Jamie Morton is the NZ Herald's science reporter.

Kaikoura quake: The power of 400 atom bombs

The escalation of the Kaikoura earthquake from 7.5 magnitude to 7.8 puts its power on a par with the energy release of 400 atomic bombs detonating.

Canterbury University physicist John Holdaway said the revised magnitude also meant that the November 14 midnight main shock accounted for more than 70 per cent of the energy released in all New Zealand earthquakes of the past seven years combined.

Its energy release was now nearly three times that than previously estimated.

Around 32 quadrillion joules of energy was released in two minutes during the magnitude 7.8 quake - the equivalent of about eight million tonnes of TNT, or detonating 400 atomic bombs.

"To put that into perspective, the same amount of energy could power the city of Christchurch for three years - or every home in the South Island for a year - or the whole of residential New Zealand for three months," Holdaway said.

"It's comparable to 10 simultaneous Darfield quakes."

The energy release of the 2011 Christchurch earthquake equated to 15,000 tonnes of TNT.

The aftershocks alone from the past three days have already released another 800 trillion joules - more energy than all New Zealand earthquakes last year.


Source: John Holdaway

Last night, GeoNet reported scientists were still calculating what the revised magnitude meant for aftershock probabilities, but these would be released within the next day.

"Based on our findings and in discussion with international researchers, early indications are that this is one of the most complex earthquakes ever recorded on land," GeoNet spokesperson Sara McBride said.

"This complexity means we have had to take extraordinary efforts to determine the magnitude, depth and locations."

The very long time it took for the faults to rupture - more than one minute - meant that the standard methods of calculating magnitude were insufficient to capture the full energy released.

"Due to the size of the quakes, we've gathered data from our entire network of seismic stations," she said.

"All of these stations would not normally need to be included in magnitude estimates."

Further, information collected by GeoNet technicians from several sites on the ground had only just been uploaded and led to a more complete understanding of the ground deformation and strong-motion data.

"Finally, our science teams have been working tirelessly, going up and down the affected areas and measuring the length of faults and how much they moved.

"Their efforts have provided us with a clearer picture as to the size and length of the ruptures."

Huge slips, caused by the 7.5 earthquake, blocking State Highway One north of Kaikoura. Photo / Mark Mitchell
Huge slips, caused by the 7.5 earthquake, blocking State Highway One north of Kaikoura. Photo / Mark Mitchell

The 7.8 magnitude was consistent with estimates from several other international agencies, specifically the US Geological Survey.

"The new magnitude just tells us what we think most people who felt the earthquake already know: it was powerful, and went on for a long time over a large distance," McBride said.

"It doesn't change what happened but it does provide us with more knowledge about how significant the event was."

The magnitude did not change any of the observations of strong ground motion, fault breaks or GPS recorded movement of the earth's surface - these were physical observations independent of the magnitude of the earthquake.

"Our GeoNet seismic network is robust and records tremors and shakes throughout the country," she said.

"However, with these very rare large events, it requires time and thought as to what all of this new data means.

"This earthquake required us to take a different approach and we have been triaging the data to reconcile all the different data sets."

- NZ Herald

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