M 2.8 - 5 km N of Rangely, Colorado

Earthquakes in the Western Colorado Plateau

The western Colorado Plateau in southeastern Utah is an area of low seismicity, and historical earthquakes have generally been small (M2-4) and widely distributed. The highest concentration of seismicity is related to underground coal mining along the northwestern edge of the western Colorado Plateau. Regional network instrumentation in the Colorado Plateau outside the coal-mining area is sparse, so the record, particularly for smaller earthquakes, may be incomplete. Another part of the Colorado Plateau where induced seismicity has occurred is the Rangely oil field in northwestern Colorado, close to the Utah border, where earthquakes as large as M4.5 were triggered in the 1960s by fluid injection. The largest historical earthquake in the Colorado Plateau in Utah was the 1988 M5.2 San Rafael Swell earthquake near Castle Dale south of Price. However, larger earthquakes (M5-6) have occurred in northern Arizona.

Because moderate and large earthquakes are rare, expected levels of strong ground shaking are relatively low. However, the stable, undeformed bedrock at the ground surface over much of the Colorado Plateau causes ground shaking to be felt and damage to occur over a much larger area than for a similar magnitude earthquake in western Utah. Ground shaking from the M5.2 San Rafael Swell earthquake near Castle Dale was felt as far away as Golden, Colorado (475 km, 295 miles) to the east, but only as far as Delta, Utah (156 km, 97 miles) to the west in the Basin and Range.

Faults

The Colorado Plateau is a large, stable block of the North American tectonic plate that extends across much of eastern Utah and western Colorado and is characterized by generally flat-lying and locally folded sedimentary rocks. It was uplifted relative to adjacent areas during Cenozoic time, but is relatively unaffected by faulting in the past 2 million years. The most recent movement on geologically young faults in the area is generally greater than 150,000 years and little evidence exists for younger movement. Although the predominant mode of deformation in the western Colorado Plateau is faulting on northwest-trending faults in response to approximate northeast-southwest stretching, data from the 1988 San Rafael Swell earthquake implies a response to stretching in a roughly east-west direction, similar to the tectonic stresses in the nearby Basin and Range and Intermountain seismic belt to the west.

The few moderate-magnitude earthquakes in the area have not been attributable to mapped geologically young faults, and most earthquakes likely occur along deeply buried pre-existing faults lacking surficial expression.