Along strike variation of seismic behavior of the Philippine fault based on historical- and paleo-seismicity

The Philippine fault is a 1250-km-long, left-lateral strike-slip fault extending NNW parallel to the Philippine archipelago. This fault has been very active in the past 100 years with several destructive earthquakes accompanied by surface rupture. There is notable along-strike variation in historical- and paleo-seismicity of the Philippine fault that seems to be composed of locked, transition, and creeping sections as first pointed out by Bacolcol (2003) and Besana and Ando (2005) based on geodetic and seismological observations. The Philippine fault in central Luzon and Mindanao Islands appears to be fully locked and produced M≥7 earthquakes. In central Luzon, four left-stepping distinct segments comprise the Philippine fault and the Digdig segment ruptured during the 1990 Mw7.7 earthquake with maximum slip of 6 m. Paleoseismic trenching suggests an average recurrence interval of 500-600 years for the Digdig fault. The Philippine fault in Mindanao Island is also composed of several distinct fault strands separated by geometric discontinuities such as dilatational steps and branching. Two historical M7-class earthquakes are known to have occurred on the Philippine fault in Mindanao Island. Paleoseismic trenching revealed that an average recurrence interval of surface-rupturing earthquakes in northeastern Mindanao is 300-400 years. Historical seismicity of the central portion of the Philippine fault on Masbate and Leyte Islands is quite different from that on Luzon and Mindanao Islands. There was no M≥7 earthquake in the past 400 years in Masbate and Leyte Islands (Bautista and Oike, 2000). In Masbate Island, the 2003 Ms6.2 earthquake ruptured the surface, producing ~18-km-long surface rupture with maximum slip of ~50 cm. Paleoseismic trenching across the surface rupture by Papiona and Kinugasa (2008) revealed stratigraphic evidence for four surface-rupturing earthquakes in the past ~680 years. In Leyte, there is no historical surface-rupturing earthquake and the largest historical earthquake along the Philippine fault was Ms 6.4 (Bautista and Oike, 2000). Several studies identified aseismic creeping of the Philippine fault on Leyte island. Perez et al. (2008) reported aseismic surface creeping in Leyte town based on continuous fissuring of concrete structures and asphalt roads. Catane et al. (2000) identified aseismic creeping at a rate of at least 26 mm/yr in a geothermal area east of Ormoc City. The along-strike variation of seismic behavior of the Philippine fault may be in part controlled by variation of thickness and rigidity of seismogenic crust along the fault. The Philippine fault crosses the volcanic front related to the Philippine Sea plate subduction at the latitude of Leyte Island where there are many geothermal fields along the fault. The seismogenic brittle crust in Leyte Island may be thin and thus elastic strain may not accumulate to produce large earthquakes.