SCEC Award Number 19107 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Evolution of the San Diego Bay pull-apart basin: New insights from reprocessed seismic reflection data
Name Organization
Jillian Maloney San Diego State University
Other Participants Drake Singleton, PhD student
SCEC Priorities 3a, 3e, 5b SCEC Groups Geology, EFP, Transitions
Report Due Date 03/15/2020 Date Report Submitted 03/20/2020
Project Abstract
The principle objective of this project was to assess the pull-apart basin evolution and potential connectivity of regional fault systems around San Diego Bay, CA. This project focused on faulting in the southern portion of San Diego Bay where several northwest oriented faults were imaged outside the ‘active’ pull-apart basin. The data used in this project include legacy multi-channel seismic (MCS) lines, borehole logs and age models, and high-resolution chirp data. The MCS and borehole data were collected as part of a geohazard study of the Coronado bridge in the late 1990s. For this project, the MCS data were reprocessed for improved resolution and imaging at depth. High-resolution chirp data were collected in 2011-2013, and 2019. All datasets were compiled in IHS Kingdom Suite software for tracing of faults, stratigraphic horizons, and comparison of MCS and chirp data. Using these data, we generated a detailed fault map and cross sections in San Diego Bay, and assessed the spatial and temporal changes in stratigraphic sequences throughout the Bay. We identified stratigraphic variability along strike of southern San Diego Bay fault zones, with complex fault geometries associated with structural highs and a depositional center. The improved fault mapping indicated more continuity in fault zones than previous work and identified southern San Diego Bay fault strands that appear Holocene active.
Intellectual Merit Fault bends and steps are important for rupture modeling, as they can act as locations of rupture initiation or termination. Typically, the likelihood of through-going rupture across a fault step is predicted solely based on the size of the step. Nevertheless, recent rupture models show that the presence of smaller faults within a step‐over can have a complicated effect on rupture propagation across a step, with some scenarios suggesting that rupture could propagate onto intermediate faults within the step. Step-over geometry also evolves over time and could result in temporally complex rupture patterns with changes to the amount of slip accommodated on various fault segments. Therefore, a detailed understanding of fault geometry near and across the San Diego Bay step-over is important for accurate hazard assessments for the densely-populated region and for improving our understanding of step-over evolution. The large size and submergence of the San Diego Bay step-over provides an opportunity to employ marine seismic reflection methods for detailed imaging of fault structure and stratigraphic architecture. We combined deeply penetrated MCS data with shallow, very high resolution chirp data to understand the long term tectonic evolution and recent history of faulting in the Bay. Fault complexities occur on faults across the southern California region, but this effort provides a more comprehensive mapping of fault geometry across a large step than is feasible with onshore methods.
Broader Impacts This project was led by PhD student Drake Singleton and the results will make up a chapter in his thesis that he is set to defend in Spring 2020. Through this project, Drake was able to work directly with collaborators at the USGS Coastal Center in Santa Cruz, CA, exposing him to the working environment of a government research institution and opening the door to potential future research projects with USGS scientists. As a direct result of this experience, Drake is currently applying for a Mendenhall Fellowship to work with our collaborators at the USGS on earthquake hazards in Alaska. Furthermore, the reprocessed legacy MCS data is a key dataset generated in this project, which is available to the earthquake science community. These legacy seismic lines were collected in areas where it is no longer feasible, due to permitting, to collect new data (San Diego Bay). Thus, both the awareness of professional advancement opportunities and the improved competency in earthquake research tools and techniques requirements of the SCEC Transitions Program were met.
Exemplary Figure Figure 2: Chirp line SSB003 (top) and MCS line T196_a792 (bottom) from the southern portion of San Diego Bay (see figure 1 for location of A-A’). The south San Diego Bay fault is well imaged in both the Chirp and MCS data in the western part of the profiles. The increasing stratigraphic dips with depth and growth strata observed in the chirp data show the faults effect on deposition in this part of the Bay. TS=Transgressive Surface, shown as green dashed line.