STRATIGRAPHIC AND STRUCTURAL CONTROLS OF LANDSLIDES IN THE SOUTHWESTERN OLYMPIC PENINSULA, WASHINGTON STATE

Mapping and GIS analysis of 2453 landslides in the Olympic subduction complex, exposed on the southwestern Olympic Peninsula, show expected correlations of landslide occurrence with logging roads, stream-bank erosion, slope angle, and timber harvesting, especially on slopes >80%. Landslide occurrence also correlates well with 1) major thrust sheets within the accretionary prism that create stepwise eastward increases in exhumation and induration, 2) deep marine lithofacies and related bed forms, and 3) fault zones characterized by unusually wide gouge and(or) diapiric clay intrusion. An abrupt eastward increase in instability occurs in the westernmost foothills of the Olympic Mountains at the Salmon River shear zone, a major thrust fault. Here, zeolite facies metasediments are thrust southwestward over porous sandstones and poorly-indurated fine siliciclastic rocks. Favorable soil/rock coupling appears to account for the low failure frequency on the lower plate. Numerous deep-seated and shallow landslides occur in a monotonous sequence of thin-bedded interdistributary units, apparently as the result of flexural slip. Contacts of massive, deep marine channel-fill sandstones with thin- to medium-bedded sandstones and finer clastic rocks are common failure planes. Gouge zones, typically 1 km in width, are the locus of numerous failures. Most frequent are deep-seated and mid-slope failures resulting from ongoing intrusion of diapiric muds along northwest-trending thrust faults and younger north-northeast trending high-angle faults. Within Pleistocene units, older compact and cemented alpine drift is remarkably stable.