The relocation of hypocenters was studied using the method of Modified Joint Hypocenter Determination (MJHD) to improve the location of hypocenters. We conducted a case study of two events that occurred in the central Philippines, which were the 1990 and the 1996 Bohol earthquakes in the southeast and the northwest of Bohol, respectively. This study is to unlock the query of the absolute location of hypocenters, fault planes and the temporal change of seismicity. Seismic phase data were obtained by the International Seismological Centre (ISC) and the Philippine Institute of Volcanology and Seismology (PHIVOLCS). The period from 24 hrs right after the mainshock up to one week period of aftershocks was targeted to identify the fault planes. We identified the fault planes of the 1990 Bohol earthquakes which consisted of two M 6 events with pure reverse faults. The fault plane of the first mainshock of Mw 6.7 was located east of the mainshock and dipped southeastward, while that of the second mainshock with Mw 6.6, which occurred 30 minutes later, was located west of the first mainshock and dipped northwestward. We also found that the fault plane of the 1996 northwest Bohol earthquake (Mw 5.2) dipped northwestward.
The results of the MJHD relocation have a constraint to the aftershocks in the studied area and showed the lineaments of the existing faults such as strike slip and reverse faulting. In addition to the hypocenter relocation, we analyzed the GPS data surveyed in the Central Philippines. The result indicated that SOLK station in Bohol Island moves northward relative to Manila, which is in agreement with the P-axis of the 1996 northwest Bohol earthquake. It was consistent with the fault mechanisms of the earthquake. All of these findings are useful for seismic hazard assessment in the study area.