Abstract

Subsidence of Summer Lake basin, situated in the northwestern Basin and Range province in south-central Oregon, has exposed a kilometer-thick Neogene sequence of dense volcanic flow rocks overlying very weak tuffaceous sedimentary rocks in the bounding Winter Ridge–Slide Mountain escarpment. Subsidence occurs along the active, 40-km-long, Winter Ridge–Slide Mountain fault, which is capable of producing Mw ≈ 7 earthquakes with near-field, maximum horizontal acceleration approaching 1g. Gigantic landslides cubic kilometers in volume scallop the southwestern part of the escarpment, and the deposits run out several kilometers, characteristic of rock avalanches. Geotechnical rock-mass characterization and slope- stability analyses confirm observations that these landslides were initiated within the weak tuffaceous sedimentary rocks along shallow, east-dipping, planar failure surfaces, are insensitive to groundwater fluctuations, and are stable under static conditions. Pseudostatic analyses reveal that strong shaking is required to trigger landsliding. The Bennett Flat landslide failed as a single event between 900–270 ka and 16.8 ka; the landslide has a depletion-zone volume of 1.1 km3, but only 0.3 km3 of debris is discernible in the accumulation zone. The Foster Creek landslide also appears to have failed as a single event between 180 and 16.8 ka; the landslide has a depletion-zone volume of 2.9 km3, but only 0.3 km3 of debris is discernible in the accumulation zone. The Punchbowl landslide evidences multiple events, the most recent of which failed in the Holocene and had a volume of 0.4 km3; 0.5 km3 of debris is found in the accumulation zone.

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