Three-dimensional Seismic Velocity Structure around Usu Volcano, Japan(＜Special Section＞The 2000 Eruption of Usu Volcano (3))

The three-dimensional P- and S-wave velocity structures around Usu volcano, Japan are investigated using traveltime data of earthquakes during the eruptive activity in 2000, in order to clarify magma plumbing system and mechanical properties of host rocks which can affect magmatic activities. A tomographic inversion is started from a three-dimensional initial model based on a density structure inferred from a gravitational study, because strong lateral heterogeneity is expected in this region and final results can depend on the initial model. As a result, the following features are obtained. (1) Deepening of upper crustal layer from the southern coast of Lake Toya to Uchiura Bay at depth of 2～4 km is found for P- and S-wave velocity structures. This feature is consistent with density and apparent electrical resistive structures revealed by previous studies and the layer is interpreted as identical to Pre-Neogene basement by comparing with surface and borehole geological information. (2) Precursory earthquakes and earthquakes after the beginning of the eruption in 2000 mainly occurred within the Pre-Neogene basement or along the uppermost depth of the basement. This implies the seismic activity was strongly constrained by the structure. (3) Vp/Vs of greater than 2.0 is detected at the surface level and the Vp/Vs decreases with increasing the depth. The decrease of the Vp/Vs within Quaternary and Neogene layer can be due to the reduction of the porosity. A fairly small value of about 1.5 is obtained at depths greater than 4 km within the Pre-Neogene basement. The low Vp/Vs is possibly explained by the existence of a quartz-rich rock such as quartzite or hot-water under pressure and temperature condition near water-steam transition.