The 28 October 1983 Borah Peak, Idaho, earthquake (MS = 7.3) occurred in an area of low historic seismicity within east-central Idaho along a segment of the Lost River fault active during the Holocene. A dense network of portable short-period seismographs (up to 45 stations, station spacings of 2 to 10 km) was installed beginning several hours after the main shock and operated for 22 days. In addition to records from the portable instrumentation, data from permanent seismograph stations operating in Idaho, Utah, Montana, Oregon, Washington, and Wyoming provide a good regional data base. No foreshock activity above MC (coda magnitude) 2.0 was detected for the 2-month period preceding the main shock. The epicenter of the main shock is ∼ 14 km south-southwest of the end of the surface faulting. This relationship suggests unilateral rupture propagating to the northwest. The distribution of 421 aftershocks of MC > 2 defines an epicentral pattern, 75 km × 10 km, trending north-northwest parallel to the surface rupture but displaced laterally southwest by 5 to 10 km. Aftershocks extend to depths of approximately 16 km and in the central and southeastern portion of the aftershock pattern define a zone, dipping approximately 45° southwest, that intersects the surface near the fault scarp. The entire aftershock zone as observed during the first 3 weeks was active shortly after the main shock occurred.
Fault plane solutions for 47 aftershocks show predominantly normal faulting with varying amounts of strike-slip motion. Tension axis orientations indicate a dominant extension direction of NNE-SSW during the aftershock sequence. There is considerable diversity among the aftershock focal mechanisms, even along the central and southeast portions of the fault where the hypocenters appear to outline the main fault break. We therefore interpret most of the aftershocks to represent complex fracturing on subsidiary structures adjacent to the main fault.