THE WEST MOUNTAIN SITE, GENOLA NORTH FAULT, UTAH: PALEOSEISMIC HISTORY AND CONNECTIVITY WITH UTAH LAKE FAULTS

Lidar analysis has led to the discovery of many previously unrecognized active faults, including a 13-km-long discontinuous network of fault scarps running along the west side of West Mountain in Utah County. This westward dipping normal fault, called the Genola North fault, aligns with the Lincoln Point West fault system, within Utah Lake, as well as the Long Ridge fault, located to the south. These faults have been postulated to be coseismic, auxiliary faults to the Provo segment of the Wasatch fault zone. If linked together, these three faults comprise an approximately 40-km-long fault system. Characterizing their seismic hazard is an important consideration for urban development along West Mountain and on Utah Lake. Based upon our lidar interpretation, we selected the West Mountain site (40.1148, -111.8406) due to a ~ 3-m high fault scarp cutting the Provo shoreline of Lake Bonneville along the northern half of West Mountain. This site is ideal for paleoseismic analysis because of the known geomorphic age for the Provo level shoreline of Lake Bonneville (~ 15 cal ka). A single 30-m-long trench across the fault scarp revealed 20° westward dipping transgressive and regressive lake deposits below a prominent angular unconformity marked by a near horizontal ~ 10-cm thick carbonate beach deposit. The displacement of the carbonate beach deposit shows a 5-m organic soil-filled fault zone with 3-m of vertical movement. We also observed clear evidence of two ground rupturing earthquakes post Lake Bonneville and at least one rupture close in time to the Bonneville highstand (~18 cal ka). Based on these shoreline ages, we infer an average recurrence rate of 7.5 ka per event and a slip rate of ~0.2 mm/a. Freshwater mollusk shells common to Lake Bonneville were sampled for C-14 dating and amino acid racemization to provide age range of the transgressive and regressive lake deposits. Bulk soil samples from fault-derived colluvium should provide additional C-14 dates to help constrain the timing of the penultimate earthquake and the maximum age of the most recent event. This study provides evidence that faults along West Mountain have been active within the Holocene and should be taken into consideration for urban planning in this rapidly growing region.