Establishing criteria for segmentation of strike-slip faults based on data from the Philippine fault

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During the 3 year period of this research project, we have visited the Philippine 10 times conducting aerial photograph interpretation, field mapping, paleoseismic trenching of the Philippine fault zone in the Luzon Island, and mapping of late Quaternary tephra in southeastern Luzon Island. We have prepared a 1/50000 scale active fault map of the study area based on interpretation of about 1/30000 scale aerial photographs. We then conducted tectonic geomorphic studies of the active faults. We were able to map previously unidentified traces of the San Jose and San Manuel faults as well as obtain new insights into branching pattern of the active faults of the PFZ. Based on fault zone geometry and historical seismicity, the PFZ in central Luzon Island can be divided into 3 earthquake segments that rupture independently of one another. In order to test this segmentation model, we have excavated multiple trenches across individual segments of the PFZ. The Digdig fault has ruptured 4 times during the past 2000 years and the penultimate earthquake preceded the 1645 earthquake that appears to have caused by slip on the Gabaldon fault. The San Jose fault has ruptured at least 3 times during the past 2500 years and the most recent faulting event may be correlated to the 1796 earthquake. Therefore, at least during the last earthquake cycle, the three segments ruptured separately, supporting the proposed segmentation model. This suggests that the careful observation of fault zone geometry, especially the branching pattern gives important clues to identify faults that rupture simultaneously during individual earthquakes. Stratigraphic and chronological survey of late Quaternary tephra from the Mayon Volcano and Irosin Caldera in southeastern Luzon Island suggest that tephra from these volcanoes can be used as marker horizons for Quaternary studies in the Luzon Island in the future.