| Summer Lake basin is located in south-central Oregon in the northwestern Basin and Range. The 40 km-long, Winter Ridge-Slide Mountain (WR-SM) escarpment that bounds the basin on the west and south, respectively, exposes a kilometer-thick, Neogene sequence of dense volcanic flow rocks overlying very weak tuffaceous sedimentary rocks that dip 10° west. Gigantic landslides scallop the southwestern escarpment and display several kilometer long runouts characteristic of rock avalanches. Geotechnical rock mass characterization and slope stability analyses confirm observations that these landslides generally initiate along shallow, east-dipping, planar failure surfaces within the weak tuffaceous sedimentary rocks, are insensitive to groundwater fluctuations, and are stable under static conditions. Pseudo-static analyses reveal that strong shaking (>1 g), resulting from Quaternary dip-slip movement on the WR-SM fault, is required to trigger landsliding. Shaking is also responsible for initiating large flow slides within Quaternary deposits along the base of the escarpment. |
