Rupture scenarios for the San Diego Trough and San Pedro Basin fault systems, offshore Southern California

Jayne M. Bormann, Graham M. Kent, & Neal W. Driscoll

Published August 15, 2018, SCEC Contribution #8624, 2018 SCEC Annual Meeting Poster #251

The hazard posed by offshore faults for coastal populations in Southern California is poorly characterized and may be considerable, especially when these communities are located near long faults that have the ability to produce large earthquakes. Recent marine geophysical surveys along the San Diego Trough and the San Pedro Basin faults demonstrate that the mapped traces of these northwest striking, right-lateral faults are separated by a gap of <5 km, a distance which may allow ruptures to propagate between fault systems. The combined length of the two systems is ~330 km, and empirical scaling relationships indicate that an end-to-end rupture of both faults has the potential to produce a M 7.7-7.9 event.

Here, we present new observations from CHIRP and multichannel seismic reflection sub-bottom profiles, multibeam bathymetric data, and coring surveys to characterize the recency of deformation on the San Diego Trough and San Pedro Basin fault systems and evaluate the potential for ruptures that span both systems. Seismic reflection data collected in 2013 image the San Diego Trough fault offsetting young sediments at the seafloor for ~130 km between the US/Mexico border and Avalon Knoll; however a 30-km data gap exists between observations of seafloor offset in high-resolution MCS surveys on the San Diego Trough fault and seafloor offsets along the northern San Pedro Basin fault in legacy seismic reflection data. In June 2018, we collected ~400-line-km of high-resolution multichannel seismic reflection data in the San Pedro Basin to fill this data gap. The new profiles image offsets along the southern San Pedro Basin fault at depth, however we find no evidence of offset in the uppermost sedimentary packages in the San Pedro Basin. The thickness of the unfaulted packages is ~70 m in the center of the basin, and the sediments thin as they onlap the basin shoulders. The presence of the unfaulted sediments constrains the timing of the most recent slip on the San Pedro Basin fault to be prior to package deposition, however we lack absolute age constraints on the unfaulted sediments at this time. Our preliminary interpretation is that the southern San Pedro Basin fault is inactive, which minimizes the likelihood of an end-to-end rupture of the San Diego Trough and San Pedro Basin fault systems. This example highlights the importance of considering the recency of deformation on offshore structures when proposing fault models for seismic hazard evaluation.

Key Words
Inner California Borderlands, offshore faulting, seismic hazard, marine geophysicss

Citation
Bormann, J. M., Kent, G. M., & Driscoll, N. W. (2018, 08). Rupture scenarios for the San Diego Trough and San Pedro Basin fault systems, offshore Southern California. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology