Planar high‐angle faulting in the basin and range: Geodetic analysis of the 1983 Borah Peak, Idaho, earthquake

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Abstract
Geodetic elevation changes record the broad‐scale deformation associated with the M = 7.0 October 28, 1983, Borah Peak, Idaho, earthquake on the Lost River fault. The crest of the Lost River Range rose 0.2 m, and adjacent Thousand Springs Valley subsided 1.0 m, in relation to reference points 45 km from the main shock epicenter. The deformation was modeled by dislocations in an elastic half‐space. A planar fault with uniform dip slip of 2.05±0.10 m, dipping 47°±2°SW and extending to a vertical depth of 13.3±1.2 km, fits the geodetic data best and is also consistent with the main shock hypocenter and fault plane solution. The geodetic moment is 2.6±0.5×1019 N m (2.6±0.5×1026 dyn cm), and the estimated static stress drop is 2.9±0.4 MPa (29±4 bars). Tests for coseismic slip on listric faults (which flatten with depth) and on detachments (horizontal faults or shear zones) showed fits to the geodetic data that are inferior to those for planar high‐angle faults. No detectable coseismic slip occurred on the Mesozoic White Knob thrust fault, although the low‐angle thrust sheet intersects the south end of Lost River fault near the 1983 mainshock epicenter. If the high‐angle Lost River fault abuts a flat‐lying detachment fault or shear zone, such a structure must lie at depths of >12 km, near the brittle‐ductile transition, where stick‐slip behavior gives way to creep. The depth and geometry of faulting at Borah Peak is similar to that inferred from seismic and geodetic evidence for the 1954 M = 7.2 Fairview Peak, Nevada, and the 1959 M = 7.3 Hebgen Lake, Montana, events, suggesting that if detachments are active at these localities, they are deep and most likely slip by creep.