PLANNING AHEAD—A M 7 EARTHQUAKE SCENARIO FOR THE CENTRAL WASATCH FRONT, UTAH

In 1860, essayist and poet Ralph Waldo Emerson wrote, “We learn geology the morning after the earthquake, on ghastly diagrams of cloven mountains, upheaved plains, and the dry bed of the sea.” Human nature remains the same now as then—we often learn our geologic lesson after an earthquake, rather than plan in advance. However, the seismically active central Wasatch Front of Utah, with a population of about 1.7 million centered upon Salt Lake City, has not experienced a strong, historical earthquake. To help reduce earthquake risks and losses in the region, 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.

We modeled the M 7 scenario event, which we consider typical of large earthquakes along this segment, with a rupture plane 46 km long, 19.5 km wide, and dipping 55^o west. Maximum surface fault displacement is 1.9 m, and may be accompanied by subsidence that could move the shoreline of Great Salt Lake into developed areas when the lake level is at or near its historic high.

Peak ground accelerations (PGAs) are highest (up to 1.0 g) in stiff gravels and sands of eastern Salt Lake Valley. Potentially damaging ground motions (greater than 0.1 g) extend up 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 soft, fine-grained soil in central portions of the basin.

Strong ground motions may initiate other geologic hazards. PGAs greater than threshold accelerations make large lateral-spread displacements (perhaps greater than 30 cm) possible in most of Salt Lake Valley, although large displacements are unlikely in all but the most susceptible soils. Smaller, vertical displacements (commonly less than 20 cm) may result from liquefaction-induced settlement. Newmark displacements greater than 10 cm are commonly restricted to slope failures near the fault, with significantly smaller displacements possible on more susceptible but distant slopes. Seiches due to ground shaking are negligible in Great Salt Lake.

Our 1:250,000-scale hazard maps will be used with HAZUS software to estimate losses from the scenario earthquake. The maps can then be used to increase awareness and improve emergency-response planning.