Paleoseismic investigation of the Taylorsville fault at the Airport East site, West Valley fault zone, Salt Lake County, Utah

Abstract

The West Valley fault zone (WVFZ) and Salt Lake City segment (SLCS) of the Wasatch fault zone comprise Holoceneactive normal faults that bound an intrabasin graben in northern Salt Lake Valley, Utah. Both fault zones have evidence of recurrent Holocene surface-faulting earthquakes. A topic of recent research is the seismogenic relation of the antithetic (subsidiary) WVFZ to the Wasatch fault zone—specifically, to what degree are WVFZ earthquakes independent of slip on the SLCS, or other adjacent segments, of the Wasatch fault zone. To improve paleoseismic data for the WVFZ and better understand the seismogenic relation between the WVFZ and Wasatch fault zone, we conducted a fault-trench investigation at the Airport East site, developed new earthquake recurrence and fault sliprate estimates for the WVFZ, and compared WVFZ earthquake timing data with data from the Wasatch fault zone. The Airport East site is near the northern end of the easternmost traces of the WVFZ, collectively referred to as the Taylorsville fault. At this site, we excavated two parallel trenches across a small (~0.5-m high) east-facing fault scarp. Shallow groundwater severely limited trench depth, and we were able to expose deposits only as old as mid-Holocene. However, the late Holocene section contained evidence for three surfacefaulting earthquakes on the Taylorsville fault, as well as earthquake-related deformation (liquefaction and folding) from a fourth earthquake that was likely sourced elsewhere. Based on OxCal modeling of radiocarbon and optically stimulated luminescence ages, the most recent earthquake, AE1, occurred at 0.4 ± 0.2 ka (mean modeled time ± 2σ), earthquake AE2 occurred at 0.6 ± 0.2 ka, and earthquake AE3 occurred at 2.0 ± 0.3 ka. Timing data from the Airport East site indicate the liquefaction and folding event (LE1) occurred at 5.1 ± 0.3 ka. Net vertical displacement across the fault is 0.6–1.1 m, and calculations of mean per-event displacement range from 0.20 to 0.37 m. Inter-event recurrence intervals for the Taylorsville fault at the Airport East site vary from 200 to 1400 yr, and the mean late Holocene (post-2 ka) recurrence interval is 800 yr. Paleoseismic (closed-interval) slip rates range from 0.23 to 1.4 mm/yr, and geologic (open-interval) slip rates range from 0.1–0.2 mm/yr over the past ~5000 yr to 0.2–0.4 mm/yr over the past ~2500 yr. Combining our new Airport East data with previous paleoseismic data for the WVFZ shows that during individual late Holocene earthquakes, surface faulting has occurred on either the Taylorsville or Granger fault, but not both faults at the same time (at least not on the parts of the faults that have been trenched). However, five mid- to late Holocene WVFZ earthquakes have mean modeled times that are either very similar or identical to mean modeled times of SLCS and Weber-segment earthquakes, and temporal correlations of two additional late Pleistocene WVFZ earthquakes with SLCS earthquakes cannot be ruled out. When comparing the earthquake chronologies of the WVFZ and Wasatch fault zone, our new data lend support to the idea that, more often than not, some part of the WVFZ moves in response to, and possibly synchronously with, slip on the Wasatch fault zone.