The 1976 August 16, Mindanao, Philippine earthquake (Ms = 7.8) - evidence for a subduction zone south of Mindanao

Summary. The Philippine earthquake of 1976 August 16, is one of the largest to have occurred world-wide in recent years (Mw=8.8; Ms=7.8; seismic moment, Mo= 1.9 × 1028dyne-cm). It is, however, associated not with the Philippine Trench, which is the dominant tectonic feature along the eastern Philippine Islands, but with a much less prominent trench system in the Moro Gulf, North Celebes Sea, south of Mindanao. In this area most of the seismicity is at depths greater than 500 km, associated with the westward dipping Benioff zones of the Sangihe and Mindanao arc systems. This event, however, has a shallow focus and caused a locally destructive tsunami. The focal mechanism of the mainshock determined in this study from long-period surface and body waves indicates a predominantly thrust mechanism with strike N 33°W, dip 22° NE and rake + 68°. A significant amount of directivity, which can be seen in the observed surface wave seismograms, is explained very well if the source rupture propagates 160 km unilaterally in an azimuth of 300° from the mainshock hypocentre, with rupture velocity 2.5 km/s. The largest aftershock (Ms= 6.8) occurred outside the main aftershock area 12 hr following the mainshock and apparently resulted from motion on a subsidiary fault since the P-wave first motion data indicate strike-slip motion for this event. Bathymetric data indicate the presence of a trench striking north-south in the region of the Moro Gulf, and seismic reflection profiling indicates disturbed sediments east of the trench showing evidence for subduction. In addition, the geological structures mapped on the island of Mindanao are consistent with this mode of deformation. The only other known large earthquake in the region on 1918 August 15 (Ms=8.0) probably occurred along the same subduction zone, on an adjacent segment, to the south of the recent event. The 1976 August 16 Philippine earthquake thus represents the first clear seismic evidence for a north-east dipping subduction zone beneath Mindanao in the Moro Gulf, North Celebes Sea.