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GeoNet has revealed why they deleted a magnitude 6.5 earthquake off the east coast of New Zealand.
The quake struck 940km northeast of Whangārei at a depth of 213km at 7:41pm on Saturday.
GeoNet said the shaking was "unnoticeable" with the majority of people who reported the quake describing it as "light" or "weak".
But on GeoNet's website, the earthquake has been deleted from its list of recent quakes.
GeoNet said their earthquake detection system is designed to pick up local earthquakes and the one on Saturday struck hundreds of kilometres away from New Zealand.
"So when the seismic waves of a distant event are rolling through our network, our earthquake detection system doesn't recognise the wave arrivals as coming from one local earthquake and instead makes up many local earthquake detections to explain the unfamiliar seismic waves," GeoNet said.
An earthquake is made up of 'P' (primary) and 'S' (secondary) waves, among others. P waves travel faster than S waves, so they arrive at a seismic station first, according to GeoNet.
"If an earthquake happens a long way from a seismic station, there can be a long delay between the P waves and the S waves arriving - so long that the automatic system thinks that the S waves are the P waves of a separate earthquake," GeoNet said.
"These false detections from distant earthquakes are what we call "ghost quakes", and almost as soon as they are created our Geohazard Analysts are working to tidy them up," GeoNet said.
GeoNet confirmed they remove the duplicate earthquakes created by their system and revise the origin and magnitude of the real earthquake using the data.
"Almost always this results in the earthquake source moving hundreds of kilometres away into the Kermadec Arc to the northeast of New Zealand, and often with a larger magnitude," GeoNet said.