THE WEST VALLEY FAULT ZONE, SALT LAKE VALLEY, UTAH – WHAT DO WE REALLY KNOW ABOUT ITS SEISMIC HAZARD

The West Valley fault zone (WVFZ) is a system of intrabasin normal faults about 4 to 10 km west of Salt Lake City, in the most populous region of Utah's Wasatch Front. Although the WVFZ has long been recognized as a potential seismic source, source-parameter modeling for the recently updated National Seismic Hazard Maps highlighted uncertainty regarding the timing and amount of fault displacement in individual past surface-faulting earthquakes, as well as subsurface fault geometry. A fundamental question is whether the WVFZ moves sympathetically with the nearby Wasatch fault zone (WFZ) or operates as an independent seismic source. The north-south extent and general trend of WVFZ fault traces are similar to those of the nearby Warm Springs and East Bench faults (subsections of the Salt Lake City segment of the WFZ), suggesting a possible structural relationship. Also, the relatively small distance between WVFZ and WFZ fault traces allows for the possibility that the WVFZ may be truncated within seismogenic depths by the WFZ, although dips of both fault zones in this area are uncertain. Paleoseismic data for the WVFZ come largely from boreholes drilled as part of two research projects undertaken in the late 1980s. By documenting cumulative vertical offset of lacustrine deposits associated with the Bonneville (28-12 ka), Cutler Dam (60 ± 20 ka), and Little Valley (140 ± 10 ka) pluvial lake cycles, these studies established long-term slip rates ranging from 0.1 to 0.5 mm/yr. However, the timing of individual surface-faulting earthquakes and slip per event remain poorly constrained or unknown, and hence recurrence intervals and late Holocene slip rates are virtually unknown. More recently, a few trenches excavated by consultants for fault-setback studies have provided opportunities for the Utah Geological Survey (UGS) to obtain samples for radiocarbon dating. The results of these analyses provide maximum limits (but no minimum limits) on the timing of two WVFZ earthquakes: one shortly(?) after ~1300-1700 cal yr B.P. and one shortly(?) after ~2200 cal yr B.P. Timing between these earthquakes and the two most recent earthquakes on the Salt Lake City segment is similar. The UGS is currently developing plans for additional paleoseismic studies to improve our knowledge of the Holocene surface-faulting history of the WVFZ.