[Abstract]

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 104,764 P- and 97,232 S-waves arrival time data observed at 521 seismic stations from 3,930 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 ÅgCross Validation TechniqueÅh. 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 10 to 14 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 0.2 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