A tremendous amount of radioactivity was discharged because of the damage to cooling systems of nuclear reactors in the Fukushima No. 1 nuclear power plant in March 2011. Fukushima and its adjacent prefectures were contaminated with fission products from the accident. Here, we show a geographical distribution of radioactive iodine, tellurium, and cesium in the surface soils of central-east Japan as determined by gamma-ray spectrometry. Especially in Fukushima prefecture, contaminated area spreads around Iitate and Naka-Dori for all the radionuclides we measured. Distributions of the radionuclides were affected by the physical state of each nuclide as well as geographical features. Considering meteorological conditions, it is concluded that the radioactive material transported on March 15 was the major contributor to contamination in Fukushima prefecture, whereas the radioactive material transported on March 21 was the major source in Ibaraki, Tochigi, Saitama, and Chiba prefectures and in Tokyo.

[1] Recent seismic structural studies in trench-outer rise regions have shown that V p within the incoming oceanic plate systematically decreases toward the trench, probably owing to bending and fracturing of the plate. To understand the mechanisms acting to reduce V p , V s is critical because the V p /V s ratio is a sensitive indicator of lithology, porosity, and the presence of fluid. In the outer rise region of the Kuril trench, we conducted an extensive seismic refraction and reflection survey that revealed systematic changes in V p , V s , and V p /V s . Our results suggest that water content within the incoming oceanic plate increases toward the trench accompanied by the development of bending-related fractures at the top of the oceanic crust, consistent with the seawater percolation. Our results support the idea that plate bending and fracturing during the bending in the outer rise of the trench play an important role in the water cycle of subduction zones. Citation: Fujie, G.,

Hydration of the subducting oceanic plate determines the amount of water transported from Earth’s surface into its interior, and plate bending-related faulting (bend faulting) just prior to subduction is considered to promote hydration. Bend faulting shows significant spatial variation, but its contribution to hydration is still poorly understood. Here we present the results of controlled-source seismic surveys around the junction of the Japan and Kuril trenches. We found structural changes caused by bend faulting before subduction differed distinctly between both trenches and were well correlated with plate hydration after subduction, suggesting the bend faulting controls spatial variations in plate hydration. Differences in bend faulting are closely related to the angle between the current trench and the ancient spreading ridge, and the hydration is more extensive where this trench-ridge angle is oblique in the study area. Thus, we propose this angle is a major factor controlling plate hydration.

SUMMARY High‐frequency S‐wave envelopes of microearthquakes well reflect the medium inhomogeneity of the Earth. Defining the peak delay time as the time lag from the direct S‐wave onset to the maximum amplitude arrival of its envelope, we use this quantity to evaluate the strength of multiple forward scattering and diffraction due to random inhomogeneities along the seismic ray path. Analysing peak delay times of many microearthquakes occurred along the subducting Pacific Plate for 2–4, 4–8, 8–16 and 16–32 Hz frequency bands, we find a clear path dependence of the peak delay time in relation to the distribution of Quaternary volcanoes in northeastern Japan. Peak delay times of less than 2 s are usually observed at most of the stations in the study area, but large peak delay times of more than 5 s are observed in the backarc side stations for the case that S wave propagates beneath Quaternary volcanoes. The large peak delay times are inferred to be generated at a depth of 20–60 km beneath Quaternary volcanoes by considering ray paths under a 1‐D velocity structure. These strongly inhomogeneous regions are located at low‐velocity and high Vp/Vs regions revealed from tomography, which suggests the inhomogeneity may be related to dykes and melts of ascending magma.

Back‐arc basins are a primary target to understand lithospheric evolution in extension associated with plate subduction. Most of the currently active back‐arc basins formed in intraoceanic settings and host well‐developed spreading centers where seafloor spreading has occurred. However, rift structure at its initial stage, a key to understand how the continental lithosphere starts to break in a magma‐rich back‐arc setting, is poorly documented. Here we present seismological evidence for structure of the southern Okinawa Trough, an active rift zone behind the Ryukyu subduction zone. We find that the southern Okinawa Trough exhibits an almost symmetric rift system across the rift axis (Yaeyama Rift) and that the sedimentary layers are highly cut by inward dipping normal faults. The rift structure also accompanies a narrow (2–7 km wide) on‐axis intrusion resulted from passive upwelling of magma. On the other hand, an active submarine volcano is located ~10 km away from the rift axis. The P wave velocity (Vp) model derived from seismic refraction data suggests that the crust has been significantly thinned from the original ~25 km thick arc crust and the thinnest part with 12 km thickness occurs directly beneath the rift axis. The velocity model also reveals that there exists a thick layer with Vp of 6.5–7.2 km/s at lower crustal levels and may indicate that mantle materials accreted at the bottom of the crust during the crustal stretching. The abrupt crustal thinning and the velocity‐depth profile suggest that the southern Okinawa Trough is at a transitional stage from continental rifting to seafloor spreading.