Katsuyoshi Kawaguchi

Real-time geophysical measurements on the deep seafloor using submarine cable in the southern Kurile subduction zone

A permanent real-time geophysical observatory using a submarine cable was developed and deployed to monitor seismicity, tsunamis, and other geophysical phenomena in the southern Kurile subduction zone. The geophysical observatory comprises six bottom sensor units, two branching units, a main electro-optical cable with a length of 240 km and two land stations. The bottom sensor units are: 1) three ocean bottom broadband seismometers with hydrophone; 2) two pressure gauges (PGs); 3) a cable end station with environmental measurement sensors. Real-time data from all the undersea sensors are transmitted through the main electro-optical cable to the land station. The geophysical observatory was installed on the continental slope of the southern Kurile trench, southeast Hokkaido, Japan in July 1999. Examples of observed data are presented. Sensor noises and resolution are mentioned for the ocean bottom broadband seismometers and the PGs, respectively. An adaptable observation system including very broadband seismometers is scheduled to be connected to the branching unit in late 2001. The real-time geophysical observatory is expected to greatly advance the understanding of geophysical phenomena in the southern Kurile subduction zone.

Monitoring system for submarine earthquakes and deep sea environment

Although more than 80 percent of earthquakes in Japan occur on the seafloor, the seafloor seismic network on the seafloor is sparse and insufficient. To increase the network, the Comprehensive Seafloor Monitoring System was deployed in Nankai Trough off Cape Muroto in March 1997. The prototype system is a combination of observatories with a cable and without a cable. The former system comprises of two seismometers, two tsunami pressure gauges and a seafloor observatory with multiple sensors and 125 km long optical cable. The data are sent in realtime to the land station at Muroto and they are also transmitted to JAMSTEC in Yokosuka and Meteorological Agency of Japan. The latter system, which could be deployed at any place, is comprised of a seafloor observatory with multiple sensors and four long-term digital ocean bottom seismometers. The data could be recovered once every month by releasing pop-up buoys to the surface through the satellite. The system with a cable was deployed on the landward slope of Nankai Trough off Muroto at water depths between 1286 m and 3572 m. The system without cable will be deployed 200 km off Muroto in Shikoku Basin at a depth of 4300 m in early 1998. Five similar systems will be deployed until the year of 2002.

The DONET: A real-time seafloor research infrastructure for the precise earthquake and tsunami monitoring

A project of submarine cabled seafloor observatory network development has been carried out in Japan since 2006. This research has aimed to establish the technology of a large scale real-time seafloor research infrastructure development for earthquake, geodetic and tsunami observation. The project scheduled to install 20 sets of cabled earthquake and tsunami observatory to an active seismogenic zone with mega-thrust earthquakes called Nankai trough. This paper describes design concept of a novel ocean observation system and the key technologies for reliable and extendable real-time seafloor monitoring.

Construction of the DONET real-time seafloor observatory for earthquakes and tsunami monitoring

The Japanese islands are located at a quadruple junction of tectonic plates. Along these plate boundaries, a large number of earthquakes occur every year, supplied with energy by tectonic plate activity. Japan has a sophisticated seismological observatory network on land to estimate the location, magnitude, mechanism and other information about the earthquakes in support of research for earthquake procedures.

Development and application of an advanced ocean floor network system for megathrust earthquakes and tsunamis

Japan is prone to great earthquakes because of its position near two different subduction zones. The Philippine Sea plate subducts from the southeast, and the Pacific plate subducts from the east. The former was the source of a series of great earthquakes, of which the Tonankai earthquake of 1944 and the Nankaido earthquake of 1946 are the latest events. The latter was the source of the 2011 earthquake off the Pacific coast of Tohoku (Tohoku earthquake) of 11 March 2011 (M9).

Scientific results from underwater earthquake monitoring using cabled observatories

Scientific results started appearing in the literature using realtime and continuous time series of data obtained on the seafloor for seismic and micro-tsunamic observations. JAMSTEC has developed three out of a total of eight cabled observatories in the Japanese water. The installed observatories are all operating to aid earthquake studies for disaster mitigation at the future potential hazards, which take place at plate boundaries surrounding the Japanese islands. Along with earthquake activity monitoring purposes, it has become clear that there are potentially and scientifically meaningful outcome from such observations. We summarize the latest scientific results from monitored data produced by our cabled observatories and, then, to demonstrate advantages of such underwater seismic and tsunamic stations. Until now, three main areas could be pointed out as meaningful scientific products from cabled observations: (1) fine mapping of offshore seismicity, (2) modeling of micro-tsunamic pressure fluctuations caused by deep earthquakes, and (3) constraining earthquake source parameters such as source depths of offshore events using tsunami simulation. They are all interesting scientific results from the observational data could also readily be exploited for earthquake studies on deformation processes at the plate boundaries and on early tsunami warning system as a part of future disaster mitigation methodologies. It is obvious that newly deployed monitoring systems have revealed, at least partially, meaningful phenomena, which have been invincible due to lack of observations in the offshore. Technical and theoretical developments for offshore earthquake monitoring must be well considered for profound perception of geophysical processes associated with offshore seismic activities.

Proposal of next-generation real-time seafloor globe monitoring cable-network

A feasibility study on a new scientific submarine cable network of next generation around Japanese Islands is presented. The proposed cable network has a mesh-like topology covering a vast area and has many observation nodes with 50-km intervals. In observation nodes, various sensors can be installed, that are exchangeable for maintenance and replacement. The cable network supplies electric power to sensors and provides continuous and long-term data to researchers and agencies. It will be used in a vast research field in such as seismology, geodynamics, marine environmentology, ecology and biology. In this paper, topology of the cable network, structure of the observation node, optical data transmission system and power feeding system are described.

A new approach for mobile and expandable real-time deep seafloor observation - adaptable observation system

Although submarine cable in-line seafloor observation systems are very effective tools for real-time/long-term geo-scientific measurements,, there are technological difficulties for deploying as many sensors as on land. To solve this problem, JAM-STEC developed an expandable and replaceable satellite measurement station called the adaptable observation system (AOS). The AOS is a battery operated mobile observatory connected to the backbone cable system by a 10 km long thin fiber cable to ensure real-time data recovery. The system consists of a branching system, a junction box, a fiber cable, and a battery system for a six-month operation. Installation and construction of the AOS will be conducted by a towed vehicle and an ROV. A thin fiber cable-laying system was developed and tested for practical operation. This observation system provides a chance to extend existing seafloor networks from an in-line area to a wider area.

Subsea engineering ROV and seafloor observatory construction

Marine science starts shifting from the age of the exploration in the age of the observation. For the abyss research, submarine cabled seafloor observation is one of the expected engineering approaches to be realized real-time and long term observation of natural phenomenon. The DONET (Dense Ocean-floor Network system for Earthquakes and Tsunamis) is a submarine cabled seafloor observatory network design for earthquakes and tsunamis monitoring at the western part of the Pacific Rim. Twenty set of high performance observatories are installed to the seafloor in this project. One of the most difficult engineering in the project is to establish construction method of complex sensor observatory on seafloor. The use of ROV is necessary in the subsea construction. This paper describes the construction method of seafloor seismic observatory, tools design for construction, and experience result in an actual field.

Off Hatsushima Island observatory in Sagami Bay: multidisciplinary long term observation at cold seepage site with underwater mateable connectors for future use

On the seafloor at the depth of 1175 m off Hatsushima Island in Sagami Bay, Central Japan,a cable-connected multi-disciplinary observatory was installed in 1993. Since then long-term real time observation has been carried out, experiencing replacement for upgrades in 2000, recovery and re-deployment for repair in 2002. This site is known as one of the most significant cold seepage sites with large chemosynthetic biological communities consisted mainly of Vesicomyid clams (Calyptogena). The upgraded second observatory is equipped with underwater mateable connectors (optical/electrical). The observatory revealed geophysical and biological events occurred on the seafloor, such as the mudflows and sedimentation generated by swarm earthquakes, spawning of clams triggered by water temperature change. However, several kinds of phenomena and technological problems yet to be neither identified nor solved still remain. As a next step, the observatory is planning to be utilized as a test bed, by using the underwater mateable connectors.