Abstract

Parts of southwestern Utah lie within the southern Intermountain seismic belt (ISB), a zone of shallow, diffuse seismicity which in southwestern Utah trends from Scipio southwest through Richfield, Cedar City, and St. George. Historical earthquakes have reached magnitude 6 to 6 1/2, mostly near Richfield and Elsinore. Geologic studies indicate that late Quaternary deformation (faults, folds, uplift) has occurred in southwestern Utah, and many potentially seismogenic geologic structures are present. Prehistoric earthquakes in the magnitude 7 to 7 1/2 range have occurred throughout the area and could occur in the future.

The principal earthquake hazards in southwestern Utah are ground shaking, surface fault rupture, tectonic subsidence, liquefaction, slope failure, and flooding. Strong ground shaking is the most widespread and frequently occurring of these hazards. The strongest shaking is expected within the ISB in the eastern and southern parts of the study area. Faults potentially capable of rupturing the surface, with accompanying tectonic subsidence and strong ground shaking, are found throughout the area, but are concentrated in the ISB. Areas of highest liquefaction potential are along stream-valley bottoms, particularly along the Sevier and Virgin Rivers. Slope failures, including rock falls and landslides, and flooding of various types, may also accompany moderate to large earthquakes in the area.