SEISMOGENIC FLOW SLIDES WITHIN SUMMER LAKE BASIN, SOUTH-CENTRAL OREGON

Three late Pleistocene- to Holocene-age flow slides have been identified in the northwestern portion of Summer Lake basin of south-central Oregon along the active Winter Ridge fault. Each flow slide exceeds several square kilometers in area and initiates within nearly flat-lying (<3°), saturated lacustrine deposits comprised of interbedded sand, silt and clay. In the headscarp areas, thick deposits of coarse-grained colluvium, derived from the bounding Winter Ridge escarpment, fans onto the basin floor. The headscarps are linear, near repose, tens of meters in height, several kilometers in length, and share the same trace as the fault rupture. The depletion zones of two slides possess large grabens defining 10s to 100 meters of lateral displacement. The accumulation zones are characterized by folded and faulted bedding distributed in a broad arcuate form that extends up to several kilometers from the headscarp. No free faces likely existed at failure. We interpret these flow slides to be seismogenic, given their gentle topography, proximity to fault rupture, and back-analyses of slope failure. Similar features in comparable geologic settings have been noted along Holocene surface ruptures of the Dixie Valley fault in central Nevada, the Deep Springs fault in southeastern California, and possibly in thick mass wasting deposits in the southwestern corner of Summer Lake.

In Summer Lake basin, the flow sliding mechanism is due to liquefaction of saturated sandy layers within the lacustrine deposits. The large lateral displacements can be explained by lower permeability clay/silt layers causing void redistribution in underlying liquefied sandy layers and the formation of water films just beneath the silt/clay. These localized zones of increased void ratio experience considerable loss in undrained shear strength, which can then accommodate the large observed displacements.

The maximum observed depth of liquefaction of the three flow slides is about 55 meters near the headscarp at Jacks Lakes. Similar flow sliding may be initiating at depths approaching 100 to 200 meters beneath thick mass wasting deposits in the southwest corner of basin. Geotechnical borings in the area suggest that liquefaction may be occurring in materials with SPT N values between 15 and 30.