[S-EM12] Electric, magnetic and electromagnetic survey technologies and scientific achievements

[SEM12-P18] Observation of the electromagnetic field on the seafloor and modelling of 3-dimensional resistivity considering the seafloor topography in Kumano-nada

*Manaka Kuroda¹, Tada-nori Goto², Hiroshi Ichihara¹, Tetsuo Matsuno³, Keiichi Tadokoro¹, Takafumi Kasaya⁴ (1.Graduate School of Environmental Studies, Nagoya University , 2.Graduate School of Science, University of Hyogo, 3.Kobe Ocean-Bottom Exploration Center (KOBEC), Kobe University, 4.Japan Agency for Marine-Earth Science and Technology (JAMSTEC))

Keywords:The Magnetotelluric (MT) method, Electromagnetic survey, Kumano-nada

The seismic ground motion and tsunami height, and the associated damage, depend on the location of hypocenter around the plate boundary and the following fault rupture process. Because pore fluids affect the properties of fault rupture (Kodaira et al., 2004), estimating the fluid distribution is important for elucidating the mechanism of earthquake. The Kumano-nada is known as the main rupture area of the 1944 Tonankai earthquake (e.g., Baba and Cummins, 2005; Kikuchi et al., 2003; Ichinose et al., 2003). A magnetotelluric (MT) survey, one of the electromagnetic survey methods, was conducted on the seafloor in Kumano-nada to estimate the resistivity structure that is highly sensitive to subsurface fluids (Kimura et al., 2005; Kasaya et al., 2005). However, the past studies have the following issues: the accuracy of the MT impedances was not high, the survey was conducted only on a single measurement line, and the resistivity structure was estimated by two-dimensional structural analysis using only the TM mode. It is, therefore, necessary to estimate a more accurate resistivity structure model.
In this study, we aimed to estimate the accurate three-dimensional resistivity structure. First, we conducted electromagnetic surveys at 7 sites in Kumano-nada. Next, we calculated MT impedances using BIRRP code (Chave and Thomson, 2004). Then we modelled three-dimensional resistivity structure based on all components of MT impedances. For the modelling, we utilized FEMTIC code (Usui, 2015) and considered seafloor topography. By the three-dimensional resistivity modelling, we explained the local increase of apparent resistivity and anomalous impedance phases with a period of around 2000 seconds at the observation sites near the Nankai Trough. On the other hand, a discussion of detailed resistivity structure requires both the careful selection of an initial model of the resistivity structure and the more detailed seafloor topography.