Pre-Seismic Deformation And Horizontal Displacements Associated With
The February 15, 2003 Masbate Earthquake

Teresito Bacolcol,Agnes Aguilar, Robinson Jorgio, and Ronald de la Cruz

Philippine Institute of Volcanology and Seismology
CP Garcia Ave., UP Diliman Campus
Quezon City, Philippines

Abstract

Global Positioning System (GPS) surveys were used to determine the pre-seismic and the horizontal deformations within the epicentral region of the 15 February 2003 Masbate earthquake (Ms=6.2) along the Philippine Fault in Masbate Island. Pre-seismic deformations include the results of the GPS campaigns done between February 1991 and February 2002 while the horizontal displacements associated with the earthquake include the readings done a year before and within 6 months after the main shock.

Pre-seismic measurements taken between 1991 and 2002 showed that the Philippine Fault in the region has a slip rate equal to about 2.3 + 1.1 cm/yr. Elastic elongation along the fault was also observed prior to the quake.

GPS readings taken a year before and within the six months after the main shock showed that as much as 62 cm parallel to the fault was displaced along the Philippine Fault near the epicenter of the earthquake. Ten centimeters of this displacement represented the afterslip movement of the fault. GPS measurements taken during this interval also showed offshore displacements during the earthquake. The GPS data likewise indicated elastic rebound movements along the Philippine Fault in the region following the earthquake.