New Zealand's earthquake alert system has been rattled again by the mysteriously-named "ghost quakes".
Early on Monday morning, GeoNet's sensors picked up a magnitude 4 quake near Whitianga, triggering a notification to GeoNet app users. The real cause? An aftershock in the Fiji region just nine minutes before.
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It comes after a massive quake struck in the Pacific Ocean near Fiji, with initial measurements from USGS suggesting it was magnitude 8.2 in strength, but 560km deep.
"Some 'ghost quakes' (not real) have been [generated] by our auto-location system 30mins ago," GeoNet said on Twitter on Sunday.
"The real quake was VERY deep (560km) magnitude 8.2, 281 km from Fiji, the location & depth means it won't have caused damage, these deep quakes don't cause tsunami."
But seeing an earthquake without feeling it or seeing in on the GeoNet app can confuse some people.
"With the Twitter alerts you can be baffled when you go to the app and don't see what might have been quite a big quake," Amy Watling wrote on Twitter.
So what are "ghost quakes"?
GeoNet seismologist John Ristau says the shakings are seismic readings picking up earthquakes from far away.
This energy can travel enormous distances before arriving in New Zealand and triggering alarms. The strength of the quake registers as locally-generated earthquakes - or "ghost quakes" - in GeoNet's systems.
"'Ghost quakes' appear on our network typically after a large regional source earthquake," GeoNet says on its website.
"We have very sensitive seismic equipment that picks up the various waves that earthquakes create and we can pick up these waves even if it is very far away.
"Our equipment gets confused by these waves and interprets these as being a smaller, locally-sourced earthquakes close by."
Due to the need for instant public notification of any quake event, this does initially send out quake reports via the GeoNet website and Twitter account.
The GeoNet team then gather all of the reports from equipment and members of the public to work out exactly what's occurred and establish the true size and location of the earthquake.
"These quakes are an unfortunate side effect of getting information out to the public as quickly as possible, instead of waiting up to a quarter of an hour for a person to locate and ensure these are authentic earthquakes," GeoNet says.
"If we make the system too picky on the quakes it reports, we might not get rapid information about real earthquakes that occur. If we let it report on whatever information it picks up, we get 'ghost quakes'."
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