GEOLOGICAL SOCIETY OF THE PHILIPPINES
The 15 October 2013 Mw7.2 Bohol Earthquake and Its Aftershock Sequence
Ishmael C. Narag
Philippine Institute of Volcanology and Seismology
On 15 October 2013, at 8:12 a.m. PST, a magnitude Mw7.2earthquake occurred in Bohol Island with its epicenter located by the Philippine Institute of Volcanology and Seismology (PHIVOLCS) at 9.859 ⁰N and 124. 070⁰E with a focal depth of 12 kilometers. Based on analysis of recorded broadband records of the main shock using SWIFT, the mainshock is calculated to be a predominantly thrust event with nodal planes having strike,dip &rake angles of (214°,47,°68°) and(65°,47°,50°) and a scalar seismic moment (Mo=6.29 x 1019 Nm) corresponding to Mw7.2. This event was followed by intense aftershock activity recorded by the Philippine Seismic Network with epicenters located along a NE-SW trending area with an approximate length of 90 kilometers with some distinct clusters. The distribution of aftershocks and CMT solutions of significant aftershocks suggest a complex rupture process associated with the main shock that suggest transitions of the predominantly thrust event to a strike-slip event.
A review of the available earthquake catalogue shows that this earthquake is the largest onland earthquake to occur in Bohol Island which excited strong ground shaking and ground failure that damaged structures of various classes and vintage within Central Philippines. Despite these damages, circumstances during the occurrence of the event may have prevented the loss of lives and further injuries.
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