Urban Paleoseismology of the Taylorsville Fault - New Data and Challenges from one of the Last Remaining Trench Sites on the West Valley Fault Zone, Utah  

Abstract / Description

The West Valley fault zone (WVFZ) and Salt Lake City segment (SLCS) of the Wasatch fault zone (WFZ) comprise Holocene-active normal faults that bound an intrabasin graben in northern Salt Lake Valley, Utah. Both fault zones have evidence of recurrent Holocene surface-faulting earthquakes. The WVFZ trends through the western part of the highly urbanized Salt Lake Valley and has had limited paleoseismic study in the past. This fault zone consists of two main fault strands—the western Granger fault and eastern Taylorsville fault. Previous paleoseismic investigations have confirmed recurrent Holocene surface faulting on the WVFZ and suggest the likelihood of synchronous rupture of the WVFZ during surface-faulting earthquakes on the SLCS. However, the WVFZ comprises a distributed zone of faulting, and detailed paleoseismic investigations have only been conducted on a few strands of the fault zone. Because of this distributed nature of surface faulting, a complete chronology of latest Quaternary surface-rupturing earthquakes cannot be reliably obtained from a single paleoseismic study site; thus, the current paleoearthquake record for the WVFZ is far from complete. Additional earthquake timing data leads to improved understanding of the degree to which the WVFZ ruptures synchronously with the Wasatch fault zone or functions as an independent source of earthquakes.

The Indiana Avenue site is near the middle of the Taylorsville fault. At this site, we excavated two trenches across a small (~1.5-m high) west-facing fault scarp. Human-caused disturbance and shallow groundwater limited trench length and depth at the site. Evidence for fault-related deformation in the trenches consisted primarily of folding and warping along the main fault trace, with some discrete faulting on a subsidiary fault. Our detailed trench exposure mapping and dating primarily support fault rupture associated with a single surface-rupturing earthquake at this site, with potential evidence for a secondary earthquake revealed in the trench based on secondary (liquefaction) deformation observed in the south trench. Based on OxCal modeling of radiocarbon and luminescence ages, our primary, most recent earthquake (IA1), occurred at 1.6 ± 1.2 ka. In our two-event modeling scenario, the second, liquefaction-based event occurred at 9.8 ± 0.8 ka.

Our new Indiana Avenue site data supports the existing earthquake chronology of the WVFZ. Our new data refines several of the per-event recurrence intervals for the WVFZ as a whole but does not change any of the earthquake chronology comparisons with the WFZ. New data from this study does show a potential earthquake correlation between the Taylorsville and Granger faults, suggesting synchronous fault rupture between the two strands of the WVFZ. This correlation has implications for seismic hazard associated with the WVFZ. As was the case before this study, comparing the earthquake chronology for the WVFZ with WFZ earthquake chronologies, shows that, more often than not, the WVFZ as a whole moves in response to, and likely synchronously with, slip on the Wasatch fault zone.