The upper mantle structure beneath the Japanese Islands inferred from
travel-time tomography
Junichi Nakajima1, and Akira Hasegawa1
1
Research Center for Earthquake Prediction and Volcanic Eruptions, Graduate School of Science,
Tohoku University, Aoba-ku, Sendai 980-8578, Japan.
nakajima@aob.geophys.tohoku.ac.jp
1.
Introduction
Since late 1990’s, a dense nationwide seismograph network has been constructed in the
Japanese Islands with an average station separation of ~ 20 km, which has produced the highest quality data in the world and contributed to enhance the understanding of seismo and volcano tectonics. Tomographic studies using the high-quality data have revealed heterogeneous
structures in the upper mantle structure beneath the Japanese Islands, with implications for
fluid transportation in subducting slabs and arc magmatism.
2. Northeastern Japan
A belt of intraslab seismicity, perhaps caused by dehydration of eclogite-forming phase
transformations, has been found in the Pacific slab crust at depths of 70-90 km parallel to
iso-depth contours of the plate interface, showing the major locations of slab dehydration
(Kita et al., 2006; Hasegawa et al., 2007). Fluids thus released from the slab may be hosted by
serpentine and chlorite just above the slab and is dragged downward. High-resolution seismic
tomography has detected this layer of serpentine or chlorite as a thin S-wave low-velocity
layer. Serpentine and chlorite thus brought down to 150-200 km depth should decompose
there. Water released is then transported upward and encounters the upwelling flow right
above, which perhaps causes partial melting of materials within the upwelling flow (Nakajima
et al., 2001, 2005). Seismic tomography studies have clearly imaged this upwelling flow as an
inclined sheet-like seismic low-velocity zone at depths of 30-150 km in the mantle wedge
sub-parallel to the subducting slab. This upwelling flow finally meets the Moho below the
volcanic front, and melts thus transported are perhaps stagnated direct below the Moho, acting
as magma sources to active volcanoes.
3. Southwestern Japan
We determined the geometry of the subducting Philippine Sea slab from Kanto to Kyushu,
with comprehensive interpretation of seismic velocity structures, seismicity, focal mechanisms, receiver function analyses, and refraction surveys (e.g., Hirose et al., 2008l; Nakajima
et al., 2009). The estimated shape of the Philippine Sea slab is very complicated and the slab
is subducting to a depth of 140 km in central Japan (Figure 1). Seismogenesis and volcanism
may be related closely to the geometry of the Philippine Sea slab. Nakajima and Hasegawa
(2007) revealed a large upwelling in the upper mantle in the Chugoku district and interpreted
it as the origin of magmas for Quaternary volcanoes. A branch of this upwelling flow appears
to extend exceptionally to the uppermost mantle beneath the Kii peninsula. High 3He/4He ra-
tions observed in the Kii peninsula and along the coastline of the Japan Sea can be attributable
to mantel-derived helium carried by the mantle upwelling.
References
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Fig. 1. Geometry of the Philippine Sea and Pacific slabs in SW Japan.