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 pe riod range from 0.01 to 10 sec. Impedance data were decomposed with th e regional strike direction NO degrees E and were analized by two-dime nsional inversions including static shifts as model parameters. The ma jor 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 resistive gap in between, consistent with the magnetic body inferred from aeromagnetic data. The two conductors probably imply tra pped free water at around 400 degrees C. The resistor in between impli es the volcanic body, which intruded the crust and may have broken the regime of trapped free water.