PALEOSEISMOLOGY AND SEGMENTATION OF THE SEVIER FAULT, SOUTHWESTERN UTAH

The Utah Geological Survey conducted a paleoseismic reconnaissance of the Utah portion of the Sevier fault in southwestern Utah to help characterize earthquake timing, recurrence, displacement, vertical slip rates, and segmentation. The west-dipping normal fault lies within the Basin and Range-Colorado Plateau transition zone. Study results showed no fault scarps 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. Complex and poorly exposed geologic relations at Black Mountain did not permit a reliable Quaternary surface-faulting displacement estimate. Therefore, we calculated a long-term vertical slip rate using a 12-15 Ma fault age estimate and a total displacement of 472 to 869 m determined from geologic cross sections, which yielded a vertical slip rate of 0.03-0.07 mm/yr.

At Red Canyon, new 40Ar/39Ar radiometric ages identified volcanic flows of two ages: 0.51+0.02 Ma and 4.96+0.03 Ma. Geochemical analyses show that both flows are correlative across the fault. A previously unrecognized hanging-wall source for the younger volcanic flow indicates that the difference in elevation of the flow across the fault is due to surface faulting and not cascading across a pre-existing fault escarpment. The older flow likely also has a western source. Displacement estimates of 192-225 m for the younger flow and 237-344 m for the older flow yield late Quaternary to present and early Pliocene to present vertical slip rates of 0.38-0.44 mm/yr, and 0.05-0.07 mm/yr, respectively. Using 900 m of displacement in basement rocks near Red Canyon indicated by seismic reflection data, we calculate a middle Miocene to present vertical slip rate of 0.06-0.08 mm/yr.

We suspect that the 250-km-long Sevier fault consists of shorter seismogenic segments that have unique rupture histories. In addition to the 2.5-km-wide left step-over in the fault near Clay Flat, previously recognized as a possible segment boundary, we propose two other possible seismogenic segment boundaries in Utah. Coincident geometric and geomorphic anomalies, differences in stratigraphic displacement, and changes in seismic activity indicate the likelihood of seismogenic segment boundaries at Hillsdale Canyon and near the town of Alton.