Title

Seismic Evidence of Conjugate Normal Faulting: The 1984 Devil Canyon Earthquake Sequence Near Challis, Idaho

Publication Date

8-1994

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Geophysics

Department

Geosciences

Major Advisor

James E. Zollweg

Abstract

Aftershock hypocenters of the 1984 Devil Canyon, Idaho earthquake indicate the sequence was associated with conjugate normal faulting on two northwest-striking normal faults that bound the Warm Spring Creek graben. The ML 5.8 mainshock occurred on August 22, 1984 and is considered a late aftershock of the MS 7.3 Borah Peak earthquake of October 28, 1983, which ruptured portions of the Lost River and Lone Pine faults. Focal mechanisms and the distribution of aftershock hypocenters suggest that the seismogenic part of the Challis segment of the Lost River fault has a planar geometry which strikes N 25° W and dips 75° SW, and the Lone Pine fault has a planar geometry which strikes N 39° W and dips 58° NE. Focal mechanisms indicate that the earthquakes on the Challis segment and Lone Pine fault were caused by predominantly normal faulting with minor components of left-lateral strike-slip. Their T-axes are consistent with the northeast-southwest extensional direction of the local Basin and Range province.

The mainshock nucleated at a depth of 12.8 ± 0.7 km on the Challis segment where the conjugate faults merge. The largest aftershock, ML 5.0 on September 8, 1984, nucleated at a shallower depth of 7.1 ± 1.9 km on the Lone Pine fault. The Devil Canyon mainshock is interpreted to have ruptured unilaterally upward and to the northwest along the Challis segment of the Lost River fault system. Rupture dimensions inferred from the aftershock distribution indicate the mainshock had a static stress drop of 14 bars. The largest aftershock may have ruptured upward and to the northwest along the Lone Pine fault. It is suggested that stress changes resulting from the rupture of the ML 5.8 mainshock along the Challis segment increased shear stresses on the Lone Pine fault which was probably close to its yield stress. This increase caused the ML 5.0 primary aftershock and most of its subsequent aftershocks to occur along the Lone Pine fault.

Characteristics of the conjugate normal faulting observed in the Devil Canyon sequence consist of normal faults that have their own orientations and sense of slip, but are mechanically coupled leading to a cause-and-effect relationship for contemporaneous slip along each fault. Similar characteristics are observed in other conjugate normal faulting sequences, as well as strike-slip and thrust conjugate faulting sequences worldwide. Comparison to these other conjugate faulting sequences suggests that the direction of rupture, location of the initiating earthquake, and the stress level on pre-existing faults may be important factors as to whether the conjugate fault becomes active.

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