Preliminary Magnetotelluric Modeling in the Nikko Volcanic Area -Potential Break of Fluid Trap by Volcanic Intrusion

We collected magnetotelluric data in the Nikko volcanic area in the period range from 0.01 to 10 sec. Impedance data were decomposed with the regional strike direction N0°E and were analized by two-dimensional inversions including static shifts as model parameters. The major model features are: (1) southern conductor at 8 km depth which is consistent with the seismic S-wave reflectors, (2) northern conductor located at 5 km depth, consistent with the seismic attenuation zones, and (3) a resistitive gap in between, consistent with the magnetic body inferred from aeromagnetic data. The two conductors probably imply trapped free water at around 400°C. The resistor in between implies the volcanic body, which intruded the crust and may have broken the regime of trapped free water.