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

The 1984 Devil Canyon sequence was a late aftershock sequence of the 28 October 1983 Ms 7.3 Borah Peak, Idaho, earthquake. The sequence began on 22 August 1984 with the ML 5.8 Devil Canyon earthquake, which nucleated at a depth of 12.8 (plus-minus) 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 ML 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 ML 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 ML 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 Ms 7.3 Borah Peak earthquake). Coulomb failure stress analysis indicates that stress increases resulting from both the Borah Peak mainshock and Devil Canyon ML 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.