URBAN PALEOSEISMOLOGY OF THE TAYLORSVILLE FAULT - NEW DATA FROM ONE OF THE LAST REMAINING TRENCH SITES ON THE WEST VALLEY FAULT ZONE, UTAH

The intrabasin West Valley fault zone (WVFZ) in Salt Lake Valley, Utah, comprises two subparallel main strands—the Granger fault (western strand) and Taylorsville fault (eastern strand)—and is antithetic to the basin-bounding Wasatch fault zone (WFZ). Both strands trend approximately N-S through urbanized Salt Lake Valley, making sites suitable for fault trenching sparse.

In 2022, we excavated two paleoseismic trenches at the Indiana Avenue site on a relatively undisturbed, small west-dipping (1.5 m) fault scarp on the Taylorsville fault. The elevation of the site suggests it was submerged by multiple pluvial lake cycles during the Late Pleistocene and Holocene, including Lake Bonneville and the Curry-cycle and latest Holocene highstand of Great Salt Lake. Our trenches exposed several deposits likely related to these lake cycles, as well as a silty-sand deposit from the paleo-Jordan River floodplain, and several clay-rich wetland deposits. Both trenches showed evidence of fault-related monoclinal warping, as well as liquefaction, but only one trench showed evidence of discrete faulting in the footwall. Trench depth was limited due to shallow groundwater.

Our investigation revealed evidence for one earthquake, with evidence of a second earthquake based on secondary liquefaction-related features. Based on OxCal modeling, the most recent earthquake occurred at 1.6 ± 1.2 ka, and the second liquefaction-based event occurred at 9.8 ± 0.8 ka. These new data support and refine the existing earthquake chronology of the WVFZ. New data from this study show a potential earthquake correlation of the 1.6 ± 1.2 ka earthquake between the Taylorsville and Granger faults, suggesting the possibility of an instance of synchronous fault rupture on the two strands of the WVFZ. Comparing the WVFZ earthquake chronology 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 WFZ.