M 2.0 - 30 km WNW of Naturita, Colorado

Earthquakes in the Western Paradox Basin

The Paradox Basin is within the Colorado Plateau and has a low rate of seismicity. Recorded earthquakes have generally been less than M2. Although regional seismograph networks operate few instruments in the Paradox Basin, much information on earthquakes in the area was obtained from a temporary network operated from 1979-1987. The most seismically active area is near the Cane Creek potash mine west of Moab; the two largest earthquakes in this area of mining-induced seismicity were M3.3 and 3.0 in 1984. Damaging earthquakes are rare in the western Paradox Basin. The largest historical earthquakes were a M4.4 in 2000 near the Paradox Valley in western Colorado and a M4.3 in 1953 near Green River, Utah. The M4.4 earthquake in 2000 and many smaller earthquakes in the Paradox Valley were induced by brine injection in deep wells.

Because moderate and large earthquakes are rare, expected levels of strong ground shaking are low. However, bedrock at the ground surface over much of the Paradox Basin causes ground shaking to be felt over a much larger area than for a similar magnitude earthquake in western Utah.

Faults

The Paradox Basin is the area underlain by thick beds of salt in the Pennsylvanian-age Paradox Formation. In contrast to most of the Colorado Plateau, the Paradox Basin is structurally complex resulting from movement and dissolution of Paradox Formation salt. The northern part of the area around Moab is a fold and fault belt characterized by northwest-trending faults with evidence for geologically young displacement. These high-angle, valley-bounding faults are associated with collapse due to salt dissolution along the crests of large salt anticlines, cored by mobilized upwelling plumes of salt from the underlying Paradox Formation. Studies of small earthquakes in the area indicate the tectonic setting is one of northeast-directed stretching.

Earthquakes in the area generally do not coincide with mapped faults, and many are mining-induced. Although the salt anticlines and faults are favorably oriented with respect to the modern tectonic stresses, geologically young faulting is due to dissolution-related collapse or remobilization of salt, which likely does not cause moderate or large earthquakes and strong ground shaking. However, the deeper-seated Moab fault extends below the Paradox Formation and may experience tectonic displacements.