All about Seismic Monitoring System

1. Unit to express the size of an earthquake

Vibration Frequency

Vibration frequency is the measurement of the number of cycles that occur per second. The main frequency component of earthquake is less than 10Hz and its vertical wave has higher frequency than horizontal wave.

Vibration FrequencyVibration Frequency
earthquake waveform data


Unit to express the acceleration of earthquake is *Gal.
So called "Mega-earthquake" once occured in Japan; Hanshin, Chuetsu and Tohoku recorded the waveform of more than 1000Gal and it was clear that the acceleration was bigger than a gravitational acceleration (1G) generated in these quakes.

Unit of acceleration, 1Gal=1cm/s2, and widely used in the field of seismology.
"Gal" is categorized as non SI (international unit) with limitation. However, it is allowed to be used freely in gravitational acceleration or earthquake related matters.


Magnitude is the measure of an earthquake size from the focus. Generally, the deeper and larger the magnitude is, the more the vibration spreads.


Seismic Intensity Scale in Japan

At present, Japan Meteorological Agency use Shindo to measure the intensity of an earthquake.
The Seismic Intensity Scale is calculated based on the ground acceleration waveform.
The Seismic Intensity Scale may change after the calculation according to the earthquake duration and vibration.

Seismic Scale in Other Countries

The value the JMA established was re-evaluated after the Great Hanshin/Awaji earthquake. It is classified into 10 scales; 0-4・5-lower・5-upper・6-lower・6 upper・7.
Each country uses a different seismic scale .

【See chart】
-1000 7 X
VII Thrown by the shaking and impossible to move at will. Water and electric supplies may stop over wide areas.  
  6-upper X Impossible to keep standing and impossible to move at all. Gas pipeline to the local district and water distribution facility are damaged.  
-500 6-lower Difficult to keep standing. Gas supply pipelines to the household and major water pipes are damaged. 400Gal: Stop all the power supply in the factory.
-400 IX
-300 IX VI 250Gal: Emerfency call for evacuation. Shut down the dangerous gas pipeline (explosive, lethal)
  5-upper Many people are considerably frightened and find it diffucult to move. Gas supply pipelines to the household and major water pipes are damaged. 150Gal: Go on a patrol thoroughly to check the situation of damage of all equipment from all departments.
-200 VIII
  5-lower Most people try to escape from danger by running outside. Gas meter with safety devices are tripped, stopping the supply of gas in same household.
In rare cases, suspension of water supply may occur.
100Gal: Go on patrol to check if there is an abnormal condition in important equipment.
  Some people find it difficult to move. 80Gal: Evacuation call. Emergency stop for the part of equipment. Shutdown part of lifeline.
-100 VII
  4 Many people are frightened. Some people try to escape from danger. - 40Gal: Make an announcement of earthquake scale.
Evacuation call if necessary. Check if there is an abnormal condition equipment by going on a quick patrol.
-50 VI
-30 VI Most sleeping people awake.
  3 Felt by many to all people indoors. - 8Gal: Make an announcement of earthquake scale.
No need to evacuate. Check if there is an abnormal condition in equipment by going on a quick patrol.
-20 V
  Some people are frightened.
-10 IV
  2 Felt by many people indoors. - -
-5 II
Some people awake.
-3 I
-2 1 Felt by only some people indoors. - -
-1 I
  0 I 0 Not felt by all or most people. - -
JMA Seismic
in 1996
in 1956
in 1964
in 2000
Human perception and reaction Utilities and Infrastructure Reference Example of Alarm Output in Japan

**Note: Information on this chart is only for reference.

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2. SI Value

SI Value・・・“Velocity response spectrum” is one of the standards to express an earthquakeʼs destructive power given on structures. If the structure has high rigidity, the velocity normal period is between 1.5-2.5 seconds. The integrated value (area of the spectral distribution) can be one of the factors to represent the destructive power of earthquakes. Therefore, it is called as “spectral strength : SI Value.”

SI Value is the standard to asses the earthquake destructive
power on structures.

Why SI Value is necessary?
Even it is at the same acceleration or magnitude, damages on buildings may differ due to frequency range or time. The real damage on structures can not be estimated with the conventional seismic monitoring system based only on acceleration (Gal).
Therefore, value should be used to understand the real damage on structures.

Why SI Value is necessary?

When the Gas and Water Dept. in Ojiya City was struck by the earthquake, there was no damage, despite the high acceleration. In Nagaoka City where the acceleration was low and SI Value was high, huge damage was observed. This indicates that SI Value should be the reference in evaluating damage due to earthquake.

”Summary of damages to gas supply business in Niigata Prefecture Chuetsu Earthquake reported by Gas Safety Subcommittee-May 2009”

Twelve gas suppliers recorded more than level 5 in seismic intensity scale. After the earthquake, only Kashiwazaki Gas and Water Dept. had stopped supplying gas due to severe damage of their establishment.
The SI and acceleration value are presented in this chart as monitored and recorded by the seismometer in each place.

Chart 1 Damage situation of each gas dept.

Company SI Value
Damage Situation
Damage Situation
Gas & Water Dept.
99* Kagami cho 999*
Fujii 746
About 34,000 houses of outage
(Recovery target houses: 30, 978**)
Nagaoka Water Dept. (Mishima) 50 293 120 houses for trouble occurrence in supply
(Recovery on 16th, July)
Ojiya Gas & Water Dept. 23 323 No damage
Jyoetsu Gas & Water Dept. (Kashiwazaki) 73 444 81 houses for trouble occurrence in supply
(Recovery on 18th, July)
Hokuriku Gas Dept. (Nagaoka branch) 46 527 No damage
Kanbara Gas Dept. 36 210 No damage
Nagaoka Water Dept. (Koshiji) 38 258 No damage
Shirane Gas Dept. 26 165 No damage
Sakae Gas Dept. 24 174 No damage
Mitsuke Gas & Water and Sewage Facility 34 254 No damage
Kawaguchi Construction Dept. 18 217 No damage
Uonuma Gas & Water Dept. 9 145 No damage

(SI Value is the maximum level which each gas institution measured.)
  *Measurement range excess
 **Target house for recovery is the calculation amount after subtracting the sum of the number of houses
   which donʼt use gas and the number of collapsed houses due to earthquake from the number of gas supply

Refrence : Advisory Committee on Energy and Natural Resources / Urban Heat Energy Association / Gas Safety Subcommittee

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3. Configuration


  • A 3dimensional (XYZ) servo pickup and arithmetic circuit are built in the monitoring unit.
  • The monitoring unit performs basic functions from digital processing to an alarm display.
  • The functions can be extended beyond basic alarming by adding an external display.
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4.Principle of Sensor

Sensors are generally classified into two categories; Electric and Mechanical types. All of IMV's seismic monitoring systems adopt the electric type sensors.

Electric type
Electric type [Servo Type]
Mechanical type
Mechanical type [Drop-ball Type]
Mechanical type [Pendulum Type]
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