In situ video observations and sediment core samplings were performed at two hadal sites in the Japan Trench on July, 2011, four months after the Tohoku–Oki earthquake. Video recordings documented dense nepheloid layers extending ~30–50 m above the sea bed. At the trench axis, benthic macrofauna was absent and dead organisms along with turbid downslope current were observed. The top 31 cm of sediment in the trench axis revealed three recent depositions events characterized by elevated 137Cs levels and alternating sediment densities. At 4.9 km seaward from the trench axis, little deposition was observed but the surface sediment contained 134Cs from the Fukushima Dai–ichi nuclear disaster. We argue that diatom blooms observed by remote sensing facilitated rapid deposition of 134Cs to hadal environment and the aftershocks induced successive sediment disturbances and maintained dense nepheloid layers in the trench even four months after the mainshock.

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Abstract.Intraplate earthquake zones in the back arc of NE Japan were imaged by wide-band magnetotelluric

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Deep-sea hydrothermal vents discharge abundant reductive energy into oxidative seawater. Herein, we demonstrated that in situ measurements of redox potentials on the surfaces of active hydrothermal mineral deposits were more negative than the surrounding seawater potential, driving electrical current generation. We also demonstrated that negative potentials in the surface of minerals were widespread in the hydrothermal fields, regardless of the proximity to hydrothermal fluid discharges. Lab experiments verified that the negative potential of the mineral surface was induced by a distant electron transfer from the hydrothermal fluid through the metallic and catalytic properties of minerals. These results indicate that electric current is spontaneously and widely generated in natural mineral deposits in deep-sea hydrothermal fields. Our discovery provides important insights into the microbial communities that are supported by extracellular electron transfer and the prebiotic chemical and metabolic evolution of the ocean hydrothermal systems.

We conducted a self-potential survey at an active hydrothermal field, the Izena hole in the mid-Okinawa Trough, southern Japan. This field is known to contain Kuroko-type massive sulphide deposits. This survey measured the self-potential continuously in ambient seawater using a deep-tow array, which comprises an electrode array with a 30-m-long elastic rod and a stand-alone data acquisition unit. We observed negative self-potential signals not only above active hydrothermal vents and visible sulphide mounds but also above the flat seafloor without such structures. Some signals were detectable >50 m above the seafloor. Analysis of the acquired data revealed these signals’ source as below the seafloor, which suggests that the self-potential method can detect hydrothermal ore deposits effectively. The self-potential survey, an easily performed method for initial surveys, can identify individual sulphide deposits from a vast hydrothermal area.

We obtained an electrical transect image of the Niigata‐Kobe Tectonic Zone (NKTZ). Several major active faults are located in this zone of concentrated deformation. The main features of the final two‐dimensional model are a thick resistive block in the upper crust, with a thinned‐out portion beneath the Atotsugawa Fault, and a strong conductor in the lower crust that intrudes upward into the upper resistor. The upper crustal resistive zone corresponds well to the spatiality of the NKTZ, and relatively conductive zones sandwiching this resistor may contribute to observed changes in displacement rates. The overlapping locations of the conductor and the low‐velocity body in the lower crust indicate that the conductor represents a zone that was weakened by fluids. Given that microearthquakes are localized in the regions between the resistive and conductive zones, we suggest that the distribution of earthquakes is influenced by intrusions of fluid derived from the conductor.