Fault-Scarp Dating Utilizing Soil Strength Behavior Techniques

Abstract

Field and laboratory studies of the alluvial and colluvial soils, of varying lithology, that comprise the 1954 Dixie Valley–Fairview Peak fault scarps and the 1915 Pleasant Valley fault scarps of west-central Nevada, show that the engineering properties of these soils play a significant role in the development of scarp morphology. Cohesive strengths of these scarp soils increase in an approximately linear manner with increasing ratio of free face to total scarp height, and comparable linear relationships exist between the bulk densities and the ratio of free face to scarp height. The correlation coefficients between the soil-engineering parameters and the scarp ratios are very high. The peak friction angles for the Dixie Valley–Fairview Peak study area also exhibit a linear trend and a high correlation coefficient; those of Pleasant Valley did not, probably due to the smaller sampling base. Such results help explain the often abrupt changes in the free face/total scarp height ratios observed along fault scarps at the boundaries between differing soil types. Studies along the Lone Pine scarp (1872 – most recent offset) of central Owens Valley, California, reveal similar relationships between the soil-engineering properties and the free face to scarp height ratios. Shear tests performed on three different age scarp bevels (due to three faulting episodes) show a consistent decrease in soil strength with increasing bevel age, indicated by graphical plots of cohesion vs age and cohesion vs bevel inclination. Use of this data with more qualitative methods can be utilized in the assessment of fault scarp ages in regions of comparable aridity and rock/soil types.