EVIDENCE FOR MULTIPLE SURFACE RUPTURES WITH 0.3-1.5 METER SLIP-PER-EVENT ALONG STRUCTURES BETWEEN THE SALT LAKE CITY AND PROVO SEGMENTS OF THE WASATCH FAULT

Segmentation along normal faults is often expressed as a mountain salient that separates fault-bounded valleys to either side. Segment boundaries are areas of lower cumulative slip and thus their faults typically cut through bedrock units making it challenging to determine the age of recent earthquakes. However, bracketing the age of such earthquakes and their kinematics is necessary to establish linkages with earthquake histories along adjacent primary faults. This type of data is important for constraining models of fault interaction and earthquake hazard scenarios. As a part of the 2013 Utah Valley University geology field camp we produced a 1:6K geomorphic fault strip map across the salient separating the Salt Lake City and Provo segments of the Wasatch Fault. Our mapping was based upon 2 m LiDAR data, 0.25 m aerial photographs, and field validation. We identified multiple sub-parallel surface traces across the 7 km segment boundary. Individual fault traces range from 0.2 – 2 km in length and trend roughly east to west. Scarps are generally south facing with less than 10 m of relief. Based upon this mapping, we reoccupied the westernmost of seven large consultant trenches which were left open following a 2006 geotechnical investigation. In this trench we documented three faulted exposures. The central exposure coincided with the most prominent fault scarp. This exposure revealed four colluvial wedges. From oldest to youngest the heights of the wedges are 0.4, 1.2, 0.8, and 1.3 m with uncertainties of about 25%. The northern exposure revealed evidence for one earthquake with 0.5 m of displacement. This event evidence appears to be older than those of the central exposure. The southern exposure revealed evidence for two surface ruptures with 0.4 and 1.0 meters of displacement. We do not know the age of these events relative to those of the central exposure. Each of these wedge deposits consisted of dark, reworked soil a-horizons. We collected bulk samples from each wedge deposit for subsequent radiocarbon analyses. In summary, we documented evidence for at least five different surface rupturing earthquakes with 0.3-1.5 meters of vertical displacement that have broken through the 7 km long boundary between the Provo and Salt Lake City segments of the Wasatch Fault.