oa Temporal Variation in Shear Wave Anisotropy Detected with ACROSS

We are now developing a seismic exploring system named ACROSS aiming at detection of temporal variation in elastic properties of the crust. We made an experiment lasting for 15 months, from January 2000 to April 2001 at a site near the Nojima fault, which ruptured during the 1995 Kobe earthquake (Mw7.2). Elastic waves generated by surface-deployed ACROSS sources were received with seismometers deployed in the bottom of 800m- and 1700m-deep boreholes near the sources. During the experiment, sudden delays in travel time for S wave were observed at the time when the 2000 Western-Tottori earthquake (Mw6.6) and the 2001 Geiyo earthquake (Mw6.7) occurred. The traveltimes of the S wave abruptly delayed and gradually recovered associated with each earthquake with little change in that of P waves. The delay was about 0.4% and 0.1% of absolute travel time for the Western-Tottori earthquake and Geiyo earthquake, respectively. The delays were polarized in a direction perpendicular to the Nojima fault for both cases. This suggests that the density of the cracks parallel to the fault increased associated with the earthquakes. These cracks can be regarded as being opened by an increase of pore-pressure. An additional experiment determining the static structure revealed that the preferred orientation of the cracks had existed and be enhanced with the strong shaking of the earthquakes in middle distance.