HR: 0800h
AN: DI41A-1260    [Abstracts]
TI: Electrical Conductivity In The Transition Zone Beneath The North Pacific And Its Implication For The Presence Of Water
AU: * Utada, H
EM: utada@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Goto, T
EM: tgoto@jamstec.go.jp
AF: Japan Agency for Marine Science and Technology, Natsusima 2-15, Yokosuka, 237-0061 Japan
AU: Koyama, T
EM: tkoyama@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Shimizu, H
EM: shimizu@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Obayashi, M
EM: obayashi@jamstec.go.jp
AF: Japan Agency for Marine Science and Technology, Natsusima 2-15, Yokosuka, 237-0061 Japan
AU: Fukao, Y
EM: fukao@jamstec.go.jp
AF: Japan Agency for Marine Science and Technology, Natsusima 2-15, Yokosuka, 237-0061 Japan
AB: Recently we obtained a model of 3-D electrical conductivity distribution in the transition zone (TZ) beneath the north Pacific region and compared it with P-wave tomography (Fukao et al., 2004). Most of anomalous features in electrical and seismic parameters were found to be consistently explained simply by a thermal effect, but a part of the TZ beneath the Philippine Sea showed significant inconsistency. Next we considered the effect of water (e.g., Karato, 1993) to account for the inconsistency (Koyama et al., 2004), and concluded this anomaly is ascribed to the excess water content of about 0.3 wt % or so. In this presentation, we show recent two sets of evidence for further discussion on this TZ anomaly. From May 2004 to May 2005, we deployed a new instrument with a 10 km long cable in the Philippine Sea to measure electric field variations. Using the obtained data from this experiment, we estimated the electromagnetic responses at periods longer than a day and compared them with those expected from the model. We found a systematic discrepancy between observed and calculated responses: observed responses tend to be at more conductive side at shorter periods and at more resistive side at longer periods. Considering the depth of field penetration at these periods, this tendency can be interpreted by the presence of conductive anomaly at shallower part and resistive anomaly at deeper part of the TZ. This result is in a good agreement with the feature of our 3-D model and confirms its accuracy. We also estimated geomagnetic induction responses at long periods (>1day) for 11 magnetic observatories in Europe and inverted them to 3-D electrical conductivity distribution in the TZ below. Then the result was compared with the result of P- and S-wave tomography in the same region, which showed a good correlation with much less inconsistency than in the Philippine Sea case. This good correlation can be ascribed to the relatively young (hot) plate age subducting below Europe so that slab material had been completely dehydrated at shallower depths.
DE: 1515 Geomagnetic induction
DE: 3914 Electrical properties
DE: 7270 Tomography (6982, 8180)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Study of Earth's Deep Interior [DI]
MN: Fall Meeting 2005