HOLOCENE PALEOSEISMOLOGY OF THE NEPHI, WEBER, AND BRIGHAM CITY SEGMENTS OF THE WASATCH FAULT ZONE, UTAH: INSIGHTS FROM RECENT TRENCH INVESTIGATIONS

The Wasatch fault zone (WFZ), Utah's longest and most hazardous fault, has evidence of repeated Holocene surface-faulting earthquakes on its five central segments. Three decades of paleoseismic studies indicate a unique earthquake history for each segment, but are generally limited to the middle and late Holocene and are too spatially restricted to determine rupture lengths for individual events. Between 2005 and 2008, we excavated trenches at six sites on the Nephi (NS), Weber (WS), and Brigham City (BCS) segments to better define earthquake timing and rupture extent on these poorly characterized WFZ segments.

Trenches at two sites on the NS refine earthquake timing on the southern strand and provide information for the previously untrenched northern strand. Three earthquakes younger than 2.6 ka ruptured the southern strand, including a most recent earthquake (MRE) at ~150–350 cal yr B.P. A single post-mid-Holocene earthquake occurred on the northern strand at 350–600 cal yr B.P. and may correlate with the ~500–650 cal yr B.P. MRE on the adjacent Provo segment. On the WS, trenches at the Rice Creek site refine earthquake timing and correlation. Five or six earthquakes occurred after 8–10 ka, including an MRE at 500–600 cal yr B.P. The Rice Creek MRE probably correlates with an ~500-yr earthquake found 9 km to the south at East Ogden, and may represent partial rupture of the northern WS. Trenches at three sites on the BCS targeted the MRE, which is thought to have an elapsed time nearly double the mean earthquake-recurrence interval for the segment. Preliminary results indicate different MRE times on the northern and southern BCS: the northern BCS MRE likely occurred before ~2000 cal yr B.P., consistent with previous data, whereas the southern BCS MRE occurred ~1100–1800 cal yr B.P.

These new paleoseismic data help reduce uncertainties in the timing and extent of paleoearthquakes on the NS, WS, and BCS, and indicate a complex history of Holocene surface faulting on the WFZ that likely includes partial-segment and spillover ruptures. Ultimately, these results will improve segmentation scenarios for the WFZ as well as probabilistic earthquake-hazard models for the Wasatch Front region.