TIMING AND MECHANICS OF THE WHITE PINE ROCKSLIDE, SALT LAKE CITY, UTAH

Little Cottonwood Canyon is a glacially carved east-west trending canyon cut into the intrusive quartz monzonite of the Little Cottonwood Stock in the Wasatch Mountains east of Salt Lake City. The White Pine Rockslide, located approximately 10 km east of the mouth of the canyon, has not been previously considered in assessing the geologic hazards present along the Wasatch Front. The White Pine Rockslide contains 2.8 x 105 m3 of rock spread over 72,000 m2 and consists of large (0.5-20 m) prismatic boulders of Little Cottonwood Stock. The source area of the rockslide rises 425 m above the canyon floor to the north of the rockslide deposit. Three orthogonal joint sets are persistent in this portion of the stock, two of which develop a wedge of rock with kinetic freedom of movement. The factor of safety (FS) was calculated with a wedge failure model to determine the effects of varying frictional properties, earthquake accelerations and water saturation levels. The location of the rockslide in the footwall of the Wasatch Fault, approximately 10 km east of the surface expression of the fault, suggests strong ground motion as a potential trigger for failure. Neither earthquake acceleration nor water uplift alone significantly altered the FS calculated for the wedge model. The model indicates that the most stable wedge conditions only provide a FS of 1.3; simultaneous presence of earthquake shaking and water pressure significantly reduces the stability of the wedge to failure at FS=1. Exposure ages of 1.09±0.31 ka and 2.67±1.14 ka from 10Be exposure ages do not closely match to provide an absolute age of the rockslide; however, the associated error from AMS measurement nearly overlap at approximately 1.4 ka, which is determined to be the most probable age of the rockslide. This age is within the range of ages found for the last surface rupturing event on the Salt Lake City segment of the Wasatch Fault, ca. 1.2 ka, indicating that the failure of the slide could be attributable to strong ground motion.