The large Mindanao earthquake (Mw=8.1) of August 16, 1976, presents a complex rupture history. The epicenter of this earthquake is located at the southern end of the 160¿80 km2 aftershock area, and the thrust mechanism with a shallow NE dipping plane indicates the subduction of the North Celebes Sea beneath Mindanao. We have characterized both temporally and spatially the rupture process of this event by deconvolving the source time functions from long-period P-wave seismograms at 20 azimuthally well-distributed stations.

The seismic moment is released in a jagged fashion in two main pulses. The observable directivity associated with these two pulses of moment release defines three segments on the fault: (1) from 0 to 54 km N-NW of the epicenter with ≈1/2 of the moment release and an apparent rupture velocity of 2.0 km/s, (2) from 54 to 72 km N-NW of the epicenter with low to no resolvable moment release and an apparent rupture velocity of 0.8 km/s, and (3) from 72 to 157 km N-NW of the epicenter with ≈1/2 of the moment release and a rupture velocity of 3.3 km/s. Although the overall moment release is comparable in the first and second pulses, the second pulse has a substantially higher rupture velocity and a higher level of short-period radiation. The sharp truncation of the second pulse generates the largest amplitudes in the P-wave seismograms and is interpreted as the abrupt termination of the rupture front against Mindanao Island, 160 km N-NW of the epicenter. An additional third pulse of moment release after the dominant truncation is identified at stations to the NW and SE. This additional moment release is located on a separate fault to the west which is probably strike-slip in nature and related to a large strike-slip aftershock along the coast of Mindanao. The jagged, multiple event moment release of the Mindanao earthquake is in sharp contrast to the smooth rupture of the 1979 Colombia subduction zone earthquake (Mw=8.2), which resulted from the rupture of a single large asperity (≈60 km). Although these two earthquakes are both underthrusting events, they occur in very different tectonic settings. The Mindanao earthquake occurred at a relatively young subduction zone in a region with rapidly evolving plate boundaries, whereas the Colombia earthquake occurred at a well-established plate boundary.