Determining where the next big Southern California earthquake may occur is complicated by the number of faults that cut the region and challenges associated with having many of these faults lie underwater. The hazard posed by submarine 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 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. Individual rupture scenarios for the SDT and SPB fault systems (https://earthquake.usgs.gov/scenarios/catalog/bssc2014/) 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. A complete rupture of all three segments would extend offshore from Ensenada, Mexico to Los Angeles, California with a length >330 km. Scaling relationships indicate that a strike-slip rupture of this length has the potential to produce a M 7.7-7.9 event. Combining individual rupture Shakemap 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, with the strongest shaking in Ensenada and the Los Angeles Basin. At present, the paleoseismic histories of both faults are unknown. Here, we present observations from on-going submarine paleoseismic investigations using CHIRP and multichannel seismic reflection sub-bottom profiles, multibeam bathymetric data, and coring surveys to characterize recent earthquakes on the SDT and SPB fault zones. We use these constraints to evaluate the potential for rupture that spans both systems, and we will perform detailed shaking analysis for coastal communities in northwestern Baja California and Southern California based on multi-segment rupture scenarios.