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Stress Triggering of Conjugate Normal Faulting: Late Aftershocks of the 1983 M_s 7.3 Borah Peak, Idaho, Earthquake

Idaho National Engineering and Environmental Laboratory
PO Box 1625
Idaho Falls, Idaho 83415-2025
msj1{at}inel.gov
(S.J.P)

Boise State University
1910 University Drive
Boise, Idaho 83725
jzollweg{at}hotmail.com
(J.E.Z.)

Idaho State University
Pocatello, ID 83209
rodgdavi{at}isu.edu
(D.W.R)

The 1984 Devil Canyon sequence was a late aftershock sequence of the 28 October 1983 M_s 7.3 Borah Peak, Idaho, earthquake. The sequence began on 22 August 1984 with the M_L 5.8 Devil Canyon earthquake, which nucleated at a depth of 12.8 ± 0.7 km between the surface traces of two normal faults, the Challis segment of the Lost River fault and the Lone Pine fault. Two hundred thirty-seven aftershocks were recorded by a temporary array during a 3-week period. Their focal mechanisms and hypocenter distribution define a cross-sectional "V" pattern whose base corresponds to the M_L 5.8 event, whose tips correspond to the exposed fault traces, and whose sides define two planar fault zones oriented N25°W, 75°SW (Challis fault segment) and N39°W, 58°NE (Lone Pine fault). This pattern describes a graben bounded by conjugate normal faults. Temporal aspects of the Devil Canyon sequence provide strong evidence that slip on conjugate normal faults occurs sequentially. Aftershocks occurred primarily along the Challis segment until the occurrence of the 8 September 1984 M_L 5.0 earthquake along the Lone Pine fault, after which aftershocks primarily occurred along this fault. These observations are consistent with worldwide seismologic and geologic observations and with physical and numerical models of conjugate normal faulting. Aftershocks of the Devil Canyon sequence occurred immediately northwest of the M_L 5.8 Devils Canyon earthquake, which itself was immediately northwest of the Thousand Springs segment of the Lost River fault (the fault that slipped in association with the M_s 7.3 Borah Peak earthquake). Coulomb failure stress analysis indicates that stress increases resulting from both the Borah Peak mainshock and Devil Canyon M_L 5.8 earthquake were sufficient to induce failure on the Lone Pine fault. These space–time patterns suggest that conjugate normal faults may transfer stress or accommodate stress changes at the terminations of major normal faults in the Basin and Range Province.