[J09-3] Geodesy and seismology general contributions III

[J09-3-02] Secular and co-seismic velocity changes in Tokai region detected by ACROSS

^○Shuhei Tsuji¹, Ryoya Ikuta², Koshun Yamaoka¹, Takahiro Kunitomo¹, Toshiki Watanabe¹, Yasuhiro Yoshida³, Akio Katsumata⁴ (1.Nagoya University, Nagoya, Japan, 2.Shizuoka University, Shizuoka, Japan, 3.Meteorological College, Kashiwa, Japan, 4.Meteorological Research Institute, Tokyo, Japan)

We detected secular and co-seismic travel time changes and their anisotropy using active seismic measurement in the Tokai region, Japan. In this study, we used seismic signals generated by ACROSS, which is a highly stabilized artificial seismic source, to measure propagation properties of seismic waves. The high stability of ACROSS signal makes us possible to detect very small changes of physical properties in the crust. The ACROSS signal is received by High sensitivity seismograph network Japan (Hi-net) stations to calculate a transfer function from the ACROSS source to each Hi-net station in the Tokai region during 2007 to 2014. The transfer functions show secular and annual change in the travel time of S-wave throughout the observation period. A large co-seismic change is also observed at the time of the 2011 Tohoku-Oki Earthquake. Considering S/N ratio, we focused on the stations near the ACROSS source. We found that the travel time was gradually advanced in the observation period and were suddenly delayed associated with earthquakes, which caused strong motion in this area such as Tohoku-Oki earthquake. Due to high S/N ratio at the nearest station, which is about 3 km from the ACROSS source, post-seismic travel time change shows a gradual recovery just after the sudden delay at the time of the Tohoku-Oki Earthquake with a time constant of one month. However travel time was not recovered to their original level. Next, we analyzed polarization anisotropy of the travel time changes. At the almost stations, the advance rate of the secular changes were the largest in NE-SW direction, and the co-seismic delay associated with the Tohoku-Oki earthquake were also the largest in NE-SW direction. The two anisotropies are very similar in direction, though the sense of travel time changes are opposite. We are trying to create a model that can explain these anisotropies by comparing with crustal strain change measured by GNSS and other geophysical observations.