OVERVIEW OF SEISMICITY IN UTAH RELEVANT TO SEISMOTECTONICS AND EARTHQUAKE HAZARDS

We review the first-order features of Utah's seismicity, both natural and human-triggered, using a refined earthquake catalog (1981–2009; 39,000 events ≤ M 5.9) based on regional seismic monitoring by the University of Utah. Utah is transected by a curvilinear, northerly-trending zone of concentrated seismicity ~100–150 km wide forming part of the Intermountain Seismic Belt (ISB). The correlation of background seismicity with mapped geologic structures in the region is well known to be problematic, even where fine-scale spatial resolution has been achieved using temporary portable seismic arrays. The regional map pattern of seismicity appears to be fundamentally influenced by lateral changes in lithospheric properties and by pre-Cenozoic thrust-belt structure. In east-central Utah, abundant seismicity (M ≤ 4.2) is induced by underground coal mining.

Although only 12% of the natural earthquake locations in the study catalog have well-constrained focal depths, there are evident variations in maximum focal depths. Notable examples are in central Utah, where the deepest foci (15-25 km) occur beneath and east of the Basin and Range-Colorado Plateau transition, and in northern Utah, where the deepest foci (15–30 km) predominate within a prominent north-northwest-trending zone of seismicity parallel to and roughly centered 20–30 km east of the Wasatch fault. Numerous moment tensors, routinely determined since the 1990s for earthquakes of M > 3.5 in the region, are consistent with roughly east-west crustal extension. Normal-faulting and strike-slip focal mechanisms predominate, in some cases with a suggestion of depth-varying change in stress state. Earthquake swarm sequences, with the largest event in an individual sequence in the upper M 4 range or less, commonly occur in central and southwestern Utah. A systematic statewide analysis of swarm sequences shows a significant concentration in an east-northeast-trending zone along the northern margin of the Marysvale volcanic field. Results of interest from regional and fault-specific recurrence modeling include: (1) an unusually high b-value (slope of earthquake frequency vs. magnitude) along the ISB between ~39 deg and 40 deg N latitude and (2) consistency of available data with either a characteristic or maximum-magnitude model for the Wasatch fault.