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Source: GNS Science

Three large earthquakes occurred offshore New Zealand beginning with a M7.3 East Cape earthquake felt widely across the country. This was then followed a few hours later by a M7.4 and M8.1 earthquake in the Kermadec Islands. All three quakes caused tsunamis.

We’ll be thinking – does this confirm what we know, or change it? What’s caused this and what can we learn?

Sally Dellow

In the early stages of GNS Science’s response, Earth Structure and Processes Manager Sally Dellow was the incident controller.

“When an event like this happens, scientists will instantly see the opportunities for new information and new research,” she says.

“We’ll be thinking – does this confirm what we know, or change it? What’s caused this and what can we learn?”

After the event response – where scientists provide emergency managers and first responders with the information they need to make decisions – comes the science response. From seismologists to geodetic specialists to social scientists, here’s some of the work our teams are already doing to find out more about an extraordinary series.

Location of the three large earthquakes that occurred last Friday 5 March.

East Cape puzzle – one quake or two?

Of last Friday’s three big quakes, the M7.3 East Cape quake has proved to be the most challenging for scientists to unravel. That’s because it was actually two earthquakes a short distance apart and separated by a just few seconds. They were also at different depths.  

What we know so far is that the earthquake broke two large faults in different locations and at different depths

Caroline Holden

Seismologists around the world are deciphering the various signals the earthquake produced. Tsunami wave amplitudes, ground shaking recordings, and GPS measurements of ground displacement are helping to build a complete picture of the event.  

“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.

Recordings from land-based GPS instruments also show that the M7.3 quake pushed large parts of East Cape and Hawke’s Bay about 1cm to the west and southwest, which is not unexpected for an earthquake of this size.

Record Felt Reports

The M7.3 East Cape earthquake generated the largest-ever number of “felt” reports on the GeoNet website – with more than 52,000 users logging the shaking. The previous record was set by the M5.8 earthquake off the coast of Levin in 2020, with nearly 37,000 reports.

For our scientists, the most exciting data comes from the “Felt Detailed” reports – a longer questionnaire which digs deeper into what people experienced during the quake and the levels of damage. We had a massive number of these too:  more than 6500 at last count, the highest tally since the 2014 Eketahuna event.

Dissecting the tsunami

It’s extremely rare for three earthquakes in quick succession to generate three tsunamis that then combine in complicated ways to impact on one country. In fact, it’s so rare that most tsunami scientists cannot recall any similar examples.
 

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

Dr David Burbidge

In the past week, tsunami specialists at GNS Science have been collecting and analysing a wealth of data collected from deep ocean tsunami sensors, tide gauges, and eyewitness accounts. They are starting to form a clearer picture of the three tsunamis and how they interacted to threaten our coasts.

In particular, the data is expected to shed light on aspects such as the length of time a tsunami threat can last and the level of hazard it represents to different stretches of coast.
 
“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. This will add to the understanding of both the seismic and tsunami hazard faced by Aotearoa New Zealand.
 
Scientists are asking the public to share any unusual sea behaviour they witnessed last Friday or Saturday. Photos and videos of the sea at that time would be especially welcome. A link to the tsunami survey is here.

Raoul Island update

A team from GNS Science, NZDF, DOC and MetService arrived at Raoul Island early on Wednesday morning. This trip had already been planned before the earthquakes struck, but after a full risk assessment GNS Science deemed it safe for technicians to go onto certain areas of the island. This allowed them to repair GeoNet monitoring instruments damaged in the quakes.

“Luckily it ended up being quite straightforward to get the instruments back online,” says GNS Remote Infrastructure Operations Co-ordinator Kris O’Brien.

The team were able to fully restore power and communications to all GeoNet instruments but they found some data from the second earthquake had been lost.

“While it’s disappointing to have lost some data from the Raoul island instruments, we can still learn plenty about the Kermadec events from the wider GeoNet and regional monitoring network, including the DART buoys,” GNS Science operations and data team leader Jonathan Hanson says.
 

View of Blue Lake on Raoul Island, showing the landslides on the crater wall with the DoC establishment and the RNZN Canterbury at anchor in the background

Volcanoes

In the wake of so much activity, our volcano team has been closely watching the active volcanoes in case they change. So far, there’s nothing significant to report.

“We haven’t seen changes at Whakaari/White Island, but at Raoul we have seen a small rise in the level of Green Lake of about 150 millimetres,” volcanologist Brad Scott says.

“We have observed a landslide into the pool at Marker Bay, which was formed during the 2006 eruption. We’re now getting data back from Raoul Island, which will help us monitor any changes.”

Aftershock forecasts

GNS Science has updated the 7 and 30-day aftershock forecasts for the M7.3 East Cape earthquake.

The continued seismic activity that we’re seeing is very much in line with what we’d expect to see following a significant earthquake, like the events last Friday

Dr Annemarie Christophersen

On the day of the quakes, we estimated a probability of one or more M5.0—M5.9 aftershocks at greater than 99 percent (extremely likely). This has now decreased to an estimated probability of greater than 85 percent (very likely).

Similarly, the probability of one or more aftershocks in the M6.0—M6.9 range has decreased from 50—75 percent (likely), to 20—35 percent (unlikely).

“We are still working on better understanding the completeness of the aftershocks so that we can provide a comparison of forecast and observations of further earthquakes,” says Earthquake Physics and Statistics Team Leader Annemarie Christophersen.

“We plan to release another probability forecast next week, and future updates will include long-term models and a one-year forecast”.

As at 1pm on Friday 12 March, The GeoNet network had recorded nearly 1700 further earthquakes in the East Cape and Kermadec regions following the mainshock events last Friday.

“The continued seismic activity that we’re seeing is very much in line with what we’d expect to see following a significant earthquake, like the events last Friday”.

 
 

More on aftershock forecasts:

Tsunami: An animation of the M8.1 Kermadec tsunami, produced by GNS Science tsunami modeller Aditya Gusman, can be seen here: https://www.gns.cri.nz/static/download/RAOUL/animate.gif

MIL OSI