Conductivity structure around seismogenic layer beneath the Chugoku and Shikoku districts

We delineated the Philippine Sea plate beneath the South-western Japan arc using the electro-magnetic method in this study.The Network-MT method is one of the magnetotelluric methods and it is suitable for surveying a conductivity structure beneath a wide area and to a deep portion。We applied the method in Chugoku and Shikoku district to reveal a subsurface structure beneath the South-west Japan arc.We focused on the where and how does the Philippine Sea Plate exist。The first reason is that a location of The Philippine Sea plate is most important factor to controlled occurrences of earthquakes，because the cold and rigid plate makes large influence on a thermal state and/or a distribution of seismic-wave velocity。Second reason is that it is hard to decide a location of the plate using only seismic methods.The precise2-dimensional conductivity model was established.This model is characterized by following points.1)North dipping resistive layer(&gt；500Ω·m)exists beneath both Chugoku and Shikoku districts.The upper depth of the resistive layer is 20公里，30公里beneath the southern and northern part of the Shikoku district，and 40公里，60公里beneath the southern and northern part of the Chugoku district，respectively。2)Conductive layer(~100Ω·m)exists beneath the north dipping resistive layer。3)Highly conductive layer is found at 40 km-depth beneath the northern most part of the Chugoku district.We interpreted the north dipping resistive layer is the Philippine Sea plate。

We delineated the Philippine Sea plate beneath the South-western Japan arc using the electro-magnetic method in this study.The Network-MT method is one of the magnetotelluric methods and it is suitable for surveying a conductivity structure beneath a wide area and to a deep portion. We applied the method in Chugoku and Shikoku district to reveal a subsurface structure beneath the South-west Japan arc. We focused on the where and how does the Philippine Sea Plate exist. The first reason is that a location of The Philippine Sea plate is most important factor to controlled occurrences of earthquakes, because the cold and rigid plate makes large influence on a thermal state and/or a distribution of seismic-wave velocity. Second reason is that it is hard to decide a location of the plate using only seismic methods.The precise 2-dimensional conductivity model was established. This model is characterized by following points.1) North dipping resistive layer (> 500Ω・m) exists beneath both Chugoku and Shikoku districts. The upper depth of the resistive layer is 20km, 30km beneath the southern and northern part of the Shikoku district, and 40km, 60km beneath the southern and northern part of the Chugoku district, respectively. 2) Conductive layer (~100Ω・m) exists beneath the north dipping resistive layer. 3) Highly conductive layer is found at 40km-depth beneath the northern most part of the Chugoku district. We interpreted the north dipping resistive layer is the Philippine Sea plate.