GEOLOGY OF SUBMARINE LANDSLIDES BELOW SANTA BARBARA CHANNEL, SOUTHERN CALIFORNIA, AND THEIR POTENTIAL TSUNAMI HAZARD

A large submarine landslide complex and four small landslides developed under the Santa Barbara Channel, suggesting a potential hazard from landslide-generated tsunamis. We integrate offshore stratigraphy and geologic structure, multibeam bathymetric information, and several kinds of seismic-reflection data to understand how and when the submarine landslides formed. These landslides directly underlie or make up the large Goleta landslide complex. Age control from ODP Site 893 together with seismic-reflection data indicate that over the past 250 ka landslides have occurred consistently along one segment of the shelf break. Seismic-reflection data show that mass failure along the slope began at least 200 ka ago. Landslides appear as zones of poor reflectivity, and these zones alternate vertically with strong parallel reflections. Our seismic-stratigraphic correlation lacks sufficient resolution to determine the sea-level ranges at which landslides typically developed. The emplacement ages of two of the three main landslide lobes are well established at 8 and 10 ka. The source material for the youngest part of the landslide complex was sediment of probable late Pleistocene and Holocene age that accumulated in a shelf-edge delta. Directly under this delta, active faults and growing anticlines tended to oversteepen the deltaic deposits. These growing structures also formed migration pathways and reservoirs for aqueous and hydrocarbon fluids from the deep basin. Tsunami deposits have not been described from low-lying areas near Santa Barbara, so the history of tsunami inundation is unknown. Nonetheless, numerical modeling of tsunamis generated by hypothetical landslides in Santa Barbara Channel indicates a moderate to severe threat (Borrero and others, 2000). Typically, modeled wave runups of 2-20 m would affect only narrow, ~10-km-long sections of the shoreline.