[S01-4] Open session IV

[S01-4-04] Long-term monitoring of of seismic velocity around a source fault of the 1995 Kobe earthquake

^○Ryoya Ikuta¹, Koshun Yamaoka², Takahiro Kunitomo², Kinya Nishigami³, Toshiki Watanabe³ (1.Shizuoka University, Shizuoka, Japan, 2.Nagoya University, Nagoya, Japan, 3.Kyoto University, Kyoto, Japan)

Temporal variation in seismic velocity structure around Nojimafault, which ruptured at the time of 1995 Kobe earthquake (Mw6.9), has been measured for 17 years from 2000 to 2017 using Accurately Controlled Routinely Operated Signal System (ACROSS). We constructed the ACROSS sources which generate horizontally polarized seismic wave on the southern end of Nojima fault and started the measurement in 1998. The generated seismic wave were detected by borehole-type seismometers deployed at the bottom of 800-m- and 1700-m-deep boreholes near the ACROSS sources. The temporal changes in travel time and amplitude of P-, S-, and later phases have been monitored.
As a result, gradual decreases in the travel times of the body waves as well as that of the later phases were detected. The decreasing rate decayed during the 22 years after the Kobe earthquake. This decay corresponds to the decay of permeability change measured by water injection test into the fault, which was repeated from 1997 to 2009. The velocity variation should be a result of crack healing around the fault after the earthquake.