Abstract

Quaternary faults are present in much of western Utah and are concentrated near the eastern edge of the Basin and Range province either in linear, en echelon zones along mountain fronts or in clusters of short segments in basin areas. Faults with clearest evidence for recurrent Holocene movement occur in the Wasatch Front area, although faults with lesser Holocene activity are found in west-central Utah. The age of most recent movement on Quaternary faults in southwestern Utah and west of the Wasatch Front is generally late Pleistocene or older. Faults with evidence for Quaternary movement are generally absent along the western edge of the State. Historical surface faulting in the Great Basin has accompanied all earthquakes greater than magnitude 6.3, and events greater that about magnitude 7 were required to form the meter or more displacements recorded in prehistoric scarps.

Historically, most of Utah’s earthquakes have occurred in the Intermountain Seismic Belt (ISB) which trends north-south through the center of the state from the Wasatch Front to Cedar City and St. George. Several 6.5+ magnitude earthquakes have occurred in the ISB during historical time, with the 1934 Hansel Valley earthquake (M_L 6.6) being the only one in Utah to rupture the surface. No direct relationship between historical seismicity and Quaternary faults is evident in western Utah, and the areas of greatest apparent fault activity, such as the Wasatch Front, generally do not coincide with areas of greatest seismicity.

Both historical seismicity and the geologic record of surface faulting must be considered in evaluating seismic hazards, which include ground shaking, liquefaction, slope failure, surface fault rupture, tectonic subsidence, and flooding. The potential for damage is greatest along the ISB where both the population and seismic hazards are greatest. Hazards generally decrease to the west and south in western Utah. The most widespread hazard is ground shaking because it occurs most often and affects the largest areas. Liquefaction generally requires earthquakes greater than magnitude 5.0, and slope failures may occur during magnitude 4.0 and greater earthquakes.