How is earthquake data measured, computed and published ?

Geonet, The New Zealand governmental office who publishes earthquake information, released a very good describing article how an earthquake is measured, monitored, computed etc. has a constant interest in explaining the background of earthquake numbers for his readers. We hope that this article will clarify a number of issues concerning earthquakes.

To make rapid locations of earthquakes GeoNet operates a country-wide network of seismic stations that transmit their data to the GeoNet Data Management Centre (DMC) where it is analysed by automated processes. If the automated processes detect an earthquake the Duty Response Team is notified and if the Duty Officer confirms that the earthquake is real and significant, the earthquake information is released.

The seismic stations operated by GeoNet consist of a seismometer and a seismograph. A seismometer is a sensitive instrument that generates a small electrical current in response to ground shaking. The electrical current is digitised by the seismograph and transmitted continuously to the DMC in real time. This digital recording of ground shaking is the raw data used to make earthquake locations. The seismic stations are supplemented by a network of strong-motion seismographs, which only transmit data whenever they detect a higher level of shaking, typically from earthquakes that will have been felt by the public.

The real-time seismic data is received by the DMC data reception computers located at Avalon (Lower Hutt) and Wairakei (near Taupo) and analysed automatically for possible earthquakes. The computer processes look for ground shaking that is distinct from the normal background activity (such as that caused by weather and oceans) and may be associated with an earthquake. These occurrences are called detections. If a detection is deemed significant, then the relevant portion of the data is parcelled up and sent to the DMC data analysis computers. They store all the detected earthquake data, grouping the detections from different stations into earthquake data sets. The detections are examined for P (primary) and S (secondary) wave arrivals from the earthquake, and the times of these arrivals are inverted against seismic velocity models for the earth to yield the best location for the event. The magnitude of the earthquake is determined at a station by measuring the maximum amplitude of the seismic signals, and relating them to the distance of the station from the event, together with the characteristics of the seismometer and seismograph. The magnitudes from all available stations are then averaged to give an overall value for the event.

It also provides locally recorded data from global earthquakes to the International Seismological Centre in the United Kingdom, and preliminary earthquake information to the National Earthquake Information Center, part of the United States Geological Survey responsible for locating major earthquakes worldwide. The waveform data and the located hypocentres are freely available to the worldwide community of researchers through the Resources section of this website.

Courtesy Geonet and GNS Science – Link to the original article of Geonet – GNS Science

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