16:45 〜 17:00
[SCG55-12] Resistivity imaging around the Japan trench based on ocean bottom electromagnetic surveys
★Invited Papers
キーワード：沈み込み直前プレート、2011年東北沖地震、海底電位差磁力計、地磁気地電流法
Electrical resistivity in the earth reflects amount, composition and connectivity of fluid, temperature, and lithology. Recent developments of seafloor observation instruments, data processing and inversion method allow us to obtain reliable images of resistivity distribution. Thus resistivity imaging is a powerful tool to discuss fluid and clay mineral distribution around the outer-rise (e.g. Naif et al., 2015) and subduction zone. In the Tohoku-oki area, Ocean Bottom Electro-magnetometers were deployed at 48 sites in order to obtain regional resistivity distribution based on magnetotelluric method. In the presentation, we introduce the OBEM surveys and resistivity models based on 2-D inversion procedures in the incoming plate zone and around the rupture zone of the 2011 Tohoku-oki earthquake (M9.0).
Beneath the survey line along N39.4o across the incoming plate zone, the resistivity profile showed a conductive zone around a bending fault area. Sensitivity tests showed that the conductive zone may widely distributed near the Japan trench although depth range of the conductor were not constrained well. It implies hydration around the bending fualt. Beneath the survey line across rupture area of the 2011 Tohoku-oki earthquake (M9.0), the resistivity profile shows a low resistivity zone in the shallowest part of plate interface where huge fault slip over 80 m was reported (e.g. Iinuma et al., 2012; Fujiwara et al., 2011). On the other hand, resistivity is relatively high in the deeper plate interface that had been known as “asperity” and initial rupture zone was located. They indicate that the huge fault slip occurred in the pore fluid rich area that may affected by hydration in the incoming plate. However, especially in the incoming plate zone, MT impedances indicated strong three-dimensionality of resistivity structure and were not explained by a simple resistivity model including 3D bathymetry. It implies strong heterogeneity of the underground resistivity distribution and thus requires 3-D resistivity analyses.
Beneath the survey line along N39.4o across the incoming plate zone, the resistivity profile showed a conductive zone around a bending fault area. Sensitivity tests showed that the conductive zone may widely distributed near the Japan trench although depth range of the conductor were not constrained well. It implies hydration around the bending fualt. Beneath the survey line across rupture area of the 2011 Tohoku-oki earthquake (M9.0), the resistivity profile shows a low resistivity zone in the shallowest part of plate interface where huge fault slip over 80 m was reported (e.g. Iinuma et al., 2012; Fujiwara et al., 2011). On the other hand, resistivity is relatively high in the deeper plate interface that had been known as “asperity” and initial rupture zone was located. They indicate that the huge fault slip occurred in the pore fluid rich area that may affected by hydration in the incoming plate. However, especially in the incoming plate zone, MT impedances indicated strong three-dimensionality of resistivity structure and were not explained by a simple resistivity model including 3D bathymetry. It implies strong heterogeneity of the underground resistivity distribution and thus requires 3-D resistivity analyses.