**3. EM view of the seismogenic zone beneath northeast Japan**

Northeast Japan is classified into the cold subduction regime and thus said to be the very spot of on-going water supply into the deep mantle (Iwamori, 2004). Injection of water into the deep mantle can produce electrical conductivity anomalies beneath back-arc regions. In order to image such kind of anomalies, we constructed a seafloor MT array in the Japan Sea (Toh et al., 2006).

The seafloor array consisted of six ocean bottom electromagnetometers (OBEMs) that are capable of measuring both vector geomagnetic and horizontal geoelectric fields in addition to horizontal tilt variations. The attitude data were used to rotate each measuring frame at the seafloor back to a common reference frame in the horizontal plane. Directions of the geomagnetic north at each site were estimated using the averages of the horizontal geomagnetic components in order to carry out azimuthal corrections. The thus corrected EM time-series were further processed by the robust remote reference MT response estimator in frequency domain (Chave et al., 1987) to yield MT impedance tensors in Eq. (3). The seafloor MT response functions, together with those at four sites on land, were used in the subsequent 2-D inversion to explain their spatial distribution as well as the frequency dependence.

To construct a 2-D electrical model of northeast Japan, a Reduced Basis OCCam's (REBOCC) inversion method (Siripunvaraporn & Egbert, 2000) was applied to the land and sea MT impedances observed at the latitude of 39.5N. The REBOCC inversion is a variant of Occam inversion (Constable et al., 1987), which works with sensitivity matrices in data space instead of the conventional model space. This significantly reduces the size of the sensitivity matrices required in the course of the inversion procedure. Because the REBOCC inversion prefers high correlation of the final model with a priori model, it was possible to build in the basic tectonic model of northeast Japan such as the presence of the thick and resistive subducting Pacific plate. The known 2-D section for crustal depths (Ogawa et al., 2001) was also included in the REBOCC inversion as another a priori information. The inversion converged at an rms of 3.55 using both TE and TM mode responses.

The derived electrical 2-D section (Toh et al., 2006), which is an EW slice of the non-volcanic part of northeast Japan, reveals a resistive shallow mantle and a conductive anomaly beneath the back-arc region at depths 150-200 km (Fig. 2). The electrical conductivity anomaly can be interpreted as a direct manifestation of slab dehydration associated with collapse of the high-temperature type serpentine (Iwamori, 1998) rather than that of a group of minor hydrous phases such as phlogopite (Tatsumi, 1989). To test the robustness of the anomaly, we examined changes in the rms when the anomaly was replaced by a normal and uniform mantle conductivity of 3.3x10-2 S/m. It turned out that the rms increase is too large to explain the observed MT data by the normal conductivity if we mask the anomaly surrounded by the black-dashed lines in Fig. 2. However, the increase was marginal if we do the same thing for the anomaly surrounded by the white-dashed lines. We, therefore, concluded that the anomaly surrounded by the black-dashed lines is required by the MT data.

Fig. 2. The 2-D electrical conductivity model of northeast Japan at the latitude of 39.5N. The inverted triangles with labels indicate the locations and names of the observation sites. The horizontal gray bar represents the island arc of northeast Japan, while dots denote hypocenters in this region. The areas surrounded by black- and white-dashed lines are those masked in the F-tests (See text for details). Reproduced from Toh et al. (2006).

The Pacific plate is subducting beneath New Zealand as well. Wannamaker et al. (2009) derived a 2-D electrical cross-section beneath the South Island of New Zealand using a wide-band MT dataset on a densely distributed profile perpendicular to the island arc strike. They found three conductivity anomalies beneath the fore-arc region, the volcanic front and the back-arc region. The fore-arc conductor can be regarded as a natural result of dehydration from a younger and thus relatively hot subducting plate. The age of the Pacific plate there is approximately twice as young as northeast Japan (70-75 Myr). The conductor at the volcanic front is no wonder if the MT transect traverses a volcanic part of the island arc. However, there is a significant difference in the depth of dehydration beneath the backarc region. They concluded the back-arc dehydration to occur at depths ranging from 75 to 100 km and attributed the process to breakdown of amphibole-zoisite. It is natural that the

Fig. 2. The 2-D electrical conductivity model of northeast Japan at the latitude of 39.5N. The inverted triangles with labels indicate the locations and names of the observation sites. The

hypocenters in this region. The areas surrounded by black- and white-dashed lines are those

The Pacific plate is subducting beneath New Zealand as well. Wannamaker et al. (2009) derived a 2-D electrical cross-section beneath the South Island of New Zealand using a wide-band MT dataset on a densely distributed profile perpendicular to the island arc strike. They found three conductivity anomalies beneath the fore-arc region, the volcanic front and the back-arc region. The fore-arc conductor can be regarded as a natural result of dehydration from a younger and thus relatively hot subducting plate. The age of the Pacific plate there is approximately twice as young as northeast Japan (70-75 Myr). The conductor at the volcanic front is no wonder if the MT transect traverses a volcanic part of the island arc. However, there is a significant difference in the depth of dehydration beneath the backarc region. They concluded the back-arc dehydration to occur at depths ranging from 75 to 100 km and attributed the process to breakdown of amphibole-zoisite. It is natural that the

horizontal gray bar represents the island arc of northeast Japan, while dots denote

masked in the F-tests (See text for details). Reproduced from Toh et al. (2006).

depth of the back-arc dehydration as well as the collapsing minerals at that depth differs in the case of northeast Japan and the South Island of New Zealand, since the different ages mean different thermal effects of the subducting plates on the wedge mantle. In the case of the South Island of New Zealand, the temperature may be too high for the hydrous minerals to penetrate deep into the mantle beneath its back-arc region.
