Abstract

We describe the seismicity of central Utah using a recently revised catalog of earthquakes (M ≤ 5.2) based on regional seismic monitoring by the University of Utah from 1981 through mid-2006. Natural seismicity in central Utah, including 16 shocks of M ≥ 5.0 since 1850, occurs chiefly within a roughly 100-km-wide zone forming part of the Intermountain seismic belt. The seismic belt follows the north-northeasterly-trending Basin and Range—Colorado Plateau transition in the northern half of the study area but swings westward south of ~39° N., tracking with the eastern (foreland) part of the ancestral Sevier thrust belt. In the eastern part of the study area, an arcuate belt of intensely clustered mining seismicity (M ≤ 4.2) is associated with underground extraction in the Wasatch Plateau—Book Cliffs coalfields. The correlation of background seismicity with geologic structure in the study area is well known to be problematic; in this study, focal-depth resolution is handicapped by an average station spacing of 40–50 km. Although less than 10% of the natural earthquakes in the catalog have well-constrained focal depths, there are evident variations in maximum focal depths, with the deepest foci (15–25 km) occurring beneath the Basin and Range—Colorado Plateau transition, notably beneath the Wasatch Plateau. Normal- to strike-slip earthquake focal mechanisms predominantly reflect WNW—ESE extension across the transition. In at least two localities, focal mechanisms indicate normal faulting in the upper crust that contrasts with oblique to pure strike-slip faulting in subjacent lower crust. Two new observations in the study area warrant future attention. First, there is an unusually high average b-value (slope of earthquake frequency vs. magnitude) of 1.10 ± 0.07 for independent mainshocks in the northern subregion of the main seismic belt that is significantly higher than the average value of 0.79 ± 0.04 in the southern part; the latter is more consistent with the historical earthquake record. Second, the spatial distribution of earthquake swarm sequences, often associated with the movement of hydrothermal or magmatic fluid, shows a significant concentration of such sequences in an east-northeast-trending zone along the northern margin of the Marysvale volcanic field (just north of the axis of the Cove Fort transverse structure).