Temporal changes of the crustal structure associated with the M6.1 earthquake on September 3, 1998, and the volcanic activity of Mount Iwate, Japan

We detected temporal changes of crustal structure by cross spectrum analyses of seismic waves excited by two artificial explosions that were carried out about one month before and two months after a M6.1 shallow earthquake. Phase spectrum analysis shows that seismic velocity of the upper crust around the focal area of the M6.1 earthquake and Mount Iwate decreased 0.3 - 1.0% during the three months. Since lager decreases of the seismic velocity were observed at the region close to the focal area and a volcanic pressure source, we conclude that stress change in the upper crust is a most plausible candidate to cause the velocity changes among other candidates. We further apply a coherence analysis to the observed seismic coda to locate regions where short-wavelength heterogeneity temporally changed. The result indicates a possibility of temporal changes of the heterogeneity beneath the volcano, where a magmatic fluid intrusion is inferred.