A magnitude 6.2 earthquake that struck off the Kermadec Trench on Wednesday evening was felt across much of New Zealand, even though GeoNet's automated system initially classified it as "unnoticeable." The agency also briefly recorded a second earthquake near Dannevirke, which was later confirmed to be a false detection, commonly referred to as a "ghost quake."
According to a report by 1News, the earthquake occurred at 7.43 pm, about 560km north-east of Whakatāne, at a depth of 100km. GeoNet received 3,494 felt reports from people across the country, with shaking reported from East Cape to Wellington.
Just three minutes after the main quake, GeoNet's monitoring network detected a magnitude 4.6 tremor east of Dannevirke. However, analysts later removed the event after determining it had never actually occurred.
Explaining the discrepancy between the automated alert and the public's experience, Earth Sciences NZ seismic duty officer Katie Jacobs said the shaking classification is calculated solely based on an earthquake's magnitude and distance, and is not updated as felt reports come in. She emphasised that "Our event-response systems are not dependent on the shaking classification tag; this automated classification is only used to prioritise how earthquakes are displayed on the GeoNet website and app," as quoted by 1News.
Jacobs said the earthquake behaved differently because it occurred deep beneath the Tonga-Kermadec Trench, close to the subducting Pacific Plate. "The location near the subducting Pacific Plate meant energy could travel through this subducting slab, which is made of strong material," she explained, 1News has quoted.
She added that "Earthquake waves travelling along the strong slab material lose less energy with distance, and this creates a mismatch between the simplified automated shaking classification calculation and the reality of what people across Aotearoa felt," 1News has quoted.
GeoNet currently labels earthquakes according to shaking intensity, ranging from weak to strong. However, Jacobs acknowledged that the wording used for the lowest level of shaking could be confusing, saying, "Unfortunately, the word for the least amount of shaking felt is ‘unnoticeable’, which is described as ‘barely sensed, only felt by a few people’. We are looking into this wording," 1News has quoted.
The agency also explained the appearance of the so-called "ghost quake," saying the automated detection system misinterpreted seismic waves from the distant Kermadec earthquake as a separate local event. According to Jacobs, "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," 1News has quoted.
GeoNet said the issue arises because primary (P) waves travel faster than secondary (S) waves. When earthquakes occur far offshore, the long delay between these waves can confuse the automated detection system into identifying a second earthquake that does not exist.
While manual verification introduced in 2019 had largely eliminated such "ghost quakes," GeoNet resumed automatic publishing during the Covid-19 pandemic to ensure faster public access to earthquake information. The agency has continued with the system, stating, "We have decided to keep the auto publishing on indefinitely to enable our vital information to go out to the public no matter what happens in the future," as quoted by 1News.