Slope Instability and the Potential for Tsunami Generating Landslides
H. Gary Greene , Norman Maher and Charlie Paull
Recently collected high-resolution multibeam bathymetric data along the active convergent continental margin of the northeastern Pacific have revealed numerous areas of past and potential sites of mass wasting. These features are examples of mass wasting events that may have produced tsunamis.
A number of landslide scars and deposits occur in several areas along the central California margin. One submarine landslide is offshore of Goleta, California, along the northern boundary of the Santa Barbara Basin. Here we have mapped a massive complex compound landslide that extends over an area of 134 km2. The Goleta slide has an overall width of about 10.5 km and a length of about 14.6 km that can be easily traced from its head scarp located in 90 m to its toe in 574 m of water. We estimate that the slide displaced ~1.75 km3 of material. The landslide scar is comprised of three distinct geomorphic components. Each component is made up of a steeper head scarp (47 to 75 m high and up to 24° in inclination), a main scar (196 to 240 m high and inclined from 7-10°), a slump block (8.3x10-3 to 25x10-3 m3 in volume, averaging 20 m thick) and a relatively thin (7 to 12 m thick) extensive (26.3 to 44.7 km2 in area) debris flow deposits. The debris flow deposits exhibit surface textures of convex down-slope compression ridges with relief of 1 to 3 m. The relative sequence of movement for each of the three components can be estimated from cross-cutting relationships between discrete events. Although the EM300 multibeam images of the Goleta slide show sharp and well defined relief, indicating a youthful geomorphological age, the timing of failures is otherwise unknown.
Visual examination and sediment sampling were undertaken with MBARI's ROV Ventana along a transect across the upper displaced mass, main scar and head scar of the middle component. This investigation indicated that the surface of the main scar consisted of stiff clay covered with unconsolidated mud. The headwall was coated with mud, shell hash and angular cobble- to small-boulder-size carbonate rocks, apparently derived from the upper head scarp and adjacent shelf edge.
Multibeam data in this area show that smaller slides exist on the adjacent slopes immediately to the west and east of the main Goleta slide scar. Further to the west of the Goleta slide is a much smaller slide (~4 km2 in area), described by Edwards et al. (1995) before multibeam data were available. This slide is located on a slope of 4.60 that is marked by numerous parallel rills and gullies. An incipient scarp (4.4 km long) appears to be propagating from the head scarp of the smaller slide eastward toward the western head scarp of the Goleta slide, which suggests a potential for additional slope failure.
Whether tsunamis were generated by the Goleta event is unknown. However, our estimates suggest that 1.8 km3 of material was displaced by the slide, but only 0.7 km3 can be accounted for in the identified zone of accumulation at the toe of the slide mass on the slope. This implies that approximately 1.1 km3 was transported downslope as a flow slide, presumably into the deeper parts of the Santa Barbara Basin. Transport events of this magnitude imply catastrophic failures, which is consistent with the potential to generate tsunamis.