Earthquakes in the Area of Coal-Mining-Induced Seismicity

Areas of underground coal mining in central Utah are the sites of mining-induced earthquakes caused by rockbursts and collapses in the mines, extensional subsidence above mine workings, and stress relief along pre-existing faults in the rock mass immediately surrounding the mine workings. Earthquakes coincide with the locations of active mines along the eastern Wasatch Plateau and western Book Cliffs. Earthquakes are generally less than M3, and most are less than M2.5. Two of the largest earthquakes were M3.8 in 1998 and 4.2 in 2000. A mining-induced earthquake of M3.9 in 2007 in the Crandall Canyon mine southwest of Price killed six miners. The western part of this area of mining-induced seismicity is within the Intermountain seismic belt (ISB); the eastern part is within the Colorado Plateau.

Because of its proximity to the ISB, expected levels of strong ground shaking are relatively high compared to the Colorado Plateau to the east. Also, the stable, undeformed bedrock at the ground surface over much of the area causes ground shaking to be felt over a much larger area than for a similar magnitude earthquake in western Utah.

Faults

The western part of the area within the ISB includes several geologically young faults in the Wasatch Plateau. The tectonic significance of these faults is uncertain because they bound north-trending down-dropped blocks with little net tectonic displacement across the fault pairs. The eastern part of this area is characterized by the stable tectonics of the Colorado Plateau and contains no recognized geologically young faults.

Nearly all earthquakes within the outlined area of coal-mining-induced seismicity are mining-related, particularly those at and above the mines. Attempts to discriminate natural earthquakes within this area have found very few that arguably might have a tectonic origin. Exceptions include a June 2, 1996 M2.9 earthquake with a normal-faulting mechanism that occurred 11 km (7 miles) deep beneath the Wasatch Plateau and the 1988 M5.2 San Rafael Swell earthquake, which occurred in the adjacent Colorado Plateau to the west less than 12 km (7.5 miles) outside of the boundary of the area of coal-mining-induced seismicity.