Takashi Yokobiki

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.

Outline of New Cabled Observation System off Toyohashi

A new cabled observation system off the coast of Toyohashi in central Japan is now being developed. The system uses a couple of former underwater optical telecommunication cables of about 60 kilo-meters long. The authors will use the cable in two ways simultaneously. One is to build a new observatory at the end of the cable. Underwater sensors including a broadband seismometer, a precise water-pressure sensor and an electro-magnetometer will be connected to the junction unit using underwater mateable connectors. The other is to use the same cable simultaneously as a long emitting antenna to monitor the electro-magnetic property of the earth crust. We have developed a new time synchronization system. It provides precise 1PPS signal, clock and NMEA data to underwater sensors. In this paper, we will describe the outline of the system. The longterm monitoring will start in this April.

Power Supply System for Toyohashi Cabled Observatory with Wide Input-range

A new cabled observatory off Toyohashi in central Japan is now being developed. In this area, mega-thrust earthquakes are anticipated, and continuous long-term monitoring is needed to promote seismic study and to mitigate disasters. It will use a pair of decommissioned underwater optical telecommunication cables of 60 kilometers in length. A new junction box will be deployed at the end of the cable which will be equipped with five underwater mateable connectors. The cables will also be used to electromagnetically study the inner structure of crust. In order to use the cable as a low-frequency emitting antenna, the authors developed a new power supply system in which supply voltage/current can be widely controlled. This paper will describe the outline of the observatory and the power supply system

Development of an automated cable-laying system for DONET construction

This paper describes an automated cable-laying system that can automatically pay out a submarine cable on the seafloor while keeping balance with the ground speed. We have installed DONET (Dense Oceanfloor Network System for Earthquakes and Tsunamis) at Kumanonada and a ROV (Remotely Operated Vehicle) was successfully used to lay the cables which connect observatories to the nodes of the DONET. However, such a cable-laying work was regarded as one of the hardest works since it takes approximately 10 hours (about 10km distance) and is manually carried out by a ROV operator(s). The automated cable-laying system can contribute to reduce the physical and mental burdens on the ROV operator and speed up the construction of DONET2 (the second phase of DONET). In this paper, the components and cable-laying control strategies are described and some experimental results are presented. In addition, we report on the first mission to connect the borehole observatory C0002 to the Node D of the DONET. This achievement has made it possible to receive the data of the borehole observatory in real-time. The paid out cable is 8470m and the travelled distance is approximately 8000m.

Background seismic noise level in DONET seafloor cabled observation network

One of the observation target of the seafloor cabled observation network called Dense Ocean Network for Earthquake and Tsunamis (DONET) in the Nankai Trough, south of Japan, is slow and weak seismic motion. The target necessitated installation of broadband seismometers in the seafloor in a very quiet background noise environment. We developed a technique to install seismometer in a buried caisson using a piston corer and a remotely operated vehicle. We installed 18 observatories in the DONET using the technique, and evaluation of the resultant seismic background noise level and its relationship with the quality of the installation, was performed.

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.

Deployment of the junction unit for Off-Toyohashi cabled observatory

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 Earths 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.

Automated Cable-Laying System for Thin Optical-Fiber Submarine Cable Installation

We have installed the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) that includes 20 observatories at Kumanonada (called DONET1) for the purpose of monitoring the seismogenic zone around the Nankai Trough. During the construction of DONET1, a remotely operated vehicle (ROV) was successfully used to lay the optical-fiber submarine cables that connect the observatories with the science nodes; however, the cable-laying operation was onerous because it takes approximately 10 h and is manually performed by ROV operators. For this reason, the cable-laying operation is usually not performed on successive days, which makes it difficult to schedule the construction work. In this paper, we present an automated cable-laying system that can automatically pay out the optical-fiber submarine cable on the seafloor at a rate that keeps pace with the ground speed and also adjusts the cable tautness. The components and control strategies are first described, and the results of some experiments and the first mission are then presented. The experiments and the first mission demonstrate that the automated cable-laying system can reduce the physical and mental burdens on the ROV operator and speed up the installation of DONET2 in 31 observatories. DONET2 is planned to be deployed off the Kii Peninsula (on the western side of DONET1).

Advanced Real Time Monitoring System and Simulation Researches for Earthquakes and Tsunamis in Japan

Mega thrust earthquakes generated large tsunamis quite often. Based on lessons learned from the 2004 Sumatra and the 2011 East Japan Earthquakes/Tsunamis, we recognized the importance of real time monitoring on the natural hazards. Monitoring systems using multi kinds of sensors such as the accelerometer, broadband seismometer, pressure gauge, difference pressure gauge, hydrophone and thermometer is indispensable not only for mitigation of damage from earthquakes and tsunamis, but also for understanding of broadband crustal activities around mega thrust earthquake seismogenic zones. Therefore, we have developed the Dense Ocean floor Network for Earthquakes and Tsunamis (DONET) to acquire the seafloor data in real time around the Nankai trough seismogenic zone, southwestern Japan. The first phase of deployment (DONET1) was completed and the second phase (DONET2) is being developed at the time of writing of the manuscript. At the 2011 East Japan Earthquake, DONET1 observatories detected offshore tsunamis 15 min earlier than onshore stations. Furthermore, DONET1 and DONET2 will be expected to monitor silent phenomena such as low frequency tremors and slow earthquakes for the estimation of seismic stage which would occur in the inter-seismic or pre-seismic stage. The recurrence cycle of mega thrust earthquakes, modeling of tsunami inundation and seismic response on buildings and cities are also important in the disaster mitigation programs and related measures. Real-time monitoring data should be integrated with the advanced simulations for precise earthquake and/or tsunami early warnings and rapid estimation of the damages.

Decision-making on seafloor surveillance infrastructure site for Earthquake and Tsunami monitoring in Western Japan

DONET (Dense Ocean-floor Observatory Network for Earthquakes and Tsunamis) is a submarine cabled real-time seafloor surveillance infrastructure for earthquake activity at assumed focus region of mega-thrust earthquake around Japan. The original system DONET1 was constructed in To-Nankai earthquake focus region and twenty seafloor observatories are working in operation beginning in 2011 to contribute the earthquake and tsunami early warning program in Japan. Development of second seafloor network DONET2 was planned in 2010 to target Nankai earthquake focus region. This paper describes a decision making approach of DONET2 observation site arrangement based on the knowledge of DONET1 development and construction.