Active Fault Mechanisms In The Central Philippines From Recent Earthquakes

Mario A. Aurelio^1* , Jane T. Punongbayan² and Teresito Bacolcol²

¹ Lands Geological Survey Division, Mines and Geosciences Bureau,
North Avenue, Diliman, Quezon City, Philippines 1104
² Philippine Institute of Volcanology and Seismology, C.P. Garcia Avenue,
University of the Philippines, Diliman, Quezon City, Philippines

Abstract

Recent earthquakes in the Bicol-Masbate region provide present-day evidence to strengthen previous models of transtensional tectonics along the central segment of the Philippine Fault, where studies have been scarce due to its predominantly offshore distribution. Focal mechanism solutions of earthquakes in the Sibuyan Sea area, the latest of which occurred in June 2004, indicate a NW-striking left-lateral fault with a significant normal component to the NE. Similar mechanisms have also been observed in the Ragay Gulf area, the most important one being the magnitude Ms 7.2 of 17 March 1973. These earthquakes occur where the Philippine Fault interact with other strike-slip faults, particularly the Sibuyan Sea in Masbate, and the Legaspi Lineament in Bicol. The geometry and relative location of these interacting faults create a transtensional regime that produce earthquakes along strike-slip faults with notable normal fault component. These observations are consistent with recent block-motion vectors derived from GPS measurements.