Electrical resistivity structure and helium isotopes around Naruko Volcano, northeastern Japan and its implication for the distribution of crustal magma

Asamori, Koichi; Umeda, Koji; Ogawa, Yasuo*; Oikawa, Teruki*

The two-dimensional electrical resistivity structure beneath Naruko Volcano was determined using magnetotelluric soundings. The resulting model shows that a prominent conductor exists in the crust beneath the volcano. The location of the conductor agrees closely with a seismic low-velocity zone. Hypocenters of low-frequency micro-earthquakes are located adjacent to the conductor. The cut-off depth of shallow micro-earthquakes becomes shallow toward the west side of the volcanic center and it is coincident with the upper boundary of the conductor. Furthermore, new helium isotope data from hot springs around the volcano were obtained. The high He/He ratios are obtained near the volcano. The maximum He/He ratio is located in proximity to this peak and decrease toward away from the volcanic center. This supports the likelihood of mantle-derived materials supplying beneath the volcano. These results indicate that the conductor beneath the volcano is due to high-temperature fluids consisting of magma and related aqueous fluids.