The three largest earthquakes in the last 25 years in southern California occurred on faults located adjacent to the southern San Andreas fault, with the M7.3 1992 Landers and M7.1 1999 Hector Mine earthquakes occurring in the eastern California shear zone (ECSZ) in the Mojave Desert, and the M7.2 2010 El Mayor-Cucapah earthquake occurring along the Laguna Salada fault in northern Baja California, Mexico. The locations of these events near to but not along the southern San Andreas fault (SSAF) is unusual in that the last major event on the SSAF occurred more than 300 years ago, with an estimated recurrence interval of 215± 25 years. The focus of this dissertation is to address the present-day deformation field along the North America-Pacific plate boundary in southern California and northern Baja California, through the analysis of GPS data, and elastic block and viscoelastic earthquake models to determine fault slip rates and rheological properties of the lithosphere in the plate boundary zone. We accomplish this in three separate studies. The first study looks at how strain is partitioned northwards along-strike from the southern San Andreas fault near the Salton Sea. We find that estimates for slip-rates on the southern San Andreas decrease from ~23 mm/yr in the south to ~8 mm/yr as the fault passes through San Gorgonio Pass to the northwest, while ~13-18 mm/yr of slip is partitioned onto NW-SE trending faults of the ECSZ where the Landers and Hector Mine earthquakes occurred. This speaks directly to San Andreas earthquake hazards, as a reduction in the slip rate would require greater time between events to build up enough slip deficit in order to generate a large magnitude earthquake. The second study focuses on inferring the rheological structure beneath the Salton Trough region. This is accomplished through analysis of postseismic deformation observed using a set of the GPS data collected before and after the 2010 El Mayor-Cucapah earthquake. By determining the slip-rates on each of the major crustal faults prior to the earthquake, we are able to model the pre-earthquake velocity field for comparison with velocities measured using sites constructed post-earthquake. We then determine how individual site velocities have changed in the 3 years following the earthquake, with implications for the rate at which the lower crust and upper mantle viscously relax through time. We find that the viscosity of the lower crust is at least an order of magnitude higher than that of the uppermost mantle, and hypothesize that this is due to mafic material emplaced at the base of the crust as the spreading center developed beneath the Salton Trough since about 6 Ma. The final study investigates crustal deformation and fault slip rates for faults in the northern Mojave and southern Walker Lane regions of the ECSZ. Previous geodetic studies estimated slip-rates roughly double those inferred via geological dating methods in this region for NW striking strike-slip faults, but significantly smaller than geologic estimates for the Garlock fault. Through construction of a detailed elastic block model, which selects only active fault structures, and applying a new, dense GPS velocity field in this region, we are able to estimate slip-rates for the strike-slip faults in the ECSZ that are much closer to those reported from geology.

L’étude des séismes géants de subduction présente un intérêt de premier ordre, car ils sontsuffisamment puissants pour exciter le manteau et déclencher sa relaxation visco-élastique. Cephénomène est caractérisé par des déformations à grande échelle spatiale (plusieurs milliers dekilomètres) et temporelle (plusieurs décennies). L’étude des déformations post-sismiques en surfacepar géodésie spatiale permet de contraindre les caractéristiques géométriques et rhéologiques del’interface de subduction, ouvrant ainsi la voie à l’étude du cycle sismique dans sa globalité.Le 27 février 2010 se produit le séisme de Mw 8.8, dans la région du Maule, au large du Chili. Lasubduction de la plaque Nazca sous la plaque continentale Sud-Américaine offre, pour la premièrefois, la possibilité de mesurer de manière continue et dense les déformations post-sismiques sur plusde 1500 km. Par ailleurs, plus de 10 ans de campagnes de mesures GPS, ont permis d’imager uncouplage très hétérogène tout au long de l’interface de subduction. L’imbrication alors visible entreles déformations post-sismiques et inter-sismiques, appuyée par l’étude de la sismicité historique,met ainsi en évidence les interactions inter-segments que seuls les modèles visco-élastiques de cyclesismique permettront de mieux comprendre.Cette thèse a été centrée autour de deux axes principaux, qui conduisent vers l’objectif finaldes modèles visco-élastiques de cycle sismique. Le premier et principal objectif est l’étude desdéformations post-sismiques du Maule. J’ai ainsi traité et analysé les cinq ans de données aprèsle séisme afin d’extraire le champ de déformation post-sismique. Ces données ont alors permis decontraindre les modèles visco-élastiques, grâce à la méthode des éléments finis. Un modèle combinéd’afterslip et de relaxation visco-élastique dans l’asthénosphère et dans un chenal à faible viscositétrès profond, permet ainsi d’expliquer le champ de déformation horizontal mais aussi verticalobservé. L’amplitude et la complexité des déformations en champ proche résulte de "l’afterslip",tandis que la relaxation dans le chenal permet de reproduire le très fort soulèvement de la Cordillèredes Andes. Enfin, la relaxation dans l’asthénosphère est responsable de l’extension sur plusieursmilliers de kilomètres des déformations post-sismiques. De plus, la continuité de l’effort de terrainet le traitement des données recueillies a permis de combler l’ultime gap de données. Il a ainsiété possible de déterminer un champ de vitesse inter-sismique continu sur la quasi totalité del’interface. Finalement, même si un modèle de cycle sismique à l’échelle de la subduction Chiliennen’a pas encore pu être réalisé, le modèle de post-sismique apporte déjà de nouveaux indices sur lesinteractions entre les différents segments de l’interface Chilienne, suite au dernier séisme. / The study of giant earthquakes on subduction zone represents a main interest. They are indeedsufficiently powerful to excite the mantle and trigger its viscoelastic relaxation, over a very largespatial (thousands of kilometers) and temporal (several decades) scale. Postseismic deformation,monitored by spatial geodesy, are a proxy to the geometrical and rheological characteristics of thesubduction interface, that will allow us to study the whole seismic cycle.On February 27th 2010 in the region of Maule, Chile, occurs the Mw 8.8 megathrust earthquake.Yet, the subduction of the Nazca plate beneath the continental South-American plate offers, forthe first time, the opportunity to measure continuously and densely the postseismic deformationfollowing the earthquake, over more than 1500 km. Otherwise, more than a decade of GPS repeatedmeasurements allowed to image a very heterogeneous coupling all along the Chilean interface. Thevisible imbrication between postseismic deformation and interseismic loading, supported by historicaland instrumental seismicity, highlights interactions between the segments. Viscoelastic modelsof seismic cycle appears to be the only way to understand these interactions.This PhD focused on two main axes, that will lead to the development of viscoelastic modelsof seismic cycle. The first part was dedicated to the study of postseismic deformation followingthe Maule earthquake. Therefore, we processed and analyzed very precisely GPS data in orderto extract the postseismic pattern and modeled it using the finite elements method. A combinedmodel of afterslip and viscoelastic relaxation in the asthenosphere and in a low viscosity channel,extending deep along the slab, can reproduce the complex deformation pattern, horizontaly and inverticaly. The amplitude and complexity of the near-field deformation result from aseismic slip onthe fault plane, while the great uplift of the Cordillera is reproduced by relaxation in the channel.The far field extension, up to 1600 km, entirely results from relaxation in the asthenosphere. Onthe other hand, the continuity of campaign measurements was the occasion to fill the ultimate gapof data, and thus estimate a continuous interseismic velocity field from the North of the Maulerupture zone up to North Chile. Finally, even if the final viscoelastic models of seismic cycle couldnot be processed yet, the present postseismic model already brings new insights on interactionsbetween the different segments of the Chilean interface, following the last Chilean earthquake.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19283号 / 工博第4080号 / 新制||工||1629(附属図書館) / 32285 / 京都大学大学院工学研究科社会基盤工学専攻 / (主査)教授 田村 正行, 教授 小池 克明, 准教授 須﨑 純一 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

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