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.