Modeling of long-period body waves of the MS = 7.3 Borah Peak, Idaho, earthquake suggests that the earthquake was a simple rupture that nucleated at a depth of about 16 km and propagated unilaterally northwestward toward the surface. The seismic moment tensor obtained from the inversion of teleseismic body wave amplitude data agrees well with the fault plane solution obtained from short-period first motion data (strike, 138 ± 3°; dip, 45 ± 3°; rake, −60 ± 5°) and the observed surface faulting. The scalar double couple moment of 2.1 × 1026 dyne-cm obtained from the inversion is comparable to a moment of 1.4 × 1026 dyne-cm estimated from the observed surface faulting. Estimates of stress drop based on these values for the moment are 17 to 12 bars, respectively.

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