The Mindanao earthquake of August 16, 1976, caused considerable destruction of life, primarily due to the tsunami. The earthquake occurred in an area of high seismicity and poorly understood tectonics. P-wave first-motion study indicates that the fault movement was of the thrust type with the fault plane trending northwest (N25°W) and dipping 24°NE. The aftershocks of this earthquake trend northwest. The location of the earthquake on the inner wall of the Celebes Sea Trench indicates its association with underthrusting. The extent of this zone of underthrusting is, however, not clear.