THE TRAVERSE RIDGE SITE, WASATCH FAULT, UTAH: AN EXAMPLE OF CHALLENGES IN INTERPRETING EARTHQUAKE RECORDS AT SEGMENT BOUNDARIES

The Wasatch fault zone (WFZ) is a ~350 km-long normal fault that dips beneath the Salt Lake-Provo-Ogden metropolitan area, posing the greatest seismic risk within the Intermountain West of the United States. How earthquake behavior is modulated by segment boundaries is an important scientific question, especially for the WFZ where the urban area extends across multiple fault segments. This investigation aims to use earthquake chronologies from the Traverse Ridge paleoseismic site (TR site: 40.492°, - 111.805°) along the 7 km-long Fort Canyon fault, which links the Salt Lake City (40 km) and Provo (59 km) segments of the WFZ, to evaluate rupture models for this segment boundary.

We re-excavated two trenches at the TR site across two parallel (northern and southern) traces of the Fort Canyon fault. Evidence from this site leads us to infer that 2 or 3 earthquakes have ruptured the Fort Canyon fault since ~14 ka. Radiocarbon results constrain the age of the most recent event (MRE) along the southern trace to 0.2-0.5 ka, the MRE on the northern trace to 0.4-3.8 ka, and a prior event on the northern trace to between 7.4 and 14.3 ka. Soil development within these MRE colluvial wedges is similar, but it is uncertain that they formed following the same event. We compared these results with previous paleoseismic data for the southern Salt Lake City and northern Provo segments and show that the TR site only records about half the number of earthquakes that have occurred on the adjacent segments over the Holocene. The MRE on the southern trace at the TR site overlaps with the youngest event identified on the northern Provo Segment, likely providing evidence of rupture into the segment boundary. The youngest event on the northern trace at the TR site overlaps with event ages on both the southern Salt Lake City and northern Provo segments, and could be evidence of a multisegment rupture. The currently published data permit a range of rupture models across this segment boundary including spill-over, segment-boundary, single-segment, and multi-segment ruptures. Forthcoming records from nearby sites on the northernmost Provo (Alpine site) and southernmost Salt Lake City (Corner Canyon site) segments will further refine the evaluation of rupture models at this segment boundary.