Walton, Maureen A. L., Conrad, James E., Papesh, Antoinette G., Brothers, Daniel S., Kluesner, Jared W., McGann, Mary, and Dartnell, Peter
It is critical to characterize submarine landslide hazards near dense coastal populations, especially in areas with active faults, which can trigger slope failure, subsequent tsunamis, and damage seabed infrastructure 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 total region between Point Conception and the United States‐Mexico border from the California coast out to the base of Patton Escarpment ∼200 km offshore. In a comprehensive compilation and interpretive mapping effort, we find evidence of seafloor failure throughout offshore southern California with nearly 1,500 submarine landslide‐related features, including 63 discrete slide deposits with debris and >1,400 slide‐related scarps. In our analysis, we highlight new mapping of submarine landslides in Catalina Basin, the Del Mar slide, the San Gabriel slide complex, and the 232 km2 San Nicolas slide, the largest area of any known submarine landslide mass offshore southern California. Analysis of the spatial distribution of submarine landslide features suggests that most mapped slide features are located relatively near coastal sediment sources, particularly during sea‐level lowstand conditions, which underscores the importance of sediment supply and sediment accumulation on low‐gradient slopes as failure preconditioning processes. Tectonically driven uplift at shelf edges and along basin flanks is another key preconditioning factor, and our results also suggest that earthquakes along active faults trigger mass wasting, especially for repeated, small‐scale failures on tectonically steepened slopes. Plain Language Summary: Submarine landslides can damage seabed infrastructure such as cables and moorings, cause tsunamis, and be triggered by shaking from earthquakes. It is important to understand the risk of submarine landslides near dense coastal populations, particularly where earthquakes also pose hazards. Offshore southern California, we have new high‐resolution seafloor and subsurface imaging data that help us to identify submarine landslide deposits in the marine environment. In our study, we map and compile evidence for submarine landslides and find nearly 1,500 slide‐related features, 63 of which feature significant debris deposits. We describe some of the larger slides in this study for the first time, including submarine landslides in Catalina Basin, the Del Mar slide, the San Gabriel slide complex, and the 232 square kilometer San Nicolas slide, which is one of the largest known submarine landslide masses offshore southern California. Our work suggests that submarine landslide failure processes offshore southern California require a combination of (a) significant sediment supply, which is enhanced during low sea‐level conditions, (b) uplift and steepening along faults, and (c) earthquake shaking to trigger slide events. Key Points: Comprehensive analysis of submarine landslides in southern California provides new metrics on their size, distribution, timing, and geologySubmarine landslide failure processes are controlled by a combination of sediment deposition, tectonic uplift, and earthquake triggeringSmall‐scale failures dominate steep areas near Quaternary faults; large slides tend to occur on lower slopes farther from faults [ABSTRACT FROM AUTHOR]