Geometry and Structure of the Philippine Fault in Ragay Gulf, Southern Luzon
Hirotake Yasuda*, Teresito Bacolcol**, Arturo Daag**,
Ericson Bariso**, Emmanuelle Mitiam**, Jaime Marjes**,
and Takashi Nakata***
*Department of Geophysics, Kyoto University, Kitashirakawa-oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
**Philippine Institute of Volcanology and Seismology (PHIVOLCS), C. P. Garcia Avenue, Quezon City 1101, Philippines
***Department of Geography, Hiroshima University, Kagamiyama, Higashi-Hiroshima 739-8522, Japan
A 15 km southward offshore extension of the Philippine Fault in the Ragay Gulf near the east coast of the Bondoc Peninsula is recognized and described with 150 newly acquired, high-resolution acoustic reflections. The vertically dipping fault strikes roughly NWSE and exhibits pressure ridges and depression structures indicative of strike-slip movement. The southern portions of the fault exhibit particularly sharp breaks on the seafloor that were probably produced by the 1973 Ragay Gulf Earthquake. Offsets of distinct acoustic layers are interpreted to indicate the strikeslip fault has slipped in earthquakes at least four, and likely more than 11 times during Holocene.
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