Ryoichi Iwase

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.

PRELIMINARY RESULTS OF A THREE-YEAR CONTINUOUS OBSERVATION BY A DEEP SEAFLOOR OBSERVATORY IN SAGAMI BAY, CENTRAL JAPAN

Abstract A comprehensive long-term deep seafloor observatory was deployed at the plate boundary between the Philippine and North American plates in Sagami Bay, central Japan in 1993 in order to investigate the relations among geophysical parameters associated with earthquake swarms and eruptions of submarine volcanoes that have occurred repeatedly from at least 0.01 Ma to the present. It is also located within the habitat range of the vesicomyid clam, Calyptogena soyoae , the presence of which suggests seepage from below. The observatory, at a depth of 1174 m off Hatsushima Island, is equipped with multi-sensors, such as a seismometer, hydrophone, heat flow temperature probes, color video cameras, a CTD and a current meter. The data and power to and from the land station at Hatsushima Island are sent in real time through an 8-km-long electro-optical cable. More than 3 years of continuous geophysical and environmental data on the deep seafloor were collected up to 1996. In this paper, we describe the system and report on significant changes in ground temperature associated with three earthquake swarms in the adjacent area.

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.

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.

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.

An expandable deep seafloor monitoring system for earthquake and tsunami observation network

The Japan Marine Science and Technology Center is developing and installing the submarine cable systems for building a series of in-line geo-scientific observation network at typical seismogenic zones around Japan since 1995. The second observation system was deployed in July 1999 on the southern Kuril subducting zone in southeastern part of Hokkaido (Off Kushiro-Tokachi area). The system consists of three in-line ocean bottom seismometers, two in-line tsunami pressure gauges, a cable end multi sensor station, and two expandable interfaces. The mobile/replaceable sensor package named Adaptable Observation System was developed to connect to the expandable interface to extend the observation network from line to wider area.

10 year video observation on deep seafloor at cold seepage site in Sagami Bay, central Japan

As a part of multidisciplinary real-time observation at cold seepage site on deep seafloor which faces swarm earthquake area at a depth of 1175 m off Hatsushima Island in Sagami Bay, central Japan, video monitoring has been carried out for about 10 years by a cabled observatory. Through the observation, outbreak event such as mudflows caused by earthquakes, spawning of clams and other biological events were observed along with long-term phenomena such as sedimentation on seafloor associated with mudflow or the fluctuation of the amount of suspended particles. Some of these video images provide important information on the fluctuation observed by other sensors of the observatory, such as sub-bottom temperature and intensity of gamma-ray, indicating that those fluctuations were primarily related to such environmental changes on seafloor as sedimentation. On the other hand, extracting meaningful events or phenomena from a huge amount of stored video images needs tremendous time and effort. Effective methodology of scene detection and indexing is necessary

Multi-disciplinary VENUS observation at the Ryukyu Trench using Guam-Okinawa geophysical submarine cable

A multi-disciplinary VENUS (versatile eco-monitoring network by undersea-cable system) observatory, which equips seven geophysical instrument groups, was installed at the depth of 2,170 meters on the slope of the Rukyu Trench. Prior to the installation of the VENUS multi-disciplinary ocean bottom (MDOBO) observatory, an ocean-bottom telemetry system, which has functions to supply electrical power to the MDOBO, and the submarine coaxial cable were installed at ocean bottom. The installation of the multi-disciplinary ocean bottom observatory was done by use of deep-towing unit and ROV Kaiko-10K. During the period of August-September 1999, seven instrument groups of MDOBO were deployed at the target position, at 80-100 meter distances from the telemetry system, with several meters allowances using a deep-towing unit. To install the instrument at the exact location, the mother ship of deep-towing unit was precisely navigated. The extension cables were also dropped from the deep-towing unit. The ROV Kaiko-10K extended multi-conductor extension cables from instrument units towards the ocean-bottom telemetry system and connected them to undersea mateable connectors on the junction box. The MDOBO collected one and half month records. Some useful data were observed since the installation.

Fin whale vocalizations observed with ocean bottom seismometers of cabled observatories off east Japan Pacific Ocean

Fin whale vocalizations were found in the archived waveform data from both hydrophones and ocean bottom seismometers (OBSs) of a cabled observatory off Kushiro-Tokachi in Hokkaido. A fin whale was localized on the basis of the incident orientation estimated with a single OBS and the time difference of multipath arrival of sound pressure data from a hydrophone. Furthermore, several fin whale vocalizations were found in the archived OBS waveform data from other cabled observatories off east Japan Pacific Ocean. These findings suggest that the cabled OBSs would be significant apparatuses for real-time monitoring of the presence of baleen whales around Japan.

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.