MULTIPLE HOLOCENE SURFACE-RUPTURING EARTHQUAKES ON THE EAST BENCH FAULT, SALT LAKE CITY, UTAH

The 43-km-long Salt Lake City segment (SLCS) of the Wasatch fault zone is composed of three left-stepping strands: the southern Cottonwood fault, the central East Bench fault (EBF), and the northern Warm Springs fault. The 12-km-long EBF strikes generally N-S but curves to the NE near its northern end. It extends through the most heavily urbanized part of Salt Lake Valley, and prior to our investigation, this urbanization had precluded detailed study of the fault’s rupture history. However, we identified a site suitable for trenching at Penrose Drive (PD) (40.77°N, 111.84°W, ~1460 m ASL) near the Univ. of Utah. This site, near the northern end of the strand, is the only undeveloped site on the EBF where the scarp is relatively unmodified.

At the PD site, we excavated a 36-m-long western trench and a 14-m-long eastern trench across the 10-m-high scarp and exposed a simple, well-defined fault plane that strikes N48°E and dips 79-88°NW. In the footwall, we exposed well-oxidized, moderately indurated alluvial-fan deposits that pre-date pluvial Lake Bonneville. In the hanging wall, we exposed Lake Bonneville deposits of sandy clay and silt overlain by Provo-stage (17.4–14.4 ka) boulder beach gravel. These lacustrine sediments are overlain by a sequence of stacked colluvial wedges, most of which are separated by very weak paleosols. We interpret this stratigraphy to document evidence of at least five and possibly six post-Provo surface ruptures. We also found evidence of an earthquake that occurred between the Bonneville high-stand (~18 ka) and the Provo stage. Each colluvial wedge is about 1 m thick, suggesting that each earthquake rupture produced 1–2 m of vertical surface offset. Because the fault’s NE strike at the PD site is oblique to the approximately E-W regional extension direction, fault displacements may have a significant, but unobserved, component of lateral slip. We document a minimum of 16–17 m of vertical offset of the pre-Bonneville fan alluvium at this site. We will use age data from 14 radiocarbon and nine luminescence samples to constrain the times of individual surface ruptures and compare these ages with results of previous studies, which have all been done on the Cottonwood fault near the southern end of the SLCS.