UC Santa Cruz

Over nearly a century, seismology has provided valuable earthquake observation and information. The locations of earthquake hypocenters have helped define the spatial distribution of faults that host earthquakes and helped identify regions that may have large and damaging events in the future. However, exactly how aseismic slip, small earthquakes, and smaller faults are related to large events and the seismic cycle remains unresolved. Recent advances in instrumentation have extended research into the oceanic realm, which is important for at least two reasons. First, it enables the detection of ever smaller earthquakes that provide information about slip behavior on offshore portions of previously unresolved faults. Second, with ever-increasing resolution, marine seismic reflection images provide increasingly precise depictions of fault zones that are unattainable in the terrestrial environment. This dissertation is dedicated to better understanding the spatial distribution of faults and their potential to influence slip behavior and seismic cycle through the use of marine seismic methods. It encompasses two distinct facets of marine seismology: passive recording of earthquakes and the interrogation of subsurface faulting through seismic reflection imaging. These dual areas of investigation offer unique insights into the subsurface structure and the interplay of seismic and aseismic processes. Chapter 1 explores the enigmatic plate interface in southernmost Cascadia with the application of a dense array of ocean-bottom seismometers complementing traditional terrestrial seismic stations. A high-quality seismic catalog is created through the meticulous analysis of continuous waveform data and advanced earthquake location techniques. The results reveal a conspicuous absence of seismic activity at seismogenic depths and the shallowest up dip section, indicative of high coupling or locking and strain accumulation increasing the magnitude potential associated with this fault. Notably, a cluster of low-magnitude earthquakes (M < 3) is identified near the plate interface, exhibiting a response to nearby strain transient observed in prior studies. These strain transients are interpreted as either the tail end of a slow slip event spanning the preceding one and a half years or a rapid change in coupling. Template matching of these plate interface earthquakes demonstrates their uniqueness, with no recurrence over the observed decade. The correlation between the sudden onset of clustered earthquake activity and strain transient suggests that the two are related, and the interpretation is made that the cluster of earthquakes is a response to local stressing rate changes. Chapter 1 provides critical insights into the southern Cascadia plate interface, shedding light on the complex interactions between seismic behavior and slow slip events. In Chapters 2 and 3, this dissertation examines the shallow near-surface portion of the fault zone, with a specific focus on the spatial distribution of secondary faults surrounding the primary fault. This damaged area around the main fault provides an important window into comprehending the inelastic response of the Earth’s crust to strain, the allocation of fracture energy in the earthquake energy budget, near-fault hydrogeology, and for near-field hazards. The dimensions, both in width and depth of the damage zone are important in addressing these science questions. The primary objective of these two studies is to gain insight on the in-situ expression of the fault damage with marine controlled source seismic reflection images, an approach which had not been fully explored prior. The advantage of this data type lies in its ability to provide rich and dense sampling of the subsurface and the capability to directly image fault offsets over substantial distances that are often unattainable through other means. To achieve a comprehensive understanding of the damage zone, ranging from the seafloor to the basement, roughly 2 kilometers below, marine active source reflection surveys of varying resolutions are employed. Both Chapters 2 and 3 focus on the San Pedro Shelf region of the Palos Verdes Fault offshore southern California because of the richness in available reflection datasets and the opportunity to address unanswered questions about the generation of fault damage. Chapter 2 focuses on examining the damage zone of the Palos Verdes Fault using 2 overlapping 3D seismic volumes and demonstrates the development of a workflow for automating fault detections. Automation is achieved through the implementation of multi-trace waveform similarity or semblance-based attribute called thinned fault likelihood (TFL). These results reveal peak fault likelihoods at the location of mapped fault strands, with fault likelihood exponentially decaying with distance from the fault. Importantly, this decay with distance intersects a relatively undamaged region or background at 2 kilometers from the fault across all depths (ranging from 450 m to 2.2 km). Lithological constraints are provided by well tied 3D horizons, and damage decay trends are for each geologic unit. Notably, the findings indicate an overall increase in background damage with increasing depth. In addition to background damage increasing with depth, it is demonstrated that the decay of damage also decreases with increasing depth. This surprisingly, results in a consistent damage zone width of 2 kilometers regardless of the variability in background damage and decay trends. Chapter 3 investigates the shallowest portion of the Palos Verdes Fault damage zone which could not be fully explored in Chapter 2. This section leverages newly collected 2D high-resolution sparker multichannel seismic lines and sub-bottom profiles (chirp). Resolving the challenges of imaging in shallow water profiles is achieved through the development of custom seismic processing workflows designed to eliminate the ringing and overprinting of seismic waves trapped in the water column. Similar to Chapter 2, a semblance-based technique is used to identify discontinuities in the seismic images associated with faults and fractures. The analysis of fault perpendicular TFL profiles yields a diverse pattern of damage in the vicinity of the fault on adjacent profiles. In this study we define and map the active fault strand determined by the offset of the near seafloor sediment beds. An average pattern of damage decreasing with distance from the fault is shown by stacking fault perpendicular profiles with respect to the active fault. The stacked profiles reveal that damage decreases with distance both east and west of the fault. Notably, the peak of stacked fault damage occurs within roughly 50 meters of the active fault strand, with splay faults manifesting as secondary peaks in the stacked results. The width of the damage zone along the slope appears to align with power law displacement scaling relationships established by previous compilation studies. However, the significant reduction in damage zone width and intensity towards the south may be better attributed to the reduced obliquity of fault in that direction. In contrast to canonical strike-slip models, the presented data suggests compelling evidence that the damage zone does not widen as it approaches the surface, a characteristic that may distinguish syndepositional submarine fault’s structure. Furthermore, there is an apparent correlation between fault damage and seafloor seeps that are visibly evident in the water column. This correlation strongly implies that damage plays a role in controlling fluid flow around the fault.