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Our 3-D electrical resistivity model clearly detects particular subsurface features for magmatism associated with subduction of the Philippine Sea Plate (PSP) in three regions: a southern and a northern volcanic region, and a non-volcanic region on the island of Kyushu. We apply 3-D inversion analyses for geomagnetic transfer function data of a short-period band, in combination with results of a previous 3-D model that was determined using Network-Magnetotelluric response function data of a longer-period band as an initial model in the present inversion to improve resolution at shallow depths; specifically, a two-stage inversion is used instead of a joint inversion. In contrast to the previous model, the presented model clearly reveals a conductive block on the back-arc side of Kirishima volcano at shallow depths of ~50 km; the block is associated with hydrothermal fluids and hydrothermal alteration zones related to the formation of epithermal gold deposits. A second feature revealed by the model is another conductive block regarded as upwelling fluids, extending from the upper surface of the PSP in the mantle under Kirishima volcano in the southern volcanic region. Third, a resistive crustal layer, which confines the conductive block in the mantle, is distributed beneath the non-volcanic region. Fourth, our model reveals a significant resistive block, which extends below the continental Moho at the fore-arc side of the volcanic front and extends into the non-volcanic region in central Kyushu.

During the 2000 eruption of the Miyakejima volcano in Japan, step-like tilt changes (TC) generally accompanied by very long period (VLP) seismic signals with a pulse-like shape and widths of ∼50 s were repeatedly observed (TC/VLP events). Kinematic GPS time series for Miyakejima were investigated in order to detect displacements associated with these events. We found that the kinematic GPS time series could be interpreted as the superposition of the following features: (1) displacement associated with the TC/VLP events, the source of which possibly corresponded to a shallow magma chamber represented by an almost vertical ellipsoidal cavity elongated NE–SW at a depth of 2–3 km; (2) displacement following TC/VLP events that may have been caused by exponential-type volume decreases of the same magma chamber with a decay constant of approximately half a day; and (3) displacements that may be caused by continuous volume decreases of the same magma chamber. These features broadly support the piston model of VLP seismic signals, in which a vertical piston of solid conduit material intermittently sinks into a magma chamber located a few kilometer beneath the edifice following deflation caused by continuous outflux of magma. The volume increase of the magma chamber associated with the TC/VLP events was found to be much smaller than that of the collapsed caldera, suggesting that most of the mass in the conduit sank into the magma chamber without generating VLP seismic waves or step-like TC.

[1] We estimated the spatial and temporal evolution of the preceding aseismic slip from January 2003 to January 2011, the coseismic slip of the Tohoku earthquake, and the postseismic slip after the earthquake based on global positioning system (GPS) data. Time-dependent analysis indicates aseismic slip offshore of Miyagi and Fukushima prefectures from 2004 associated with a series of subduction earthquakes that overlap the aseismic slip area. These preceding aseismic and coseismic slip areas are centered between the centers of the coseismic and afterslip areas of the Tohoku earthquake offshore of Miyagi prefecture, while they overlap the coseismic and afterslip areas of the Tohoku earthquake off Fukushima prefecture. The timing of moment magnitude nine (Mw9) -class earthquakes appears to be controlled by multiple preceding slip events, smaller earthquakes and their afterslip. The preceding aseismic and coseismic slip decreased the coupling rate off the Tohoku coast, suggesting the possibility that the preceding slip represented a precursive stage of the Tohoku earthquake. The afterslip of the Tohoku earthquake occurred in an area where the coseismic slip was not large, complementing the large coseismic slip zone. The afterslip along Iwate-Miyagi extends up to 80 km in depth and is currently the sole mechanism of strain release in this depth range. The source region of the anticipated Miyagi-Oki earthquake shows small postseismic slip after the Tohoku earthquake, reflecting the energy release at the time of the earthquake. Aftershock activity is roughly governed by an afterslip process.