Large magnitude 6.6 shake triggered 'ghost quakes' in the North Island, but why?
The magnitude 6.6 quake that struck off the coast of the North Island on Tuesday was widely felt and triggered false reports of shakes in New Zealand.
It is not uncommon for earthquakes to confuse seismic readings as energy from tremors travels large distances.
These "ghost quakes" register as local earthquakes when the GNS Science system starts to receive data. The earthquake was centered near the Kermadec's Raoul Island, about 1,000km northeast of New Zealand.
What happened on Tuesday?
A large magnitude 6.6 quake centered about 850 kilometers north of Whakatane at a depth of 360km.
The epicenter was near the Kermadec Islands and the Kermadec Trench. The United States Geological Survey located the earthquake 120km north-northwest of a barren outcrop known as L'Esperance Rock.
It marks the point where the Australian plate meets the Pacific plate.
Geographically, the entire region is one of the most seismically active areas in the world.
New Zealanders felt the quake's energy, but it wasn't in New Zealand.
Seismographs interpreted the waves as locally sourced and triggered alerts for the North Island.
"Ghost" quakes sometimes appear on the seismic monitoring system, the GeoNet array of around 200 seismograms dotted around New Zealand, after a large regional earthquake.
Sensitive equipment picks up the seismic waves created by earthquakes. Equipment gets confused and pushes out an earthquake alert interpreted as locally sourced to the public, when the shake could be hundreds of kilometres away.
|The location of the quake. Deep quakes off the coast can be initially recorded as multiple quakes.|
Yes, the earthquake was around 1000km away and deep.
This is all a bit confusing.
As the seismic waves travelled south from the source they are picked up progressively by detectors.
First Raoul Island, which has a webcam by the way, picked up the earthquake then the network detected the waves as they quickly moved southwards.
The "ghost" or "false" quakes, reported on Tuesday as three severe quakes in the Bay of Plenty region, were removed from the GeoNet alert system after initial reports.
This kind of thing has happened before.
In 2013, a quake near the location of Tuesday's shake triggered ghost earthquake readings in New Zealand.
Why don't scientists wait to confirm an earthquake's location?
In short, it's important to get information and data out to the public quickly.
As soon as readings start coming in from the seismic network that information is automatically publicly notified.
Later, once GNS Science review data, they can revise the reported magnitude and pinpoint the precise location.
Let's get technical shall we?
OK, it's all about different types of seismic waves, known as P-waves and S-waves, and the types of detection equipment.
|Cr: Geonet: Below the North Island, the Pacific plate is subducting beneath the Australian plate, which causes deeper quakes.|
Broadly, the equipment confuses the secondary waves for primary shaking, hence the three severe shakes initially reported by GeoNet and felt by New Zealanders.
GNS seismologist Dr John Ristau says:
"People actually felt the quake. It was quite deep.
"Our automatic system, generally, for 99 per cent of the time it actually works quite well.
"We know there's a problem when we have large earthquakes north of the North Island, particularly when they are deep.
"Our automatic system gets fooled.
"Primary waves are the fastest. That triggers the system. Then you have the secondary waves. The S-waves come in well defined...so the automatic system gets fooled into thinking it's another earthquake."
Can scientists do anything about the confusion?
The current system is the best available.
Ristau says GNS Science would rather have the system detect earthquakes as false than re-calibrate it and risk failing to detect a locally-sourced shake.
Plus, it's important to get information out to the public as quickly as possible in New Zealand and the Pacific.
Seismograms are extremely sensitive aren't they?
They can pick up wind and traffic noise, so it's no surprise when they detect tremors from earthquakes hundreds, or even thousands, of kilometres away.
The Kermadec Islands and the surrounding area lie within a marine sanctuary created in 2015.