Temporal variation in the shear wave anisotropy detected using the Accurately Controlled Routinely Operated Signal System (ACROSS)



[1] Temporal variation in shear wave anisotropy was detected in a monitoring experiment using an accurately controlled, routinely operated signal system (Accurately Controlled Routinely Operated Signal System, ACROSS). We conducted an experiment that lasted for 15 months between January 2000 and April 2001 at a site near the Nojima fault, which ruptured during the 1995 Kobe earthquake (Mw 7.2) [Yamaoka et al., 2001; Ikuta et al., 2002]. Two vibration sources that generated 2 × 105 N with centrifugal force, which were firmly fixed on the ground, were used to emit elastic waves. Seismometers deployed at the bottom of 800-m- and 1700-m-deep boreholes near the ACROSS sources were used to receive the signal. We extracted small temporal changes in the travel time of the P and S wave by calculating cross-spectral density among the records every hour. During the experiment, sudden delays in travel times for the S wave were observed when the 2000 western Tottori earthquake (Mw 6.6) and the 2001 Geiyo earthquake (Mw 6.4) occurred. Their epicenters were 165 and 215 km away from the site, respectively. The travel times of the S waves between the surface and the bottom of 800-m-deep borehole abruptly slowed down and gradually speeded up with each earthquake. The delay in S was about 0.4% and 0.1% of the absolute travel time for the western Tottori earthquake and the Geiyo earthquake, respectively. The delays were polarized in a direction perpendicular to the Nojima fault in both cases. This indicates that the density of the cracks parallel to the fault increased in association with the earthquakes. These cracks can be regarded as opened by the increase in pore pressure. The small changes in P wave velocities support this interpretation. An additional experiment to determine the static anisotropy revealed that there was a preferred orientation of the cracks that was enhanced with the strong shaking of earthquakes in the middle distance.