Waveform modeling and first motion analysis are used to determine the source parameters of six 5.8 ≦ M ≦ 6.8 earthquakes that occurred between 1925 and 1964 within the northern Intermountain Seismic Belt of Montana. Results of this study suggest that the 1925 Clarkston earthquake occurred along an oblique normal fault with a trend similar to the southern end of the Clarkston Valley fault. The two largest earthquakes of the 1935 Helena sequence occurred along right-lateral strike-slip faults with trends similar to the Bald Butte and Helena Valley faults. The 1947 Virginia City earthquake occurred along a northwest-southeast trending segment of the Madison fault. Movement at depth was along a fault with strike similar to that of the 1959 Hebgen Lake main shock. A reanalysis of a M = 6.0 aftershock of the 1959 Hebgen Lake sequence suggests the earthquake occurred at a depth of 8 km along a fault that is not seen at the surface. An M = 5.8 earthquake in 1964, located about 10 km from the 1959 aftershock, may have occurred along steeply dipping fault planes (48° to 80°) at depths of 8 to 14 km. Most events could be modeled as simple ruptures.