Contributionof subsurface fluids to tectonic activities beneath the Kii Peninsula, southwestern part of Japan

The Kii Peninsula in the forearc region of southwestern Japan has distinct structural and tectonic features due to the subduction of the Philippine Sea (PHS) slab. To solve this problem and investigate the fluid supply process, deeper shape of plutonic rocks, and their tectonic influence, we reanalysed all Network-MT data acquired in the Kii Peninsula to construct a wide and deep resistivity structure model by conducting 3-D inversion analysis. The 3-D resistivity structure model obtained contains highly resistive zones that are widely distributed in the eastern part of the Kii Peninsula shallower than 20 km below the surface. The highly resistive zones reach depths of approximately 30–40 km in their deepest portions and correlate well with previously identified high seismic velocity zones and gravity anomalies. This correspondence implies that the highly resistive zones are interpreted as acidic rock bodies because the high resistivity of a rock body implies a lack of interstitial fluid in the body. High-resistivity zones were surrounded by low-resistivity zones. Based on the obtained 3-D resistivity structure, the Kumano and Ohmine Plutons have a relatively low permeability. This may prevent the upward flow of fluids supplied by the PHS slab and control earthquake occurrence with deep fluids. Comparison with DLFE distributions shows most of the DLFEs occur in the area where the Kumano and Ohmine plutons reach above the top of the PHS slab. A comparison with regular earthquake distributions shows that microseismicity epicentres are concentrated at the margins of the high-resistivity zone. This may be due to differences in local lithological characteristics, such as permeability. This indicates that the local lithological characteristics of the upper plate, such as permeability and brittleness, in the vicinity of the plate boundary control deep fluid transport and influence seismic activities such as DLFEs or micro-seismicity.