Spatial variation in the crustal anisotropy and its temporal variation associated with a moderate-sized earthquake in the Tokai region, central Japan

Abstract

Spatial and temporal variations in seismic anisotropy in the crust are investigated using earthquakes in the crust and the subducting Philippine Sea Plate beneath the Tokai region, central Japan. We use a total of 351 high-quality waveform data recorded from 1986 December until 1999 August by the microearthquake observation network of the Research Center for Seismology and Volcanology, Nagoya University. The leading shear waves are polarized in an approximately E–W direction, independent of the focal depth, showing consistency with the direction of regional horizontal maximum compressive stress in the Tokai region. The time delay increases for greater focal depth down to 30 km. These results indicate that the regional compressive stress controls anisotropy not only in the upper crust but also in the lower crust. Assuming a uniform distribution of anisotropy, the degree of anisotropy is estimated to be 0.6 per cent in the lower crust. An increase in time delay is found after the Aichi-ken Tobu earthquake (M = 5.7) in 1997 at station STN. This variation is statistically significant and is not an apparent change due to the variation in hypocentre distribution or any kind of artificial effect. On the other hand, no temporal variation in time delay is found at station INU. The spatial pattern of the temporal variation in time delay is consistent with the change in the Coulomb failure function (ΔCFF) caused by both coseismic and post-seismic fault slips. We therefore conclude that the observed temporal change in time delays might be caused by increasing crack density as well as pore fluid pressure due to the static stress change accompanied with the Aichi-ken Tobu earthquake.