RIDGE-TOP DEPRESSIONS, LANDSLIDES, AND EARTHQUAKES, CAPE MENDOCINO REGION, CALIFORNIA

Numerous ridge-top depressions (sackungen) occur on and just below the crests of steep-sided ridges in the Cape Mendocino region (CMR). Based on the interpretation of 1941 and 1954 airphotos, 74 localities with anomalous ridge-top features were mapped and described between Cape Mendocino and Point Delgada. These features range in character from narrow linear troughs along ridge axes to more complex landforms on broad ridges that appear to be flattened and laterally spread. An analysis of mappable landforms shows that 84% of the localities included ridge-top troughs, 59% closed depressions, 31% scarps (including uphill-facing scarps and sidehill benches), and 14% swales. All but three localities occur in penetratively deformed terranes of the Franciscan Complex. Two-thirds of the localities are associated with deep-seated landslides. All gradations appear to exist between ridge-top depressions with no associated slides to features that clearly mimic or are part of adjacent landslides, suggesting a continuum and a common origin for ridge-top depressions and landslides. A similar conclusion was reached by McCalpin and Hart (1999) in the San Gabriel Mountains.

The chief origin of ridge-top depressions in the CMR very likely is seismic shaking, based on the following: 1) The CMR lies astride the highly seismic Mendocino Triple Junction; 2) ridge-top depressions in the CMR are quite similar to landforms reactivated by fracturing during the 1989 M7.1 Loma Prieta and previous earthquakes in the Santa Cruz Mts. (Nolan and Weber, 1998); and 3) small ridge-top grabens and cracks mapped after the 1992 M7.1 Petrolia earthquake (Dunklin, 1992; Stein et al, 1993) appear to coincide with some of the mapped features in the CMR. Based on airphoto reconnaissance, ridge-top depressions are relatively widespread in California's seismically active Coast and Transverse Ranges and constitute potential seismic hazards to structures. They may also be useful paleoseismic sites as they tend to form only in large earthquakes and may be particularly useful in areas of blind faults.