6-6）　Earthquake Observation Center

 

The Earthquake Observation Center was established by integrating the observation divisions, the earthquake prediction observation center and the observatories when the Earthquake Research Institute was reorganized in 1994.  At present, the center is composed of the four observational research groups for earthquakes, crustal movements, and strong motions.  The center has ten observatories and the large-scale regional observation networks covering the districts of Kanto-koshin'etsu, Kii Peninsula, Shikoku, and the western Seto inland sea regions.  The center, in order to approach earthquake prediction and disaster mitigation, has been actively conducting observational researches to clarify the mechanisms and the processes of earthquake generation, and has been developing observational techniques and instruments.  The Earthquake Observation Center has been also conducting the　collaborative observational researches with universities and other related institutions to promote earthquake-prediction program.

 

Earthquake Research Group

  Seismic activities, rupture processes of earthquakes and heterogeneous crustal structures have been investigated using the high-sensitivity seismic networks of more than one hundred stations covering the areas of Kanto-koshin'etsu, Kii Peninsula, and western Seto inland(Fig.1).

 

Fig.1. Map of the seismic stations of the Earthquake Observation Center.

 

  A data telemeter system utilizing a satellite has been developed and has been in operation since 1996, which has ensured to establish a real-time transfer of a huge amount of earthquake waveform data. The data telemeter system has been also effectively utilized in mobile seismic observations(Figs.2,3).

 

Fig.2. Satellite dishes at the ERI hub of the satellite seismic telemetry system.

Fig.3. A VSAT station of the satellite seismic telemetry system.

 

  The Earthquake Observation Center is responsible for the cooperative researches using the satellite-telemeter system. Earthquake swarms, having repeatedly occurred off the East coast of the Izu Peninsula, have been intensively studied to clarify the mechanisms using the dense network of land stations and the optic-cable linked ocean-bottom seismometer network system that was deployed in 1994. The Earthquake Observation Center has been the responsible organization since 2002 for the extensive researches composed of the well-organized seismic refraction/reflection profiling and the seismic studies with an off-line dense array in Tottori-Shimane area.

 

Marine Earthquake Research Group

Marine Earthquake Research Group has researched crustal deformation processes, heterogeneous structures and earthquake activities in the sea and the plate boundary region using the optic-cable linked ocean-bottom seismic stations and tsunami stations and pop-up ocean-bottom seismometers. The cabled geophysical observation systems are deployed off Sanriku, Tohoku district and off Ito, Izu district.  Moreover, seismic activities and heterogeneous structures around Japan have been studied with using a pop-up ocean-bottom seismometer (OBS) array system.

 

1. Optical Fiber Cabled Ocean Bottom Seismometers and Tsunami Gauges

The crust off the eastern Izu Peninsula has had many earthquake swarms estimated to be results of magma activities. Observations just above the earthquakes swarm are essential for a detailed study of seismic activities. The earthquake Observation Center deployed a cabled OBS network system in the swarm area in 1994 (Fig.4). Data from three sensors at sea floor are transmitted on-line by using optical fibers to a land station. Precise location of microearthquakes has become possible using the data from the cabled OBSs.

 

Fig.4. Positions of the cabled OBS system and seismicity near the cabled OBS system (April 20th - May 11th, 1998). Colors of dots denote origin times of events.

 

The Pacific plate is subducting beneath the Japan islands from the Japan Trench and the plate boundary has generated destructive earthquakes in past. In 1996, an ocean bottom geophysical observatory using sea floor optical fiber cable was installed off Sanriku, northeastern Japan to observe earthquakes and tsunamis as a part of the Earthquake Prediction Program of Japan (Fig.5). The system has three ocean bottom seismometers and two tsunami gauges. The data from sensor are transmitted to Kamaishi land station by a sea floor optical fiber cable in real time. The sea floor optical fiber cable is buried in area where the water depth is less than 1,000m. The system provides an integrated observation in land and sea for earthquake prediction. A real-time monitoring of earthquakes and Tsunamis in marine area plays an important role in studies of plate subduction dynamics.

 

Fig.5. Position of seismometers and tsunami gauges of the cabled ocean bottom geophysical observation system and route of the sea floor optical fiber cable. Circle and triangle indicate ocean bottom seismometer and tsunami gauge, respectively.

 

2. Marine seismic observations using pop-up type long-term ocean bottom seismometers

Seismic observation in marine area is important to research earthquake activity and heterogeneous structure around Japan. The high-quality pop-up ocean-bottom seismometers with the life of more than one year continuous recording have been developed (Fig.6).  From 2000, long-term ocean bottom seismic observations have been carried out in plate boundary regions and back-arc basins using the newly developed ocean bottom seismometers. Temporal variations of earthquake activities and heterogeneous structure of deep regions are expected to be revealed by these observations.

 

Fig.6. Newly developed high-quality pop-up ocean-bottom seismometer with a recording period of more than one year.

 

3. Studies of heterogeneous structure using pop-up type ocean bottom seismometers

Some important regions in Earth sciences such as a plate boundary are under the sea. For example, detailed seismic structures in seismogenic zones are important information to study the earthquake generation mechanism. We have performed seismic experiments using ocean bottom seismometers, a hydrophone streamer and controlled sources to obtain heterogeneous seismic structures (Fig.7). The seismic experiment has been also carried out in back arc basins, ocean basins and ridge areas to understand plate dynamics. A seismic structure survey around a volcanic island was carried out in cooperation with the research group of volcano in Japan in 1999.

 

Fig.7.  Ocean bottom seismometers and airgun (energy source, behind OBSs) on a research vessel for a seismic structure survey.

 

Crustal Movements Research Group

 

 Crustal dynamics and relationship between crustal movements and earthquake occurrence have been investigated by using data obtained from observation stations monitoring continuous crustal movements, and EDM and GPS array stations, together with groundwater measurements. Multi-component borehole observation instruments have been developed and enabled us to record six components of strain, two components of tilt, temperature, and three components of seismic waves.  They are equipped with a newly developed gyro for positioning the instruments when installed in deep boreholes.  They have been deployed at areas where earthquake swarms occur and next destructive earthquakes may occur in the future as Kanto-Tokai and Nankai areas, being installed in deep boreholes at depths from 150 m to 800 m. The single frequency GPS system is newly developed to realize the dense observation. It is enough low power consumption and light weight for out-door observation. Since September, 1997, dense GPS network was constructed in Ito City. Our network detected the crustal deformation when the swarm activity became high in April, 1998. Mobile GPS observation by using this new system makes come true in 1999 Chi-Chi Earthquake and 2000 Tottori-ken Seibu Earthquake.

　Precise and continuous measurements of sound velocity are carried out at the vaults of Aburatsubo (Kanagawa, central Japan), Kamaishi(Iwate, northeastern Japan) and Mizunami (Gifu, central Japan) by using an accurately controlled pulse transmission method. The highest resolution of the sound velocity is 100 ppm, 1 ppm and 10 ppm for Aburatsubo, Kamaishi and Mizunami, respectively. The reliability of the estimated sound velocity depends on the system clock (0.01 ppm/yr). This method has an advantage that the long-term reliability in the temporal variation of the sound velocity should be fundamentally the same for short-term one. Gradual change in the stress state and water content in rocks can therefore be estimated by the monitored sound velocity change.

 

Strong Motion Research Group

 

 Major objectives are: (1) to understand the generation and the propagation of strong ground motions from large earthquakes through strong motion observations at rock sites surrounding Suruga Bay where a large earthquake of M8 is anticipated and Izu Peninsula regions, and (2) to evaluate the effects of surface geology on strong motions using the array observation data at Ashigara valley (Figs.8,9). A quasi real-time information system for strong ground motions is under development. This system will focus on dispatching additional data that will allow a local government or other organization to respond quickly. The group collaborates with the Division of Disaster Mitigation Science.

 

Fig.8.  Strong motion observation network of the Earthquake Observation Center in the Kanto-Tokai area plotted on the geological map by the Geological Survey of Japan.

Fig.9.  Array strong motion observation in Ashigara valley.

 

Fig.10 Wakayama Seismological Observatory

Fig.11 Local seismic observation network in the Kii peninsula operated by the Wakayama Seismological Observatory.

 

Fig.12 Hiroshima Seismic Observatory

 

Fig.13 Shin-etsu Seismological Observatory

 

 

Fig.14 Nokogiryyama Geophysical Observatory: Watertube tiltmeter and extensometer (center), the inside view of the observation vault (right lower), and the entrance of the observation vault (left upper).

 

Fig.15 Fujigawa Geophysical Observatory

Fig.16  Okuyama observation vault, Fujigawa Geophysical Observatory