Earthquake

GeoNet quickly locates the earthquakes that you feel in New Zealand, as well as routinely analysing more than 15,000 others each year.

Damaged farm structures in the Edgecumbe earthquake, 1987.

GeoNet and Earthquakes

GeoNet analyses, locates and archives over 15,000 earthquakes each year within the New Zealand region. 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.

For the over 250 significant earthquakes felt in the New Zealand region each year, GeoNet provides the location and magnitude of the events within one hour of their occurrence. Other significant events within the greater south-west Pacific region may be detected by our equipment, but we do not routinely locate these as the geometry of our recording network does not allow accurate epicentres and magnitudes to be determined. Details about these earthquakes may be available from the United States Geological Survey.

Did you feel an earthquake? 

If you have felt an earthquake recently, we would like to know where you were and what happened. This information will help us understand how your area might respond in future earthquakes. Your input will be used to make maps of how the intensity of shaking was distributed over the area which felt the earthquake. The questionnaire should only take a few minutes to complete:

• View the page of Recent Quakes to see if the event is listed there; if so, click on the felt it? button and make your report.
• If the event is not listed in Recent Quakes, please make an unlisted GeoNet Felt Earthquake Report. You will need to make a reasonable estimate of the date and time you experienced the shake. Sometimes earthquakes occur where the shaking was too slight to alert our duty team, or it is possible that you have experienced something that was not an earthquake. If you make a report and we are able to establish its location, it may be significant enough to be added to the Recent Quakes list.

It can take between fifteen minutes and one hour after a significant earthquake for the location and magnitude to be reviewed and the event to be listed on this web site. During this time, it will be necessary to make an unlisted report.

Seismic traces showing P and S arrivals.

How GeoNet Locates 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.

Preliminary information for significant Recent Quakes is posted to this web site, whilst the locations and magnitudes of other earthquakes are available through the Quake Search facility as soon as they become available.