Timing and Extent of Submarine Landslides in Southern California

Abstract Submarine landslide deposits occur in many locations throughout the Southern California Borderland and indicate the potential for continued slope failure. Future landslide activity may constitute a direct hazard to offshore facilities and an indirect hazard to coastal communities through landslide-induced tsunamis. Evaluating the risk of these hazards requires information on the scale of landslides that can occur and their recurrence rate. In this study, five mass transport complexes are described and volumes are estimated. Two of these complexes, the Palos Verdes debris avalanche and the Goleta slide contain the remains of many past events. Using dated cores and tracing stratigraphy to nearby ODP borings, we have estimated ages of the most recent failures in the five complexes and some of the ages of earlier failures in the Goleta slide. These results show that the volumes of the failed masses vary over several orders of magnitude with the largest of the masses having volumes on the order of 0.5 km3. The ages of the failures range from a few hundred years to over 100,000 years. The two complexes that show repeated failure represent the largest landslides we evaluated and probably are the largest complexes on the mainland slope in outhern California. We estimate that these large failures on the mainland slope probably reoccur with an interval that has an order of magnitude of 10,000 years. Introduction The Southern California Borderland is marked by a system of basins and ridges, some of which form offshore islands. The area is complex geologically and affected by considerable seismicity. Earthquakes having magnitudes in the range of 7.0 have been recorded and there is historical evidence that tsunamis related to local earthquakes have occurred. Within this environment there have been many studies conducted to identify geologic hazards and, specifically, underwater landslide deposits. Field and Clarke (1979) and Field and Edwards (1993) presented maps showing the extent of observed submarine landslide deposits. The maps show that most of the basin slopes that have been surveyed contain landslide deposits and that the landslides vary greatly in scale. Recent mapping with multibeam technology (Gardner and Mayer, 1998, Gardner et al., 1999, Gardner et al., 2003) has greatly improved our knowledge of the morphology of these features and allows us to begin to calculate the scale of the deposits. This sort of information can help us to understand the direct risk posed by undersea landslides to offshore structures and the indirect risk posed to coastal communities by undersea landslide-induced tsunamis. Without knowledge of the ages of the landslides and the recurrence intervals of failures within major landslide complexes, however, the hazard estimates can only be qualitative. To improve our ability to evaluate hazards, several studies have been conducted recently to estimate the ages of observed landslide deposits. These resulting dates and corresponding estimates of the volumes of the deposits can be used eventually to produce relations between landslide volume and recurrence interval for the Southern California Borderland. This paper summarizes currently available data and includes information on five significant mass transport complexes within the Southern California Borderland (Fig. 1). At least two of these complexes show multiple events extending far back into the Pleistocene.

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