Assessing evidence for connectivity between the San Diego Trough and San Pedro Basin fault systems, offshore Southern California.

Jayne M. Bormann, Graham M. Kent, Neal W. Driscoll, & Alistair J. Harding

Published August 15, 2016, SCEC Contribution #7018, 2016 SCEC Annual Meeting Poster #109

The seismic hazard posed by offshore faults for coastal communities in Southern California is poorly understood and may be considerable, especially when these communities are located near long faults that have the ability to produce large earthquakes. The San Diego Trough fault (SDTF) and San Pedro Basin fault (SPBF) systems are active northwest striking, right-lateral faults in the Inner California Borderland that extend offshore between San Diego and Los Angeles. Recent work shows that the SDTF slip rate accounts for ~25% of the 6-8 mm/yr of deformation accommodated by the offshore fault network, and seismic reflection data suggest that these two fault zones may be one continuous structure.

Here, we use hull-mounted Knudsen sub-bottom profiles, high-resolution multichannel seismic (MCS) reflection profiles, and legacy USGS and industry MCS profiles in combination with multibeam bathymetric data to characterize recent deformation on the SDTF and SPBF zones and to evaluate the potential for an end-to-end rupture that spans both fault systems. The SDTF offsets young sediments at the seafloor for ~130 km between the US/Mexico border and Avalon Knoll. The northern SPBF has robust geomorphic expression and offsets the seafloor in the Santa Monica Basin. The southern SPBF lies within a 25-km gap between high-resolution MCS surveys. Although there does appear to be a through-going fault at depth in industry MCS profiles, the low vertical resolution of these data inhibits our ability to confirm recent slip on the southern SPBF.

Empirical scaling relationships indicate that a 200-km-long rupture of the SDTF and its southern extension, the Bahia Soledad fault, could produce a M7.7 earthquake. If the SDTF and the SPBF are linked, the length of the combined fault increases to >270 km. This may allow ruptures initiating on the SDTF to propagate within 25 km of the Los Angeles Basin. At present, the paleoseismic histories of the faults are unknown. We identify three locations where thin lenses of sediment mantle the SDTF, providing the ideal sedimentary record to constrain the timing of the most recent event on the fault. Characterizing the paleoseismic histories of the fault systems is a key step toward defining the extent and variability of past ruptures, which in turn, will improve maximum magnitude estimates for the SDTF and SPBF systems.

Key Words
Inner California Borderland, offshore fault system, San Diego Trough fault, San Pedro Basin fault, marine geohazards, fault geometry, seismic reflection

Bormann, J. M., Kent, G. M., Driscoll, N. W., & Harding, A. J. (2016, 08). Assessing evidence for connectivity between the San Diego Trough and San Pedro Basin fault systems, offshore Southern California.. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology