by Dozens of aftershocks in the wake of Friday’s earthquakes and tsunami warnings aren’t bad news.
In fact, the activity is in line with the scenario monitoring agency GNS Science pegged as most likely.
That is, clusters of aftershocks of a smaller magnitude smaller than Friday’s quakes, becoming less frequent over the coming month.
But GNS is warning people to stay on their guard as recent activity has still raised the probability of something larger occurring.
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Obviously, the longer we go without [something larger] happening, the less likely it is to happen,” GNS Science seismologist Matt Gerstenberger said.
Three large earthquakes rattled Aotearoa on Friday, and tsunami warnings and evacuations were triggered for Northland and coastal Bay of Plenty towns.
Director of Civil Defence Roger Ball speaks about the earthquake swarm and tsunamis on Friday.
The three scenarios
GNS had three possible scenarios for what might happen next.
The chance of another similar-sized earthquake of around magnitude 8.0 is classed as very unlikely.
An even bigger quake of magnitude 8.5 or more is classed as extremely unlikely.
So far, the numerous activity fits nicely into scenario one: aftershocks that fall away over time.
“If anything, it’s even on the lower end of aftershocks we would expect. We don’t know that for sure, but it’s following that [scenario one].
That means quakes like the magnitude 6.1 reported on Saturday afternoon are “perfectly expected, Gerstenberger said.
By early Sunday afternoon, there had been about 185 aftershocks of magnitude 4 or above in the East Cape area.
One was greater than magnitude 6.
Aftershock numbers are about what GNS Science expected, under the scenario it considered most likely to follow Fridays quakes. This image shows aftershocks recorded up until 1.10pm on Sunday 7 March.
It’s harder for GNS to record quakes in the Kermadecs because of the distance, but more would be expected there because it was the site of the most powerful quake.
So there would have been more than the 37 aftershocks of at least magnitude 4 – and four of magnitude 6 and up that GNS recorded, Gerstenberger said.
A global seismographic network had recorded 100 magnitude four and up, but even that won’t be the full picture, he said.
Aftershocks tend to happen in clusters as opposed to steadily declining, Gerstenberger said.
“You get a bunch for a little while, then they’ll go quiet, and then you get a bunch for a little while.
Most often, there’s not something larger than the main shock. That’s kind of what those scenarios are showing – that there are a whole lot of earthquakes, but they’re all smaller. But there always is some probability for something larger to occur.
Scientists are monitoring two separate sequences after the quakes on Friday, and are still working out how they’re related.
One stems from East Cape shock the early-morning magnitude 7.1 earthquake.
The other is linked to later quakes in the Kermadecs, which measured magnitude 7.4 and 8.1.
There’s still a heightened probability of tsunami, so people need to stay vigilant. Pictured are Northland residents on Friday.
And tsunamis could be in the mix as well.
Normally, scientists would start looking closely at the possibility when earthquakes are magnitude 7.1 or 7.2 or more, Gerstenberger said.
“What ultimately matters is displacement, so offset of the sea floor … Distance from the shore doesn’t really matter because tsunami waves can travel huge distances.
However, there are rare cases of tsunami earthquakes, which register as a lower magnitude but release enough energy to cause a wave.
And because there’s still increased probability a bigger quake or tsunami, Gerstenberger reminded people to stay vigilant and remember the “if it’s long or strong, get gone” message.
