Strong Earthquake Motion Observation in Japan
The strong motion observation in the field of engineering began in the 1950s in Japan. Professors, researchers and engineers organized a committee in order to develop the original strong motion instruments and to promote observation projects and related studies. The steady progress has been made in the accumulation of valuable records and the improvement in the instrumental technology.
With the 1995 Hyogo-ken-nanbu Earthquake, which is known as Kobe Earthquake, the situation of strong motion observation underwent drastic changes. Several projects were planned and conducted in order to reinforce the strong motion network. More than 4,000 instruments have been newly placed by national organizations and municipalities.
On the other hand, Building Research Institute (BRI) is operating three strong motion networks at present. "Nationwide Strong-Motion Network of BRI" has 47 observation sites in major cities throughout Japan. "Dense Strong-Motion Instrument Array in Sendai" is the array observation that is focused on the amplification effect of surface geology to seismic motions. "Strong-Motion Instrument Network in the Metropolitan Area" is a dense instrumental array, which consists of eighteen sites in the Tokyo metropolitan area. Recent installation was made with more specific aims, i.e. effect of surface geology, soil-structure interaction, and other relevant problems to evaluation of seismic input to buildings.
This report generally outlines the up-to-date state of strong motion networks in Japan. Strong motion observation activities of BRI are also introduced briefly.
Brief History of Strong Motion Observation in Japan
Japan has often suffered from earthquakes. Major destructive earthquakes in the twentieth century are listed in Table 1. Figure 1 shows epicenters of those earthquakes. Any city in Japan cannot escape from earthquake hazards.
The 1948 Fukui Earthquake made researchers realize the necessity to conduct strong motion observation. Some professors, researchers and engineers organized the Strong Motion Accelerometer Committee to develop strong motion instruments in 1951. The prototype instrument was manufactured in 1953 and was named SMAC from initials of the committee. Specifications of typical instruments are shown in Table 2. In the early stages of strong motion observation, most of sites were instrumented and were operated by universities and national research institutes.
The Strong-Motion Earthquake Observation Committee was established in 1956 in order to maintain strong motion instruments, to process data, and to publish records. The Committee was mainly operating instruments placed in buildings, but the expenses were not officially budgeted. In 1965, Strong Earthquake Motion Observation Center was organized in the Earthquake Research Institute, University of Tokyo. The Center obtained the certain budget and took over part of the Committee's task.
In 1967, the Strong-Motion Earthquake Observation Councila) was organized and followed the Committee. The secretariat of the Council was constituted in the National Research Center for Earth Science and Disaster Prevention. The Council consisted of representatives of Earthquake Research Institute (ERI), University of Tokyo, Building Research Institute (BRI), Public Works Research Institute (PWRI), Port and Harbor Research Institute (PHRI), Japan Railway (JR), Nihon Telegram and Telephone (NTT), and so on. The Council still keeps on functioning as the coordinator on strong motion observation activities in Japan.
Table 1 List of destructive earthquakes in the twentieth century
Figure 1 Location of destructive earthquakes in Japan in the twentieth century
Table 2 Specifications of Typical Strong Motion Instruments
Recent Strong Motion Networks in Japan
In the light of the tragic disaster of the 1995 Hyogo-ken-nanbu (Kobe) Earthquake, the Special Measure Law on Earthquake Disaster Prevention (implemented on July 18, 1995) was passed to protect the people's lives and properties from disasters caused by earthquakes. According to the law, the Headquarters for Earthquake Research Promotion was established under the Prime Minister's Office for unified promotion of earthquake research b).
The Headquarters is comprised of the Director (Minister of State for Science and Technology) and its staff (Vice-Ministers of relevant Ministries and Agencies). Under the Headquarters, there are two subsidiary committees, each is comprised of the staffs of relevant Ministries and those of people of experience or academic standing, who are conducting the following mandate concerning to earthquake research.
On the other hand, the situation of strong motion observation also underwent drastic changes. Several projects were planned and conducted in order to reinforce the strong motion network. Japan Meteorological Agency (JMA) deployed about 600 seismic intensity meters throughout Japan. National Research Institute for Earth Science and Disaster Prevention (NIED) established the nationwide network "K-Net" with 1,000 observation sites. NIED is also constructing another strong motion instrument network "KiK-Net". Every prefecture equipped the seismic intensity information network system to concentrate data from all containing municipalities. About 2,600 seismic intensity meters were newly installed in municipalities that have neither JMA station nor K-Net station. Those up-to-date networks can gather and announce the seismic information rapidly.
2. JMA Seismic Intensity Network1)
The Japan Meteorological Agency (JMA) is a unique national agency that is responsible for tsunami forecasts, short-term prediction of a large earthquake, and information service on earthquakes, tsunamis and volcanic activities c). JMA is operating a network made up of about 180 seismographs for continuous earthquake monitoring and 600 seismic intensity meters covering the whole of Japan as shown in Figure 2. The Earthquake Phenomena Observation System (EPOS) at the headquarters of JMA in Tokyo and the Earthquake and Tsunami Observation System (ETOS) at six District Meteorological Observatories collect the observational data.
After an earthquake occurs, JMA immediately processes the observational data and quickly announces information on epicenter, magnitude and the distribution of seismic intensity to the public through the media as well as to the disaster prevention organizations. Information from 2,000 seismic intensity meters, which are set up by local governments, is also compiled together.
The JMA seismic intensity scale was originally assessed by the human feeling and the damage examination. In 1996, JMA introduced the new seismic intensity scale, which can be calculated from acceleration records, and developed the seismic intensity meter for prompt estimation of the instrumental seismic intensity. Table 3 provides situation and damage caused by earthquakes correspondent to the JMA seismic intensity scales. Acceleration data files are distributed by Japan Meteorological Business Support Center at cost later on.
Figure 2 Distribution of JMA seismic intensity meter stations
Table 3 Explanation of JMA Seismic Intensity Scale c)
3. K-Net and KiK-net
National Research Institute for Earth Science and Disaster Prevention (NIED) d), Science and Technology Agency, constructed a large network, which is called K-Net e), of strong motion instruments in 1996. K-NET consists of 1,000 observation stations that were deployed all over Japan with spaces of about 25 km. Each station has a digital strong-motion instrument with a broad frequency-band and a wide dynamic range on the free field and connects with the control center in NIED, Tsukuba, through the Integrated Services Digital Network (ISDN) line. After the occurrence of an earthquake, the distribution of peak ground accelerations is quickly reported by facsimile and e-mail. Digital acceleration records are posted on the web site within a few days.
NIED is also deploying the high sensitivity seismograph (Hi-net) and the digital strong-motion seismograph (KiK-net) f) across the all of Japan, as part of the activities of the Headquarters for Earthquake Research Promotion. A high sensitivity seismograph and an acceleration sensor are installed on the firm bedrock at the bottom of a well. An additional acceleration sensor is placed on the ground surface. The project plans to construct a network of more than 500 stations, and about 450 installations have been completed at present. Strong earthquake motion data are opened on the Internet web server.
Figure 3 Distribution of K-Net (blue) and KiK-Net (green) sites
4. Seismic Intensity Information Network of Local Governments 2)
The Fire Defense Agency, Ministry of Home Affairs, subsidized local governments to construct the network system that gathers information on seismic intensity promptly. The system assists emergency measures and disaster relief activities by transmitting the information to organizations concerned with disaster measures. A seismic intensity meter is installed in every municipality and each local government collects seismic intensities from containing municipalities. All information finally concentrates at the Fire Defense Agency. About 200 JMA stations and 500 K-Net stations have been already placed on the premises of municipal offices. Therefore, 2,600 seismic intensity meters were newly installed at remaining municipalities. Forty-seven local governments (prefectures) and about 3,300 municipalities (cities, towns and villages) are enrolled in this huge network in total. Seismic intensity scales at a large part of the stations are included in the JMA announcement.
Figure 4 Sample of distribution of JMA seismic intensity scale provided by the Seismic Intensity Information Network (a case of Osaka Prefecture g))
5. Other National Research Institutes and Public Bodies
(1) Public Works Research Institute, Ministry of Construction h)
Public Works Research Institute (PWRI), Ministry of Construction, is a national institute in the field of civil engineering. PWRI developed an essential tool for grasping damage outline immediately after an earthquake by providing rough estimation on damage of road facilities. The tool, which is called Seismic Assessment Tool for Urgent Response and Notification (SATURN), consists of about 700 strong motion instruments placed along highways and rivers with an interval of 20 to 40 kilometers 3). Peak ground accelerations and spectrum intensities are transmitted to the headquarters in real-time, and are used to estimate liquefaction possibilities and damage of highway bridges.
On the other hand, PWRI is operating the Dense Instrument Arrays for Strong Motion Monitoring in nine sites 4). In order to investigate the seismic effect of geological and topographical conditions, many accelerometers are three-dimensionally arrayed at each site. PWRI is also maintaining traditional strong motion observation sites. The network possesses 1672 instruments at 361 sites and targets dynamic behavior of bridges, river embankments and dams.
(2) Port and Harbour Research Institute, Ministry of Transport i)
Port and Harbour Research Institute (PHRI), Ministry of Transport, is committed to researching on broad range of themes concerning ports and harbors as well as airports by tying-up closely with the bureaus of the ministry in charge of constructing ports and airports. Strong motion observation of PHRI has been carried out since 1963, and nowadays observation stations reach 57 ports including high-density earthquake observation network using seismic meter distributed intensively in the Haneda Airport and the Kushiro Airport 6). The results of the observation are periodically reported as the annual reports and widely used for seismic design and study on earthquake disaster prevention measures.
(3) Earthquake Research Institute (ERI), University of Tokyo j)
Earthquake Research Institute (ERI), University of Tokyo was a member institute of the Strong Motion Accelerometer Committee that developed the original Japanese strong motion instruments, and has long history of strong motion observation. ERI has deployed strong motion observation stations from southern Kanto area to Suruga Bay, and densely arranged instruments in the Ashigara Plain 7) as shown in Figure 5. Observational records are provided on the Internet web site k).
Figure 5 Location of Observation Stations operated by Earthquake Research Institute (ERI), University of Tokyo k)
(4) Tokyo Gas Co. Ltd. l)
Tokyo Gas Co. Ltd. supplies gas to customers in the Tokyo metropolitan area. Tokyo Gas launched development of the Seismic Information Gathering and Network Alert System (SIGNAL) in 1986 and put it into operation with 331 SI (Housner's Spectral Intensity) sensors in 1994. In addition, Tokyo Gas commenced preparation of the most extensive ultra-high-density real-time seismic motion monitoring and disaster mitigation system in 1998. The system, which is called Super-Dense Real-Time Monitoring of Earthquakes (SUPREME), will install 3,600 new SI sensors 8).
Figure 6 Sample of PGA distribution map posted on the SIGNAL web site l)
(5) Yokohama City m)
Yokohama City, the second largest city in Japan, founded the Dense Strong Motion Network as a part of the READY (Real-time Assessment of Earthquake Disasters in Yokohama) system 9). The network consists of 150 ground surface stations and nine borehole stations distributed at an average interval of 1.7 kilometers. Information on earthquake ground motions, e.g. peak ground acceleration and JMA seismic intensity, is transmitted to three centers through the ISDN line within three minutes. The seismic information is reported to organs concerned disaster countermeasures and is utilized for damage estimation by READY. Distribution of seismic intensity is also uploaded to the web site of Yokohama City m).
Figure 7 Sample of Seismic intensity distribution map posted on the web site of Geophysics and Seismology Lab., Yokohama City University n).
String Motion Observation of BRI
Building Research Institute (BRI) o), Ministry of Construction, is a national institute engaging in researches on architecture and building engineering. BRI has started the installation of strong motion instruments more than 40 years ago and is now in charge of three networks p). The aim of the observation is contribution to the enhancement of earthquake-resistant design technology by means of experimental investigation of strong ground motion characteristics and building seismic response. Dynamic soil-structure interaction is also the essential target of observation. Our three networks and the intensive strong motion installation at BRI are outlined hereinafter.
1. Nationwide Strong Motion Network
The nationwide strong motion network, which has the longest history, has observation sites in major cities throughout Japan. Forty-seven observation sites are equipped with digital strong-motion instruments as shown in Figure 8, and connected to BRI through the telephone line. The objects of observation are mainly buildings, and acceleration sensors are usually placed both at the top floor and at the basement floor of the building. In addition, a sensor is also set up on the ground surface at the newly equipped sites.
The network has obtained a number of noteworthy records. For example, in the 1964 Niigata Earthquake, the change in the characteristics of the seismic motion caused by liquefaction was clearly recorded in the building next to the collapsed apartment house in Kawagishi-cho. Also, in the 1978 Miyagi-ken-oki (Off Miyagi Pref.) Earthquake, acceleration of more than 1,000 cm/s2 was recorded at the top floor of the nine-story school buildings of Tohoku University. Recently, the peak ground acceleration of 711 cm/s2 was recorded at the Kushiro Local Meteorological Observatory by the 1993 Kushiro-oki (Off Kushiro) Earthquake 10). In addition, in the 1994 Sanriku-haruka-oki (Far Off Sanriku) Earthquake, an enormous acceleration record was obtained in the building next to the severely damaged old Hachinohe city hall building 11).
Figure 8 Site location of the nationwide strong motion observation of BRI.
2. Dense strong-motion instrument array in Sendai
It has been considered that the sub-surface layer especially influences characteristics of the seismic motion acting upon buildings. From 1984 to 1989, eleven observation stations were established on grounds with various conditions in the Sendai area, under the name of joint research by governmental and private bodies 12). Each observation station holds three accelerometers, placed on the ground surface, in the base rock, and in the intermediate layer. Figure 9 shows configuration of observation stations.
The observation systems of all stations are collectively controlled by the control center in Sendai City. An NTT exclusive line connects the observation stations with the control center, and an NTT public line connects the control center to BRI in Tsukuba. The information on the earthquake records and the change of instrumental conditions will be sent immediately.
The project as joint research with private bodies has been completed in 1999. BRI reduced the network and updated recording equipment in order to continue the operation.
Figure 9 Site configuration of the dense strong-motion instrument array in Sendai
3. Strong-motion instrument network in the Metropolitan Area
The 1995 Hyogo-ken-nanbu (Kobe) Earthquake awakened us again to the importance of disaster prevention measures for large-scale urban areas. It is important to predict the probability of a future earthquake and its impact, and make as many preparations as possible in anticipation of such an event. It is also very essential to grasp the damage situation immediately to put in effect the necessary countermeasures.
In 1996, BRI established eighteen new observation sites that are deployed radially in the Tokyo metropolitan area as shown in Figure 10. Tokyo stands on the extremely thick sediment at the center of the Kanto Plan, which is the largest on in Japan. Therefore, the site configuration was planed in consideration of the effect of the sedimental layers and the influence of extensive Kanto Plain on seismic motions. At typical sites, two or three accelerometers are installed in a building. The system immediately collects information on the seismic intensities through the telephone line at the time of an earthquake occurrence.
Figure 10 Site location of the strong-motion instrument network in the Tokyo metropolitan area
4. Strong-motion observation at Urban Disaster Prevention Research Center, BRI
The project to observe the complicated behavior of the building and the effect of the soil-structure interaction during earthquakes was drafted with the construction of the Urban Disaster Prevention Research Center (Annex) building of Building Research Institute, Tsukuba. The installation was completed in 1998, and the observation is now under way 13).
The annex building has eight stories with single basement floor and supported by the mat foundation on the clayey layer of 8.2 meters below. The observation system has eleven sensors in the annex building, seven sensors in the surrounding ground, and four sensors in the main office building. The farthest sensor on the ground is placed 100 meters away from the annex building, and the deepest sensor is set up 89 meters in depth. The amplification process by the ground surface layers and the three-dimensional behavior of the buildings are recorded using twenty-two sensors in total. All sensors are connected to the recording equipment in the observation room in the annex. The system has 24-bit A-D converters, high-performance digital signal processors and 40 MB flash memory storage.
Figure 11 Accelerometer configuration at the Urban Disaster Prevention Research Center, Building Research Institute (BRI)
History of strong motion observation reaches a half of a century in Japan. The instrumentation technology has been steeply improved and the observation density has been extremely getting higher. Especially the 1995 Hyogo-ken-nanbu (Kobe) Earthquake accelerated installation of strong motion instruments as a principal part of the real-time disaster information system. In such system, information on the intensity of earthquake motions is gathered primarily. Peak ground accelerations, JMA seismic intensity scales and/or SI values are adopted as values representing the ground motion intensity. Acceleration wave data will be afterwards collected. Characteristics of recent strong motion instruments and observation work can be summarized as follows.
However, the most of strong motion instruments, which were expanded after the 1995 Kobe Earthquake, are placed on the ground surface. Strong motion observation for structures is also essential to rationalization of seismic design. More reinforcement of the strong motion observation targeted at building structures is earnestly expected.
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