Permalink : http://hdl.handle.net/2115/38146

JaLCDOI : 10.14943/gbhu.72.79

In the 2000 eruption of Usu volcano, it is widely believed that magmatic intrusion has reached the shallow subsurface to produce significant ground deformation. We performed a magnetotelluric survey over this area aiming for prospecting the intruded magma. The resistivity section exhibits a High-Low-High three-layered structure. The surface layer with a resistivity around 100 Ωm is likely to correspond with the lower Pleistocene andesite. The second layer is very conductive (0.1 to 1 Ωm), thinning and shallowing in the middle. It is difficult to explain such a low resistivity by anything but the Neogene units containing conductive clay minerals with high salinity ground water. Below it the resistivity gradually increases. The top middle part of the third layer shows the resistivities around 10 Ωm. It looks that this part is pushing the second layer upwards. We investigated whether this block is explainable by intruded magma or not from a viewpoint of the resistivity values. We at first calculated the resistivities of rhyolitic melt by using Gaillard (2004) 's model. The melt resistivities are in a range from 1 to 10 Ωm for 800 to 600 ℃. Secondly, we examined the partial melt by using the MBL model of Schilling et al. (1997). Given that the resistivities of melt and solid are 1 and 10,000 Ωm, respectively, the melt fraction corresponding to the bulk resistivity of 10 Ωm should be 15 vol%. Higher melt resistivity requires a larger melt fraction. The discussion above predicts that the bulk resistivity of the intruded magma will increase by some orders of magnitude in the course of cooling and solidification and that this process can be monitored by MT repeat measurements.