Earthquakes in the Intermountain Seismic Belt (Southern Utah)

Intermountain Seismic Belt, Southern Utah

The Intermountain seismic belt (ISB) is a prominent north-south-trending zone of recorded seismicity in the Intermountain West. A modern catalog of instrumentally located earthquakes in Utah begins in mid-1962, and historical earthquake records date back to the 1850s. The ISB in southern Utah is characterized by scattered seismicity with locally dense clusters of small- to moderate-sized earthquakes. The largest earthquake in the ISB in southern Utah was a M6.5 earthquake in 1901 in Richfield. A group of three M5 and 6 earthquakes occurred in Elsinore in the Sevier Valley in 1921. To the south in southwestern Utah, a damaging earthquake (M5.9) occurred in 1992 near St. George. Earthquake swarms (clusters of earthquakes with no outstanding main shocks) of maximum magnitude 3 to 4 are common in the area.

Because moderate and large earthquakes are likely, expected levels of strong ground shaking are relatively high. Ground shaking from a M5 earthquake can cause significant damage at distances up to 10 km (6 miles), and a M7 at distances up to 50 km (30 miles) and more. Geologic site conditions such as deep sediment-filled basins may locally amplify and prolong ground shaking.

Faults

The ISB in southern Utah coincides with a transition between east-west-directed stretching in the Basin and Range to the west and more stable crust of the Colorado Plateau to the east. Tectonic movement on generally north-trending, east- and west-dipping range- and plateau-bounding normal faults, which results in horizontal extension, characterizes this part of Utah. The Sevier Valley is an area of variable and complex deformation involving significant components of folding and both normal and strike-slip faulting. The most prominent geologically young faults in southwestern Utah are the Hurricane and Sevier faults. The Hurricane fault forms the west-facing Hurricane Cliffs, which define the eastern edge of the Basin and Range within the ISB. Faults in the ISB in southern Utah locally show evidence of displacement younger than 10,000 years, but average recurrence intervals are generally longer than those on faults in the ISB in northern Utah. Recurrence intervals for surface faulting on the most active segments of ISB faults in southern Utah are generally many thousand to tens of thousands of years.

Moderate to large earthquakes in the Sevier Valley area and other small to moderate earthquakes in the ISB in southern Utah generally cannot be correlated with particular mapped faults, although the 1992 M5.9 St. George earthquake may have occurred on the west-dipping Hurricane fault.