Earthquake How

Computers do the ceaseless job of monitoring seismic data looking for possible earthquakes. Once one has been initially detected, the location and magnitude become refined over a number of minutes as more data from further stations arrives. For the bigger quakes, we get a Duty Officer to confirm the final details.

Seismic traces showing P and S arrivals.

Seismic traces showing P and S arrivals.

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. The earthquake is then located by SeisComP3 and information is released onto the Quakes part of the website. The seismic stations operated by GeoNet consist of a seismometer and a digitiser. A seismometer is a sensitive instrument that generates a small electrical current in response to ground shaking. The electrical current is then digitised 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 are received by the DMC data reception computers located at Avalon (Lower Hutt), Wairakei (near Taupo) and central Auckland, 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 human activity) and is most likely to have been caused by an earthquake.

These occurrences are called detections. If a detection is deemed significant, then SeisComP3 tries to group it with detections from different stations. Once we have detections from at least ten stations, we declare a seismic event. The detections are further analysed 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.

Preliminary information for earthquakes are posted to the Quakes pages soon after their occurrence; this is updated as more data becomes available.

The system 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 Applications and Data section of this website.