Series of smaller earthquakes likely
GNS scientists predict further earthquakes of a smaller magnitude will occur but will decrease in frequency over the next 30 days, following a morning of large offshore quakes that rocked the country.
The most likely place for future earthquakes to occur is close to the three mainshocks that occurred today. Locations farther away from the mainshock, including Hawke's Bay, have a much lower probability of experiencing increased shaking.
Three large earthquakes struck offshore New Zealand this morning all triggering tsunami alerts. The first was a magnitude 7.1 earthquake that struck at 2.27am - 105km east of Te Araroa at a depth of 90km.
GNS received over 52,000 felt reports, easily breaking the last record holder, a magnitude 5.8 earthquake offshore Levin in 2020, which had 36,867.
This was then followed a few hours later by a magnitude 7.4 earthquake that struck at 6.41am, and an 8.0 magnitude that struck at 8.28 north-east of New Zealand near the Kermadec Islands.
Seismologist Dr Matt Gerstenberger says that while they do not know for certain what will happen next, they do know that there will be more earthquakes. How large these will be or what time they will happen is uncertain.
Based on their forecast models, they have developed three scenarios for what earthquakes may happen over the next 30 days. Earthquakes that generate tsunami impacting the northern part of Aotearoa-New Zealand are possible in any of these scenarios.
"We expect there to be more felt earthquakes, particularly in the East Cape," Dr Gerstenberger said.
The region considered for these scenarios extends from the central Kermadec region north of this morning's M8.1 earthquake to offshore East Coast Aotearoa within the next 30 days, because this area of activity is closest to the land and people, he said.
Dr Gerstenberger said a tsunami that can impact Hawke's Bay can occur in all three scenarios. In scenario one, Hawke's Bay is unlikely to experience anything greater than moderate shaking. In the very unlikely, and extremely unlikely scenarios two and three, Hawke's Bay may experience much stronger shaking, including the potential for a rupture on the Hikurangi Subduction Zone.
The first scenario is considered very likely - up to 90 per cent within the next 30 days. "The most likely scenario is that further earthquakes of a smaller magnitude than occurred today will occur but will decrease in frequency over the next 30 days."
This includes the potential for earthquakes in the magnitude 7.0-7.9 range, with more than 80 per cent probability within the next 30 days. Larger earthquakes are more likely to occur in the central Kermadecs, near the northern part of the sequence.
Similar sequences to these have occurred in this region in the last 50 years including a central Kermadecs M8.0 in 1976 and M7.7 in 1986.
Scenario two has a 15 per cent or less likelihood within the next 30 days, making it "very unlikely".
"The next most likely scenario is a similar-sized earthquake of around M8.0. This scenario is similar to what occurred in the 2014 Iquique, Chile earthquake sequence. Should this occur, it is much more likely to be in the central Kermadecs, but it is still possible for it to happen near the East Cape," Dr Gerstenberger said.
Earthquakes of this size can occur on either the subduction interface or farther away as an "outer rise" earthquake on the incoming Pacific plate, east of the subduction zone.
The final scenario is considered "extremely unlikely" with a one per cent or less likelihood within the next 30 days.
"A much less likely scenario than the previous two scenarios is that the recent earthquakes will trigger a significantly larger earthquake (M8.5 or greater) within the next 30 days.
"For example, an earthquake could occur on the Kermadec subduction interface in the central Kermadecs. This scenario is very complex and when combined with the current uncertainty in our models, we cannot confidently put a probability estimate on it occurring; however, our models suggest this is even less likely to occur near the location of the March 5th East Cape M7.1 earthquake."
This scenario is similar to what occurred in the Tohoku Earthquake in Japan in 2011. Although it is still extremely unlikely, the chances of this occurring have increased since the M8.1 earthquake.
Dr Gerstenberger said large events like today's earthquake are a good reminder to always be prepared. "Protect yourself from the earthquake first and drop cover and hold during shaking. And then, if it's Long or Strong, Get Gone."