A COMPREHENSIVE ASSESSMENT OF SUBMARINE LANDSLIDES AND MASS WASTING PROCESSES OFFSHORE SOUTHERN CALIFORNIA

It is critical to characterize submarine landslide hazard near dense coastal populations, especially in areas with active faults, which can trigger slope failure during earthquake shaking. Offshore southern California, numerous marine geophysical surveys have been conducted over the past decade, and high-resolution bathymetric and subsurface data now cover about 60 percent of the region between Point Conception and the U.S.-Mexico border from the coast to the base of Patton Escarpment ~250 km offshore. In a comprehensive compilation and interpretive mapping effort, we find evidence of seafloor failure throughout offshore southern California with >1500 landslide-related features, including 60 discrete landslide deposits and >1400 landslide headwall scarps. Mapped landslides cover ~16% (~1,250 km²) of the aforementioned area, with individual slides up to 230 km² in size.

We present analysis of the spatial distribution of landslide features in relation to regional geology. Over half of the 60 discrete landslides occur within 20 km of coastlines, within 10 km of mapped Quaternary faults, and in relatively low-slope (<10°) areas, including all but 3 of the 21 largest (>10 km²) landslides. Of the 21 largest landslides, 17 failed in uplifted, pre-Quaternary sediment. Failure ages exist for only 8 of the 60 landslides, and generally range from latest Pleistocene to early Holocene. Like the discrete landslides, the majority (~80%) of the ~1400 mapped headwall scarps are also located within 10 km of Quaternary faults and 20 km of coastlines. The locations of landslide features near coastal sediment sources underscores the importance of sediment supply, as well as sediment accumulation on low-gradient slopes, as failure preconditioning processes. Mapped headwall scarps often cluster around canyons and channel heads, attesting to the role of mass wasting in sediment dispersal processes. Our results also suggest a dependence of slope failures offshore of southern California on earthquake-triggering mechanisms due to proximity to faults and the prevalence of late Pleistocene failures, when sea-level rise may have led to increased fault activity. Future work that prioritizes dating additional landslide features is a critical next step toward better understanding landslide hazards offshore southern California.