Three-dimensional deep electrical resistivity structure beneath the Kii Peninsula, southwestern Japan: fluids and their contribution to seismic activities
Abstract
Abstract 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. Diverse seismic activities, including deep low-frequency earthquake (DLFE), slab boundary earthquakes, and seismic swarms, have been observed beneath the Peninsula. These seismic activities may have been caused by fluids dehydrated from the subducting PHS slab. Because subsurface fluids are thought to play an important role in seismic activities, understanding the distribution of deep fluids is essential to elucidate the earthquake generation mechanism. Therefore, we estimated the deep 3-D electrical resistivity structure beneath the Kii Peninsula, using the Network-MT method, to investigate the fluid supply system and distribution and its relationship with seismic activities. The obtained 3-D resistivity structure contains highly resistive zones that are widely distributed at depths shallower than 20 km in the eastern part of the Kii Peninsula. The highly resistive zones reach depths of approximately 30–40 km in their deepest portions and spatially correlate well with high seismic velocity zones and large positive gravity anomalies. This correspondence suggests that the highly resistive zones are the Kumano and Ohmine Plutons, consisting of acidic rock bodies. Most DLFEs occur in the area where the Kumano and Ohmine plutons extend down to the top of the PHS slab, implying that those plutons act as an…
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