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

[SEM14-P20] Anisotropic 1-D electrical resistivity structure of the lithosphere-asthenosphere system beneath the West Philippine Basin

*Tetsuo Matsuno¹, Kiyoshi Baba² (1.Kobe Ocean-Bottom Exploration Center, Kobe University, 2.Earthquake Research Institute, The University of Tokyo)

We reveal an anisotropic 1-D electrical resistivity structure of the upper mantle beneath the West Philippine Basin from ocean bottom electromagnetic array data to investigate the physicochemical state of the lithosphere-asthenosphere system (LAS). The array consists of three sites with spacing of 100 km or less, and is located at ~55 Ma seafloor, where the fossilized seafloor spreading direction is N30°E-S30°W and the absolute plate motion direction of the Philippine Sea plate is N60°W. Magnetotelluric (MT) impedances observed at the three sites were corrected for topographic distortions, and then were averaged to derive a full impedance tensor that is representative for the array. The anisotropic 1-D electrical resistivity structure was inferred through an inversion code (Matsuno et al., 2020) using the averaged MT impedance and an isotropic 1-D electrical resistivity structure for the West Philippine Basin (Baba et al., 2010). The resultant anisotropic 1-D inversion model shows three notable features; (1) an anisotropic layer at 40-60 km depths with ~30 Ω-m for the most conductive direction (ρ_c) and ~300 Ω-m for the most resistive direction (ρ_r), respectively, and with N70°E-S70°W for the azimuth of ρ_c, (2) an anisotropic layer at 90-300 km depths with ~20 Ω-m resistivities for ρ_c and ~30 Ω-m for ρ_r, respectively, with N20° W-S20°E for the azimuth of ρ_c, and (3) an isotropic or weakly anisotropic layer at 60-90 km depths, which is sandwiched between the two anisotropic layers. These model features were tested by a series of forward modeling by changing the resistivities and the azimuth, and they were confirmed to be supported by the data. We also compared the response predicted from our anisotropic 1-D model and that predicted from an isotropic 3-D electrical resistivity structure of the study area (Tada et al., 2014), and a better fit of the former to the data than the latter was confirmed to support plausibility of our anisotropic 1-D model. Our anisotropic 1-D model was first interpreted by dry and wet isotropic olivine and wet anisotropic olivine with plausible thermal profiles of the oceanic upper mantle for the study area. This reveals a thermal state and dry or wet condition of the LAS and a depth range of melt existence. At depth where melt exists, an influence of melt on bulk resistivity was investigated using three mixing models for two phases (melt and background olivine), the Hashin-Shtrikman upper bound, the parallel circuit model, and the series circuit model. The interpretation of our model will be discussed with respect to the LAS, referring to seismic results for the study area (Kawakatsu et al., 2009; Isse et al., 2010) and other MT and seismic results on the LAS at similar seafloor age but different locations (e.g., Sarafian et al., 2015; Russel et al., 2019).