New evidence for a low-velocity layer on the subducting Philippine Sea plate in southwest Japan

Abstract

A distinct precursory phase is sometimes found prior to S arrival, when deep-focus and intermediate-depth earthquakes are observed at seismic stations in the Chugoku district, southwest Japan. The phase from intermediate-depth earthquakes has apparent velocity close to that of S wave in the Philippine Sea (PHS) plate, but at epicentral distance of 100–250km it arrives earlier by about 2.5s than the S wave propagated as a head wave along the upper boundary of the HPS plate. Thus we interpreted the phase as Sp wave into which the S wave refracted upward after traveling in the high-velocity PHS plate was converted at a boundary while propagating on a ray path between the plate and seismic station. The precursory phase to S wave from deep-focus earthquakes was also identified as Sp wave that was caused by conversion of the direct S wave into P wave, because the S–Sp times were independent of epicentral distance and focal depth and the direction of horizontal particle motion of the phase almost agreed with the azimuth direction of the epicenter. The Sp conversion interface was located at the upper boundary of the PHS plate. Further, it was demonstrated that a thin low-velocity layer should overly just above the PHS plate in order to explain both the observed difference in polarity of the S and Sp waves and the amplitude ratio of the Sp wave to the direct S wave.