白山火山周辺の三次元地震波速度構造

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(3) ,**. . ,2 ,**. +* +. . Three-Dimensional Seismic Wave Velocity Structure around the Hakusan Volcano Naoki TAKAHASHI῍,῍῍, Hiroaki NEGISHI῍῍῍ and Yoshihiro HIRAMATSU῍ We determine the three-dimensional P- and S-waves velocity structures around the Hakusan volcano in the Ryohaku mountains area, where there are some Quaternary volcanoes, using the travel-time tomography method. We use +*.,10. P- and 31,,-, S-waves arrival time data observed at /,+ seismic stations from -,3-* earthquakes in and around this region. The initial model, hypocenters and one-dimensional velocity structure, for the three-dimensional inversion are simultaneously determined by the Joint Hypocenter Determination method. Since dumping parameters and iteration number a#ect the absolute value of velocity perturbation, we determine these values quantitatively by applying “Cross Validation Technique”. A checkerboard resolution test is applied to the dataset to examine the resolution scale of this dataset. We find a velocity anomaly zone with low-VP, low-VS and high-VP/VS beneath the Hakusan volcano at the depth of +* to +. km, while there is no such a region beneath other volcanoes in the Ryohaku mountains area. Seismicity is shallower beneath the Hakusan volcano than beneath the surrounding areas and no earthquake occurs within the velocity anomaly zone beneath the Hakusan volcano. These features support that the velocity anomaly is a partially melting rock matrix with high temperature. The Hakusan volcano is one and only active volcano in recent *., Ma in the Ryohaku mountain area. This geological feature is consistent with our tomographic result. The existence of magma body, which can be shown as low-velocity body, may indicate that Mt. Hakusan is still noteworthy active volcano. Key words : tomography, Quaternary volcanoes, Hakusan volcano, VP/VS +ῌ ῐ ῎ ῑ ῏. yz{|)}jn pq 6~y{AB ! "

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(147) Fig. 0. P-wave velocity perturbation (upper) and the checkerboard resolution pattern (lower) on the eight sections in depth. Note that the perturbations are measured from the average velocity at each depth. The Hakusan volcano is indicated by a solid triangle (῏). Open triangles (῎) show the Quaternary volcanoes. White pluses (ῌ) show seismic stations used in this study. Seismicity is also shown with open circles.. Fig. 1. S-wave velocity perturbation (upper) and the checkerboard resolution pattern (lower). All symbols are the same as those in Fig. 0..

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(179) Fig. 2. The perturbations of VP, VS and VP /VS on the north-south cross section across the Hakusan volcano (along A-A’ in the index map). Red and blue indicate slower (higher) and faster (smaller) velocity (Vp/Vs) of seismic wave, respectively. Seismicity is shown as open circles. Low-frequency earthquakes identified by JMA are also shown as open stars (ΐ). Mt. Hakusan and the other volcanoes in this area are shown as a solid triangle and gray triangles, respectively.. Fig. 3. The perturbations of VP, VS and VP /VS on the east-west cross section along B-Bῌ in the index map. All symbols are the same as those in Fig. 2..

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