'Ghost quakes' in Geonet machine after Kermadecs quake

Monday, 10 September 2018

A cluster of earthquakes have been recorded across New Zealand on Monday. Earlier, a large quake near the Kermadec Islands triggered four 'ghost quakes'.

On Monday afternoon, four moderate earthquakes were recorded on Geonet's monitoring website and people throughout New Zealand reported feeling the shaking.

But those four quakes never happened. They were what are known as 'ghost quakes'.

Instead, one magnitude 6.9 quake had struck near the Kermadec Islands, north of New Zealand, at 4.19pm at a depth of about 130 kilometres.

Due to its depth and distance from New Zealand, it's impact for people here was minimal.

GNS Science seismologist John Ristau said the four ghost earthquakes were a result of the way the monitoring system worked to try to rapidly locate quakes.

LATEST: M6.9 quake 785km north-east of Whakatane has been reviewed by our duty officer. The Kermadec area quake was ~130km deep. It triggered a number of ghost quakes locally. Some smaller, real quakes have occurred since too. #eqnz https://t.co/KA7C71EiiE

— GeoNet (@geonet) September 10, 2018

'When we get these large earthquakes up north of New Zealand - it happens quite often actually with events even further north in the Pacific Islands - our system records them and the first stations that start to get the signal, they try to locate [the quake] but they go under the assumption that the earthquake is actually close to New Zealand.

'So they mis-locate it as being close to New Zealand and then further south, stations start to record it and they try to associate it with the previous earthquake but since the previous earthquake was located really badly, it can't associate it so they think it's a new earthquake and … this starts happening all over the country.'

At that point, someone at GNS Science had to manually step in to find real earthquake and delete the ghost quakes from the system.

The system could be changed to eliminate a lot of the ghost quakes, but that would mean waiting until more data had arrived, Ristow said

'If we have a local earthquake then it takes that much longer to release that information. We have to make this trade-off between the ghost quakes and getting information out there as fast as we can.

'We think it's better to have the ghost quakes than delay putting out information about real local quakes.'

After the Monday quake at the Kermadecs, Civil Defence said there was no tsunami risk for New Zealand.

Later, a series of smaller, real earthquakes did occur.