A magnitude 6.2 earthquake off the Kermadec trench was reportedly felt by thousands last night, despite GeoNet's automated system rating it "unnoticeable" — and moments later, its network logged a second quake that never happened at all.
The quake hit at 7.43pm on Wednesday, 100km deep and 560km north-east of Whakatāne, and drew 3494 felt reports from East Cape to Wellington.
Three minutes later, Geonet logged a magnitude 4.6 tremor just east of Dannevirke, a "ghost quake" analysts later deleted.
Earth Sciences NZ seismic duty officer Katie Jacobs said the gap between the automated classification and the volume of felt reports was something the agency sometimes observed.
Jacobs said the tag was worked out from magnitude and distance alone and did not change as felt reports arrived. She was quick to note it played no part in how the agency responded to events.
"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."
Usually, seismic waves fade as they travel, and people don't feel small or moderate quakes from far away. Someone more than 500km from a quake the size of Wednesday's would usually feel nothing.
However, the quake broke that pattern because it was deep and occurred along the Tonga-Kermadec trench, near 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," Jacobs said.
"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."
Geonet uses shaking intensity to label earthquakes: weak, moderate, strong.
"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."
'Ghost quakes'
The Dannevirke "ghost quake" was a phantom event the monitoring network invented while trying to make sense of waves from far offshore.
Jacobs said the detection system was designed to pick up local quakes and could misfire when a distant one passed through.
"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."
The problem lies in the timing of primary (P) and secondary (S) waves. P waves travel faster and arrive first but, when a quake is a long way off, the delay before the S waves grows so long that the system mistakes them for a fresh event.
Ghost quakes had briefly disappeared in 2019, when analysts at the new National Geohazard Monitoring Centre vetted quakes before they were published. The agency reversed that during Covid-19 and has kept auto-publishing on since.
"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."
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