Earth Planets Space, Vol. 53 (No. 9), pp. 861-871, 2001
Tetsuzo Seno1, Dapeng Zhao2, Yoji Kobayashi3, and Masao Nakamura1
1Earthquake Research Institute, University of Tokyo, Tokyo 113-0032, Japan
2Department of Earth Sciences, Ehime University, Matsuyama 790-0826, Japan
3Faculty of Earth Sciences, Tsukuba University, Tsukuba 305-0006, Japan
(Received January 23, 2001; Revised June 4, 2001; Accepted June 12, 2001)
Abstract: The seismicity in the subducting Philippine Sea slab (PHS) beneath southwest Japan shows a variety of modes of occurrence. We try to explain this variety on the basis of dehydration embrittlement in the subducting oceanic crust and/or mantle. The PHS subducting along the Nankai Trough shows commonly a single narrow seismic zone shallower than 60 km, which may reflect dehydration embrittlement in the hydrated subducting oceanic crust only, implying the lack of hydrated slab mantle. The PHS beneath Kanto, however, shows a double seismic zone (Hori, 1997) in the mantle part. Here the serpentinized mantle wedge of the Izu-Bonin fore-arc is subducting, and the double zone can be explained by its dehydration. Beneath Kii Peninsula and Kyushu, seismic events within the slab mantle have also been detected. This indicates that the PHS mantle beneath these areas is also hydrated, which may have resulted from subduction of the serpentine stable in the Izu-Bonin back-arc area. Aqueous fluids released from the serpentinized mantle beneath Kii Peninsula may have initiated partial melting in the mantle wedge, as indicated by the presence of high 3He/4He ratios in the natural gasses and the shallow seismic swarms in this region (Wakita et al., 1987).