Coseismic Displacement and Recurrence Interval of the 1973 Ragay Gulf Earthquake, Southern Luzon, Philippines
Hiroyuki Tsutsumi**, Jeffrey S. Perez**, Jaime U. Marjes**,
Kathleen L. Papiona**, and Noelynna T. Ramos***
*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
***National Institute of Geological Sciences, University of the Philippines, Diliman, C. P. Garcia Avenue, Quezon City 1101, Philippines
The 1973 Ragay Gulf earthquake produced an onshore surface rupture approximately 30 km in length along the Guinayangan segment of the Philippine fault in southern Luzon Island. Through geologic mapping and paleoseismic trenching, we have characterized the amount of coseismic offsets, the average recurrence interval, and the slip rate of the segment. The coseismic offsets we identified in the field were fairly constant along the fault, ranging from 1 to 2 m. Paleoseismic trenching at the Capuluan Tulon site exposed stratigraphic evidence for three or possibly four surfacerupturing events after the deposition of strata dated at AD 410–535. The average recurrence interval was calculated to be 360–780 years, which is close to that for the Digdig fault, the source fault of the 1990 central Luzon earthquake. The slip rate, based on the calculated recurrence interval and offsets during the 1973 earthquake, has been calculated to be 2.1–4.4 mm/yr. This rate is significantly smaller than the geodetic slip and creep rates of 20–25 mm/yr estimated for the Philippine fault on the islands of Masbate and Leyte. The slip rate deficit may be explained by the possibilities of underestimation of the recurrence interval due to possible missing paleoseismic events within the stratigraphic records, the occurrence of larger earthquakes in the past, and the aseismic fault creep between the surface-rupturing earthquakes.
-  C. R. Allen, “Circum-Pacific faulting in the Philippines-Taiwan region,” Journal of Geophysical Research, Vol.67, pp. 4795-4812, 1962.
-  T. Nakata, H. Tsutsumi, R. S. Punongbayan, R. E. Rimando, J. A. Daligdig, A. S. Daag, and G. M. Besana, “Surface fault ruptures of the 1990 Luzon earthquake, Philippines,” Research Center for Regional Geography, Hiroshima University, Special Publication No.25, p. 86, 1996.
-  PHIVOLCS Quick Response Team, “The 15 February 2003 Masbate Earthquake,” PHILVOCS Special Report No.5, p. 30, 2003.
-  E. M. Morante, “The Ragay Gulf earthquake of March 17, 1973, southern Luzon, Philippines,” Journal of the Geological Society of the Philippines, Vol.28, No.2, pp. 1-31, 1974.
-  H. Tsutsumi and J. S. Perez, “Large-scale active fault map of the Philippine fault based on aerial photograph interpretation,” Active Fault Research, No.39, pp. 29-37, 2013.
-  H. Yasuda, T. Bacolcol, A. Daag, M. Cahulogan, E. Bariso, E. Mitiam, J. Marjes, T. Ueki, T. Kobayashi, and T. Nakata, “Geometry and structure of the Philippine Fault in Ragay Gulf, southern Luzon,” Journal of Disaster Research, Vol.10, No.1, 2015 (this issue).
-  M. L. P. Bautista and K. Oike, “Estimation of the magnitudes and epicenters of Philippine historical earthquakes,” Tectonophysics, Vol.317, pp. 137-169, 2000.
-  H. Tsutsumi, J. A. Daligdig, H. Goto, N. M. Tungol, H. Kondo, T. Nakata, M. Okuno, and N. Sugito, “Timing of surface-rupturing earthquakes on the Philippine fault zone in central Luzon Island, Philippines,” EOS Transactions AGU, Vol.87, p. 52, 2006.
-  T. Bacolcol, E. Barrier, T. Duquesnoy, A. Aguilar, R. Jorgio, R. de la Cruz, and M. Lasala, “GPS constraints on Philippine fault slip rate in Masbate Island, central Philippines,” Journal of the Geological Society of the Philippines, Vol.60, No.1-2, pp. 1-7, 2005.