PALEOSEISMIC RECONNAISSANCE OF THE SEVIER FAULT, SOUTHWESTRN UTAH

The Utah Geological Survey made a reconnaissance of the Utah portion of the Sevier/Toroweap fault in southwestern Utah to identify sites where future paleoseismic studies may provide information on earthquake timing, recurrence, displacement, and vertical slip rate. Study results showed no fault scarps formed on unconsolidated deposits on the main fault in Utah; however, Quaternary basalts are displaced at two locations (Black Mountain and Red Canyon) on the northern section of the fault. Geologic relations at Black Mountain are complex and poorly exposed. Vertical slip rates based on displaced 0.57 Ma basalt flows range from 0.04 to 0.40 mm/yr, depending on the amount of displacement attributed to surface faulting. At Red Canyon a previously unidentified hanging-wall source for the displaced basalt indicates that the 200 m difference in elevation of the basalt across the fault at that location results from surface faulting and not flows cascading across a pre-existing fault escarpment. Based on an existing K-Ar age for the volcanic rocks of 0.56+0.07 Ma, this reconnaissance confirmed a vertical slip rate at Red Canyon of 0.36 mm/yr.

Near Panguitch, Utah the fault hanging wall contains numerous scarps and folds formed on Pleistocene to late Tertiary basin-fill deposits. The scarps range from less than a meter to about 25 m high. These scarps may be genetically related to the main Sevier/Toroweap fault to the east, but some scarps in a northeast-trending fault and fold zone south and east of Panguitch are likely related to ongoing aseismic folding in the hanging wall.