**4. EM view of the seismogenic zone beneath southwest Japan**

Southwest Japan makes a good contrast to northeast Japan in terms of Volcanology, Seismology and Geomagnetism in the sense that:


In the following, we will try to illustrate how the magma source can be multiple in southwest Japan and how the subducting young slab influences the seismogenic zone beneath southwest Japan by reviewing an EM study in and around the Kyushu Island and the ongoing field work in the back-arc region of southwest Japan.

#### **4.1 Mantle plume in the west of Kyushu Island**

It has been long known that if one calculates GDS responses using short-period vector geomagnetic variations observed in southwest Japan, they end up with |*p(f)*|<<|*q(f)*| and *q(f)* > 0 where *p(f)* and *q(f)* are the transfer functions (i.e., the GDS responses) appeared in Eq. (1). This implies that there exists a prominent electrical conductivity anomaly in the west of southwest Japan. In order to identify the intensity of the conductivity anomaly and its spatial extent, a genetic algorithm inversion (e.g., Sambridge & Drijkoningen, 1992) of the observed GDS responses using non-uniform thin sheet approximation (McKirdy et al., 1985) was applied.

The genetic algorithm inversion converged at an rms of 3.20 after 112 iterations. Because we worked with 50 models per iteration, the best model in Fig. 3 is the result of more than 5000 forward calculations using the non-uniform thin-sheet approximation. It is evident that the model can give no constraints on the spatial extent both in westward and southward directions. This is due to the spatial distribution of the original GDS dataset that were mainly collected on land in southwest Japan. Direct EM measurements at the seafloor both in the south and west of the model in Fig. 3 will be indispensable for any further improvement in spatial resolution of the derived model.

The surface conductance (a product of the layer thickness and its electrical conductivity) anomaly model in Fig. 3 has two intriguing features in terms of arc volcanism if the distribution can be interpreted as that of a subsurface mantle plume:


Fig. 3. Result of the genetic algorism (GA) inversion in search for the optimized conductance distribution within the surface thin sheet. Additional conductance [S] required by the GA inversion is shown by the white-to-red scale. Bathymetric contours are also shown. Triangles denote Quarternary volcanoes in the East Asia (yellow) and on the small volcanic branch (magenta) such as the Unzen Volcano and the Cheju Island. The focal mechanism of the main shock of the west off Fukuoka Prefecture earthquake (Mw 6.7) is also shown. Reproduced from Toh & Honma (2008).

1. The plume can be one candidate of the multiple magma sources suggested by Iwamori

2. There seems to exist a short volcanic chain that branches out from the main volcanic

Fig. 3. Result of the genetic algorism (GA) inversion in search for the optimized conductance distribution within the surface thin sheet. Additional conductance [S] required by the GA inversion is shown by the white-to-red scale. Bathymetric contours are also shown.

Triangles denote Quarternary volcanoes in the East Asia (yellow) and on the small volcanic branch (magenta) such as the Unzen Volcano and the Cheju Island. The focal mechanism of the main shock of the west off Fukuoka Prefecture earthquake (Mw 6.7) is also shown.

(1991).

front on the Kyushu Island.

Reproduced from Toh & Honma (2008).

Presence of a regional-scale mantle plume in the middle of the East China Sea has been favoured by many researchers (e.g., Ichiki et al., 2006) since Miyashiro (1986) first claimed its existence. It is noteworthy that the geological strike of the volcanic branch is nearly parallel to the northeastern boundary of the partly discovered anomaly. A vertical slice of a seismic tomography result (Zhao et al., 2000) cutting through the northern Kyushu Island from the northwest to southeast direction has also imaged a low velocity branch toward the back-arc region. Another interesting evidence is the occurrence of the west off Fukuoka Prefecture earthquake near the edge of the electrical anomaly. Even though the direct cause of the earthquake is probably due to the extensional regional stress field in the back-arc region of the Kyushu arc induced by the mantle upwelling (Wei & Seno, 1998), the focal mechanism implies a lateral motion between the Eurasian plate and the Amurian plate.

#### **4.2 2-D electrical section of southwest Japan**

One major characteristic of the seismicity in southwest Japan is that the epicenters tend to concentrate within a belt of about 4-9 km wide parallel to the coast line of the Japan Sea (Kawanishi et al., 2009). Most of the focal depths there are shallower than approximately 10km and thus the earthquakes are occurring in the upper crust. In the seismic belt, several large earthquakes of M6.2-7.4 also occurred in 1943, 1983 and 2000, respectively. Furthermore, quaternary volcanoes, such as the Daisen and Oginosen Volcanoes are located in the seismic belt while the basalt that formed the Oki Islands in the back-arc region is much older (> 5 Myr) and of different composition (Kimura et al., 2003).

Wide-band MT observations have been made along a number of north-south profiles on land since 1998 so as to reveal high conductivity regions beneath the seismic belt on each MT profile. It is noteworthy that the earthquakes seem to occur on the boundary between the upper resistive crust and the highly conductive body in the lower crust. The high conductivity regions found beneath each wide-band MT profile may form a connected conductive zone extending in an almost east-west direction. Coincidence of the hypocenter distribution with the upper surface of the conductive zone as well as the presence of deep low-frequency events suggests that crustal fluid must involve the focal mechanism in the seismogenic zone.

In order to clarify the relation among the mantle dynamics in the back-arc region, the lower crustal conductors found on land and the complicated volcanism, seafloor EM observations were conducted off southwest Japan together with MT measurements on land. We laid out two seafloor MT arrays, one traversing the non-volcanic region in the eastern part of southwest Japan and the other running through a volcanic ridge including the Oki Islands. These seafloor arrays are indispensable to image the subducting Philippine Sea plate, a possible source of the crustal fluid and seismicity of the region.

Figure 4 shows the result of 2-D finite element forward modeling of the eastern profile, which ended up with an rms of 3.3. Note that the electrical section is a product of not inversion but forward modeling, although the finite element code was improved to give high precision even at locations very close to the coastline as well as those on the rugged bathymetry/topography. It was not until a superior differential scheme (Li et al., 2008) and triangular elements suitable for describing complicated bathymetry/topography had been adopted that the good precision in forward calculation was achieved.

The 2-D model shows that the lower crustal conductor has seaward extension at least more than 30 km further north of the coastline. Because we were unable to identify the tip of the 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., Nakajima & Hasegawa, 2007).

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 Philippine Sea plate is also shown by thick dashed lines.

#### **5. Conclusion**

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 beneath northeast Japan is governed by its regional stress field rather than by the presence of 'geofluid'. Major earthquakes of this region are mostly related to the processes involved with the plate boundary. However, the geofluid in northwest Japan is important for generation of hazardous inland earthquakes as well as water circulation in the wedge mantle.

The scenario by the warm and thin subducting plate beneath southwest Japan is more complicated than that of northeast Japan. In terms of Volcanology, the structure as well as the magma source is 3-D in the sense that there are several firm evidences for the presence of multiple magma sources (Iwamori, 1991; Toh and Honma, 2008) and 3-D seismic structures (Nakajima & Hasegawa, 2007). Because the subducting plate is too warm to carry the surface water deep into the mantle, there occurs major dehydration from the slab beneath the fore-arc region that causes much more geofluid-related seismic activities in southwest Japan (e.g., Obara & Hirose, 2006) than in northeast Japan. The young slab commenced its subduction several million years ago to have the penetration of its edge as close as just beneath the Japan Sea coast. The shallow penetration resulted in production of adakite magmas, which is a signature of slab melting and gives the volcanism in southwest Japan further complexity. However, it also turned out that the mantle upwelling in the backarc region of southwest Japan is governed by not the warm slab but the cold slab further below the warm slab.
