Fujiwara Paper

Strong-motion Observation Networks of NIED: K-NET and KiK-net

Hiroyuki Fujiwara, Shin Aoi, Takashi Kunugi, Shigeki Adachi National Research Institute for Earth Science and Disaster Prevention

3-1 Tennodai Tsukuba Ibaraki 305-0006, JAPAN

ABSTRACT After the Hyogoken-nanbu (Kobe) earthquake on January 17, 1995, new national projects

were started to improve the Japanese seismic observation systems. Numerous numbers of strong-motion seismographs were newly installed to realize uniform networks covering whole Japan. NIED (National Research Institute for Earth Science and Disaster Prevention) has constructed the nation-wide seismic observation networks. K-NET (Kyoshin network) consists of 1034 strong-motion seismographs settled on ground surface covering whole Japan. KiK-net (KIBAN kyoshin network) is the sub-set of Hi-net (High-sensitivity seismograph network) and consists of 660 strong-motion observation stations installed both on the ground surface and at the bottom of boreholes. The strong-motion records obtained by the K-NET and the KiK-net are available on the internet. 1. Introduction

The Great Hanshin-Awaji Earthquake Disaster caused by Hyogoken-nanbu (Kobe) earthquake on January 17, 1995 killed more than 6,400 people. Following on the lessons learned from this disaster, Earthquake Disaster Management Special Act was enacted in July 1995 to promote a comprehensive national policy on earthquake disaster prevention. In accordance with this act, Japanese government established the Headquarters for Earthquake Research Promotion (HERP). Under the direction of the HERP, new national projects were started to improve the Japanese seismic observation systems. As the first project, in the next year of the Hyogoken-nanbu earthquake, NIED has completed a strong-motion network consisting of 1,000 stations covering whole Japan (Figure. 1(a)). The network is called as K-NET (Kyoshin network) (Kinoshita [1]), where Kyoshin is the Japanese word to mean strong-motion.

The HERP set up the Fundamental Survey and Observation Plan for Earthquake Research in August 1997. In short, this plan is called as the KIBAN project, where KIBAN is a Japanese word to mean fundamental or infrastructure. The goal of this plan is to evaluate long-term earthquake occurrence probability through understanding the earthquake phenomena and complete a national seismic hazard map through understanding the generation mechanism of strong-motions. The followings were selected as the core projects to be promoted with higher priorities.

(1) Seismic observation (high-sensitivity observation and broadband observation) (2) Observation of strong-motion (3) Observation of crustal movement (continuous GPS observation) (4) Survey of active faults in land and coastal regions In the basis of this plan, NIED has constructed the following nation-wide seismic observation

networks. Hi-net (High-sensitivity seismograph network) (Obara [2]) is the network for microearthquake observation consisting of 740 stations. The seismometers are installed at the bottom of boreholes to reduce the noise. F-net (Full-range seismograph network) (Fukuyama [3]) is the network for broadband seismic observation. The seismometer of the F-net is installed in a vault for stable observation. KiK-net (Kiban-Kyoshin network) is the strong-motion observation

(a) (b)

Figure.1. Distribution of the strong-motion observation stations in Japan. The observation stations of K-NET are shown in (a) and those of KiK-net are shown in (b). network consisting of 660 stations (Figure. 1(b)) and is the sub-set of the Hi-net. The strong-motion seismographs are installed both on the ground surface and at the bottom of boreholes. NIED has a role not only to operate these networks but also to work as a data center providing the data obtained from these networks.

2. K-NET

Just after the 1995 Hyogoken-nanbu (Kobe) earthquake, NIED decided to construct a nation-wide strong-motion observation network: K-NET (Kyoshin network) as an original project of NIED. The K-NET consists of 1,034 strong-motion observation stations with a spacing of 25km, the Data Management Center of NIED in Tsukuba, and a mirror station in Osaka. The spacing of the K-NET enables us to record strong-motions in the epicentral region of a crustal earthquake with more than magnitude of 7 anywhere in Japan. The observatory is installed in a site of 3 meters square. It consists of a house made of fiber reinforced plastic, a concrete base on which a seismograph is installed, facilities for electric power and a telephone line (Figure 2). The strong-motion seismographs (K-NET95) used in the K-NET are three component accelerographs and a data logger having a 24-bit A/D converter and a 8MB flash memory that can record waveforms (sampling frequency of 100Hz) of 2.5 hours. The event recording is started with a threshold level of 2.0 Gal and ended with 30 seconds continuation of the level lower than 0.1Gal keeping minimum recording length of 120 seconds. Resolution of the K-NET95 is more than 108dB and the maximum measurable acceleration is 2,000 Gal. The overall frequency response characteristics of the K-NET95 is flat from DC to 30Hz. The frequency response characteristics of higher than 30Hz is approximated by the characteristics of 3-pole Butterworth filter with a corner frequency of 30Hz. Timing in the K-NET95 is calibrated by GPS signal in the accuracy of

Figure 2. The observatory of the K-NET is installed on a site 3 meters square. It commonly consists of a house made of fiber reinforced plastic, a concrete base on ehich a seismograph is installed, facilities for electric power and telephone line (ISDN) with lightning arresters, and fence. At sites where the temperature becomes less than 20C, the concrete base is constructed 80cm below the ground surface. 5msec. At each K-NET station, the velocity structure beneath the site down to a depth of 20m was investigated by downhole measurement. The information on soil structures including N-values, bulk density, P and S wave velocity profiles and soil profile is provided. 3. KiK-net

At most of Hi-net stations, strong-motion seismographs are installed both on the ground surface and at the bottom of the observation boreholes (the depths are more than 100m), which form the KiK-net. KiK-net consists of 660 stations covering whole Japan (Figure 3). While the K-NET stations are mainly constructed at the thick sedimentary sites, most of the KiK-net stations locate on the rock or thin sedimentary sites because the Hi-net is primarily designed for the purpose of highly sensitive seismic observation. Although many of KiK-net stations have boreholes of 100-200m in depth, deeper observation wells were drilled at some specific sites if necessary. Instrumentation of the KiK-net is basically the same as that of the K-NET95 used in the K-NET. The difference is a sampling frequency of 200Hz and 85MB memory storage that can hold 6 component waveforms of 6.5 hours in total. The event trigger system of the data

Figure 3. Observatory of the Hi-net and KiK-net. To realize stable high sensitivity seismic observation avoiding surface noise, seismographs are settled at the bottom of borehole of 100m or more at each station. Short period three component velocity seismometers, three component accelerometers and two component tiltmeters are installed at the bottom of the observation well. Accelerometers are installed on the ground surface as well as at the bottom of the wells. recorder is controlled using the signal of the downhole seismometers. The event recording is started with a threshold level of 0.2Gal and ended with 30 seconds continuation of the level lower than 0.1Gal, keeping minimum recording length of 120 seconds. Downhole loggings were performed at most stations and the velocity structures as well as soil conditions are available for those stations.

Figure 4. The procedure of the K-NET data retrieval and release.

4. Data Management Center of NIED

The technical staffs of the Data Management Center of NIED in Tsukuba maintain the K-NET and KiK-net systems. Strong-motion records obtained from K-NET and KiK-net are collected through the dial up operations using 30 telephone lines (INS64) from the Data Management Center of NIED in Tsukuba. When an earthquake occurs, the Japan Meteorological Agency (JMA) determines and issues a quick report on JMA intensities and source parameters of the earthquake through a JMA satellite if the JMA observatories record JMA intensity more than 3. Upon receiving the quick report, the distribution of maximum acceleration is estimated automatically by using an empirical attenuation relation and acquisition starts to retrieve the strong-motion records of necessary stations. The procedure of the K-NET data retrieval and release is shown in Figure 4.

Dial up operation often encounters troubles by jamming of communication traffics and the data acquisition considerably delays especially in case of large earthquakes. To overcome this problem, the renewal of the K-NET to improve the data acquisition system that adopts dial out operation is currently in progress.

The collected waveform records of all K-NET and KiK-net stations are stored in the strong-motion database at the Data Management Center of NIED and all data are provided to researchers, engineers and general public through the Internet under fully open policy (Figure 5). Users can select specific events or stations to download the waveform data. They can also browse, select, and retrieve various information such as distribution map of acceleration, paste

up of waveforms, station maps, site information including seismic velocity profiles and soil condition, and so on. The URL of K-NET is http://www.k-net.bosai.go.jp/, and KiK-net is http://www.kik.bosai.go.jp/.

There is no restriction on using the K-NET or the KiK-net data, but if you make a dissertation or article through the using of the K-NET or the KiK-net data, please accede following. (1) Specify in the article that the K-NET or the KiK-net data are used to make the article. (2) Send a copy of the publication to following address;

Strong-Motion Network Laboratory, National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba, Ibaraki 305-0006, Japan.

Figure 5. K-NET and KiK-net data are available on the Internet. The URL of K-NET is http://www.k-net.bosai.go.jp/, and KiK-net is http://www.kik.bosai.go.jp/. Users can select specific events or stations to download the waveform data. They can also browse, select, and retrieve various information such as distribution map of acceleration, paste up of waveforms, station maps, site information including seismic velocity profiles and soil condition.

5. Examples of strong-motion records 5.1 The 2000 Western Tottori earthquake (MJMA7.3)

The western Tottori earthquake (MJMA7.3) occurred on 6th October 2000 at 13:30 (JST). The 2000 Western Tottori earthquake was the first large crustal earthquake of magnitude more than 7 since the operation of nation-wide strong-motion networks K-NET and KiK-net started.

In the Data Management Center of NIED, the data acquisition for K-NET and KiK-net begun from a few minutes after the earthquake occurred. The dial up operation, however, encountered troubles by jamming of communication traffics. The communication troubles had continued for more than 3 hours and the data acquisition from the stations near the epicentral region considerably delayed. Most of the acceleration records were collected in midnight, about 12hours after the earthquake occurred. Finally, we obtained the acceleration records from 307 stations of K-NET and 217 stations of KiK-net.

The nearest station to the earthquake fault, TTRH02 (KiK-net, Hino-station, epicentral distance 8km) recorded peak vector acceleration of 1135Gal on the ground surface and 653Gal at the bottom of a 100m borehole. These acceleration waveforms are shown in Figure 6. The distribution of observed peak accelerations obtained from K-NET and KiK-net is shown in Figure 7.

Figure 6. Acceleration waveforms recoded at TTRH02 (KiK-net, Hino-station, epicentral distance 8km). Waveforms at the bottom of the borehole (depth of 100m), and waveforms on the ground surface are shown.

Figure 7. The distribution of observed peak accelerations obtained from K-NET and KiK-net. The star shows the epicenter. White circles show locations of aftershocks.

5.2 The 2003 Tokachi-oki earthquake (MJMA8.0) The Tokachi-oki earthquake (MJMA8.0) struck the northern Japanese island of Hokkaido on

26th September 2003 at 4:50 (JST). The 2003 Tokachi-oki earthquake was the first large interplate earthquake recorded by the nation-wide strong-motion networks, K-NET and KiK-net. Most of the acceleration records from 655 stations (K-NET and KiK-net) were collected in 5 hours by dial up operation from the Data Management Center of NIED. The maximum peak acceleration of 988Gal, obtained by vector summation of three components, recorded at station HKD100 (K-NET). The distributions of observed peak acceleration and peak velocity on the ground surface are shown in Figure 8(a) and 8(b), respectively. The rupture process was estimated from strong-motion data of 15 stations in the source region using multi-time linear inversion method (Honda [4]). Estimated total slip distribution is shown in Figure 9.

(a) (b)

Figure 8. The distribution of observed peak acceleration and peak velocity on the ground surface are shown in Figure 8(a) and 8(b), respectively.

Figure 9. Estimated total slip distribution.

Figure 10. Distribution of the stations installed the new strong-motion seismograph, K-NET02. The blue circle indicates a station in the area where the strong-motions due to the Nankai or Tonankai earthquakes are warned. The red circle indicates a station in the area where the strong-motion due to the Miyagi-oki earthquake is warned.

Figure 11. The current system for K-NET95 (top). Dial-up from the DMC to observation stations. Dial-up operation is automatically triggered with information from the JMA via satellite line. The new system for K-NET02 (bottom). Dial-out from observation stations to the DMC. Data transmission is automatically triggered by detection of strong-motion at each station.

6. Renewal of the K-NET

The K-NET was constructed after the Hyogoken-nambu earthquake in January 1995 and the operation of the K-NET began in June 1996. Eight years have passed since the K-NET started and many strong-motion records for large earthquakes are obtained. Many researchers have used strong-motion records of the K-NET in their studies. During these eight years the progress in information technologies is remarkable and new technologies become to be available.

The NIED has developed the K-NET using the new technologies. The new seismographs for the K-NET have been developed and installed at 443 observatories mainly in the southwestern Japan where strong-motion due to the Nankai and Tonankai earthquakes are warned (Figure 10). The main features of the new strong-motion seismograph are functions for real time data communications (Figure 11) and processing JMA seismic intensity. We expect that the time to acquire strong-motion records from observatories to the Data Management Center in Tsukuba is shortened by using the new seismographs. The operation will be started in June 2004.

6.1 Outline of the new strong-motion seismograph The new strong-motion seismograph (K-NET02) is consists of three modules of a sensor

module, a measurement module, and a communications module (Figure 12). It connects with the Data Management Center through an ISDN line. The main features of a new strong-motion seismograph are functions for processing JMA seismic intensity and near real time data communications. Although the strong-motion seismograph currently used (K-NET95) does not calculates seismic intensity, a new strong-motion seismograph functions as a formal seismic intensity meter that suited the official approval of the Japan Meteorological Agency. If a new strong-motion seismograph detects a strong-motion, it will communicate with the Data Management Center automatically in dozens of seconds. Furthermore, the full-scale was improved from 2000gals to 4000gals. The dynamic range of an AD conversion is 132dB. Since it is programmable, the contents of processing of a measurement module and a communication module have various functions, such as accumulation of continuous data, and are flexibly easy to extend.

UPSRouterGPS

Digitaltelephone line

10BASE

10BASE

AC100V

DC12V

DC5V

CPU1(Linux OS)

Communication moduleMeasurement module

A/D converter sensor controll recording event data battery back-up basic function of seismograph

calculation of JMA intensity recording event data recording continuous data realtime data communication advanced processing

CPU2(Linux OS)

Sensor module

3 componentaccelerarometer

Figure 12. Block diagram of the new strong-motion seismograph, K-NET02. The K-NET02 is consists of a sensor module, a measurement module and a communication module. An UPS, a GPS antenna and a dial-up router are also installed with K-NET02. A tri-axial accelerometer, FBA-ES-DECK (Kinemetrics Inc.) is built into a sensor module. The measurement module functions as a conventional strong-motion seismograph with high precision of observation. The communication module processes advanced tasks, for example, calculation of the JMA seismic intensity, recording continuous data and real-time data transmission. At the time of a power failure the measurement module can control power supply for the router and the communication module to save a battery power.

6.2 Difference in records of old and new strong-motion seismograph With change of the full-scale, a scale factor changes by the old and new strong-motion

seismograph. Since the scale factor is specified in the header of data, users should refer to this for analysis. Moreover, the anti-alias filter was changed into the FIR filter with a cutoff frequency of 30Hz from the IIR filter of the Butterworth characteristic. Some difference may arise in record length by the difference in trigger algorithm. Comparison of K-NET95 and K-NET02 are shown in Table 1.

Table 1: Comparison of K-NET95 (Old type) and K-NET02(New type) Seismograph K-NET95Old type K-NET02New type Maximum measurable Acc. 2000Gal 4000Gal Dynamic range (RMS/F.S.) 19bit (114dB) 22bit (132dB) Scale factor 0.24 mGal/COUNT 0.63 mGal/COUNT Power consumption 16W 35W (5W in low power mode) Sensor (Accelerometer) V403-BT (Akashi) FBA-ES-DECK (Kinemetrics) JMA seismic intensity Data communication RS-232C IP ( + RS-232C) Recording capacity 8MB 512MB Programmable by user (Linux OS) Recording continuous data Data Communication Old system for K-NET95 New system for K-NET02 Data acquisition Dial up from the DMC to

observation stations Dial out from observation stations to the DMC

Type of communication Serial communication using ISDN modem

IP connection using ISDN router

Measures for congestion None Make connection before congestion

Number of lines at DMC 24 lines (INS64) 460 lines (INS1500: 20 lines) The time required collecting data from observatories to the DMC.

Data acquisition is begun after receiving a quick report from JMA. The time required collecting data depends on the magnitude of earthquake. It usually takes several hours.

Data acquisition is begun automatically just after triggering and all of data are collected within 5 minutes.

The time required for the data release

Several hours half a day Within 10 minutes

ACKNOWLEDGEMENTS The technical staffs in the Data Management Center of NIED curry out the maintenance of the

K-NET and the KiK-net. We wish to thank them for their cooperation.

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