A MORE COMPLETE PALEOSEISMIC RECORD FOR THE WASATCH URBAN CORRIDOR– NEW DATA AND CHALLENGES FROM THE WEST VALLEY FAULT ZONE, SALT LAKE CITY, UTAH

The Wasatch fault zone (WFZ) is a 386 km (240 mi) long active normal fault zone at the base of the Wasatch Range in Utah. Over 80% of Utah’s population lives near the WFZ putting a majority of the state’s population at risk of earthquakes. The West Valley fault zone (WVFZ) is antithetic to the WFZ, creating an intrabasin fault zone that consists of two strands: the western Granger fault and the eastern Taylorsville fault. Previous paleoseismic investigations of the WFZ and WVFZ indicate recurrent Holocene surface faulting at statistically similar times, suggesting synchronous or triggered ruptures. However, this record is incomplete for the Taylorsville fault and, more broadly, we cannot refute the WVFZ acting as an independent source of earthquakes. To better understand the history of earthquakes on the Taylorsville fault, we excavated, logged, and sampled paleoseismic trenches across a scarp at the Indiana Avenue site in Salt Lake City. The site was previously a dump so trench locations were limited to where we were reasonably confident that we could find undisturbed earthquake deposits. Two trenches were excavated, and although both showed evidence of fault-related monoclinal warping, only one showed evidence of discrete faulting. Pedogenic and stratigraphic units exposed in the trenches include a modern soil A horizon, scarp-derived colluvium deposits, wetland, floodplain, lacustrine clay, post-Lake Bonneville sand, and clay interpreted to be associated with the Lake Bonneville highstand (~18 ka). In the southern trench, we interpret zones of faulting at the base of the fault scarp and in the hanging wall and monoclinally warped stratigraphy as evidence for paleoearthquakes. Our exposure was limited due to shallow groundwater in the hanging wall. To constrain earthquake timing, nine optically stimulated luminescence samples and six bulk radiocarbon soil samples were collected. We plan to show the available geochronology results and preliminary Oxcal modeling results of earthquake timing at the Indiana Avenue site, and compare that data to other paleoseismic trenching investigations on the WVFZ and WFZ. The results of this study will inform future updates to the U.S. Geological Survey National Seismic Hazard Map used in building codes to prevent catastrophic loss and disruption to the state of Utah.