[S-EM14] Electric, magnetic and electromagnetic survey technologies and scientific achievements

[SEM14-P03] Re-visit of the upper mantle electrical conductivity structure beneath the Philippine Sea

*Chen-Hsiang Hung¹、PeiYing Patty Lin²、Kiyoshi Baba³、Hisashi Utada³ (1.Department of Physics, National Taiwan Normal University, Taipei, Taiwan、2.Department of Earth Sciences, National Taiwan Normal University, Taipei, Taiwan、3.Earthquake Research Institute, University of Tokyo, Tokyo, Japan)

キーワード：Electrical conductivity, Marine magnetotellurics, Philippine Sea, Upper mantle, Ocean Bottom Electro-Magnetometer

Seafloor electromagnetic field observations recorded by Ocean Bottom Electro-Magnetometers (OBEMs) enable us to understand the electrical conductivity structure in the oceanic upper mantle. The electrical conductivity of the lithosphere and uppermost mantle provides key constraints on the chemical compositions and the dynamics as well. We re-visit the OBEM data recorded in the Philippine Sea and western edge of the Pacific Ocean (Baba et al., 2010) to explore the back-arc basin dynamics where the Pacific plate subducts to the Philippine Sea plate. In this study, we re-analyzed magnetotelluric (MT) time series of a total of 47 instruments installed at 27 sites with different data-lengths during 1999-2009. We applied the generalized remote reference technique with two-stage processing (Chave and Thomson, 2004) using on-land geomagnetic stations as the reference sites to improve the quality of MT response. In the first stage, we estimated the transfer functions of the horizontal magnetic fields between an OBEM site and a geomagnetic observatory, Kakioka (KAK), Kanoya (KNY), or Guam (GUA). In the second stage, the MT responses were estimated by the OBEM electric field and the horizontal magnetic field predicted from the first stage transfer function and the observatory magnetic field. The generalized remote reference technique significantly reduces the correlated noise contaminated in both OBEM electric and magnetic data and then improves the quality of the MT responses up to ~2.5 × 105 seconds at the sites. Furthermore, we analyzed all available data, in which some sites have two- or three-years long records, in this study, although the previous study by Baba et al. (2010) analyzed only selected one-year data. The increase of available data length also contributed to the improvement. The coherence between OBEM observed and predicted electric fields become higher. Similarly, error bars of the apparent resistivity and impedance phase become smaller. In addition, we confirmed the apparent resistivity of the diagonal elements in MT tensors significantly vary in different periods at a few OBEM sites as reported previously (Baba et al., 2010). The values of swift skew, which is the ratio of diagonal and the off-diagonal components in MT tensors, are larger than 0.3, suggesting lateral heterogeneity in the conductivity.