Contemporary tectonics of the Wasatch front region, Utah, from earthquake focal mechanisms

We have completed a comprehensive study of focal mechanisms of digitally recorded earthquakes (M, -_< 4.4) that occurred in the Wasatch front region in Utah during 1980 to 1986. Single-event solutions for 24 events were determined using recently revised crustal models and a computerized grid-search technique. Overall, the mechanisms show predominantly normal faulting on N-S-striking nodal planes of moderate to steep dip (>30°). Tension-axis azimuths average 96 ° _+ 12% Thus, in general, the mechanisms indicate E-W to ESE-WNW crustal extension and vertical crustal shortening. Oblique slip, when observed, is characterized by left-lateral motion on planes striking N to NE or right-lateral motion on planes striking N to NW. Most of the mechanisms with significant amounts of oblique-slip motion occur in the southern part of the study area, where compression- axis orientations range from near vertical to near horizontal. Thus, the mechanisms suggest a possible change in stress regime from north to south along the Wasatch front. Despite geologic evidence for low-angle faults in the study area, shallowly dipping nodal planes are relatively uncommon.