Contemporary seismicity, faulting, and the state of stress in the Colorado Plateau

Abstract

The contemporary seismicity of the Colorado Plateau based on seismic monitoring in the past 30 yr can be characterized as being of small to moderate magnitude, and contrary to earlier views, of a low to moderate rate of occurrence with earthquakes widely distributed. Concentrations of earthquakes have been observed in a few areas of the plateau. The most seismically active area of the Colorado Plateau is the eastern Wasatch Plateau-Book Cliffs, where abundant small-magnitude seismicity is induced by coal mining. The largest earthquakes observed to date, of estimated Richter magnitude (M L ) 5-6, have generally occurred in northern Arizona. Although very few earthquakes can be associated with known geologic structures or tectonic features in the Colorado Plateau, seismicity appears to be the result of the reactivation of pre-existing faults lacking surficial expression but favorably oriented to the tectonic stress field. The small to moderate size of the earthquakes and their widespread distribution are consistent with a highly faulted Precambrian basement and upper crust, and a moderate level of differential tectonic stress. Earthquakes in the plateau generally occur in the upper crust from the near-surface to a depth of 15-20 km, although events have been observed in both the lower crust and uppermost mantle in areas of low to normal heat flow. The latter suggests that temperatures are sufficiently low at these depths that brittle failure and hence earthquakes are still possible. The predominant mode of tectonic deformation within the plateau appears to be normal faulting on northwest- to north-northwest-striking faults with some localized occurrences of strike-slip faulting on north-west- or northeast-striking planes at shallow depths. The contemporary state of stress within the plateau is characterized by approximate northeast-trending extension in contrast to the previous belief that the plateau was being subjected to east-west tectonic compression. One area of the plateau, the eastern Wasatch Plateau and Book Cliffs, may still be characterized by compressive stresses; however, the nature of these stresses is not well understood.