The San Andreas fault forms the right lateral transform boundary between the North American and Pacific tectonic plates. At various locations along the San Andreas fault the geometry of the fault surface is much more complex than a straight, vertical, plane. The San Bernardino Mountain segment of the San Andreas fault, in the San Gorgonio Pass region has one of the most complex active fault geometries in southern California due to a left-stepping restraining bend in the San Andreas fault. The evolution of the actively faulting pass has created an intricate network of active and formerly active, dipping and vertical, three-dimensionally irregular fault surfaces. The purpose of this research is to gain a better understanding of the mechanics of the present day active fault geometry and the evolution in the San Gorgonio pass region, through numerical modeling. We use the three-dimensional Boundary Element Method modeling code Poly3D to simulate different fault configurations. We see that fault geometries that include geologically observed and inferred fault dips match geologic data more accurately than simplified, vertical faults in the San Gorgonio Pass region of the San Andreas fault. The evolution of the San Andreas Fault in the San Gorgonio Pass region over the past million years may follow the principle of work minimization in the Earth’s crust up until the present day configuration.

abstract: There is a need to understand spatio-temporal variation of slip in active fault zones, both for the advancement of physics-based earthquake simulation and for improved probabilistic seismic hazard assessments. One challenge in the study of seismic hazards is producing a viable earthquake rupture forecast—a model that specifies the expected frequency and magnitude of events for a fault system. Time-independent earthquake forecasts can produce a mismatch among observed earthquake recurrence intervals, slip-per-event estimates, and implied slip rates. In this thesis, I developed an approach to refine several key geologic inputs to rupture forecasts by focusing on the San Andreas Fault in the Carrizo Plain, California. I use topographic forms, sub-surface excavations, and high-precision geochronology to understand the generation and preservation of slip markers at several spatial and temporal scales—from offset in a single earthquake to offset accumulated over thousands of years. This work results in a comparison of slip rate estimates in the Carrizo Plain for the last ~15 kyr that reduces ambiguity and enriches rupture forecast parameters. I analyzed a catalog of slip measurements and surveyed earth scientists with varying amounts of experience to validate high-resolution topography as a supplement to field-based active fault studies. The investigation revealed that (for both field and remote studies) epistemic uncertainties associated with measuring offset landforms can present greater limitations than the aleatoric limitations of the measurement process itself. I pursued the age and origin of small-scale fault-offset fluvial features at Van Matre Ranch, where topographic depressions were previously interpreted as single-event tectonic offsets. I provide new estimates of slip in the most recent earthquake, refine the centennial-scale fault slip rate, and formulate a new understanding of the formation of small-scale fault-offset fluvial channels from small catchments (<7,000 m2). At Phelan Creeks, I confirm the constancy of strain release for the ~15,000 years in the Carrizo Plain by reconstructing a multistage offset landform evolutionary history. I update and explicate a simplified model to interpret the geomorphic response of stream channels to strike-slip faulting. Lastly, I re-excavate and re-interpret paleoseismic catalogs along an intra-continental strike-slip fault (Altyn Tagh, China) to assess consistency of earthquake recurrence. / Dissertation/Thesis / Doctoral Dissertation Geological Sciences 2016

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第22113号 / 理博第4540号 / 新制||理||1652(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)教授 福田 洋一, 教授 岩田 知孝, 准教授 深畑 幸俊 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

京都大学 / 新制・課程博士 / 博士(理学) / 甲第24171号 / 理博第4862号 / 新制||理||1695(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 宮﨑 真一, 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

The geomorphic expression, sedimentation, and near-field deformation of a fault system may be characterized to obtain an understanding of its kinematic evolution and potential seismic hazards. The dynamics and deformation history of the Eastern California shear zone (ECSZ), a wide and complex network of right-lateral strike-slip faults, is not well understood, despite hosting three large (>Mw 7.0) earthquake ruptures in recent decades. The low-net slip faults of the ECSZ (each with <10 km) offer a unique opportunity to assess strain distribution in a developing, kinematically immature strike-slip system. To do so, I conducted field-based investigations of these faults within the Mojave Block of the ECSZ. First, I investigated the morphology, structure, and controls of restraining bend growth along the numerous faults of the ECSZ via field mapping and numerical deformational modeling. I found that the ECSZ restraining bends are small (kilometer-scale), exhibit high-angle, doubly fault-bound geometries with positive flower structures, and have self-similar morphologies characterized by a "whaleback" longitudinal profile and an arrowhead shape in map view. Gradual changes in form with increasing restraining bend size suggest a common growth mechanism influenced more by the kinematics of local fault geometries than by the fault's obliquity to plate motion. Modeling results indicate that concentrated shear strain at single transpressional bends facilitates the development of new secondary faults with cumulative strain as a mechanism to accommodate horizontal shortening via uplift between the faults. The ECSZ restraining bends contribute minimally to regional contractional strain due to their small size, steep fault angles, and shallow crustal penetration (< 5 km), which also suggests that they are unlikely to obstruct large earthquake ruptures. Second, I conducted a spatiotemporal slip rate analysis of the Calico fault with new mapping and geochronology of offset alluvial fans from North Hidalgo Mountain. From this work I obtain several findings. 1) The slip rate along North Hidalgo Mountain ranges from 1.5-2.1 mm/yr in the Holocene and 0.8-2.0 mm/yr in the late Pleistocene. 2) The similarity in slip rates between North Hidalgo Mountain and the Rodman Mountains suggests that this 38 km stretch is a kinematically coherent fault segment with a relatively steady slip rate of 1.7 +0.4/-0.3 mm/yr over the past 60 ka. Faster rates reported from Newberry Springs suggest either a significant increase in slip rate from the Rodman Mountains to Newberry Springs or temporal variations in slip rate. 3) The new rates support previous work which showed the central section of the Calico fault has the highest slip rate in the Mojave Block. However, it does not resolve the discrepancy between ECSZ geodetic and geologic slip rates, implying that transient changes in slip rate, or the contribution of off-fault deformation or other structures may be required. Additionally, the lack of geological slip rate data might contribute to this discrepancy if significant spatial and temporal variations exist on other ECSZ faults. / Doctor of Philosophy / The topography and geology within a fault system may be studied to understand tectonic plate motion over time and assess earthquake hazards. The Eastern California shear zone is a complex network of strike-slip faults within the Mojave Desert, which has hosted three large earthquakes (>Mw 7.0) in recent decades. Despite this significant seismic activity, the mechanisms of motion across the numerous faults in the Eastern California shear zone remain poorly understood. The individual faults have accumulated relatively little strike-slip motion since their inception (less than 10 kilometers), offering a unique opportunity to investigate the early-stage kinematics and seismic hazards of a strike-slip fault system. To do so, I conducted field-based investigations of the faults within the Eastern California shear zone. First, I investigated the early evolution and controls of compressional strike-slip fault bends in the Eastern California shear zone. From mapping and numerical modeling, I characterized the shape, structure, and uplift of numerous small compressional bends dispersed across the faults. From these efforts, I found that uplifted crust in the fault bends exhibit self-similar forms with shallow crustal depths (<5 km). Small changes in the shape of these structures occur with increasing size indicating a predictable pattern of growth with increasing cumulative slip that appears to be partially controlled by local fault conditions. Numerical modeling of simple compressional fault bends indicate that shear strain concentrates at bend corners, which may facilitate the growth of a new fault that more efficiently accommodates contraction in the bend via uplift of the crust between the two faults. The compressional strike-slip fault bends in the Eastern California shear zone are too small to significantly impact regional contractional strain and are therefore also unlikely to impede large earthquake ruptures. Second, I studied the slip rate (or rate at which the fault moves) of the Calico fault via new mapping and age data of displaced alluvial fans. I found that 1) the Calico fault at North Hidalgo Mountain slips at a rate of 0.8-2.0 mm/yr since ~70,000 years ago. 2) The slip rates from North Hidalgo Mountain and the Rodman Mountains are similar, indicating that the 38 kilometers between them behaves consistently, with a steady rate of ~1.7 mm/yr over the last ~60,000 years. However, faster slip rates reported at Newberry Springs suggest either a significant increase in slip rate from the Rodman Mountains to Newberry Springs or that it varies over time. 3) These findings confirm that the central Calico fault has the fastest slip rate in the Mojave Block but does not reconcile regional differences between rates from geodetic and geological measurements. The difference between the slip rates measured by geodetic methods and those from geological studies in the Eastern California shear zone suggests that there could be temporary changes in slip rates or that deformation might be occurring in areas away from the main fault. Also, the lack of geological slip rate data might contribute to this discrepancy if significant spatial and temporal variations exist on other Eastern California shear zone faults.

The primary objective of this study is to evaluate the seismic hazard in the Eastern Marmara Region using improved seismic source models and enhanced ground motion prediction models by probabilistic approach. Geometry of the fault zones (length, width, dip angle, segmentation points etc.) is determined by the help of available fault maps and traced source lines on the satellite images. State of the art rupture model proposed by USGS Working Group in 2002 is applied to the source system. Composite reoccurrence model is used for all seismic sources in the region to represent the characteristic behavior of North Anatolian Fault. New and improved global ground motion models (NGA models) are used to model the ground motion variability for this study. Previous studies, in general, used regional models or older ground motion prediction models which were updated by their developers during the NGA project. New NGA models were improved in terms of additional prediction parameters (such as depth of the source, basin effects, site dependent standard deviations, etc.), statistical approach, and very well constrained global database. The use of NGA models reduced the epistemic uncertainty in the total hazard incorporated by regional or older models using smaller datasets. The results of the study is presented in terms of hazard curves, deaggregation of the hazard and uniform hazard spectrum for six main locations in the region (Adapazari, Duzce, Golcuk, Izmit, Iznik, and Sapanca City Centers) to provide basis for seismic design of special structures in the area. Hazard maps of the region for rock site conditions at the accepted levels of risk by Turkish Earthquake Code (TEC-2007) are provided to allow the user perform site-specific hazard assessment for local site conditions and develop site-specific design spectrum. Comparison of TEC-2007 design spectrum with the uniform hazard spectrum developed for selected locations is also presented for future reference.

Understanding spatial and temporal variations in strain accumulation and release along plate boundaries is a fundamental problem in tectonics. Short-term and long-term slip rates are expected to be equal if the regional stress field remains unchanged over time, yet discrepancies between modern geodetic (decadal time scale) slip rates and long-term geologic (10^3 to 10^6 years) slip rates have been observed on parts of the Pacific-North American plate boundary system. Contemporary geodetic slip rates are observed to be ~2 times greater than late Pleistocene geologic slip rates across the southern Walker Lane. I use a combination of GPS geodesy, detailed field geologic mapping, high-resolution LiDAR geodetic imaging, and terrestrial cosmogenic nuclide geochronology to investigate the observed discrepancy between long- and short-term slip rates. I find that the present day slip rate derived from GPS geodesy across the Walker Lane at ~37.5°N is 10.6 ± 0.5 mm/yr. GPS data suggest that much of the observed discrepancy occurs west of the White Mountains fault zone. New dextral slip rates on the White Mountains fault zone of 1.1 ± 0.1 mm/yr since 755 ka, 1.9 +0.5/-0.4 mm/yr since 75-115 ka, 1.9 +0.5/-0.4 mm/yr since 38.4 ± 9.0 ka, and 1.8 +2.8/-0.7 mm/yr since 6.2 ± 3.8 ka are significantly faster than previous estimates and suggest that slip rates there have remained constant since the middle Pleistocene. On the Lone Mountain fault I calculate slip rates of 0.8 ± 0.1 mm/yr since 14.6 ± 1.0 ka and 0.7 ± 0.1 mm/yr since 8.0 ± 0.5 ka, which suggest that extension in the Silver Peak-Lone Mountain extensional complex has increased dramatically since the late Pleistocene.

This dissertation focuses on the active tectonics of Afghanistan|slip-rate estimation of the Chaman fault and assessing seismic hazard in the Kabul basin. Afghanistan is a tectonically complex zone developed as a result of the collision between the Eurasian plate and the Indian plate to the southeast and the Arabian plate to the south. For seismic hazard mitigation, there is no large-scale active fault map in Afghanistan. I, therefore, mapped active and presumed active faults mainly based on interpretation of 1-arcsecond SRTM anaglyph images, and calculate the slip rate of the Chaman fautl based on 10Be TCN dating. / 博士(理学) / Doctor of Philosophy in Science / 同志社大学 / Doshisha University