著者

雑賀 敦 平松 良浩 大井田 徹 山岡 耕春

出版者

東京大学地震研究所

雑誌

東京大學地震研究所彙報 = Bulletin of the Earthquake Research Institute, University of Tokyo (ISSN:00408972)

巻号頁・発行日

vol.76, no.1, pp.75-86, 2001-07-25

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Spatial and temporal variations in seismic anisotropy in the crust are investigated using earthquakes in the crust and at the upper boundary of the subducting Philippine Sea plate in the Tokai region, central Japan. We use waveform data from December 1986 to August 1999 recorded by the micro-earthquake observation network of the Research Center for Seismology and Volcanology, Nagoya University. The method of Silver and Chan (1991) is applied to evaluate splitting parameters, a leading shear-wave polarized direction and a delay between two polarized waves. A total of 362 high-quality waveform data within the incident angle of 35° are analyzed in this study. The leading shear-wave polarized directions are approximately E-W, in spite of differences of focal depths. This direction is consistent with the direction of regional horizontal maximum compressive stress in the Tokai region reported before. The time delay increases in proportion to the focal depth. These results indicate that the regional compressive stress controls anisotropy not only in the upper crust but also in the lower crust. Assuming the uniform distribution of anisotropy, the degree of anisotropy is estimated to be 0.5% in the lower crust. An increase in time delays between two-polarized waves is found before and after the Aichi-ken Tobu earthquake (M = 5.7) in 1997 at the station STN. This variation is statistically significant with the confidence level of 99.999% and is not an apparent change due to a variation in hypocenter distribution. No temporal variation of splitting parameters is found at the station INU. These results can be explained by a change of the volumetric and areal strain in the most-upper part of the crust due to the postseismic slip of the Aichi-ken Tobu earthquake.