Evidence of a Large-Magnitude Recent Prehistoric Earthquake on the Bear River Fault, Wyoming and Utah: Implications for Recurrence

Abstract:

Trenching across the antithetic strand of the Bear River normal fault in Utah has exposed evidence of a very young surface rupture. AMS radiocarbon analysis of three samples comprising pine-cone scales and needles from a 5-cm-thick faulted layer of organic detritus indicates the earthquake occurred post-320 CAL yr. BP (after A.D. 1630). The dated layer is buried beneath topsoil and a 15-cm-high scarp on the forest floor. Prior to this study, the entire surface-rupturing history of this nascent normal fault was thought to consist of two large events in the late Holocene (West, 1994; Schwartz et al., 2012). The discovery of a third, barely pre-historic, event led us to take a fresh look at geomorphically youthful depressions on the floodplain of the Bear River that we had interpreted as possible evidence of liquefaction. The appearance of these features is remarkably similar to sand-blow craters formed in the near-field of the M6.9 1983 Borah Peak earthquake. We have also identified steep scarps (<2 m high) and a still-forming coarse colluvial wedge near the north end of the fault in Wyoming, indicating that the most recent event ruptured most or all of the 40-km length of the fault.

Since first rupturing to the surface about 4500 years ago, the Bear River fault has generated large-magnitude earthquakes at intervals of about 2000 years, more frequently than most active faults in the region. The sudden initiation of normal faulting in an area of no prior late Cenozoic extension provides a basis for seismic hazard estimates of the maximum-magnitude background earthquake (earthquake not associated with a known fault) for normal faults in the Intermountain West.