The Agua Blanca fault is a major transverse structure of northern Baja California, extending more than 120km east from the Punta Banda ridge near the city of Ensenada to the San Matais Pass in central Baja. Through much of its eastern extent slip on this fault appears to be pure strike slip, however, at the Valle de Santo Tomás the fault makes a ~25°; change in orientation, which coincides with the formation of extensional basins on the fault. Recent evidence of the independent movement of the Baja Micro-plate relative to a stable Southern California Black leads to several possible hypotheses to explain this including: 1)That basins are localized structures, the result of a series of right steps or bends along the dextral Agua Blanca fault. 2)Basins are transtensional, possibly as a result of complexities associated with the northern boundary of the Baja Micro-plate . To test between these hypotheses it was necessary to constrain the fault kinematics on both basin bounding and in-basin faults, well as the basin geometry. This was accomplished through combined structural and geophysical surveys. Data collected suggest that the majority of dip-slip is confined to the Santo Tomás fault bounding the basin to the south, while the Agua Blanca fault bounding the basin to the north is primarily strike slip. This orientation typical in transtensional basins, suggesting that although Valle de Santo Tomás formed at a step over it is not a pull apart basin. Possible explanations for transtension in this area come from the orientation of the Agua Blanca fault in relation to the Baja Micro-plate. Where the fault is close to aligned with the relative motion of the plate there is little transtension, such as in Valle de Agua Blanca, however, where the fault makes a 25° change in orientation and becomes more oblique the motion of the Baja Micro-plate transtension is present.

Three-dimensional mechanical simulations of the San Andreas fault (SAF) within the Coachella Valley in California produce deformation that match geologic observations and demonstrate the impact of fault geometry on uplift patterns. Most models that include the Coachella Valley segment of the SAF have assumed a vertical orientation, but recent studies suggest that this segment dips 60-70° northeast. We compare models with varied fault geometry and evaluate how well they reproduce observed uplift patterns. Our model with a dipping SAF matches geologic observations, while models containing a vertical fault do not. This suggests that the active Coachella Valley segment of the SAF dips 60-70° northeast. Since ~1.5 Ma, the SAF in this region has undergone a major reorganization that entailed initiation of the San Jacinto fault and termination of slip on the West Salton detachment fault. The trace of the SAF itself has also evolved, with several shifts in activity through the San Gorgonio Pass. Despite a rich geologic record of these changes, the mechanisms that controlled abandonment of faults, initiation of new strands, and shifting loci of uplift are poorly understood. We model snapshots in time through the evolution of the fault system, and assess the mechanical viability of our snapshots by comparison with uplift patterns inferred from the stratigraphic record. Model results are compared with vertical axis rotation. We examine incipient faulting using maps of strain energy density, and explore changes to the mechanical efficiency of the system to better understand the evolution of this fault system.

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Damage zones adjacent to or associated with faults are important to the geologic community because of their implications to hazards and their ability to preserve evidence for, and show history of, slip, fluid flow, and deformation associated with large strike-slip faults. We examine two fault zones in southern California where fault zone damage is expressed. We revisit the drilled crystalline core from the Cajon Pass California drill hole, 4 km northeast of the San Andreas fault (SAF), and 1 km north of the Cleghorn fault, to perform a systematic structural analysis of deformation and alteration associated with strike-slip faulting at the site. The core preserved 19 fault zones, 11 of which were not previously identified. The most significant fault is a fully intact steep-dipping fault zone at 3,402 m depth with potassium feldspar and epidote alteration. This fault correlates well with the nearby left-lateral Cleghorn fault. The extent of deformation varies within the core, and is controlled by the size of the fault zones intersected by the core. The extent of deformation varies and is controlled by the size of the faults the core intersected. We also examined the nature of right separation across the Clark fault damage zone along the Santa Rosa segment using a marker assemblage of biotite, hornblende-bearing tonalite - marble - bearing metasedimentary rocks - migmatite located in Coyote Mountain and the southeast Santa Rosa Mountains. Separation measured from this study is 16.8 km + 3.67 km / -6.03 km. Our measurement uses the updated location of the Clark fault in Clark Lake Valley and matches a distinctive lithologic contact across the fault instead of matching the diffuse western boundary of the Eastern Peninsular mylonite zone as previously used. We calculate the errors associated with projecting the contacts across Quaternary cover to the trace of the Clark fault, and consider a range of projections. Additional strain may have been accommodated in folds and small faults within the damage zone of the San Jacinto fault zone. Two large map-scale folds deform the marker assemblage near the San Jacinto fault zone and we tested whether Cretaceous ductile deformation or brittle late Quaternary right slip produced the folds.

The aim of the San Andreas Fault Observatory at Depth (SAFOD) project, a component of the NSF Earthscope Initiative, is to directly observe active fault processes at seismogenic depths through the drilling of a 3 km deep (true vertical depth) inclined borehole across San Andreas fault. Preliminary subsurface models based on surface mapping and geophysical data predicted different lithologies than were actually encountered. At 1920 meters measured depth (mmd), a sequence of well-indurated, interbedded arkosic conglomerates, sandstones, and siltstones was encountered. We present a detailed lithologic and structural characterization as a step toward understanding the complex geologic history of this fault-bounded block of arkosic sedimentary rocks. We divide the arkosic section into three lithologic units with different compositional, structural, and sedimentary features: the upper arkose, 1920-2530 mmd, the clay-rich zone, 2530-2680 Illtlld, and the lower arkose, 2680-3150 mmd. We interpret the section to have been deposited in a Salinian transtensional basin, in either a subaqueous or subaerial fan setting. We suggest four different possibly equivalent sedimentary units to the SAFOD arkoses, the locations of which are dependent on how the San Andreas fault system has evolved over time in the vicinity of the SAFOD site. Detailed analysis of three subsidiary faults encountered in the arkosic section at 1920 mmd, 2530 mmd, and 3060 mmd, shows that subsurface faults have similar microstructures and composition as exhumed faults at the surface, with less evidence of alteration from extensive fluid flow.

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The launch of Sentinel-1A/B satellites in 2014 and 2016 marked a pivotal moment in Synthetic Aperture Radar (SAR) technology, ushering in a golden era for SAR. With a revisit time of 6–12 days, these satellites facilitated the acquisition of extensive stacks of high-resolution SAR images, enabling advanced time series analysis. However, processing these stacks posed challenges like interferometric phase degradation and tropospheric phase delay. This study introduces an advanced Small Baseline Subset (SBAS) algorithm that optimizes interferometric pairs, addressing systematic errors through dyadic downsampling and Delaunay Triangulation. A novel statistical framework is developed for elite pixel selection, considering distributed and permanent scatterers, and a tropospheric error correction method using smooth 2D splines effectively identifies and removes error components with fractal-like structures. Beyond geodetic technique advancements, the research explores geological phenomena, detecting five significant slow slip events (SSEs) along the Southern San Andreas Fault using multitemporal SAR interferometric time series from 2015-2021. These SSEs govern aseismic slip dynamics, manifesting as avalanche-like creep rate variations. The study further investigates Managed Aquifer Recharge (MAR) as a nature-engineering-based solution in the Santa Ana Basin. Analyzing surface deformation from 2004 to 2022 demonstrates MAR's effectiveness in curbing land subsidence within Orange County, CA. Additionally, MAR has the potential to stabilize nearby faults by inducing a negative Coulomb stress change. Projecting into the future, a suggested 2% annual increase in recharge volume through 2050 could mitigate land subsidence and reduce seismic hazards in coastal cities vulnerable to relative sea level rise. This integrated approach offers a comprehensive understanding of geological processes and proposes solutions to associated risks. / Doctor of Philosophy / The launch of Sentinel-1A/B satellites in 2014 and 2016 marked a big step forward in radar technology, especially Synthetic Aperture Radar (SAR). These satellites, which revisit the same area every 6-12 days, allowed us to collect many high-quality radar images. This helped us study changes over time in a more advanced way. However, there were challenges in handling all these images, like errors in the radar signals and delays caused by the Earth's atmosphere. We devised a smart algorithm based on the Small Baseline Subset (SBAS) to tackle these challenges. It helps optimize how we use pairs of radar images, reducing errors. We also developed a new method to pick the best pixels in the images and corrected errors caused by the atmosphere using mathematical methods. Moving beyond just technology, our research also looked at interesting Earth events. We found five major slow slip events along the Southern San Andreas Fault by studying radar data from 2015 to 2021. These events are like slow-motion slips along the fault, influencing how the ground moves. We also explored Managed Aquifer Recharge (MAR) as a solution in the Santa Ana Basin. By studying ground movement from 2004 to 2022, we found that MAR helped prevent the land from sinking in Orange County, California. It even has the potential to make nearby faults more stable. Looking ahead, increasing MAR activities by 2% each year until 2050 could protect against land sinking and reduce earthquake risks in coastal cities facing rising sea levels. This combined approach gives us a better understanding of Earth's processes and suggests ways to tackle related problems.

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