Title: Electrical Resistivity Structures around the Japan Trench and the Nankai Trough

Abstract:

Magnetotelluric (MT) imaging of subsurface electrical resistivity is highly sensitive to the presence and distribution of pore fluids, making it a key method for characterizing physical conditions in and around seismogenic zones. Despite the importance of megathrust earthquakes at subduction margins, MT studies of these environments remain scarce worldwide because of observational and inversion challenges and the small number of research groups engaged in marine EM study. This presentation summarizes our ocean-bottom electromagnetic (OBEM) observations and the resulting resistivity models for the source region of the 2011 Tohoku earthquake (Mw 9.0) along the Japan Trench, and for the Kumano-nada and Hyuga-nada segments of the Nankai Trough. In each area, clear spatial correlations emerge between resistivity patterns and seismic slip behavior. In Hyuga-nada in particular, the deep slow earthquake zone is characterized by low resistivity, whereas the regular (fast) earthquake zone corresponds to higher-resistivity and transitional domains. These results, consistent with findings from inland earthquake zones, suggest that pore fluids exert a first-order control on earthquake style along the plate interface.