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

Abstract

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.

The spatial distribution of earthquakes, focal mechanism solutions, and subducted lithosphere in the Philippine and Northeastern Indonesian Islands

“…Aftershocks clearly define the underthrust plane in section E of Figure 5. Stewart and Cohn [1977] suggested that the main aftershock (event 48 in Figure 6) can be interpreted as tear faulting at the northwestern end of the Cotabato Trench, although the geometry implied by this is not clear. Further south along the trench and beneath the Sangihe Ridge are two high-angle thrust faults (events 64 and 67 in Figure 6) that could be interpreted as the initiation of eastward underthrusting, but the lack of a seismic zone makes this interpretation uncertain.…”

“…The lack of an active volcanic arc, along with the short length of the inclined seismic zone, suggest that subduction began relatively recently. Stewart and Cohn [1977] and Acharya [1978] demonstrated that eastward directed underthrusting occurred in the large (M., = 7.8) earthquake of 1976 (event 58 in Figure 6 and in section E of Figure 5) . Aftershocks clearly define the underthrust plane in section E of Figure 5.…”

“…A zone of earthquakes less than 200 km deep, lying west of and parallel to the Philippine Trench, represents subducted lithosphere of the Philippine Basin. From beneath the center of the Molucca Sea there is a third zone of mantle earthquakes that [1970] , SK is Seno and Kurita [1978] , and SC is Stewart and Cohn [1977] . The numbers after the letters are the authors' original numbers for the earthquakes.…”

“…The Sulu Ridge separates the Cotabato Trench from the Negros Trench. The Cotabato Trench is a west-facing subduction system along southwest Mindanao and the western side of the Sangihe Arc [Stewart and Cohn, 1977;Hamilton, 1979] .…”

“…Rowlett and Kelleher [1976] made a special study of the large earthquakes in the Philippine region, and Kelleher and McCann [1976] studied the large earthquakes in the Indonesian region. Both Stewart and Cohn [1977] and Acharya [1978] investigated the large = 7.8) earthquake of August 1976 that occurred in the Cotabato Trench off southwestern Mindanao. Recently Seno and Kurita [1978] and Wu [1978] published earthquake focal mechanism solutions, seismicity maps, and cross sections of earthquake hypocenters for the Philippine-Taiwan region.…”