[J02-P] Poster

[J02-P-10] Changes of S-wave velocity and polarization anisotropy associated with the 2011 Tohoku Earthquake detected by the observation of the seismic ACROSS signals

^○Takahiro Kunitomo^{1, 2}, Yasuhiro Asai¹, Hiroshi Ishii¹ (1.Tono Research Institute of Earthquake Science, Mizunami, Gifu, Japan, 2.Nagoya University, Nagoya, Aichi, Japan)

We analyzed the seismic ACROSS ( Accurately Controlled, Routinely Operated , Signal System ) signals using the data of the borehole observation network of Tono Research Institute Earthquake Science (TRIES) and detected clearly changes of S-wave velocity and polarization anisotropy associated with the 2011 off the Pacific coast of Tohoku Earthquake (March 11, 2011 Mw 9.0, hereinafter called Tohoku Earthquake) in Tono area of epicentral distance approximately 600km, located in the southwest Japan. Seismometers and strain meters or stress meters at the borehole stations are installed in the Toki granite at a depth of 110m - 1030m, whose direct distance from the seismic ACROSS transmitter (the Toki station) to seismometers are 210m - 8.5km. We estimated daily changes of the direct S-wave travel times by the cross spectrum method for two years from April, 2010 to March, 2012. Coseismic travel time delays of the direct S-waves associated with the Tohoku Earthquake were 0.3 - 7.3ms, and the detailed processes to gradually recover to the original travel times over a year were observed. Coseismic decreases in S-wave apparent velocity were 0.2 - 0.6%, and decreases of the velocity observed by stations in the southeastern direction from the transmitter were remarkable. At these stations, changes of the S-wave polarization anisotropy that the SH- wave velocities were decreased considerably in comparison with the SV-wave were observed. The crust in this area was elongated in direction of NE-SW to NNE-SSW with the change in the order of 10e-6 strain or 10kPa associated with the Tohoku Earthquake. As a result, the S-wave velocity decreased due to the opening of cracks in the rock. Vertical (high angle) cracks in the WNW - ESE and NW - SE direction were opened, so that the SH-wave propagated in NE direction was considerably delayed in comparison with the SV-wave.