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

Abstract

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 seismograph 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.

<b>Improved Algorithms of Seismic Parameters Estimation and Noise Discrimination in Earthquake Early Warning</b>

“…The magnitudes varied from 4.5 to 9.0, and the focal depths were less than 112 km. Earthquake data for points within 200 km of the epicenters was collected from K-NET operated by NIED [10]. Far field earthquakes are not targets for rapid estimation of seismic parameters, which explains why the distance threshold from the epicenter was set to less than 200 km.…”

“…Waveform data was recorded at K-NET stations operated by NIED [10]. This offline simulation used typical parameters for railway EEW systems and recorded data from the "off-Noto peninsula earthquake (M j 6.9)" which occurred on March 25, 2007.…”

“…This report uses earthquake waveform data from K-NET and KiK-net operated by NIED [10]. The map was drawn using Generic Mapping Tools [16].…”

Visualizing field-measured seismic data

“…A major energy release was defined as an event that emerged at the surface in an area of nearly 10 km in radius with accelerations of 350 gal and larger. They pointed out that major energy releases happened at 11,14,15,16,18,19,20,21,23,27,29,30,32,34,43, and 50 seconds from origin time. Some of the major energy releases were from adjacent active faults triggered by the bursts of extensive energy releases of the mainshock.…”

“…There are approximately 700 digital accelerographs in free-field stations, and more than 50 real-time seismic arrays installed in buildings and bridges. With 3 km station spacing of free-field accelerographs in urban area, Taiwan has the densest strong-motion instrument network in the world [20] compared to 25 km spacing of K-net strong motion seismograph network in Japan [15]. Typical accelerographs have ±2 g scale, 200 or higher sampling rate, 16-bit precision, and 20 second pre-event recording with a Global Positioning System (GPS) receiver that can be used to synchronize the internal clock to Coordinated Universal Time (UTC) [23].…”