日本海中部地震の震源域直下における最上部マントルのP波速度
1987 年 40 巻 3 号 p. 435-444
詳細
1987 年 40 巻 3 号 p. 435-444
The Pn velocity just beneath the source area of the 1983 Japan Sea earthquake was investigated using arrival times of aftershocks of the Japan Sea earthquake observed at two stations, Matsushiro (MAT) and Okushiri (OKS). These stations and aftershocks are lined up in a direction of the strike of the aftershock distribution. This alignment has allowed us to estimate the Pn velocity for an azimuth of the strike (N190°E) without suffering from the effect of a regional variation of the Pn velocity. In order to reduce the effect of errors of hypocentral parameters, the Pn velocity was determined from a slope of differential arrival-times against the differential epicentral distances between the two stations. The Pn velocity was estimated in a range of 8.20-8.27km/s with an error of less than 0.09km/s. It is significantly greater than the Pn velocity for the corresponding azimuth of the anisotropic model proposed by OKADA et al. (1978); it agrees with the velocity for an azimuth of the maximum wavespeed (N141°E) in their model and also with the Pn velocity of 8.2km/s estimated for the Oga-Kesennuma profile which trends almost perpendicular to the present azimuth intersecting the southern part of the source area. Therefore we conclude that the uppermost mantle beneath the eastern margin of the Japan Basin does not show such an obvious anisotropy as predicted by OKADA et al. This result together with the recent result obtained in the Yamato Basin [HIRATA et al. (1987)] suggests a re-examination of the now-prevailing idea of velocity anisotropy in the Japan Sea.
