HR: 0800h
AN: GP11A-0820    [Abstracts]
TI: Two-dimensional Electrical Section Beneath the Backarc Region of Northeast Japan
AU: * MOTOBAYASHI, T
EM: s011652@ems.toyama-u.ac.jp
AF: Dept Earth Science, Toyama University, 3190, Gofuku, Toyama, 9308555 Japan
AU: TOH, H
EM: toh@sci.toyama-u.ac.jp
AF: Dept Earth Science, Toyama University, 3190, Gofuku, Toyama, 9308555 Japan
AU: BABA, K
EM: kiyoshib@jamstec.go.jp
AF: Institute for Earth Evolution, JAMSTEC, 2-15, Natsushima-Cho, Yokosuka, 2370061 Japan
AU: ICHIKI, M
EM: ichiki@jamstec.go.jp
AF: Institute for Earth Evolution, JAMSTEC, 2-15, Natsushima-Cho, Yokosuka, 2370061 Japan
AB: According to Iwamori (2002), water transported by a cold plate such as the Pacific plate beneath Northeast Japan can subduct deep into the Earth to reach the backarc side and dehydrates at depths of 150-200 km, which is one of the main supply phases of water to the wedge mantle. However, seismic tomography conducted so far has not resolved the region where the proposed dehydration occurs. In this paper, we aim at resolving the water supply to the wedge mantle by constructing an east-west electrical section of the arc-backarc region beneath Northeast Japan since electrical conductivity is sensitive to presence of water. A seafloor magnetotelluric (MT) profile had been occupied by six ocean bottom elecrtomagnetometers from OCT/'02 through JUL/'03 at 39.5 N in order to clarify upper mantle dynamics beneath Japan Sea. High quality MT data were obtained at four sites from the 10-month long geoelectromagnetic time-series. Two-dimensional (2D) preliminary inversions were conducted by Toh et al. (2003) for TE mode, which successfully resolved a resistive lithosphere beneath the Sado ridge, the very eastern margin of Japan Sea where large earthquakes occur. Baba et al. (2004) constructed a new MT dataset by adding four land sites and removing bathymetry/topography effects from all the sites to yield a 2D electrical section for both modes. However, the new dataset was found still affected by severe surface distortions and TM mode data have not been fitted by our 2D inversions. In this paper, we report a new 2D electrical section that explains vertical gradient sounding (VGS) responses belonging to TM mode. Since the VGS responses are the ratios of horizontal geomagnetic components between land and the seafloor, they are free from surface distortions for the electric field. Three-component geomagnetic data observed at Mizusawa Geodetic Observatory was used to evaluate the VGS responses for northward geomagnetic components, which corresponds to TM mode in magnetotellurics. The VGS-TM responses were obtained at 22 periods ranging from 480s to about 7.5 day whose coherences were higher than 0.9. The high quality TM mode data will be inverted by a modified version of Uchida's (1993) 2D ABIC inversion, in which 2D bathymetry can be included.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 7218 Lithosphere and upper mantle
DE: 1515 Geomagnetic induction
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting