Temporal Change of Seismic-wave Velocity Associated with the 1998 Northern Iwate Prefecture, Japan Earthquake

We analyze seismic waveform data from three artificial chemical explosions conducted before and after the 1998 northern Iwate Prefecture earthquake (M 6.1), on September 3, 1998. These explosions were carried out at the same location ; about one month before, two months after, and two years after the earthquake. The waveforms of the three explosions are quite similar at three stations deployed on and around the hypocentral area of the M 6.1 earthquake. In order to detect small differences of travel time and waveform of coda, we calculate their coherence and phases by using the cross-spectral analysis. The result of phase spectral analysis shows that the seismic-wave velocity decreased 0.1-0.9% during the period including the earthquake occurrence. On the other hand, seismic velocity change is not found between the waveform data for the two explosions conducted after the earthquake. This observation strongly suggests that the velocity change took place associated with the occurrence of the M 6.1 earthquake.