CoseismicDisplacementandRecurrence Intervalof the1973RagayGulfEarthquake,SouthernLuzon,Philippines

The 1973 Ragay Gulf earthquake produced an onshore surface rupture approximately 30 km in length alongtheGuinayangansegmentofthePhilippinefault in southern Luzon Island. Through geologicmapping andpaleoseismictrenching,wehavecharacterizedthe amountofcoseismicoffsets,theaveragerecurrenceinterval,and the slip rateof the segment. The coseismic offsets we identi?ed in the ?eld 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 averagerecurrence interval was calculated to be 360–780 years, which is close to that fortheDigdigfault,thesourcefaultofthe1990central 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 rateissigni?cantly smallerthanthegeodeticslip and creep rates of 20–25 mm/yr estimated for the Philippine fault on the islands of Masbate and Leyte. The slip rate de?cit 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 betweenthesurface-rupturingearthquakes.