CONTRASTING PALEO-SEISMICALLY INDUCED LATERAL SPREADING MECHANISMS IN DIXIE VALLEY, NV – A GEOTECHNICAL ANALYSIS

A 20 square kilometer zone of lateral spreads in Dixie Valley, within the Central Nevada Seismic Belt of the Great Basin of Nevada and dated to approximately 2,500 years before present, was previously interpreted as seismically-induced liquefaction features (Caskey et al. 1996). This investigation of one lateral spread, located east of the range front fault system of the Stillwater Range and extending into the basin playa, found the subsurface (from trenching) to be composed of inter-fingering alluvial fan material washed down from the westward bounding Stillwater Range and of beach gravel, sands and cohesive fines from the eastward bounding pluvial Lake Dixie.

Results from mapping, subsurface drilling, CPT analyses, geotechnical soil characterization, and stability analyses indicate a potentially more complex failure mechanism as a consequence of a thick clay deposit within the sands. Analyses indicate failure can occur by: 1) sand liquefaction 2) cyclic clay softening 3) translational shear within clay or 4) these mechanisms in combination. The differences in geotechnical characterization of cohesive and non-cohesive soils have led to different procedures for soil strength analysis. This investigation found at least two failure morphologies within the zone of failures. Our current hypothesis is that failure morphology is a result of subsurface geotechnical properties and in the future it may be possible to use morphology to interpret subsurface geology and failure mechanism.