Earthquake swarm/tsunami risk: Weeks of aftershocks expected after big shake off East Cape, Kermadecs

An “impressed” Emergency Management Minister Kiri Allan spoke with Newstalk ZB’s Simon Barnett and Phil Gifford after the tsunami threat was downgraded. Audio / Newstalk ZB

Scientists are expecting weeks of aftershocks after this morning's big 7.1 East Cape quake - but the chances of it triggering something bigger than 8.5 are less than 1 per cent.

GeoNet scientists have used forecast models to develop 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," GeoNet said.

"We expect there to be more felt earthquakes, particularly in the East Cape."

The region considered for these scenarios extends from the central Kermadec region - north of where the 8.1 quake hit, hours after the East Cape jolt - to offshore of the East Coast.

The most likely scenario - with a 90 per cent chance within the next 30 days - is that further earthquakes, but of smaller magnitudes, will drop off over the coming month.

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That includes the potential for earthquakes in the 7.0 to 7.9 range, with a 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," GeoNet said.

"Similar sequences to these have occurred in this region in the last 50 years including a central Kermadecs 8.0 in 1976 and 7.7 in 1986."

Following the three large earthquakes that struck offshore New Zealand this morning, our scientists have developed three scenarios for what earthquakes may happen over the next 30 days. Read all the information here: https://t.co/8Wz5yMVBeH #eqnz pic.twitter.com/jFX6sMHoNy

— GeoNet (@geonet) March 5, 2021

Less unlikely, with a chance of 15 per cent or less within the next 30 days, is another similar-sized quake to the largest Kermadec shake, of about 8.0.

"This scenario is similar to what occurred in the 2014 Iquique, Chile earthquake sequence,"GeoNet said.

"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."

Earthquakes of this size could occur on either the subduction interface or farther away as an "outer rise" earthquake on the incoming Pacific plate, east of the subduction zone.

Scientists are expecting weeks of aftershocks following this morning's big 7.1 East Cape quake. Image / GeoNet

The third scenario - and an extremely unlikely one, with a probability of less than 1 per cent within the next 30 days - is a much larger quake, measuring 8.5 or greater.

"For example, an earthquake could occur on the Kermadec subduction interface in the central Kermadecs," GeoNet said.

"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 7.1 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 8.1 earthquake."