SEISMOGENIC LANDSLIDE MODIFICATION OF FAULT SCARPS, SUMMER LAKE BASIN, LAKE COUNTY, OREGON

Situated in the northwestern Basin and Range province, Summer Lake basin is a west-dipping half-graben bound on the west by the kilometer-high, Winter Ridge escarpment. Since its inception in the Miocene, the basin has served as a sink for eolian sediment and for deposits derived from mass wasting, colluvial and alluvial processes that act on the surrounding escarpments. The Winter Ridge fault cuts most mass wasting and other surficial deposits along the base of the escarpment. Water-filled grabens are common along the base of fault scarps. In several locations, hummocky topography and small concentric ridges and depressions within large, lobate forms are preserved basin-ward of these grabens. Additionally, scarps are minimally dissected and near repose, even though most scarps are tens to hundreds of meters in height. This fresh scarp morphology suggests greater youthfulness than would otherwise be implied by such large scarp height.

We interpret this scarp and slope morphology to result, in part, from secondary, seismogenic landsliding. High groundwater conditions, evidenced by numerous springs and seeps along the base of the escarpment, predispose the soft, fine-grained basin sediments and overlying mass wasting and colluvial deposits to seismogenic slope failures. Such deformation has been described by Langridge (1998) at the Ana River fault in the north end of the basin. We interpret the aforementioned lobate forms to be compressional features and the grabens to be extensional features of large lateral spreads associated with faulting events. Perhaps similar processes have modified the 120m and 270 m fault scarps that cut mass wasting deposits at the Foster Creek and Bennett Flat landslides in the southern portion of the escarpment.