PALEOSEISMIC SLIP MODEL FOR THE WASATCH FAULT

The Wasatch fault is a normal fault that stretches through all the main cities of Utah up to the lower region of Idaho. The 390 km stretch is split up into 5 segments that were originally thought to be earthquake barriers. We used paleoseismic event data to construct a rupture catalog for the Wasatch fault and test whether large, multi-segment events are required to match the observed slip rates. Slip rates for the Wasatch fault range from 1.3 to 2.0 mm/yr. We compiled Oxcal-calibrated event records for 19 paleoseismic sites, spanning up to 12 kyr before present, and used these to construct candidate event histories. Two models were then constructed: one where no ruptures breach a segment boundary, and a second where events crossed one or more segment boundaries, producing larger events with greater slip. Confining events to single segments underpredicts the required slip rate by a factor of two to three. We conclude that the majority of earthquakes must rupture more than one segment to match the observed slip rates.