An estimate of the long-term seismicity of Utah is obtained through the calculation of seismic moment rates from available geologic data on Quaternary faulting. The recurrence rates calculated from these geologic seismic moment rates agree well with recurrence rates estimated from historical earthquake data. A return period of 80 to 190 yr for a magnitude 7.0 to 7.5 earthquake in the Utah region is predicted by historical, seismic, and geologic data.
The estimation of recurrence rates from geologic data requires the use of a moment versus magnitude relationship for earthquakes characteristic of the Utah region. Spectral studies of 19 Utah earthquakes define this relationship. In addition, moment tensors and other source parameters are estimated for these earthquakes. A resultant strain tensor for Utah indicates the amount of horizontal east-west extensional strain is nearly equal to the amount of vertical compressional strain in the region.