Abstract

We deduce the fault geometry, coseismic slip, and moment for two of the largest historic earthquakes that have occurred in the Basin and Range of the Western United States: the M = 7.3 1959 Hebgen Lake, Montana, earthquake and the M = 6.9 1983 Borah Peak, Idaho, event. Newly augmented data sets of vertical deformation from geodetic leveling and from lake shoreline changes were modeled by simple dislocations in an elastic half-space. The rms signal-to-noise ratio is 12 for the Hebgen Lake data and 38 for the Borah Peak set. The residuals for both models are about twice as large as the noise. The Hebgen Lake earthquake struck on the 15- to 25-km-long en-echelon Hebgen and Red Canyon faults, dipping 45° to 50° and extending to a depth of 10 to 15 km. The 7.0 and 7.8 m of dip-slip on these faults produced a combined moment of 1.2 × 1020 N-m. The dip of the Red Canyon fault may decrease slightly with depth (in a listric manner), abutting the planar Hebgen fault at a depth of 8 km. In addition, up to 1 m of deep slip occurred on the Holocene segment of the adjacent Madison Range fault, 10 km west of the Hebgen fault. The Borah Peak segment of the Lost River fault was found to dip 49°. Slip of 2.1 m occurred at the south fault end, extending to a depth of 14 km; 1.4 m of slip occurred at the north end, where the fault reached only to 6 km depth. A listric fault shape is not permitted by the geodetic data at Borah Peak. Both the Hebgen-Red Canyon and the Lost River faults are high-angle and nearly planar, despite the much greater age and length of the Lost River fault in comparison to the en-echelon Hebgen faults. The chief difference between the earthquakes is the 3- to 4-fold higher slip at Hebgen relative to Borah Peak and all other well-studied Basin and Range shocks. Thrust faults located close to these active normal faults must either dip steeply at depth or were not reactivated.

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