後藤 忠徳

The temperature of materials would be raised when the materials are exposed to the sunlight. Recently, it has been experimentally confirmed that such temperature rise may be restrained when coating the materials with paint admixed with fine silica spheres. Experimental consideration of this type of paint has been conducted, but how the paint controls the temperature rise has merely been clarified theoretically. The best diameter of the silica spheres to be admixed is not well understood, either. In this study, we hypothesized that the scattering of light would be attributed to restrain the temperature rise and tried to estimate the optimum size of the silica spheres. We confirmed that our hypothesis would be justified. In the calculation of the scattering intensity, the diameter of spheres in conjunction with the wavelength of incident lights would be the predominant parameter to the scattering effects. Our results might explain that our experimentally observed phenomenon is caused by the scattering of light, i.e., electromagnetic waves. (C) 2011 Elsevier Ltd. All rights reserved.

We performed a three-year seafloor electromagnetic survey in the Philippine Sea, including the western edge of the Pacific Ocean, to image electrical features of a deep mantle slab stagnating in the transition zone and the surrounding mantle in three dimensions (3-D). The project iterated one-year deployment of ocean bottom electromagnetometers (OBEMs) using a total of 37 instruments installed at 18 sites. The data obtained have been analyzed in the order of their recovery based on a magnetotelluric (MT) method. In this study, we attempt to obtain a one-dimensional (1-D) electrical conductivity model beneath the Philippine Sea and the Pacific region separately that can be used as a reference model in the first step toward the 3-D analysis. The resultant 1-D models show three main features: (1) The conductivity in the shallower 200 km of the upper mantle depths of the two regions contrasts sharply, which is qualitatively consistent with the large difference in lithospheric age. (2) The conductivity at 200-300 km depth in both regions is more or less the same at approximately 0.3 S m(-1). (3) The conductivity just below 400 km depth is higher for the Philippine Sea mantle than for the Pacific mantle. The conductivity structure can be interpreted in terms of the thermal structure, mantle hydration, and existence of partial melt using experimental results for the conductivity of mantle minerals. If the conductivity is interpreted simply as the effect of temperature, the mantle beneath the Philippine Sea could be hotter than the dry solidus of mantle peridotite and thus partially molten. However, beneath the Pacific region, the present analysis suggests that the partial melting is not required under the assumed peridotitic composition even if we consider mantle hydration. (C) 2010 Elsevier B.V. All rights reserved.

This paper reports on a magnetotelluric (MT) survey across the central Mariana subduction system, providing a comprehensive electrical resistivity image of the upper mantle to address issues of mantle dynamics in the mantle wedge and beneath the slow back-arc spreading ridge. After calculation of MT response functions and their correction for topographic distortion, two-dimensional electrical resistivity structures were generated using an inversion algorithm with a smoothness constraint and with additional restrictions imposed by the subducting slab. The resultant isotropic electrical resistivity structure contains several key features. There is an uppermost resistive layer with a thickness of up to 150 km beneath the Pacific Ocean Basin, 80-100 km beneath the Mariana Trough, and 60 km beneath the Parece Vela Basin along with a conductive mantle beneath the resistive layer. A resistive region down to 60 km depth and a conductive region at greater depth are inferred beneath the volcanic arc in the mantle wedge. There is no evidence for a conductive feature beneath the back-arc spreading center. Sensitivity tests were applied to these features through inversion of synthetic data. The uppermost resistive layer is the cool, dry residual from the plate accretion process. Its thickness beneath the Pacific Ocean Basin is controlled mainly by temperature, whereas the roughly constant thickness beneath the Mariana Trough and beneath the Parece Vela Basin regardless of seafloor age is controlled by composition. The conductive mantle beneath the uppermost resistive layer requires hydration of olivine and/or melting of the mantle. The resistive region beneath the volcanic arc down to 60 km suggests that fluids such as melt or free water are not well connected or are highly three-dimensional and of limited size. In contrast, the conductive region beneath the volcanic arc below 60 km depth reflects melting and hydration driven by water release from the subducting slab. The resistive region beneath the back-arc spreading center can be explained by dry mantle with typical temperatures, suggesting that any melt present is either poorly connected or distributed discontinuously along the strike of the ridge. Evidence for electrical anisotropy in the central Mariana upper mantle is weak.

Sagami Bay is an active tectonic area in Japan. In 1993, a real-time deep sea floor observatory was deployed at 1,175 m depth about 7 km off Hatsushima Island, Sagami Bay to monitor seismic activities and other geophysical phenomena. Video cameras monitored biological activities associated with tectonic activities. The observation system was renovated completely in 2000. An ocean bottom electromagnetic meter (OBEM), an ocean bottom differential pressure gauge (DPG) system, and an ocean bottom gravity meter (OBG) were installed January 2005; operations began in February of that year. An earthquake (M5.4) in April 2006, generated a submarine landslide that reached the Hatsushima Observatory, moving some sensors. The video camera took movies of mudflows; OBEM and other sensors detected distinctive changes occurring with the mudflow. Although the DPG and OBG were recovered in January 2008, the OBEM continues to obtain data.

We have developed a new cost-effective scientific underwater cable system named Tokai Submarine Cabled Network Observatory for Nowcast of Earthquake Recurrences (Tokai-SCANNER) using a decommissioned optical underwater telecommunication cable. We have used this cable in two ways simultaneously: 1) to construct an ocean-bottom observatory at the end of the cable, and 2) to use the cable as a long emitting antenna to sense electromagnetic properties of the Earth's crust. We have also developed a new time-synchronization system that sends a one-pulse-per-second (1PPS) signal and NMEA data to underwater sensors. To vary the supply voltage and current to the observatory and to emit a low-frequency electromagnetic field around the underwater cable, we have also developed an underwater power unit that has a wide input voltage and current range. Tokai-SCANNER has been functioning since April 2007.

A number of extensive methane plumes and active methane seeps associated with large blocks of methane hydrates exposed on the seafloor strongly indicate extremely high methane flux and large accumulations of methane hydrate in shallow sediments of the Umitaka spur and Joetsu knoll of the Joetsu basin 30 km off Joetsu city, Niigata Prefecture. Crater-like depressions, incised valleys, and large but inactive pockmarks also indicate methane activities over the spur and knoll. These features imply strong expulsions of methane gas or methane-bearing fluids, and perhaps lifting and floating-up of large volumes of methane hydrate to the sea surface.High heat flow, similar to 100 mK/m, deposition of organic-rich strata, similar to 1.0 to 1.5% TOC, and Pliocene-Quaternary inversion-tectonics along the eastern margin of the Japan Sea facilitate thermal maturation of organic matters, and generation and migration of light-hydrocarbons through fault conduits, and accumulation of large volumes of methane as methane hydrate in shallow sediments. Microbial methane generation has also contributed to reinforcing the methane flux of the Joetsu basin. Regional methane flux as observed by the depth of the sulfate-methane interface(SMI) is significantly high, < 1 m to 3 m, when compared to classic gas hydrate fields of Blake Ridge, 15 to 20 m, and Nankai trough, 3 to 15 m. delta C-13 of methane hydrate and seep gases are mostly within -30 to -50%, the range of thermogenic methane, while dissolved methane of the interstitial waters a few kilometers away from seep sites are predominated by microbial with delta C-13 of -50 to -100%.Seismic profiles have revealed fault-related, well-developed gas chimney structures, 0.2 to 3.5 km in diameter, on the spur and knoll. The structures are essential for conveying methane from deep-seated sources to shallow depths as well as for accumulating methane hydrate (gas chimney type deposits). The depth of BSR, which represents the base of gas hydrate stability (BGHS), on the spur and knoll is generally 0.20 to 0.23 seconds in two-way-travel time, whereas the BSRs in gas chimneys occur at 0.14 to 0.18 seconds, exhibiting a sharp pull-up structure. The apparent shallow BGHS is due to the accumulation of large volumes of high-velocity methane hydrate in gas chimneys.The depth to BGHS is estimated to be 115 m on an experimentally determined stability diagram, based on an observed thermal gradient of 100 mK/m. Then the velocity of the sediments on the Umitaka spur is calculated to be 1000 m/s, which is anomalously low compared to normal pelagic mud of 1600. 1700 m/s. This exciting finding leads to the important implication that sediments of the Umitaka spur contain significant amounts of free gas, although the sediments are well within the stability field of methane hydrate. The reasons for the existence of free gas in the methane hydrate stability field are not fully explained, but we propose the following possible mechanisms for the unusual co-existence of methane hydrate and free-gas in clay-silt of the spur. (i) High salinity effect of residual waters,(ii) degassing from ascending fluids,(iii) bound water effect and deficiency of free-waters, and(iv) micro-pore effect of porous media. All of these processes relate to the development of gas hydrate deposits of the Umitaka spur.Increased accumulation of methane hydrate(specific gravity similar to 0.91 g/cm(3)) in shallow sediments should have caused a gravity imbalance of methane hydrate bearing sediments, and eventually the methane hydrate blocks lifted and floated up to the sea surface(auto-collapse). Crater-like depressions and valleys are the heritage of such an auto-collapse process.Dark colored, thinly laminated units with a very low abundance of benthic foraminifers occur in 27 to 18 kyrBP, approximately the period of the LGM, indicating low-oxygen, euxinic conditions. Furthermore, delta C-13 of benthic foraminifers from the dark laminated unit exhibits sharp negative excursion toward similar to 21 kyrBP. A sea-level fall of similar to 120 m toward the LGM released the pressure of gas hydrate-bearing sediments, and presumably triggered the dissociation of subsurface methane hydrate, which, in turn, destabilized the entire gas chimney hydrate system, collapsing the gas chimney and leaving large and deep pockmarks.

The existence of fluid in seismogenic zones plays a key role in great earthquakes. Data on electrical conductivity structures obtained by electromagnetic surveys across the great earthquake zones show that the seismically locked zones correspond to low conductive zones. This low conductivity is possibly interpreted as relatively low fluid content in rock or sediment. For more discussion on the role of fluid in earthquake occurrence, we have recently started an electromagnetic and seismological monitoring project using long submarine cables off Toyohashi, in the southwest part of Japan's main island. The cables are located on the Tokai seismogenic zone, where both slow-slipping and locked zones are obvious by GPS observation. We constructed a seafloor observatory at the tip of the western cable, including a seismometer, pressure gauges, a magnetometer, voltmeters, and thermometers. These sensors measure and transfer data on the seafloor environment in real time. The data will be used for passive and active monitoring below the seafloor, including in the Tokai seismogenic zone.

During the past five years, the electro magnetic (EM) team of the Japan Agency for Marine Science-Technology (JAMSTEC) has conducted many observations to investigate the crustal and mantle structure in pursuit of various scientific aims in collaboration with other universities and institutes. Around the Nankai subduction zone, we constructed a crustal and regional resistivity model, and detected the low resistivity zone with subduction. Moreover, we developed new geophysical survey tools—the small ocean bottom electro magnetometer (OBEM) and the deep-towed DC resistivity survey system—to investigate the shallow crustal structure. We also conducted an ocean bottom EM array study in the Philippine Sea and other areas to investigate the mantle structure using long-term OBEMs, thereby obtaining high-quality EM data.

Methane hydrate (MH) is expected to be a new energy resource because a large amount of methane gas may be contained in the MH layer. MH has high resistive feature, so that the marine EM surveys will be a useful tool for detecting subseafloor MH. We introduce two case studies of marine electromagnetic surveys off Japan coast to detect MH below the seafloor. The first case is carried out in the Sea of Japan, 2005. Our marine deep towed EM streamer cable could image the subseafloor resistivity distribution to the depth of 100m below the seafloor, and successfully detected MH zones as high resistivity. The second case, a marine CSEM experiment, with a deep-towed cable and an ocean bottom electromagnetometer (OBEM), is done off the Tokai area, along the Pacific side of Japan, 2006. A thin and deep MH zone (with thickness of about 30m and depth of about 200m below the seafloor) recognized in the borehole is not clearly imaged by our survey. The reason is mainly due to fluctuation of the track and direction of deep-towed source dipole. Further and careful analysis of CSEM data with acoustic navigation data will allow us to image the deep MH zone..

Natural magnetic fields are attenuated by electrically conductive water. For that reason, marine magnetotelluric surveys have collected data at long periods (1000-100 000 s). The mantle structure has been the main target of seafloor magnetotelluric measurements. To ascertain crustal structure, however, electromagnetic data at shorter periods are important, e. g. in investigations of megathrust earthquake zones, or in natural resource surveys. To investigate of the former, for example, electromagnetic data for periods of less than 1000 s are necessary. Because no suitable ocean bottom electromagnetometer (OBEM) has been available, we have developed a small OBEM and ocean bottom electrometer (OBE) system with a high sample rate, which has an arm-folding mechanism to facilitate assembly and recovering operations. For magnetic observation, we used a fluxgate sensor.Field observations were undertaken to evaluate the field performance of our instruments. All instruments were recovered and their electromagnetic data were obtained. Results of the first experiment show that our system functioned well throughout operations and observations. Results of other field experiments off Tottori support the claim that the electromagnetic data obtained using the new OBEM and OBE system are of sufficient quality for the survey target. These results suggest that this device removes all instrumental obstacles to measurement of electromagnetic fields on the seafloor.

We have developed a new deep-towed marine DC resistivity survey system. It was designed to detect the top boundary of the methane hydrate zone, which is not imaged well by seismic reflection surveys. Our system, with a transmitter and a 160-m-long tail with eight source electrodes and a receiver dipole, is towed from a research vessel near the seafloor. Numerical calculations show that our marine DC resistivity survey system can effectively image the top surface of the methane hydrate layer. A survey was carried out off Joetsu, in the Japan Sea, where outcrops of methane hydrate are observed. We successfully obtained DC resistivity data along a pro. le similar to 3.5 km long, and detected relatively high apparent resistivity values. Particularly in areas with methane hydrate exposure, anomalously high apparent resistivity was observed, and we interpret these high apparent resistivities to be due to the methane hydrate zone below the seafloor. Marine DC resistivity surveys will be a new tool to image sub-seafloor structures within methane hydrate zones.

A new cabled observatory Tokai SCANNER (Tokai Submarine Cabled Network Observatory for Nowcast of Earthquake Recurrences) was completed in April 2007. The system is located off the coast of Toyohashi-city where huge earthquakes are anticipated and continuous long-term monitoring is needed to promote seismic study. [11 [2][31 A new junction box which is equipped with underwater mateable connectors was installed at the end of the cable. The cable itself will be also applied to electromagnetically study for the inner structure of crust. This paper describes the outline of the Tokai SCANNER and initial evaluation results of the component units.

A new cabled observatory was constructed off Toyohashi in central Japan using a pair of decommissioned underwater telecommunication cables. While providing electric power and communication line to the underwater observatory, the cables will be used as a long active antenna to monitor the electro-magnetic properties of the earth's crust that are related to water contents.To secure a safe deployment of the junction unit at the end of the cable, we conducted computer simulations before the deployment. The junction units must usually be placed exactly as planned. This paper presents that the simulation results coincide with the actual construction result.

Western part of Sagami Bay is one of the active tectonic areas in Japan. In this area, Teishi Knoll, volcanic seamount, erupted in 1989 and the earthquake swarms occurs repeatedly every few years in the eastern coast of the Izu Peninsula. The real-time deep sea floor observatory was deployed about 7 km off Hatsushima Island, Sagami Bay, at a depth of 1174 m in 1993 to monitor seismic activities, underwater pressure, water temperature and deep currents. The video camera and lights were also mounted in the observatory to monitor the relations among biological activities associated with the tectonic activities. The observation system including submarine electro-optical cable with a length of 8 km was completely renewed in 2000. The several underwater-mateable connectors are installed in the new observatory for additional observation instruments. A ocean bottom electro-magnetic meter(OBEM), precise pressure sensor and ocean bottom gravity meter were installed using ROV Hyper-Dolphin in the cruise of R/V Natsushima from January 9 to 14, 2005. We started to operate them at February 10, 2005 after checking those of data qualities. Observed data have been sent to Yokohama institute, JAMSTEC. Around the sagami bay, seismic activity is very high. A large earthquake (M5.4) occurred off Izu peninsula at April 21, 2006, and submarine land slide was then generated. Generated mud flow reached to the Hatsushima station, and moved positions of some sensors. The video camera was able to take a movie of mud flow. A OBEM and other sensors also detected some distinctive changes with the mud flow.

A new time-synchronization system for cabled underwater observation systems was developed and evaluated. It provides accurate GPS time signal (1PPS Signal) and accurate clock to underwater sensors. 1PPS signal is essential for longterm and continuous acoustical geodetic monitoring with cabled observation system. Geodetic observation on the seafloor is important in Japan for disaster mitigation, as most plate boundaries which cause mega-thrust earthquakes are placed under seafloor. The system will be installed in the Tokai-SCANNER which will be constructed off Toyohashi in central Japan. The evaluation tests show it can provide 1PPS signal of about 10 nanoseconds accuracy which surpasses the required accuracy for acoustical geodetic monitoring. This paper will describe the outline of the system and the results of evaluation test.

Intraplate earthquake zones in the back arc of NE Japan were imaged by wide‐band magnetotelluric (MT) soundings. The 90km long MT profile of 34 stations extends over the two topographic features, the Dewa Hills and the Ou Backbone Range, which were uplifted by thrust faults. MT data show two‐dimensionality and strong TE/TM anisotropic responses at the periods around 100 s. After tensor decompositions with regional strike of N12°E, two‐dimensional inversion was carried out where static shift was also a model parameter. The final model is characterized by conductive blocks in the mid‐crust to account for the anisotropic responses. Correlation of the conductors to the seismic scatterers and to the low velocity anomalies suggests that the conductors represent fluids. High seismicity clustering near the rims of conductors suggests that the intraplate seismicity results either from the migration of the fluids to less permeable crust or from local stress concentration near the structural boundary.