Fourteen new fault-plane solutions were determined for earthquakes in the region, and seismicity maps are presented for the periods January 1934 to January 1972 and January 1962 to January 1972. For fault-plane solutions determined for the Commander Islands, the horizontal component of the slip vectors is consistent with relative plate motion occurring parallel to the trend of the Aleutian arc. Along the southern margin of the Commander Islands, the dip of the fault planes along which this motion occurs is nearly horizontal; along the northern margin, the dip is nearly vertical. Fault-plane solutions for earthquakes along the east coast of Kamchatka south of the arc-arc junction indicate northwesterly underthrusting of the Pacific plate. Solutions north of the junction indicate that compressive stress normal to the east coast of Kamchatka continues to 58° or 59° N., but they are not proof that the Kamchatka Basin is underthrusting northeast Kamchatka. The rough basement topography, reduced sediment thickness, and high heat flow in the Kamchatka Basin north of the Commander Islands are the apparent results of a Cenozoic emplacement of basaltic magma. Extensive magmatism in the Kamchatka Basin was initiated 30 to 40 m.y. ago when subduction along the Aleutian arc ceased west of 176° E. The resultant release of compressive stress along the plate margin in the Commander Islands is proposed as the mechanism that mobilized a mantle diapir from an oceanic slab that dipped northward beneath most of the Kamchatka Basin. Migration and intrusions of this diapir may have involved generation of oceanic crust.