Shoji Sekiguchi

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.

Relationship Between the Detailed Hypocenter Distribution and the Velocity Structure in the Western Nagano Prefecture, Central Japan, Derived from Travel Time Tomography Using Dense Array Data

We conducted three-dimensional travel time tomography in and around the source region of the 1984 Western Nagano Prefecture Earthquake to investigate the generation process of the mainshock and swarm activity near the source. As many as about 250,000 travel time data of good quality (1 ms error) from a dense network were compiled and we obtained hypocentral distribution and three-dimensional P wave velocity structure with high accuracy and resolution. Most of the estimated hypocenters were aligned in lines or planes, not in the form of masses. We found the hypocenter distribution corresponding to the mainshock fault plane, and it was sandwiched by high-velocity regions on both horizontal sides. A low-velocity region spreading horizontally at the bottom of this hypocenter alignment was also seen. In the northeastern side of the mainshock fault, where there is swarm activity, we detected several other alignments of hypocenters, most of which may be located at the boundaries of geological strata. A low-velocity region was also found at the bottom of some of these alignments. This low-velocity region may be due to fluids from below the seismogenic zone. The generation of both the mainshock and the swarm activity might be related to fluid intrusions from the lower side of the hypocenter alignments.

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.