Yoshimitsu Okada

Recent progress of seismic observation networks in Japan —Hi-net, F-net, K-NET and KiK-net—

After the disastrous 1995 Kobe earthquake, a new national project has started to drastically improve seismic observation system in Japan. A large number of strong-motion, high-sensitivity, and broadband seismographs were installed to construct dense and uniform networks covering the whole of Japan. The new high-sensitivity seismo-graph network consisting of 696 stations is called Hi-net, while the broadband seismograph network consisting of 71 stations is called F-net. At most of Hi-net stations strong-motion seismographs are also equipped both at depth and the ground surface. The network of these 659 stations with an uphole/downhole pair of strong-motion seismographs is called KiK-net, while another network consisting of 1034 strong-motion seismographs installed at the ground surface is called K-NET. Here, all the station numbers are as of April 2003. High-sensitivity data from Hi-net and pre-existing seismic networks operated by various institutions have been transmitted to and processed by the Japan Meteorological Agency since October 1997 to monitor the seismic activity in and around Japan. The same data are shared to university group in real time using satellite communication for their research work. The data are also archived at the National Research Institute for Earth Science and Disaster Prevention and stored in their database system for public use under a fully open policy.

A densely distributed high-sensitivity seismograph network in Japan: Hi-net by National Research Institute for Earth Science and Disaster Prevention

Seismic observations to retrieve various information from the Earth are the basis of seismology. A seismic observation system requires various technologies for vibration sensors, analog-and-digital measurement, data transmission, and computing for mass data analysis, for example. New developments in technology are adopted whenever possible in the construction of seismic observation systems. In Japan, after the disastrous Kobe Earthquake in 1995, a high-density and high-sensitivity seismograph network was constructed. The seismic network, called the National Research Institute for Earth Science and Disaster Prevention (NIED) Hi-net, uniformly covers the Japanese Islands with a spacing of 20–30km. As a result, the detection capability for microearthquakes has been greatly improved, and various research using Hi-net data has indicated that this seismic network has a great potential to resolve the underground structure and various geophysical phenomena as a radar-array oriented toward the Earth. Equipped with...

Coswarm and preswarm crustal deformation in the eastern Izu Peninsula, central Japan

Remarkable crustal deformation has repeatedly been observed in the eastern Izu Peninsula, central Japan, synchronized to the occurrence of extensive earthquake swarms in the past 20 years. Among the instrumentation, a borehole tiltmeter installed at station KWN, which is located within a few kilometers of the swarm region, has faithfully recorded large signals of ground tilt associated with more than 10 swarm sequences in the past 10 years. In addition to coswarm signals it became clear that precursory tilt signals preceded major swarm activities. The amplitudes of the precursory tilt signals are of order of 0.1 μrad, and they precede the onset of major swarms by several hours to half a day. This signal is convincing because the same phenomena appeared at least 7 times and were also detected by other independent observations. We speculate that a dike intrusion at depth causes crustal deformation and only as it reaches depths shallower than ∼10 km does seismic activity begin. The precursory tilt signal can be utilized for practical prediction of the start of swarm activity in the eastern Izu Peninsula.

Earthquake of 1987, off Chiba, central Japan and possible triggering of eastern Tokyo earthquake of 1988

Abstract A moderate earthquake (M 6.7) occurred off eastern Chiba, central Japan, on December 17, 1987 as a consequence of the internal deformation of the Philippine Sea plate. This event helped in clarifying the geometry of the easternmost part of the Philippine Sea plate as well as the mode of earthquake occurrence in this region. Three months later, another moderate earthquake (M 6.0) took plate beneath eastern Tokyo. This event was generated at the slab-slab collision zone at about 90 km depth, and is considered to be triggered by the former earthquake. A similar phenomenon was observed for the Izu (M 6.7) of June 29, 1980 and the mid Chiba (M 6.1) of September 25, 1980 earthquake pair, and the off Ibaraki (M 7.0) of July 23, 1982 and the southern Ibaraki (M 6.0) of February 27, 1983 earthquake pair. These are the only three earthquake pairs of M ≥ 6 which occurred in the Tokyo metropolitan area since 1974. Thus, over these 15 years, all M-6 interplate earthquakes in this specific region are preceded by nearby M-7 earthquakes. This suggests that M-7 shallow events enhance stress in the deeper plate collision zone beneath the Tokyo metropolitan area and trigger M-6 interplate events.

On the Cause of Long-Period Crustal Movement

Abstract It has been found by continuous observation in the vaults of crustal-movement observatories that the progress of crustal deformation is accompanied by a long-period fluctuation. The migration of this fluctuation between the adjacent stations and its simultaneity over a wide area has also been recognized. Some interpretations of the cause have been proposed from the viewpoint that fluctuations originate substantially from the tectonic source. In this paper, it is concluded that this long-period strain fluctuation is fairly well explained by precipitation changes at each site; the simultaneity may be explained by the identical precipitation conditions over a wide area, while the migration may be deceptive, attributed to the differences in the response to the precipitation at each station. In other words, prior to the discussion of the long-period crustal movement in relation to tectonics, the effect of precipitation on the crustal strain at that station should be carefully examined.

Some Results in the Preliminary Data Analyses of the Fixed-Point GPS Baseline Determination Network in Central Japan

We present preliminary results of data analyses of GPS fixed point geodetic measurements of 70–260 km baselines in the Kanto-Tokai district, central of Japan. The system was introduced for the purpose of earthquake prediction researches in the district. Firstly, the analysis employs the determination of initial positions of ten sites by means of the result of the experiment using one VLBI point and one SLR point. Next, one of the station was fixed to determine other receiver locations and the wet term tropospheric delays using the orbital relaxation technique, based on the dual frequency carrier phase and surface meteorological data.