Rupture scenarios for the San Diego Trough and San Pedro Basin fault systems, offshore Southern California: New constraints combining marine geophysical, paleoseismic, and ground motion modeling investigations.

The hazard posed by offshore strike-slip faults for coastal populations in Southern California is poorly characterized and may be underappreciated as the structural relationships between faults may allow ruptures to propagate between neighboring fault systems. Recent marine geophysical investigations on the San Diego Trough (SDT) and the San Pedro Basin (SPB) faults demonstrate that these two northwest striking, right-lateral faults are separated by a gap of <5 km, which may allow ruptures to propagate between the two fault systems. Deterministic rupture scenarios for the SDT and SPB fault systems included in the U.S. Building Seismic Safety Council 2014 Event Set include a M 7.3 event on the southern SDT fault, a M 7.3 event on the northern SDT fault, and a M 7.1 event on the SPB fault; however, none of the scenarios considers events that may propagate between fault segments. A complete rupture of all three segments would extend offshore from Ensenada, Mexico to Los Angeles, California with a length >330 km. Empirical scaling relationships indicate that a strike-slip rupture of this length has the potential to produce a M 7.7-7.9 event. At present, the paleoseismic histories of both faults are unconstrained. Here, we present new observations from on-going submarine paleoseismic investigations using CHIRP and multichannel seismic reflection profiles, multibeam bathymetry, and coring surveys to characterize recent earthquakes on the SDT and SPB fault zones, and evaluate the potential for rupture that spans both systems. We will perform detailed shaking analysis for coastal communities in northwestern Baja California and Southern California based on multi-segment rupture scenarios constrained by the new paleoseismic observations and models of Coulomb stress change caused by individual fault segment rupture. Combining existing Shakemap rupture scenarios with coastal population data suggests that >12,500,000 people would be exposed to strong (MMI VI) shaking in an end-to-end rupture of both fault systems, with the strongest shaking in Ensenada, Mexico and the Los Angeles Basin.