The August 1959 (MS=7.5) Hebgen Lake, Montana, earthquake is the largest earthquake to have occurred in the intermountain region in historic time. Studies of waveforms at regional and teleseimic distances indicate that the main shock of the sequence was a double event consisting of a shock of mb=6.3 (M0=3¿1018N m) followed 5 s later by one of mb=7.0 (M0=1¿1020Nm). Comparisons between fault plane solutions from short-period first motion data, seismic moment tensors determined from the inversion of long-period body wave data, and observed surface faulting indicate that rupture occurred along one or more fault planes with strike orientations slightly discordant with the trace of surface faulting. A close association between the surface scarps and Laramide thrust faults also suggest that the events may represent normal faulting along reactivated older thrust faults. Focal mechanisms from short-period first motion data for aftershocks with MS>5.5 in northwestern Yellowstone National Park showed strike-slip, normal, and reverse mechanisms with a variety of nodal plane orientations that reflect the complex tectonics of the Yellowstone Plateau. Limited focal depth information suggests a decrease in the maximum focal depth of earthquakes from 15 km at Hebgen Lake to 6-10 km in northwestern Yellowstone National Park.