Keizo Sayanagi

Trial of Multidisciplinary Observation at an Expandable Sub-Marine Cabled Station “Off-Hatsushima Island Observatory” in Sagami Bay, Japan

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.

Interplate locking condition derived from seafloor geodetic data at the northernmost part of the Suruga Trough, Japan

We observed seafloor crustal deformation at two observation sites on opposite sides of the Suruga Trough off Japan from 2005 to 2011 to investigate the interplate locking condition at the source region of the anticipated great subduction earthquake, named Tokai earthquake. We estimated the displacement velocity vectors relative to the Amurian Plate on the basis of repeated observations. Our results at the two points, Suruga northeast and Suruga northwest (SNW) were 42 ± 8 mm/yr toward N94 ± 3°W and 39 ± 11 mm/yr toward N84 ± 9°W, respectively. These directions are the same as those measured at on-land GPS stations. The magnitudes of the velocity vectors indicate a significant shortening of approximately 4 mm/yr between SNW and on-land GPS stations located to the west of the Suruga Trough. The results show that the plate interface is strongly locked (no slip) shallower than the source region of the anticipated Tokai earthquake.

Scientific survey and monitoring of the off-shore seismogenic zone with Tokai SCANNER: submarine cabled network observatory for nowcast of earthquake recurrence in the Tokai region, Japan

Existence of fluid on seismogenic zones has a key role on great earthquakes. The electrical conductivity structures obtained by electromagnetic survey across the great earthquake zones show that the seismically locked zones correspond to the low conductive zones. The low conductivity is possibly interpreted as relatively low fluid content. For more discussion on the role of fluid to earthquake occurrence, we have just started an electromagnetic and seismological monitoring by using long submarine cables off Toyohashi, the southwest Japan Island. The cables are located on the Tokai seismogenic zone, where both slow-slipping and locked zones are obvious by GPS observation. Here, we introduce the recent and upcoming situations of the project.

Development of a New Cabled Observatory "Tokai SCANNER"

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. 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.

Multidisciplinary observations at an expandable sub-marine cabled station off the Hatsushima island, the Sagami bay, Japan

Western part of Sagami Bay is one of the active tectonic areas in Japan. In this area, Teishi Knoll, volcanic scamount, 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. An 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. An OBEM and other sensors also detected some distinctive changes with the mud flow.

Electromagnetic survey around the seafloor massive sulfide using autonomous underwater vehicle

The recent growth of world-wide requirement of metals demands advanced explorations for finding metal mine and deposits. The feasibility studies demonstrated that the electromagnetic responses are very sensitive to the conductive layer simulating the submarine massive sulfide (SMS) deposits, which is buried at the depth of several tens meters. On the basis of the results, we developed instruments for the marine controlled-source electromagnetic (CSEM) survey with autonomous underwater vehicle (AUV), on which a transmitter was attached. For the real field test, R/V Yokosuka and AUV Urashima were used. The target region is a real deep-sea mine in a caldera structure called Bayonnaise, located in the Izu-Bonin island arc, south of Japan. We succeeded in the test experiment along four survey lines with current shooting from AUV. Six ocean-bottom receivers (OBEM) simultaneously recorded those signals. The maximum source-receiver distance, in which we can detect the artificial current signals, exceeds to about 500m. Therefore, the inferred maximum sounding depth will be 150m or more below the seafloor. For evaluating the anomalous attenuation or amplification of received electric field at OBEMs, the three-dimensional forward modeling including the real bathymetry and a simple subsurface structure having an uniform resistivity (1 Ohm-m) was employed. Comparison between the observed and synthesized received field gives us a three-dimensional pseudo-section of anomalous received field, which can visualize heterogeneity of sub seafloor structure qualitatively. On the basis of the preliminary result of our AUV-CSEM survey around the SMS, high conductive features are observed not only in the SMS exposed area, but also the surrounding area of SMS. It would reflect both the mineral deposits in and around the SMS and highly conductive pore water below the surface due to warm temperature by hydrothermal activities below the SMS. We conclude that our new technology imaging the near sub-seafloor structures will be useful for discussion about the geological background of SMS, and also be a powerful tool for the SMS detection and developments.

Methane hydrate detection with marine electromagnetic surveys: Case studies off Japan coast

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 deeptowed source dipole. Further and careful analysis of CSEM data with acoustic navigation data will allow us to image the deep MH zone..