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Summary of the High-Sensitivity Seismograph Network, Hi-net
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1. Objectives and Outline of "NIED Hi-net"
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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.
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- 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
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Among these plans, the National Research Institute for Earth Science and Disaster
Resilience (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]
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.
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2. Location of Seismic Stations
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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.
- Seismic stations should be basically deployed in the 20-km mesh.
The number of stations will be about 1000.
- 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.
- 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).
* 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.
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3. Design of the Hi-net Station and Data Collection
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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.
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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.
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).
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4. Sharing and Distribution of the Hi-net Data
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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.
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Deployment of NIED Hi-net Seismic Station
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1. Overview of Deployment of NIED Hi-net Seismic Station
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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:
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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.
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Grid points which are located within 12-15 km from the existing high-sensitivity
seismograph stations are removed from the list of candidate locations.
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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.
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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.
- Geological and Geographical Conditions
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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.
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Sites near fault fracture zones, deep weathering layer
(especially granite terrain) and landslide area should be avoided.
- Social and Environmental Conditions
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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.
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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.
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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.
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Power supply and telephone service are needed. Length of the branch line
should be less than 100 m
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Site under high-voltage power line should be kept away to avoid electrical noise.
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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.
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◆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.
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[Click to enlarge]
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2. Future Deployment Policies of the NIED Hi-net
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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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