Abstract
AbstractWe have examined temporal changes in seismic velocity of the crust through repeated active seismic experiments at Iwate volcano, Japan, where a significant volcanic activity and an M6.1 earthquake were observed in 1998. We apply a cross spectrum moving window technique to seismic data recorded at eight stations for the six explosions detonated from1998 to 2003. The seismic velocity at the frequency range of 3-9 Hz decreased by about 1% during the three months including the occurrence of M6.1 earthquake. The seismic velocity gradually increased, and about one third of the decrease was recovered by 2002. Then, the seismic velocity decreased again in 2003. Spatio-temporal changes in the volumetric strains predicted from the M6.1 fault mechanism and the volcanic pressure sources are well correlated with the seismic velocity changes observed in 1998. However, the predicted stress fields are not completely matched with the observed velocity changes from 1998 to 2003. This inconsistency may be due to unknown regional tectonic stress field and/or localized stress fields induced by volcanic pressure sources. It should be noted that the observed velocity changes indicate frequency dependent characteristics although the mechanism is not yet understood.
Highlights
Temporal changes of the crustal structure have been a major topic of the solid earth, and many seismologists have attempted to detect a small change of the structure associated with seismic and volcanic activities
6.1 Comparison of seismic velocity changes with spatio-temporal changes in volumetric strains around the M6.1 fault and Iwate volcano By using the M6.1 fault model of Miura et al (2000) and the spatio-temporal distributions of the volcanic pressure sources from 1998 to 2002 by Tohoku Univ. (2003), we calculate volumetric strains around the M6.1 fault and Iwate volcano
Summary We have shown remarkable temporal changes in the seismic velocity of the crust around the M6.1 fault and Iwate volcano, by measuring the phase difference in the seismic waves excited by six artificial explosions
Summary
Temporal changes of the crustal structure have been a major topic of the solid earth, and many seismologists have attempted to detect a small change of the structure associated with seismic and volcanic activities. Poupinet et al (1984) showed that the seismic velocity around the hypocentral area of the Coyote Lake earthquake (M5.9) decreased down to 1% by applying a cross spectrum method to multiplet seismic events. He and his colleagues further examined temporal changes of shallow structure at Merapi volcano and reported a 1% increase of seismic velocity several months before the 1992 eruption (Poupinet et al, 1996; Ratdomopurdo and Poupinet, 1995). Furumoto et al (2001) examined the first arrival time data of repeated active seismic experiments at Izu Oshima, Japan, which have been planned by Geological Survey of Japan for monitoring the structure around the focal zone of a future Tokai earthquake.
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