The U. S. Geological Survey, Menlo Park, deployed and maintained a network of twelve digital instruments over the 2 weeks following the October 28, 1983, Borah Peak, Idaho, earthquake. The network recorded 45 events with M greater than equivalent to 3. 0, and 6 events with M less than equivalent to 4. 0. The seismic moments of the aftershocks increase with increasing hypocentral depth below 12 km. The dynamic stress drops of the events do not show any systematic variation with depth, however. Most of the events with large stress drops are clustered in the northwest limb of the aftershock distribution; the average stress drop of the southern events is 31 plus or minus 16 bars, while the average stress drop of the events in the northwest limb is 77 plus or minus 52 bars. This clustering of events with large stress drops marks an apparent stress concentration, possibly associated with the arrest of the main shock rupture propagation by a fracture barrier at depth.
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CHARACTERISTICS OF THE AFTERSHOCK SEQUENCE OF THE BORAH PEAK, IDAHO, EARTHQUAKE DETERMINED FROM DIGITAL RECORDINGS OF THE EVENTS.