TSUNAMIGENIC POTENTIAL OF SEDIMENT ACCUMULATIONS AT SUBMARINE CANYON MOUTHS, SANTA MONICA BASIN, OFFSHORE SOUTHERN CALIFORNIA

Extensive seismic-reflection profiling and sediment coring studies, which include Ocean Drilling Program Site 1015, have demonstrated that the sediment accumulation rate in the 900-m-deep Santa Monica Basin is the highest in the southern California offshore area. Recent observations and sampling using the Monterey Bay Aquarium Research Institute ROV Tiburon have provided core samples from outcrops of sediment on the walls of Hueneme and Redondo Canyons, which are at the western and eastern ends of Santa Monica Basin, respectively. The ROV push-cores were taken along dive transects up the near-vertical canyon walls where horizontally-bedded sections of poorly consolidated sediment are exposed.

Samples from the eastern wall of Hueneme Canyon were collected at water depths between 446 to 556 m where sediment of the lower part of the Santa Clara river delta is exposed. Radiocarbon dating results to date show that the lower delta slope was deposited during the interval from the last glacial maximum (~18 ka) until about 5.36 ka (calibrated ages). At the eastern end of Santa Monica Basin, the Redondo Canyon feeds sediment downslope that ultimately passes into the San Pedro Basin. The south wall of the canyon, where it enters deep water between these two basins, is a horizontally-bedded section of poorly consolidated sediment that is 150 m thick and 1000 m wide. Initial radiocarbon dates on samples taken from exposures on the steep canyon wall are similar in age to those from the Hueneme Canyon exposures at the west end of the basin.

The near-vertical outcrops of these sequences indicate that extensive erosion has occurred along Hueneme and Redondo Canyons during the latest Holocene leaving over-steepened side-walls within these rapidly accumulated deposits. The locations of these sequences are close to the heavily populated coastal areas of Los Angeles. In addition, they are adjacent to either the Santa Monica-Dume-Malibu Coast fault system, which underlies much of the Santa Clara river delta (Sorlien and others, 2006), or the San Pedro Basin fault zone in the case of Redondo Canyon. The observation that a neutrally buoyant ROV could take push cores from the canyon walls suggests that these young and poorly-consolidated sediment sequences could be subject to earthquake-induced failure and result in damaging localized tsunamis.