The Earthquake

A strong earthquake with Ms6.2 struck the province of Masbate at 7:01 in the evening of 15 February 2003. Preliminary determination of epicenter indicated that the event was generated along the Masbate Segment of the Philippine Fault Zone (PFZ) in central Philippines. The epicenter was located offshore of Magcaraguit Island (12.2ºN, 123.8ºE) and about 22 kilometers deep, which is approximately 28 km east of Masbate City. Initial reports from nearby stations implied that the earthquake was felt all over the island of Masbate including the nearby provinces of Bicol, Leyte, Panay, Cebu, Negros and Romblon (Figure 1).

The focal mechanism solution released by PHIVOLCS suggests a dominantly left-lateral slip along a vertical NW-SE fault (Figure 2). This left-lateral slip correlates well with the known structure in the area as observed in Dimasalang, Palanas and Cataingan along its mapped trace. Figure 3 shows the historical earthquakes along the Masbate Segment of the Philippine Fault Zone that dates back in the year 1800’s. Magnitues 6.5 and 7.0 earthquakes were reported on 16 August 1869 and on 19 October 1897 on the island respectively (Lanuza, 1994). Another reported earthquake struck the province of Masbate on 23 October 1877 between 1:00 and 2:00 in the afternoon (SEASEE Vol. IV). Other reported earthquakes for the island of Masbate are as follows: 1915 (Ms 6.6); 1970 (Ms 6.3); and 1971 (Ms 6.3). In 07 May 1998, a magnitude Ms 5.7 quake was felt at intensity VI all over the City of Masbate and the municipalities of Mobo and Uson. This event was preceded by a magnitude Ms 5.1 with reported intensity VI at Masbate City (Torrevillas et. al., 1998).

A Quick Response Team (QRT) was deployed to Masbate on 18 February 2003 based on the extent of the preliminary reported damages submitted by the Masbate Seismic Station (Dela Cruz and Torrevillas, 2003, unpublished internal report). The QRT’s primary purpose was to evaluate the extent of damages due to the earthquakes in the province of Masbate especially those municipalities located near the epicenter. The team was also tasked to verify and map the reported ground rupture, conduct intensity survey, disseminate correct information regarding the event, and install additional seismographs in order to monitor aftershocks of the Ms 6.2 earthquake. This paper reports the overall findings of the QRT during its 10-day investigation in Masbate. All intensity reports in this paper are according to the Philippine Earthquake Intensity Scale (PEIS) as shown in Figure 4.