Seismicity Along The Philippine Fault Zone – Masbate Segment And The Sibuyan Sea Fault: A Review

Baby Jane T. Punongbayan

Philippine Institute Of Volcanology And Seismology

Abstract

A review of the seismicity along the Masbate segment of the Philippine Fault and the Sibuyan Sea fault is presented in light of the two damaging earthquakes of 15 February 2003. The first occurred at 05h47m (GMT) with Ms=5.9. Maximal intensities reached VI (PEIS) with reported minor damages in several towns of Masbate. The Philippine Institute of Volcanology and Seismology (PHIVOLCS) located its epicenter at 123.827 ºE, 12.121 ºN with a 2-km depth. Five hours later, it was followed by an Ms=6.2 event at 11h02m (GMT). Its hypocenter was located at 123.811 ºE, 12.244 ºN with depth of 23 km. Maximal intensities reached VII (PEIS) and co-seismic ruptures were mapped to have reached more than 20 km. Although the ground ruptures were clear in the island of Masbate, its total length and continuation cannot be ascertained as mapping was constrained by its disappearance undersea north of the island. Here, we use the aftershocks to infer the total affected area for this crisis. Aftershocks during the first two days indicated that seismic activity continued offshore north of the Masbate Island. Aftershocks were confined within latitudes 12.0 to 12.4 ºN, along the observed surface ruptures and offshore north of the mainshock. Aftershocks up to three months later remained along this general area.

However, on May 23, a series of events (Ms=5.7, 5.0, 4.4, 4.1) concentrated further south of this area (11.8 to 12.0ºN). On June 26, an Ms=6.0 occurred just a little north of the mainshock and was followed by many smaller events. But more interesting to note is the development of a series of earthquakes that broke off the general trend of the Masbate Segment of the Philippine Fault and followed the identified trace of the Sibuyan Sea Fault. This series started on September 2003 and within a year, six Ms>4 were located by PHIVOLCS. The largest of these is an Ms=5.2 that occurred on June 28, 2004. The connection or independence of this series with the 2003 February aftershocks will be evaluated using available seismic data.