across this segment that reveals a broad 50 X 40+ km, ~2.5-km-high subducted Oceanic Massif (OM), which dips landward 2-4°, which coincides with the strong (Vp= 5 km/s) oceanic margin basement. In contrast, the decoupled southern segment shows a highly disrupted outer-wedge seafloor. The plate interface dips landward ~6-7°, and is spotted by isolated seamounts separated by a ~1 km-thick subduction channel that may act as a lubricant favoring inter-plate decoupling. A 3-step scenario is put forward for the subduction of a low-drag shaped OM beneath the resistant margin wedge of La Plata Island. Moreover, a kinematic model accounting for the uplift history of La Plata Island is proposed as a result of the OM subduction over the last 1.3-1.4 Myr.

The close proximity of the Nicoya Peninsula to the Cocos-Caribbean Subduction zone plate boundary makes it a prime location to use GPS to study episodic tremor and slip. Nicoya Peninsula currently has operating networks of both continuous GPS (CGPS) and seismic stations designed to identify and characterize the pattern of episodic tremor and slip (ETS) events along the seismogenic zone under Costa Rica's Pacific Margin. The occurrence of slow slip events has been previously postulated in this region based on correlated fluid flow and seismic tremor events recorded near the margin wedge in 2000 and from sparse GPS observations in 2003. Paucity of data prevented details of these events from being resolved. In May 2007 a slow slip event was recorded on our densified GPS network. This slow slip event was also accompanied by seismic tremor, worked up by colleagues at the University of California - San Diego. I will present the GPS time series, correlated with the seismic tremor for the event in May 2007. I will also present the inferred pattern of slip on the plate interface from elastic half space inversion modeling compared with the tremor and Low Frequency Earthquake (LFE) locations. The geodetic slip and seismic tremor co-locate temporally very well. Spatially the seismic tremor and LFE locations are offset but not independent of both the up dip and down dip patches of geodetic slip. The identification of these slow slip events enhances our understanding of the nuances of the interseismic period. Previous studies of the interseismic strain accumulation patterns in the region of the Nicoya Peninsula have not accounted for the occurrence of slow slip, thus underestimating the magnitude of locking on the fault plane. My study resolves this bias by using our CGPS network to estimate the interseismic surface velocity field, accounting for the May 2007 slow slip event. I will present the results of this velocity field estimation and the results of inversions for locking patterns on the fault plane. My study has also elucidated a potential temporal variability in the locking pattern on the fault plane beneath Nicoya.

We first describe the methodology for a two-dimensional, elastic deformable microplate modeling approach for continental plate boundaries. Deformable microplate models combine discrete slip on microplate boundaries (faults) with continuous deformation in block interiors. Two idealized models simulating continental collision are presented, one with two microplates and one with four microplates. / Earth and Planetary Sciences

Localized geodetic deformation of an approximately arctangent shape around locked strike-slip faults is widely reported, but there are also important exceptions showing distributed interseismic deformation. Understanding the controlling mechanism is important to the interpretation of geodetic observations for hazard assessment and geodynamic analysis. In this thesis, I use simple finite element models to separately study the two major contributors to the deformation: far-field loading and previous earthquakes. The models feature a vertical strike-slip fault in an elastic layer overlying a viscoelastic substrate of Maxwell or Burgers rheology, with or without weaknesses representing extensions of the fault either along strike or to greater depth. If the locked fault is loaded only from the far field without the effects of previous earthquakes, localized deformation occurs only if local mechanical weaknesses below the fault and/or somewhere along strike are introduced. I first show that the effects of far-field loading are rather limited even in the presence of extreme weaknesses. Then I use idealized earthquake cycle models to investigate the effects of past seismic events in a viscoelastic Earth. I demonstrate that, after a phase of fast postseismic deformation just after the earthquake, the localization of interseismic deformation is controlled mainly by the recurrence interval of past earthquakes. Given viscosity, shorter recurrence leads to greater interseismic localization, regardless of the rheological model used. The presence of a low-viscosity deep fault zone does not change this conclusion, although it tends to lessen localization by promoting faster postseismic stress relaxation. Distributed interseismic deformation, although less reported in the literature, is a natural consequence of very long recurrence and in theory should be as common as localized deformation. The apparent propensity of the latter is likely associated with the much greater quantity and better quality of geodetic observations from higher-rate and shorter-recurrence faults. Using viscoelastic earthquake-cycle models, I also explore the role of nearby earthquakes and creeping segments along the same fault. For faults of relatively short recurrence, frequent ruptures of nearby segments, modelled using a migrating rupture sequence with or without temporal clustering, further enhance localization. For faults of very long recurrence, faster near-fault deformation induced by a recent earthquake may give a false impression of localized interseismic deformation. / Graduate

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

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

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第22256号 / 理博第4570号 / 新制||理||1656(附属図書館) / 京都大学大学院理学研究科地球惑星科学専攻 / (主査)准教授 西村 卓也, 教授 福田 洋一, 准教授 深畑 幸俊 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

A tsunami generated by the C.E. 1707 Hōei earthquake is largely thought to be the flood event of record for southwestern Japan, yet historical documentation of the event is scarce. This is particularly true in northwestern Shikoku within the Bungo Channel, where significant inconsistencies exist between historical records and model-derived tsunami heights. To independently assess flooding from the C.E. 1707 Hōei tsunami in the context of the region’s long-term flooding history, we present complementary reconstructions of extreme coastal inundation from three back-barrier lakes in the northern Bungo Channel: Lake Ryuuoo, Lake Amida, and Lake Kamega. At all sites, the most prominent marine overwash deposit of the past ~1,000 years, as defined by grain size, density, and geochemical indicators, is consistent with the timing of the 1707 tsunami, providing strong evidence that the event caused the most significant flooding of the last millennium in this region. At Lake Ryuuoo, modern barrier beach elevations and grain sizes in the tsunami’s resultant deposit provide ~4 m as the first physically based height constraint for the 1707 tsunami in the northern Bungo Channel. Around 1,000 years ago, a concurrent and abrupt transition in lithology observed at all three sites is also consistent with rapid, regional geomorphic change. At Lake Ryuuoo, a marine overwash deposit comparable to the 1707 deposit directly overlies this transition. A 1,000-year-old lithological transition or deposit has not been observed at sites closer to the mouth of the Bungo Channel, suggesting that the deposit in Lake Ryuuoo is more consistent with a tsunami generated by local seismicity along the Japan Median Tectonic Line than with a Nankai Trough-derived tsunami. Our findings are significant in that they provide three new millennial-scale tsunami inundation reconstructions for a relatively understudied region of Japan, along with the first physically based height constraint for the Hōei tsunami in the northern Bungo Channel.