TSUNAMI GENERATING LANDSLIDES ALONG THE CALIFORNIA MARGIN: ORIGINS, MECHANISMS AND CONSEQUENCES

Submarine landslides are plentiful along the continental slope and in submarine canyons of the California margin. With the advent of high-resolution multibeam swath bathymetric mapping, mass movement features are becoming better imaged and defined. Although much more mapping needs to be done, distinct areas offshore of central and southern California have been identified as possible past or potential tsunamis generating mass wasting sites. We have initiated a study to inventory these potential tsunami-generating landslides, to determine their origin, mechanisms, probable periodicity, and to define potential tsunamis generation zones.

Many of the landslides we have recently investigated are concentrated within submarine canyons and along steep continental slopes and range from distinct down-dropped or rotational slumps to fluidized debris flows. Hydrodynamic models of tsunamis that may have occurred in these regions indicate that a distinct wave focus occurs in the direction of landslide propagation. Therefore, we hypothesize that locally generated tsunamis that have the potential to deposit marine sediment on land occur from landslides whose direction of propagation is landward or whose failure surface is close to shore. We are in the process of calculating the critical parameters in regard to size, depth, and runout ratios that are necessary for the prediction of a significant local tsunami.

Based on our assessment of past submarine mass movements that could have generated tsunamis and the associated geologic conditions (i.e., tectonic activity, sedimentation rate, steepness of slopes, erosional processes), we predict areas where the likelihood of past and present locally derived tsunamis may have, or may, occur to impact land areas. Our analyses are identifying the most probable terrestrial areas where impacts from future tsunamis should be considered and where tsunamis deposits might be found.