POTENTIAL GEOLOGIC HAZARDS OF A M 7 EARTHQUAKE ALONG THE SALT LAKE CITY SEGMENT, WASATCH FAULT ZONE, UTAH

We mapped geologic hazards posed by a M 7 earthquake along the Salt Lake City segment of the Wasatch fault zone, a major active zone of normal faulting. This event is characteristic of surface-rupturing earthquakes in the central Wasatch Front region of northern Utah, with a population of about 1.7 million centered upon the Salt Lake City metropolitan area. We modeled the scenario earthquake with a rupture plane 46 km long, 19.5 km wide, and dipping 55° to the west.

Peak ground accelerations (PGA) from our scenario earthquake exceed 0.7 g, with the highest PGA in stiff gravels and sands on the east side of Salt Lake Valley. Potentially damaging ground motions of 0.1 g and greater extend along the Wasatch Front from Provo to Ogden at distances of 50 to 60 km from Salt Lake City. The pattern of ground shaking for short-period spectral accelerations resembles that for PGA, but long-period spectral accelerations are greatest in the deeper central portions of the basin.

Our scenario earthquake results in surface fault rupture with a maximum displacement of 1.9 m and is accompanied by liquefaction, landsliding, and tectonic subsidence. Liquefaction is most likely where saturated fine sandy soils are subject to strong ground shaking in central and eastern Salt Lake Valley. Although some landsliding occurs on mountain slopes near the strongest ground shaking, deposits susceptible to slope failure are more common in areas of more moderate ground shaking to the north and south of Salt Lake Valley. Tectonic subsidence could displace the shoreline of Great Salt Lake into areas around Salt Lake City. Seiche hazards due to ground shaking appear to be negligible in Great Salt Lake, and current programs to modify large, high-hazard dams are designed to provide satisfactory performance with no uncontrolled releases during an earthquake similar to our scenario.

Our 1:250,000-scale hazard maps will be used with software such as HAZUS to estimate the consequences of the scenario earthquake. This estimate may describe the extent of damage and disruption, approximate number of casualties, repair and replacement costs, losses of function for critical facilities, and extent of induced hazards, providing an important tool for emergency-response planning and preparedness.