1990年7月16日フィリピン地震の地震学的研究

Abstract

The Philippine earthquake of 16 July 1990 (Ms=7.8) occurred in central Luzon. Surface fault trace of left-lateral offset were observed for a distance of 120 km along the Philippine fault that had been identified as one of the most active transcurrent faults around the western Pacific. The maximum left.lateral offset is 5.5 m in the northern portion of the fault trace.Temporary seismic stations were set up through a joint collaboration between the ResearchCenter for Earthquake Prediction, Kyoto University and the Philippine Institute of Volcanology and Seismology, in order to locate hypocenters of aftershocks accurately. The field observations were made twice for periods August 29 to September 5 with 11 stations and for one month with 5 ones in November, 1990. The results of the observations reveal that the fault of the earthquake extended over a distance of 150 km, longer than the traceable suface breaks, and that subfaulting occurred immediately beneath Baguio and Agoo cities, which are 30 to 60 km away from the fault and received sever damage due to strong ground motions during the main shock. Composite focal mechanism diagrams of these aftershocks show nearly the same solution as that of the main fault, suggesting the subfault a right.lateral slip, conjugated with the main fault.The result of inversion of P and SH waveforms are summarized as follows:(1) Rupture propagates predominantly unilaterally toward the north.(2) Strike-slip sources are predominant in the moment release.(3) Strong rupturing occurred near the initial break during the first 20 sec and near the northern end at 60 and 80 sec.(4) Tensor sum of all subevents mechanism is essentially strike-slip and a scalar moment of 3.6X10^27, which is almost the same as the result of surface waves by Abe and Yoshida (1990).The recurrence interval of the Philippine fault in Luzon is estimated as more than 500 y, on the basis of analogical inference from the Median Tectonic Line and of the oblique subduction rate at the subduction of the Philippine Sea plate. This estimation is consistent with recent history of large earthquakes in the island.