SEISMIC RISK IN THE WASATCH FRONT REGION, UTAH, FROM THE EAST GREAT SALT LAKE NORMAL FAULT: ESTIMATES FROM HIGH-RESOLUTION SEISMIC REFLECTION DATA

Geopulse and Chirp high-resolution seismic reflection profiles crossing the submerged East Great Salt Lake normal fault (EGSLF) define the fault's surface trace, segmentation, and late Quaternary seismogenic rupture history. Angular unconformities and stratigraphically limited auxiliary faults delineate earthquake event horizons in the hanging wall of the EGSLF within 1-2 km of its active trace. Each of two 30- to 35-km-long, NNW-striking segments south of Promontory Point has produced at least three major (~M 6.8-7.2) earthquakes with 3- to 5-meter surface offsets in post-Bonneville time (13-14 ka), corresponding to a best-estimate average recurrence interval for such events of 4500 (+1100/-800) yr/segment. The southern (Antelope) segment of the EGSLF has a lakebed scarp as high as 3.6 m. Auxiliary faults associated with the scarp-producing rupture (A3) are buried by ~0.4-0.6 m of sediment in the hanging wall. By contrast, the most recent event horizon (F3) associated with the northern (Fremont) segment, is covered by ~4 m of sediment and the related scarp is completely buried. We estimate a Holocene depositional rate of ~1 mm/yr in the proximal hanging wall of the EGSLF, implying an age of ~400-600 yrs for the A3 rupture and ~4000 yrs for the F3 rupture. Thus, the Fremont segment appears to pose the greater seismic risk to the Ogden-Salt Lake City-Provo urban corridor. Direct dating of event horizons A1, A2, and A3 (Antelope segment), and F1, F2, and F3 (Fremont segment) from core samples collected in August, 2000 will enable testing and refinement of these preliminary results.