**6. Acknowledgment**

194 Earthquake Research and Analysis – Seismology, Seismotectonic and Earthquake Geology

subducted Philippine Sea plate beneath the volcanic front of the profile, it is unlikely that the lower crustal conductor beneath the volcanic front stems from the slab melting at least for this particular portion of the island arc. Another major feature of the model is that it has a deep (> 100 km) conductor in the back-arc region. This conductor may be attributed to the magma source for the volcanism that made the Oki Islands. However, it is difficult to relate the conductor to the slab melting or the dehydration from the Philippine Sea plate, since the plate initiated its subduction too recently to allow itself enough penetration toward the back-arc region. It is more appropriate to regard it as a result of mantle upwelling from the deeper slab, i.e., the Pacific plate, whose subduction beneath the back-arc region of southwest Japan has been clearly imaged by recent seismic tomography studies (e.g.,

Fig. 4. The 2-D electrical section around the land-sea boundary of southwest Japan. The red vertical arrow indicates the location of the coastline of the Japan Sea. The red and black inverse triangles are the seafloor and land EM observation site, respectively. Small black dots show the distribution of hypocenters. Estimated location of the edge of the subducted

The very cold and thick subducting plate beneath northeast Japan can supply water deep into the back-arc region (Iwamori, 1998), which forms 3-D counter flows to generate arc volcanism of that part of the Japanese Islands (Tamura et al., 2002). Because of this scenario, the magmatic source of northeast Japan can be simple enough to be approximated by 'uni-source' magmatism even though the magmatic structure itself can remain 3-D. The seismogenic zone

Philippine Sea plate is also shown by thick dashed lines.

**5. Conclusion** 

Nakajima & Hasegawa, 2007).

We are indebted to captains and crew members of R/V Tansei-Maru, Ocean Reserach Institute, University of Tokyo for their skilful help at the time of the sea experiments in both back-arc regions of northeast and southwest Japan. The geomagnetic data presented in the subsection 4.1 were kindly provided by Geographical Survey Institute, Japan. Earthquake Research Institute, University of Tokyo is acknowledged for its continuous support throughout this work. The land and seafloor EM data necessary for obtaining the 2-D electrical section of southwest Japan were kindly provided by Disaster Prevention Research Institute at Kyoto University, Japan Agency for Marine-Earth Science and Technology and Kyushu Polytechnic College.
