We deployed the vertical seismic observation array in the Mizunami Underground Research Laboratory (MIU) constructed by Japan Atomic Energy Agency (JAEA). The vertical array consists of 4 seismic stations whose depths are 100, 200, 300, 400 meters, respectively. We installed 3 components accelerometer (JEP-6A3-10, Mitutoyo co.) in each station.
Although the seismic activity around MIU in the Tono region, Japan, is not high, some inland earthquakes occurred around MIU. We observed the seismic waves from these events in our vertical array. For an example, we observed the seismic waves from the event occurred at 21:10 JST on September 19, 2016. The JMA magnitude and the depth of this event are 3.0 and 7.3 km, respectively. The hypocentral distance between this event and MIU is about 30 km. The amplitude of the initial P wave at the shallow station is larger than the one at deeper station. We calculated the Fourier spectra of the direct P and S waves of these seismic waves. We found that the spectra in the 100-meter depth have more low-frequency (about 3 Hz) components than the ones in other depth. There is the geological boundary with the Mizunami group and Toki granite between 100 and 200-meter depth. We consider that the differences of the spectra in our array were caused by the geological boundary.
We have a plan of installing another seismic station to the stage 500 m in MIU (The depth of stage 500 m is 500 meters from the surface).