We use waveform cross-correlation to study clustering of small earthquakes that occurred from 1981 to 1983 within 20 km of (1) an ML 4.3 main shock near Salt Lake City, Utah, on 8 October 1983, and (2) an ML 4.0 main shock near Richfield, Utah, on 24 May 1982. No clusters of earthquakes were observed prior to the Salt Lake City event during the time period studied. However, a cluster of four nearly identical preshocks within 80 m of each other, possibly representing the failure of a critical asperity, occurred 5 km northwest of the Richfield event during a 4-hour period 10 months before the main shock. We found no similar clusters among the other preshocks within the Richfield study area, all of which were located more than 10 km from the main shock.
The aftershocks in both study regions during the first 17 days after the respective main shocks occupied areas smaller than the rupture areas estimated for the main shocks from initial pulse width measurements. Both aftershock sequences showed a spatial migration of the initial events that could only be discerned from the waveform data. One possible explanation for this migration could be propagating stress changes caused by the occurrence of the aftershocks themselves or by postseismic creep.