Source mechanism of the 1998 Mw 7.4 intraplate strike-slip earthquake in the West Philippine Basin revealed by Coulomb stress changes

On April 11, 2012, Mw 8.6 and Mw 8.2 earthquakes occurred off the west coast of northern Sumatra as a result of strike-slip type events within the Indo-Australia plate. It is suggested that these two earthquakes were linked to the occurrence of the 2004 great Sumatra-Andaman earthquake. Over the last few decades, several strike-slip type earthquakes have been observed within the West Philippine Basin (WPB). However, little investigation has been conducted to understand these strike-slip earthquakes and the possible geohazard potential that they may bring. In our study, to determine the rupture plane for these strike-slip events, we calculated the static Coulomb Failure Stress changes (ΔCFS) induced by the 1998 Mw 7.4 strike-slip earthquake and compared them with the distribution of aftershocks. Our results show that when the fault plane is trending in the NW-SE direction, the aftershocks that occurred in the region show a good agreement with the maximal positive ΔCFS. Consequently, instead of sliding along the pre-existing oceanic fracture zones, the 1998 strike-slip event that occurred in the WPB should have ruptured in the NW-SE direction. Moreover, we also observed a correlation between the activities of the normal faulting type earthquakes along the trench and the positive ΔCFS induced by the 1998 Mw 7.4 event, which suggests that the occurrence of intraplate strike-slip events in oceanic plates might affect the distribution of earthquakes along the nearby subduction systems.