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 What is Hi-net?

Summary of the High-Sensitivity Seismograph Network, Hi-net
1. Objectives and Outline of "NIED Hi-net"

After the Hyogo-ken Nanbu (Kobe) earthquake (M7.3) in January 1995 and the occurrence of the Grate Hanshin-Awaji Earthquake Disaster, Japanese government enacted the Special Measure Law on Earthquake Disaster Prevention in July 1995 to promote a comprehensive national policy on earthquake disaster prevention. Based on this law, the Japanese government established the "headquarters of earthquake research promotion" (HERP) whose director is the minister of Science and Technology Agency (now the minister of Education, Culture, Sports, Science and Technology).
The HERP is composed of the policy committee and the earthquake research committee. One of the important tasks of the HERP is the determination of the fundamental plans for earthquake observation. The followings are the basic plan for survey and observation to mitigate earthquake disasters.
  • Basic objective: Promotion of research into earthquakes with the goal of strengthening disaster prevention measures, particularly for the reduction of damage and casualties from earthquakes.
  • Purpose: Improving the accuracy of the prediction of earthquake occurrence and seismic motion.
  • Fundamental plans for survey and observation: As the core of the fundamental plans, the following survey and observation is promoted;
    • (1) Seismic observation
      • a) High-sensitivity seismic observation (microearthquake observation)
      • b) Broadband seismic observation
    • (2) Strong motion observation
    • (3) Crustal movement observation (continuous GPS observation)
    • (4) Survey of active faults in inland and coastal regions
Among these plans, the National Research Institute for Earth Science and Disaster Prevention (NIED) has contracted the deployment of the nation-wide high-sensitivity seismograph network, because we had already accumulated experience for the Tokyo metropolitan deep borehole array and the Kanto-Tokai seismic network. This new nation-wide high-sensitivity seismograph network is called 'NIED Hi-net'.

To detect non-human-sensitive weak ground shaking by micro-earthquakes, a high-sensitivity seismic station is located in quiet place and a seismometer is installed at the bottom of a borehole at depths of over 100 m. Since small earthquakes occur frequently, these high-sensitivity seismometer observations provide new knowledge about seismic activity, mechanisms of earthquake occurrence, and underground structure.

[Results of the high-sensitivity seismic observation]

FIG2      FIG3
The maps above show the epicentral distribution in 1998 (left) and 2001 (center) around the Kii Peninsula, central Japan. Since the NIED Hi-net was established in Oct. 2000, the older (left) and newer (center) maps correspond to without and with the Hi-net data, respectively. In the map of 2001, many earthquakes occurred beneath the Wakayama city surrounded by the rectangle. Was the seismicity in 2001 higher than that in 1998?
The right-hand figure above indicates the accumulated number of earthquakes that occurred within the box with respect to their magnitude. Generally, smaller earthquakes occur more frequently, and the logarithmic values of the accumulated number of the earthquakes with respect to the magnitude are negatively proportional to the magnitude. This relationship is broken at magnitudes less than 1.5 in 1998. This means that the detection capability of small earthquakes was M1.5. However, the relationship is extended to M0.9 in 2001. The Hi-net revealed that small earthquakes occurred frequently there. Moreover, you can see many aligned epicenters in the map of 2001. These straight lines correspond to active faults.

2. Location of Seismic Stations ▲Go to the top of this page

The magnitude of an earthquake depends on both size of a fault plane and the dislocation amount. If the deeper limit of the crustal seismicity is well known at a specified area, we can empirically estimate the maximum size of an earthquake in the area using a scaling law, even if it is difficult to estimate fault length and dislocation amount. Since the depth limit of the seismogenic zone of inland earthquakes is usually 15-20 km in Japan, the seismic network was constructed with a spacing of 15-20 km to assure precise focal depth determination of the earthquakes at such a depth.

According to this concept, we determined the location of the Hi-net stations with the following fundamental policies.
  1. Seismic stations should be basically deployed in the 20-km mesh. The number of stations will be about 1000.
  2. We avoided the vicinity of the existing seismic stations operated by the Japan Meteorological Agency (JMA) and universities. We constructed seismic stations in the area where seismographs are rather sparse first.
  3. After the deployment mentioned above, we evaluated capability of the seismograph network, and we try to update an observation system if needed.

The figure below shows the progress of high-sensitivity seismograph station distribution. Colors in the lower-left map correspond to the organizations who maintain the seismograph station. Before the establishment of Hi-net (lower-left map), the seismograph stations were distributed inhomogeneously: A dense seismograph network was installed in the Kanto-Tokai districts, central Japan, though the stations was very sparse in the Chugoku district, SW Japan. The NIED Hi-net is a uniform network of high sensitivity seismographs covering the whole of Japan (lower-right map).

FIG4
* See "Deployment of NIED Hi-net Seismic Station" if you want to know how to determine the location of a Hi-net station in detail.

3. Design of the Hi-net Station and Data Collection ▲Go to the top of this page

NIED Hi-net seismic stations are composed of a borehole to install seismometers and an observation house containing data recorders and other equipment for data transmission. The figure at right shows a schematic illustration of the design of a Hi-net station. To achieve stable high-sensitivity seismic observation avoiding surface ground noise, we select a place where the ground noise is relatively small and place seismometers at the bottom of the observation well (> 100 m). Short-period three-component velocity seismometer, its natural frequency is 1 Hz and sensitivity is 200 V/m/s, are installed at the bottom of the borehole together with a three-component strong motion accelerometer. We also deploy a three-component strong motion accelerometer at the ground surface. Waveform data from the pair of two strong motion accelerometers are accumulated by KiK-net, Kiban-Kyoshin seismograph network. A sensor unit at the bottom of the borehole is not buried with cement and the unit is able to be pulled up and be repaired when the unit has problems. Thus, we can provide stable seismic observation data for a long time at Hi-net stations.
FIG6

FIG5

The depth of a borehole is determined with consideration of the thickness of sediment layers and the ground noise. The figure below shows the number of stations with respect to the depth of the borehole. The boreholes with depth deeper than 1000 m are located near the large cities such as Tokyo, Osaka, and Nagoya region. The deepest borehole is Iwatsuki (N.IWTH) station located in Saitama city. Its depth is 3510 m.

FIG7

Hi-net waveform data are transmitted to not only the data management center (DMC) of NIED but also the JMA and other related organizations in real-time. Earthquake information such as hypocenter location and magnitude are sent to organizations involved in disaster prevention as a quick announcement from JMA. Hi-net waveform data is used for the monitoring of seismic activity in and around the Japan Islands and for research and investigation of earth science and earthquake disaster mitigation.

Motion of these seismometers are converted to weak electronic signal and transmitted to an analog-digital converter in an observation house through a signal cable. Hi-net waveform data is digitized at 1000 samples per second. After a decimation procedure, waveform data are formed as digital data with sampling frequency of 100 Hz and 27-bit resolution. Each one-second increment of data is collected as 'packet' and is sent to the DMC with time stamps obtained from GPS.

Each Hi-net station is connected to the NIED DMC located in Tsukuba city, Ibaraki prefecture. Packet data generated at each station are continuously transmitted to the DMC through IP-VPN network for 24 hours/365 days. At the DMC, we collect and accumulate all continuous waveform data and analyze them for the real-time monitoring of seismic activity and other geo-scientific studies in various fields. See the page of (research).

FIG8

4. Sharing and Distribution of the Hi-net Data ▲Go to the top of this page

NIED Hi-net data has been formally established since October 2000. At the same time, we opened this Hi-net website and began to provide observed waveform data and preliminary hypocenter information estimated by the NIED Hi-net automatic hypocenter determination system via the Internet. Before the establishment of the Hi-net system, the JMA started to provide the standard hypocenter catalog since October, 1997. JMA determines the hypocenters using waveform data provided by not only JMA own but also by many organizations (e.g. NIED and universities) operating high-sensitivity seismic networks. This catalog is called 'JMA unified hypocenter catalog'. NIED Hi-net follows this framework. We provide the Hi-net waveform data to the JMA in real-time. The Hi-net waveform data contribute to the improvement of the unified hypocenter catalog. The data from densely deployed Hi-net stations play an important role in the system of the earthquake early warning which was opened to the public from October, 2007. NIED also archives the waveform data of the other organizations as well as the Hi-net data and opened those data to the public from June 2003.

On this website, you can access the waveform data of the Hi-net and the other organizations, preliminary results of the JMA unified hypocenter catalog and other earthquake information. In order to collect the statistics of the provided data, we ask you to register before you access the waveform data and some additional information. See the page of user registration.





Deployment of NIED Hi-net Seismic Station
1. Overview of Deployment of NIED Hi-net Seismic Station ▲Go to the top of this page

In this chapter, we introduce how to construct the Hi-net seismic station as a new nation-wide high-sensitivity seismograph network.

◆1-1. Paper Plan for Station Distribution
Before construction of high-sensitivity seismograph stations, we discussed where we should construct new stations. First, we looked for suitable sites for high-sensitivity seismic observation with the following steps:
  1. Based on a map of the Japan Islands with the Gauss-Kruger projection (datum point: 137° 42’44“E, 37° 30’52“N), we create 20-km-spacing triangular grids and make a list of coordinate of grid points except for the ocean bottom region. The isolated island locations are out of this concept, and we consider them separately.
  2. Grid points which are located within 12-15 km from the existing high-sensitivity seismograph stations are removed from the list of candidate locations.
  3. By checking grid points on a large scale map, we move unsuitable points for high-sensitivity seismic observation (e.g., mountain area without telephone line and/or electric power supply, and area near noise sources like railways, highways and coastlines) to suitable sites within 5 km from original grid points. If there are no suitable sites within 5 km, these points are excluded from the list of locations.
  4. At each proposed site, we check geology and environment (*). Based on this information, we estimate required borehole depth, construction method, cost and so on.
    *) Nature conservation area (national park), protected forest, steep slope, landslide area, urban area, source of large noise (e.g., giant plant, quarry).
Drilling depth may be determined with considerations to the depth of the pre-tertiary basement rock. Depth of anobservation borehole is 100 m at least even though bedrock exists near the ground surface in order to reduce the surface noise due to weather condition and/or human activity. At sites with unsuitable geological conditions and/or high-level noise observed, we select to construct deeper observation borehole (> 200 m). In the case of urban area, a 1000-m-class observation well may be required.

◆1-2. Selection of Definite Station Locations
Each NIED Hi-net station requires long-term stable observation. In order to achieve this requirement, not only technical matters but also long-term (semi-permanent) useable observation sites are quite important. We, NIED, ask local governments to introduce suitable sites for long-term high-sensitivity seismic observation.
  1. Geological and Geographical Conditions
    1. The most suitable geological condition is sedimentary rocks in the early Miocene or older, or plutonic rocks existing above 50 m depth. A site with limestone rocks should be avoided because there is a possibility limestone caves exist.
    2. Sites near fault fracture zones, deep weathering layer (especially granite terrain) and landslide area should be avoided.
  2. Social and Environmental Conditions
    1. The following site should be avoided:
      * Distances from the Shinkansen super-express railways and/or highways are less than 3 km.
      * Distances from other railways and/or main roads are less than 1 km.
      * Distances from quarries, electric power plants, large factories and great waterfalls are less than 500 m.
      * Distances from community roads and rapid rivers are less than 30 m.
      In mountain area, a dead-end site along a branch river or valley region with low-traffic is preferable. Mountain top should be avoided.
      In plain area, a site away from main roads as far as possible is preferable.
    2. Desirable sites are public or quasi-public facilities such as schools, shrines, temples, parks and so on from a point of leases of observation site.
    3. To construct and keep a stable seismic observation, we need sufficient space (20 - 40㎡) for an observation space. During station construction, we need additional space about 200㎡ and water supply.
    4. Power supply and telephone service are needed. Length of the branch line should be less than 100 m
    5. Site under high-voltage power line should be kept away to avoid electrical noise.
    6. For long-term observation, area where regional developments are planed should be avoided.

◆1-3. Field surveys and Negotiations
Geological specialists investigate technical requirements such as difficulty of station construction, environmental and geological conditions at each candidate site. Based on the results of this survey, scientists visit the candidate locations and select the most appropriate site for a high-sensitivity seismograph station. They explain the significance of earthquake observation to the landowner and start negotiations to use the site.

◆1-4. Determination of Observation Sites
After field surveys, we construct an implementation plan of the fiscal year, and determine the location of the observation facilities.

◆1-5. Construction of the Observation Facility
For a construction of the Hi-net observation facility, we prepare work space with
200 - 250㎡ and equip a drilling derrick there. We use the drilling equipment as shown in the figure at right. The size of the drilling derrick is 5 m width and 15 m height. Triple casing pipe are inserted into the drilled hole, and the outside of the whole pipe is closed with cement. Therefore, the observation well doesn't affect the groundwater system near the site.

After completion of the observation well, we measure the key orientation installed at the bottom of a casing pipe in order to direct two horizontal seismometers to north-south and east-west directions correctly. The borehole sensor unit is about 3 m in length. The bottom of the sensor unit is seated on the mount, and the head is fixed to the surrounding casing pipe with a spring. The sensor unit is attached with not only signal cables but also a hanging wire. If we drag up the wire, the fixator is unhooked, making it possible to retrieve the sensor unit.

After completion of the facility, the observation site is unmanned. NIED regularly patrols and maintains the facilities and equipment.
[Click to enlarge]
FIG10
2. Future Deployment Policies of the NIED Hi-net ▲Go to the top of this page

NIED Hi-net operates and manages about 800 stations from Hokkaido to Kyushu (as of the end of 2009). The observation network covers the whole of Japan at the moment. In some areas, however, the density of the Hi-net stations is not as high as initially planned because of various factors such as availability of public services and budget constraints. Moreover, we should evaluate the ability of old high-sensitivity seismograph networks operated by the NIED, the JMA, universities, and other organizations since areas near these stations were excluded from the candidates of new Hi-net station in the early stage. If their ability is insufficient, we should replace these stations. The isolated islands, which were excluded to be considered separately in the first stage, must be deployed in the near future.





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We, NIED, do not deliver any e-mails of immediate seismic information obtained by the AQUA system.
In addition, please understand that we prohibit the redistribution of the seismic data. For more information, please read How to use the Hi-net data and FAQ.
High Sensitivity Seismograph Network Laboratory, Earthquake and Volcano Data Center, Earthquake and Volcano Research Unit, Monitoring and Forecast Research Department, National Research Institute for Earth Science and Disaster Prevention.
3-1 Tennodai, Tsukuba City, Ibaraki Prefecture, 305-0006, JAPAN
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