日高山脈直下の浅発地震に見られる顕著な相について

Abstract

For the earthquakes occurring in depths less than 35km under the Hidaka mountain region in Hokkaido, two remarkable phases with sharp onsets and large amplitudes are sometimes recorded on the only seismograms at station IWN. Station IWN is one of the six telemetering seismic stations of Research Center for Earthquake Prediction of Hokkaido University located in the Hidaka mountain region about 80km from east to west and about 100km from north to south. These stations have observed more than 700 earthquakes occurring in the region from 42.25°N to 43.40°N and from 142.50°E to 144.20°E during the period from June 1976 to October 1978, among which eight earthquakes were selected as producing the two remarkable phases at station IWN which are referred to as the I1 and I2 phases, respectively.The two phases are always recorded at the station as clear later arrivals; that is, the I1 phase arrives in 1.7-2.6sec after the direct P wave arrival and the I2 phase, in 0.8-1.1sec after the direct S wave arrival. In this paper, the analysis made on wave motions and travel times of these two phases is presented.The principal component analysis applied to the phases recorded in three component seismograms shows that the prevailing wave motion of the I1 phase is in the longitudinal direction while that of the I2 phase is in the transverse direction and the I1 phase arrives with much smaller incident angle. In addition to the wave motions, the spectra of the phases were obtained: the spectra show that the amplitudes die out at frequencies lower than about 15Hz for the I1 phase and at frequencies lower than about 3Hz for the I2 phase. The wave motions and spectra thus obtained strongly suggest that both phases can safely be identified with a kind of reflected waves associated with a boundary possibly existing in the lower crust. The I1 phase may be assumed as a converted wave which is generated by S wave originated from the source reflecting as P wave at the boundary and the I2 phase, as a purely reflected S wave. From travel time analysis applied to both phases, the boundary related to these phases was inferred to exist in depths of 30km beneath a point about 20km south of station IWN and to dip roughly toward the south with the surprisingly large dip angle of 33°. Spatial extent of the boundary with such a large dip angle may be limited probably in a small portion under station IWN, because the other five stations in the region considered have not observed such remarkable later phases.