Thesis (Ph. D.)--University of Washington, 2002. / Vita. Includes bibliographical references (p. 144-159).

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.

No description available.

Thesis (Ph. D.)--University of Oregon, 1999. / Typescript. Includes vita and abstract. Includes bibliographical references (leaves 251-279). Also available for download via the World Wide Web; free to University of Oregon users. Address: http://wwwlib.umi.com/cr/uoregon/fullcit?p9957567.

Thesis (Ph. D.)--University of Oregon, 1999. / Typescript. Includes vita and abstract. Includes bibliographical references (leaves 251-279). Also available for download via the World Wide Web; free to University of Oregon users. Address: http://wwwlib.umi.com/cr/uoregon/fullcit?p9957567.

In this thesis, we investigate and develop the optimal data processing procedures necessary to recover Green’s functions for body waves propagating among a network of borehole seismometers near Parkfield, CA. Applying these procedures, we detect P-waves propagating among these stations, which allows us to produce a first-order crustal velocity model for the San Andreas Fault in the Parkfield region. We also discuss under what conditions body wave phenomena such as reflections and mode conversions (P to S) may be observed, as further observing these would provide a dramatic improvement in our ability to characterize seismic velocity structures. Finally, we discuss the potential of seismic interferometry to produce time-lapse body wave characterizations of the San Andreas Fault, in which properties of the fault can be seen to change in time.

This project examines the composition, structure, and geophysical properties of rocks sampled within the San Andreas Fault Observatory at Depth (SAFOD) borehole drilling experiment near Parkfield, California. Cuttings, sidewall cores, spot-core, and whole-rock core are examined from the meso- to micro-scale to characterize the nearfault environment at shallow crustal levels (0-4 km) along the central segment of the San Andreas fault. The central segment deforms by contiuous aseismic creep and microseismicity. An integrated approach utilizing core-logging, detailed structural core mapping, petrology, microstructural analyses, whole-rock geochemistry, borehole geophysics, and analog field studies is followed. At SAFOD, fractured granitic rocks and arkosic sediments are identified west of the San Andreas fault zone on the Pacific Plate; whereas sheared fine-grained sediments, ultrafine black fault-related rocks, and serpentinite-bearing fault gouge are present within and northeast of the fault zone on the North American Plate. Here, the fault consists of a broad zone of variably damaged rock containing localized zones of highly concentrated shear that often juxtapose distinct rock-types. Two zones of serpentinite-bearing clay gouge, each meters-thick are found in two locations where active aseismic creep was identified in the borehole. The gouge is composed of Mg-rich clays, serpentinite (lizardite ± chrysotile) with notable increases in magnetite, and Fe-, Ni-, and Cr-oxides/hydroxides and Fe-sulfides relative to the surrounding host rock. Organic carbon is locally high within fractures and bounding slip surfaces. The rocks adjacent to and within the two gouge zones display a range of deformation including intensely fractured regions, blockin- matrix fabrics, and foliated cataclasite structure. The blocks and clasts predominately consist of competent sandstone and siltstone embedded in a clay-rich matrix that displays a penetrative scaly fabric. Mineral alteration, veins, fracture-surface coatings, and slickelined surfaces are present throughout the core, and reflect a long history of syndeformation and fluid-rock reaction that contributes to the low-strength and creep in the meters-thick gouge zones. Evaluation of borehole geophysical data and elastic modulii for the lithologic and structural units identified in the SAFOD Phase 3 core reveal a correlation between composition and textures and the structural and/or permeability architecture of the SAF at SAFOD. Highly reduced velocity and elastic modulii surround the two serpentinitev bearing gouge zones, the Buzzard Canyon fault to the southwest, and another bounding fault to the northeast. Velocity and elastic moduli values on the Pacific Plate or southeast of the active fault trace intersected by SAFOD are much higher relative to the values measured on the North American Plate, or northeast of the fault trace. Within and adjacent to the two active gouge zones, the rock properties are highly variable over short distances, however, they are significantly lower relative to material outside of the fault zones. This research contributes critical evidence for rock properties and slip behavior within an active plate boundary fault. Results from this research and the SAFOD experiment help to constrain numerous hypotheses related to fault zone behavior and earthquake generation within central California.

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.

Spot core from the San Andreas Fault Observatory at Depth (SAFOD) borehole provides the opportunity to characterize and quantify damage and mineral alteration of siliciclastics within an active, large-displacement plate-boundary fault zone. Deformed arkosic, coarse-grained, pebbly sandstone, and fine-grained sandstone and siltstone retrieved from 2.55 km depth represent the western damaged zone of the San Andreas Fault, approximately 130 m west of the Southwest Deforming Zone (SDZ). The sandstone is cut by numerous subsidiary faults that display extensive evidence of repeating episodes of compaction, shear, dilation, and cementation. The subsidiary faults are grouped into three size classes: 1) small faults, 1 to 2 mm thick, that record an early stage of fault development, 2) intermediate-size faults, 2 to 3 mm thick, that show cataclastic grain size reduction and flow, extensive cementation, and alteration of host particles, and 3) large subsidiary faults that have cemented cataclastic zones up to 10 mm thick. The cataclasites contain fractured host-rock particles of quartz, oligoclase, and orthoclase, in addition to albite and laumontite produced by syn-deformation alteration reactions. Five structural units are distinguished in the subsidiary fault zones: fractured sandstones, brecciated sandstones, microbreccias, microbreccias within distinct shear zones, and principal slip surfaces. We have quantified the particle size distributions and the particle shape of the host rock mineral phases and the volume fraction of the alteration products for these representative structural units. Shape characteristics vary as a function of shear strain and grain size, with smooth, more circular particles evolving as a result of increasing shear strain. Overall, the particle sizes are consistent with a power law distribution over the particle size range investigated (0.3 µm < d < 400 µm). The exponent (fractal dimension, D) is found to increase with shear strain and volume fraction of laumontite. This overall increase in D and evolution of shape with increasing shear strain reflects a general transition from constrained comminution, active at low shear strains to abrasion processes that dominate at high shear strains.

It has always been enigmatic which processes control the accretion of the North American terranes towards the Pacific plate and the landward migration of the San Andreas plate boundary. One of the theories suggests that the Pacific plate first cools and captures the uprising mantle in the slab window, and then it causes the accretion of the continental crustal blocks. The alternative theory attributes the accretion to the capture of Farallon plate fragments (microplates) stalled in the ceased Farallon-North America subduction zone. Quantitative judgement between these two end-member concepts requires a 3D thermomechanical numerical modeling. However, the software tool required for such modeling is not available at present in the geodynamic modeling community. The major aim of the presented work is comprised basically of two interconnected tasks. The first task is the development and testing of the research Finite Element code with sufficiently advanced facilities to perform the three-dimensional geological time scale simulations of lithospheric deformation. The second task consists in the application of the developed tool to the Neogene deformations of the crust and the mantle along the San Andreas Fault System in Central and northern California. The geological time scale modeling of lithospheric deformation poses numerous conceptual and implementation challenges for the software tools. Among them is the necessity to handle the brittle-ductile transition within the single computational domain, adequately represent the rock rheology in a broad range of temperatures and stresses, and resolve the extreme deformations of the free surface and internal boundaries. In the framework of this thesis the new Finite Element code (SLIM3D) has been successfully developed and tested. This code includes a coupled thermo-mechanical treatment of deformation processes and allows for an elasto-visco-plastic rheology with diffusion, dislocation and Peierls creep mechanisms and Mohr-Coulomb plasticity. The code incorporates an Arbitrary Lagrangian Eulerian formulation with free surface and Winkler boundary conditions. The modeling technique developed is used to study the aspects influencing the Neogene lithospheric deformation in central and northern California. The model setup is focused on the interaction between three major tectonic elements in the region: the North America plate, the Pacific plate and the Gorda plate, which join together near the Mendocino Triple Junction. Among the modeled effects is the influence of asthenosphere upwelling in the opening slab window on the overlying North American plate. The models also incorporate the captured microplate remnants in the fossil Farallon subduction zone, simplified subducting Gorda slab, and prominent crustal heterogeneity such as the Salinian block. The results show that heating of the mantle roots beneath the older fault zones and the transpression related to fault stepping, altogether, render cooling in the slab window alone incapable to explain eastward migration of the plate boundary. From the viewpoint of the thermomechanical modeling, the results confirm the geological concept, which assumes that a series of microplate capture events has been the primary reason of the inland migration of the San Andreas plate boundary over the recent 20 Ma. The remnants of the Farallon slab, stalled in the fossil subduction zone, create much stronger heterogeneity in the mantle than the cooling of the uprising asthenosphere, providing the more efficient and direct way for transferring the North American terranes to Pacific plate. The models demonstrate that a high effective friction coefficient on major faults fails to predict the distinct zones of strain localization in the brittle crust. The magnitude of friction coefficient inferred from the modeling is about 0.075, which is far less than typical values 0.6 – 0.8 obtained by variety of borehole stress measurements and laboratory data. Therefore, the model results presented in this thesis provide additional independent constrain which supports the “weak-fault” hypothesis in the long-term ongoing debate over the strength of major faults in the SAFS. / Seit jeher rätselhaft sind die Prozesse, die die Akkretion der Nordamerikanischen Terranen in Richtung der Pazifischen Platte sowie die Wanderung der Plattengrenze der San-Andreas-Verwerfung in Richtung Festland bestimmen. Eine Theorie besagt, dass sich die Pazifische Platte erst abkühlt und den aufsteigenden Mantel im „Slab Window“ fängt und somit die Akkretion der kontinentalen Krustenblöcke bewirkt. Die andere Theorie geht von einer Akkretion durch das Fangen von Teilen der Farallon-Platte (Mikroplatten) aus, die in der inaktiven nordamerikanischen Farallon-Subduktionszone fest stecken. Die quantitative Beurteilung dieser beiden gegensätzlichen Konzepte erfordert eine thermomechanische numerische 3-D-Modellierung. Das dafür benötigte Software Tool steht jedoch der geodynamischen Modellierung derzeit noch nicht zur Verfügung. Das Hauptziel der vorliegenden Arbeit umfasst im Wesentlichen zwei miteinander verbundene Aufgaben. Die erste besteht in der Entwicklung und Erprobung des Finite-Element-Codes, dessen Eigenschaften den hohen Anforderungen an die Ausführung der dreidimensionalen Simulationen lithosphärischer Deformation auf geologischer Zeitskala gerecht werden müssen. Die zweite Aufgabe ist die Anwendung des entwickelten Tools auf die neogenen Deformationen der Kruste und des Mantels entlang der San-Andreas-Verwerfung in Zentral- und Nordkalifornien. Die Modellierung auf geologischer Zeitskala lithosphärischer Deformation bringt für die Software Tools in Bezug auf Konzept und Durchführung zahlreiche Herausforderungen mit sich. Unter anderem gilt es, den Brittle-Ductile-Übergang in einem einzigen Modell sowie die Gesteinsrheologie in einer breiten Spanne unterschiedlicher Temperaturen und Spannungen adäquat darzustellen und die extremen Deformationen der freien Oberfläche und internen Grenzen aufzulösen. Im Rahmen der vorliegenden Arbeit erfolgte die erfolgreiche Entwicklung und Erprobung des neuen Finite-Element-Codes (SLIM3D). Dieser Code beinhaltet eine gekoppelte thermomechanische Behandlung von Deformationsprozessen und ermöglicht eine elasto-visko-plastische Rheologie mit Diffusion, Dislokation, Peierls Kriechmechanismen und Mohr-Coulomb-Plastizität. Der Code verbindet eine Arbitrary Lagrangian-Eulerian kinematische Formulierung mit freien Oberflächen- und Winkler-Randbedingungen. Das entwickelte Modellierungsverfahren wird für die Untersuchung der Aspekte verwendet, die die neogene lithosphärische Deformation in Zentral- und Nordkalifornien beeinflussen. Die Modellanordnung konzentriert sich auf die Interaktion zwischen drei großen tektonischen Elementen in dieser Region: die Nordamerikanische Platte, die Pazifische Platte sowie die Gorda-Platte, die sich in der Mendocino-Triple-Junction treffen. Unter anderem verdeutlicht die Modellierung den Einfluss des Aufsteigens der Asthenosphäre in das sich öffnende „slab window“ der übergelagerten Nordamerikanischen Platte. Die Modelle beziehen auch die angelagerten Überreste der Mikroplatten in der fossilen Farallon-Subduktionszone, die vereinfachte subduzierende Gorda-Platte sowie markante Heterogenitäten der Kruste, wie beispielsweise den „Salinian Block“, mit ein. Die Ergebnisse zeigen, dass die Erwärmung der Mantellithosphäre unter den älteren Störungszonen sowie die Transpression eine Abkühlung im „Slab Window“ als alleinige Begründung für die Ostwärtsbewegung der Plattengrenze nicht zulassen. Aus Sicht der thermomechanischen Modellierung bestätigen die Ergebnisse das geologische Konzept, welches durch das mehrmalige Fangen von Mikroplatten den Hauptgrund für die Wanderung der Plattengrenze der San-Andreas-Verwerfung in Richtung Festland über die letzten 20 Millionen Jahre sieht. Die Überreste der Farallon-Platte, die in der fossilen Subduktionszone gefangen sind, verursachen im Mantel eine wesentlich stärkere Heterogenität als die Abkühlung der Asthenosphäre und stellen somit den effizienteren und direkteren Weg für die Anlagerung der nordamerikanischen Gebiete an die Pazifische Platte dar. Die Modelle demonstrieren, dass ein hoher effektiver Reibungskoeffizient an großen Störungen nicht in der Lage ist, die eindeutigen Zonen der Dehnungslokalisierung in der spröden Kruste vorherzusagen. Die Größe des Reibungskoeffizienten, die sich aus der Modellierung ableitet, beträgt etwa 0,075 und ist damit wesentlich kleiner als die durch unterschiedliche Bohrlochmessungen und Labordaten ermittelten Spannungswerte zwischen 0,6 und 0,8. Daher liefern die in dieser Arbeit präsentierten Ergebnisse der Modelle in der seit langem geführten Debatte über die Stärke von großen Störungen in der San-Andreas-Verwerfung eine zusätzliche unabhängige Begründung der „Weak-Fault“-Hypothese.