Shiveluch volcano (Kamchatka, Russia) is an active andesitic volcano with a history of explosive activity, dome extrusion, and structural collapse during the Holocene. The most recent major (>1 km3) dome collapse occurred in November 1964, producing a ~1.5 km3 debris avalanche that traveled over 15 km from the vent and triggered a phreatic explosion followed by a voluminous (~0.8 km3) eruption of juvenile pyroclastic material. Seismic records suggest that the collapse was likely triggered by a magnitude 5.1 earthquake. The geomechanical properties of the pre-1964 dome are unknown; accordingly, the mechanics of the collapse are poorly understood. Here, we employ slope stability modeling using the finite element method to constrain probable ranges of geomechanical properties for the materials involved in the collapse, considering earthquake loading as the most likely triggering mechanism. Model results show good agreement with the 1964 collapse geometry considering Geological Strength Index and seismic coefficient ranges of 30 to 60 and 0.05 to 0.15 g, respectively, representing variably fractured and altered dome material under moderate earthquake loading. Deep-seated rotation is the dominant failure mode, but local extension within the dome appears to play an important role in the development of the collapse.