Kermadec earthquake: Why last week's tsunami warning has scientists surprised

Scientists at GNS say last weeks events were "extremely rare."
Scientists at GNS say last weeks events were "extremely rare." Photo credit: Geonet / Getty

Scientists say it's "extremely rare" for three earthquakes in quick succession to generate three tsunamis, combining in "complicated ways" to impact one country - as they did in New Zealand last week.

Three big quakes struck the Tonga Kermadec subduction zone on March 5, beginning with a magnitude 7.3 East Cape quake that was felt widely across the country, followed by a 7.4 and 8.1 earthquake in the Kermadec Islands. 

Tsunami warnings were issued for many North Island coastal regions following the 8.1 quake. Scientists said all three quakes could cause a tsunami, with footage of strong ocean surges and other unusual activity circling on social media. 

Three quakes struck the Tonga Kermadec subduction zone last Friday.
Three quakes struck the Tonga Kermadec subduction zone last Friday. Photo credit: GNS Science

In the week since, tsunami specialists at GNS Science have been analysing the data collected from deep ocean tsunami sensors, tide gauges and eyewitnesses. The series of events last Friday are considered so rare, most tsunami scientists "cannot recall any similar examples".

The 7.3 East Cape quake quickly became the most challenging of the three to decipher. Scientists have found it was in fact two earthquakes in quick succession, separated by just a few seconds. They also struck at different depths. 

"What we know so far is that the earthquake broke two large faults in different locations and at different depths," says GNS Science seismic modeller Caroline Holden.

The quake was so powerful, it pushed large parts of East Cape and Hawke's Bay about 1cm west and southwest.

Scientists hope the data will help paint a clearer picture of how the tsunamis interacted to pose a threat to Aotearoa's coasts - and how long the tsunami threat can last. 

"Having a large amount of new data to work with is going to be very useful in advancing the understanding of the threat tsunamis pose to New Zealand," said David Burbidge, Tsunami Team Leader at GNS Science.

They are also investigating how much the seafloor was deformed by each of the three earthquakes.

The East Cape 7.3 quake generated the largest-ever number of "felt" reports on the GeoNet website, with more than 52,000 users logging the shaking.

GNS is asking the public to share "any unusual sea behaviour they witnessed last Friday or Saturday" here.

They say photos and videos of the sea at this time would be "especially welcome" in furthering their investigations. 

The probability of one or more aftershocks in the magnitude 6.0 to 6.9 range has decreased from 50-75 percent (likely) to 20-30 percent (unlikely).

As of 1pm on Friday, the GeoNet network has recorded nearly 1700 further earthquakes in the East Cape and Kermadec regions following the mainshock events last Friday.