Abstract
The Philippine Earthquake (Ms=7.8) broke out in July 16, 1990 along the Philippine Fault in Central Luzon. The Philippine Fault is seismically very active and large earthquakes of M 7 class have occurred during this century along this fault. However large earthquakes have not taken place along the active traces of the fault in the Central Luzon during this century, while two large historical earthquakes occurred along its southern trace in 1645 and its northern trace in 1796. Therefore it is considered that the 1990 earthquake was caused by the surface faulting in the seismic (aseismic) gap along the Philippine Fault.The total length of the surface fault is over 120 km and the fault is divided into two segments by the major bend near Rizal. The surface fault is rather straight and linear and general orientation of the northern segment is N 25 W and the southern segment N40W. Left-lateral displacement is dominant along most of the fault traces and the maximum horizontal displacement is about 6 m in the 60-km-long northern segment and the maximum vertical displacement is 2.0 min the 50-km-long southern section. Sense of vertical displacement changes in places and is consistent with the sense of the displacement along the pre-existing active fault traces. Average displacement along the northern segment is 5-6 m, while 2-3 m along the southern segment.Along most of the surface fault, ruptures appear exactly along the pre-existing active fault traces. Offsets of roads, foot-pass, streams are common earthquake-induced features. Local extensional and compressional jog forms related to slight change in fault strike creates characteristic features such as depressions, trenches, mole tracks, bulges etc.The rupture propagated bilaterally northward and southward from hypocenter east of Bongabon near the major bend. The source process of the earthquake deduced from the slip distribution along the surface fault from the epicenter well coincides with that deduced seismologically from the source time function.
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