Seismic characterization of the Wasatch fault system beneath Salt Lake City using a land streamer system

Abstract:

We characterize the active Wasatch fault system beneath downtown Salt Lake City by measuring p- and s-wave velocities and seismic reflection profiling. Our focus was on the segment boundary between the Warm Springs and East Bench faults. We collected 14.5 km along 9 west-east profiles in 3 field days using a 60 m aperture seismic land streamer and 200 kg weight drop system. From a p-wave refraction analysis, we measure velocities from 230-3900 m/s for the upper 20-25 meters. Shear wave velocities for the upper 30 m, derived from a multi-channel analysis of surface waves (MASW) approach, show velocities that range from 100-1800 m/s. P-wave reflection images from the upper 100 m depth indicate offset and truncated (mostly) west-dipping strata (Bonneville Lake deposits?) that suggest active faults extend beneath the downtown urban corridor. We identify saturated sediments on the lower elevation (western) portions of the profiles and shallow high velocity (dry) strata to the east of the mapped faults. We observe slow p-wave velocities near identified faults that may represent the fault’s colluvial wedge. These velocity results are best highlighted with Vp/Vs ratios. Analyzing shear wave velocities by NEHRP class, we estimate soft soil (NEHRP D) limited less than 1 m depth along most profiles, and stiff soil (NEHRP C) to up to 25 m depth in some locations. However near steep topographic slopes (footwall deposits), we identify NEHRP Class D stiff soil velocities to less than 2 m depth before transition to NEHRP Class C soft rock. Depth to hard rock (velocities >760 m/s) are as shallow as 20 m below the land surface on some steep slopes beneath north Salt Lake City and greater than our imaging depths along the western portions of our profiles. Our findings suggest large variations in seismic velocities beneath the Salt Lake City corridor and that multiple fault strands related to the Warm Springs fault segment extend beneath downtown.