Regional variability of slope stability: application to the Eel margin, California

Relative values of downslope driving forces and sediment resisting forces d etermine the locations of submarine slope failures. Both of these vary regi onally, and their impact can be addressed when the data are organized in a Geographic Information System (GIS). The study area on the continental marg in near the Eel River provides an excellent opportunity to apply GIS spatia l analysis techniques for evaluation of slope stability. In this area, swat h bathymetric mapping shows seafloor morphology and distribution of slope s teepness in fine detail, and sediment analysis of over 70 box cores delinea tes the variability of sediment density near the seafloor surface. Based on the results of ten geotechnical studies of submarine study areas, we devel oped an algorithm that relates surface sediment density to the shear streng th appropriate to the type of cyclic loading produced by an earthquake. Str ength and stress normalization procedures provide results that are conceptu ally independent of subbottom depth. Results at depth an rigorously applica ble if sediment lithology does not vary significantly and consolidation sta te can be estimated. Otherwise, the method applies only to shallow-seated s lope failure. Regional density, slope, and level of anticipated seismic sha king information were combined in a GIS framework to yield a map that illus trates the relative stability of slopes in the face of seismically induced failure. When a measure of predicted relative slope stability is draped on an oblique view of swath bathymetry, a variation in this slope stability is observed on an otherwise smooth slope along the mid-slope region north of a plunging anticline. The section of slope containing diffuse, pockmarked g ullies has a lower measure of stability than a separate section containing gullies that have sharper boundaries and somewhat steeper sides. Such an as sociation suggests that our slope-stability analysis relates to the stabili ty of the gully sides. The remainder of the study area shows few obvious in dications of slope instability except for a feature that has become known a s the 'Humboldt Slide,' but it is too deep-seated to be amenable to the slo pe-stability-prediction techniques presented herein. In general, few slope failures have been mapped in the Eel margin study area despite the high lev el of seismicity, the relatively high rates of sediment accumulation, and t he extent of gas charging observed by others. (C) 1999 Elsevier Science B.V . All rights reserved.