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Observation network


Free Field Strong Earthquake Observation Network

The establishment of CWB's free field strong earthquake network is to collect full records of strong earthquakes to understand the seismic activities under different geological conditions to serve as the basis for economic and safe anti-earthquake design norms to effectively reduce and prevent earthquake disasters.

Metropolitan area: establish stations in major metropolises of Taiwan, starting from less densely distributed ones.
Fault belt: establish stations around various active faults in Taiwan.
Environment of different geological conditions: establish stations in different geological environments including rock, soft soil and hard soil.
Major facilities: establish stations in major facilities such as nuclear power plant, railway, reservoir, and the Ministry of Defense.。

 
Six division map of Free Field Strong Earthquake Observation Network

All the observation stations are equipped with accelerometer, which is a type of acceleration seismograph recording signals of the acceleration of ground vibration caused by the earthquake. As the vibration acceleration can directly reflect the vibration of the local ground to provide the disaster relief units with an important basis for rescue and the acceleration is as well one of the major parameters for anti-earthquake design norms, therefore, the data recorded by the acceleration type seismograph are an extremely important source for earthquake prevention and engineering applications.

 

 

Station outside view and instruments of Free Field Strong Earthquake Observation Network

 

The instrument used in the free field strong earthquake observation network is the embedded force balance accelerometer. Its sampling rate can be determined according to the actual needs (for example, 200 points/second or 250 points/second), and its definition is at least 16 bits, being able to record strong earthquakes within negative/positive 2 G. The accelerometer determines the occurrence of the earthquake by Trigger Mode. When the accelerometer senses vibration above certain level, it starts to trigger the recording until the vibration dies down below the standards. At present, most of the observation stations of the strong earthquake observation network use 2/1000 of the maximum amplitude as the trigger standard (about 3.92 gal). Meanwhile, the pre-event time and post-event time records are kept to ensure the completeness of the seismic waves. (at present, the pre-event time of general observation station is set at 20 seconds, and the post-event time is set at 15 seconds) , the longest duration of recording is determined according to the instrument types at 90 seconds generally. The times of earthquake depend on the built-in memory or the plug-in storage such as CF card of the instrument with the minimum one being able to record 58 times of earthquakes at least. Most of the observation stations are equipped with GPS for timing by receiving the satellite signals. The hardware is composed of 5 parts including the acceleration sensor, signal processing device; memory unit, timing device and power supply with the dynamic recording scope more than 96 dB. The free field strong earthquake data are regularly downloaded by personnel of the observation stations. Meanwhile, the simple adjustment and parameter verification of the instruments are carried out. If any instruments are found malfunctioning or having big DC offset, professional maintenance service providers are brought in for repairing to ensure the data quality and instrument properness. When processing the data, first remove records of non-seismic signals, then compare the recorded time and position of the strong earthquake with earthquake parameters in the earthquake database to determine the related parameters of the present earthquake record to get it integrated and added into the strong earthquake database ultimately.