PROBABLE SEISMOGENIC ORIGIN OF PLEISTOCENE LANDSLIDES ON THE EAST TRAVERSE MOUNTAINS, WASATCH FRONT, UTAH

The East Traverse Mountains (ETM) is a 10 km-long, 400 m-high bedrock ridge separates Salt Lake (Jordan) Valley (to the north) from Utah Valley (to the south). The central part of the Traverse Mountains is underlain by Tertiary volcanic rocks (block-and-ash flows, andesite flows, and sandy tuffs) that are deeply weathered, altered to clay to a depth of over 250 m, and prone to landsliding. The Landslide Inventory Map shows 53 landslides within the 4000-acre Suncrest Development, six of which (Slides A through F) were studied in detail by trenching and core drilling. All slides are composed of unstratified, poorly-sorted diamicton deposits that closely resemble the Tertiary block-and-ash flows. Slide A is a planar rockslide controlled by a 2 m-thick clay bed (altered ash) that dips 12 degrees SE, in the direction of slope. Slides B-D are planar or wedge failures controlled by SE-dipping strata and Neogene faults. Slides E and F are deeper-seated rotational rock failures on the steep north-facing scarp slope. All slides are 15-25 m deep, based on drilling, shear wave velocity profiles, and seismic tomography. Nine oriented, undisturbed samples of shear zone material were taken from trenches at the heads and toes to measure residual strength. Residual friction angles range from 16 to 34 degrees (mean 24 degrees) and vary inversely with cohesion, which ranges from 80 to 600 psf. Groundwater on the mountain is everywhere deeper than 50 m, except where locally perched on altered clays. Stability analyses are based on presumed pre-slide togopography, and the following conservative values: friction of 16-24 degrees, cohesion of zero, and perched water in the middle of the future slide mass. With these inputs, static factors of safety are consistently greater than 1.5-2.0, suggesting these were not static failures. The Wasatch fault zone lies 1.5-3 km east of the slides, dips west beneath the slides, and has characteristic earthquakes of M7.1 every ca. 1400 years. Predicted ground motions at the slides are 0.23g (475-yr return period), 0.40g (1400 yr return period, characteristic EQ), and 0.62g (2500-yr return period). The landslides are predicted to be marginally stable at 0.23g and 0.40g, and become unstable at ca. 0.65g. This result suggests that the slides were triggered by wet-season characteristic earthquakes on the WFZ.