2021 New Zealand Earthquake

A magnitude 8.1 earthquake near the Kermadec Trench was the third and largest earthquake of the three. This event is near the magnitude 7.4 earthquake that occurred earlier this afternoon, some 600 miles (950 km) north of New Zealand. While large, these earthquakes are remote and the USGS PAGER report is Green for fatalities and economic losses. The 8.1 Mw earthquake is about 11 times larger than the earlier 7.4 Mw and occurred at a shallower depth (early estimates about 12 miles, or 20 km). The larger size and shallower depth increase the tsunami potential, and NOAA have released tsunami warnings for many islands in the southwest Pacific. The National Emergency Management Agency of New Zealand have also released a tsunami warning.

Map shows shaking intensity of the March 4, 2021 New Zealand earthquake.

(Credit: USGS. Public domain.)

Like the preceding 7.4 Mw, the 8.1 Mw earthquake occurred as the result of thrust faulting at shallow depth, the quake was initially reported as an 8.0 Mw and downgrade to 7.8 Mw, then third update to 8.1 Mw, likely on the subduction zone interface between the Pacific and Australia plates. Large earthquakes in this region are common. While the 7.4 Mw earthquake was a downgrade from a 7.5 Mw, it was unlikely to have been triggered by static stress changes caused by the prior 7.3 Mw near New Zealand this morning, the 8.1 Mw and 7.4 Mw are directly related. The 7.4 Mw event can be considered a foreshock of the 8.1Mw.

Further and updated information about the earthquake can be found here:

M 8.1 - Kermadec Islands, New Zealand (usgs.gov)

M 7.4 - Kermadec Islands, New Zealand (usgs.gov)

M 7.3 - 174 km NE of Gisborne, New Zealand (usgs.gov)

USGS scientists expect that these events will trigger aftershocks, but these will decrease in frequency over time.

If you felt the M7.3 earthquake, report your experience on the “USGS Did You Feel It?” website for this event.

Three notable earthquakes struck in the southern Pacific Ocean near the Kermadec Islands and New Zealand today, March 4, 2021.

There are currently as 10 March there are over 750 aftershocks by the geonet notifications predicted so far by the megathrust earthquake.

A tsunami watch for Hawaii and an advisory for American Samoa prompted by a major earthquake off New Zealand were lifted Thursday evening.

"There is no further tsunami threat to American Samoa from this earthquake although minor sea level fluctuations may continue," the Pacific Tsunami Warning Center said in a bulletin sent at 11:16 a.m. Samoa Standard Time Thursday (5:16 p.m. EST).

American Samoa is about 1,700 miles northeast of New Zealand, where a series of earthquakes had struck earlier.

The center also said Hawaii was in the clear.

"There is no tsunami threat to Hawaii," Gov. David Ige tweeted at 12:22 p.m. Hawaii Standard Time (5:22 p.m. EST). "This is a good reminder to stay vigilant and be prepared."

Coastal areas from southern Mexico down to Chile remained under threat as of 7 p.m. EST, according to the center.

The U.S. West Coast was never considered in danger.

New Zealand’s National Emergency Management Agency, meanwhile, also lifted warnings and said evacuees could return home, although there was still a threat of strong currents and waves.

Hundreds of students, workers and residents had fled to higher ground on New Zealand's North Island after the strongest of the earthquakes, rated at 8.1 magnitude, struck at 8:28 a.m. Friday (2:28 p.m. Thursday EST) near the country's Kermadec Islands, the M 8.1 resulted in a slight damage but no deaths and injuries reported.

"A tsunami may have been generated by this earthquake," the warning center said in a post.

People "must MOVE IMMEDIATELY to the nearest high ground, out of all tsunami evacuation zones, or as far inland as possible. DO NOT STAY AT HOME," the agency warned after the 8.1 magnitude temblor.

With tsunami sirens sounding, roads on the North Island became congested. Whangārei Mayor Sheryl Mai encouraged people to walk or ride to ease traffic congestion, TV New Zealand reported.

"Within the tsunami grounds sirens and alerts have been going off. Right now there is a degree of traffic chaos in the communities of Mangawhai and at Lang's beach near Waipu, all across the lower part of Northland," Kaipara Mayor Jason Smith told Stuff New Zealand.

Schools, businesses and homes evacuated and people moved to higher ground, the New Zealand Herald reported.

"This is definitely one to watch," Tim Stern of Victoria University told the New Zealand Herald.

Stern said the 8.1 magnitude quake was significant for its rare size and shallow depth, which the USGS said was about 12 miles deep.

"That puts it in the zone where it's a thrust, which means it's definitely possible that you can get a tsunami from there," he said.

The evacuation warnings were the second time Friday that residents on the North Island were warned about possible tsunamis. The first came after a major earthquake struck off the country’s North Island early Friday and caused severe shaking.

A 7.3-magnitude earthquake struck 147 miles northeast of Gisborne, New Zealand, at 2:27 a.m. Friday (8:27 a.m. EST), according to the U.S. Geological Survey.

New Zealand’s National Emergency Management Agency issued a tsunami warning shortly after that first earthquake. About 5 a.m., the agency said in an update, "Based on current scientific advice and information from coastal tsunami gauges there is no longer a threat to land areas. People who have evacuated can now return home."

Earlier the Emergency Management Agency had warned residents from Cape Runaway to Tolaga Bay to move to higher ground.

Hundreds of people sought shelter in a school on higher ground above Hawke's Bay on the central coast of North Island, TV New Zealand reported.

Gisborne Mayor Rehette Stoltz told TV New Zealand, "I've heard reports of little bits of damage, nothing major at this stage."

Crews will check for damage to buildings, roads and pipes in the city, Stoltz said.

Train service was stopped across eastern parts of the North Island so the tracks could be examined for possible quake damage, the New Zealand Herald reported.

At least eight aftershocks have been reported since the first quake, including one of magnitude 5.4, according to the USGS.

So far there have been no reports of injuries or major damage.

"Hope everyone is ok out there — especially on the East Coast who would have felt the full force of that earthquake," New Zealand's Prime Minister Jacinda Ardern said on Facebook.

Thousands of people reported feeling the quake, some even as far away as Christchurch and Dunedin on the country's South Island, more than 500 miles from the epicenter. Some of the shaking was even characterized as "severe," the second most intense level.

The New Zealand Herald quoted several callers to Newstalk ZB who had experienced the quake.

"She was a beauty, it really shook. I'm quite frightened, I've got no idea if there's going to be a tsunami, it was massive," Rex from Gisborne told Newstalk ZB. "It's the biggest I've felt in a long, long time and I'm 80."

Helen in the Chatham Islands said, "It's the biggest one I've ever felt. It went on and on and on. I'm in the old stone house and I didn't know where to stand because it's all rock. I've never felt one so big — it must be massive across New Zealand. It died down and then went on and on again."

Thursday's earthquake came 10 days after the 10th anniversary of a magnitude-6.2 quake that killed 185 people in Christchurch, New Zealand's second-largest city, the New York Times reported. The earthquake on Feb. 22, 2011, caused high-rise buildings to crumble and left parts of the city uninhabitable.

Five months earlier, on Sept. 4, 2010, a magnitude-7.1 quake had also caused severe damage in Christchurch, but no one was killed by the quake.

New Zealand is prone to seismic and volcanic activity because it sits at the edge of two tectonic plates, the Pacific Plate and the Australian Plate.

## East Cape and Kermadec aftershocks - what's happening?

It’s natural to ask the question about how many aftershocks there have been since the M7.2, but the answer isn’t so simple.

After a large earthquake, such as Friday’s M7.2 East Cape event, we expect further earthquakes in the region, some of which may be widely felt. But after such a dramatic series of earthquakes, counting the number of aftershocks is not an easy task.

GNS Science seismologist Matt Gerstenberger explains:

“We're dealing with events that are happening hundreds of kilometres offshore, far from the GeoNet seismograph network. There are so many earthquakes happening right now – ranging from big earthquakes (M6.1 just after 1pm on Saturday) to small ones – and they are overlapping in time. The distance from the network and the overlapping signals from these earthquakes makes it trickier to identify them from one another.

“We have had three large earthquakes: M7.2 in the East Cape around 2:30 on Friday morning, and two in the Kermadecs, M7.4 and M8.1 several hours later. These are creating two separate sequences of earthquakes, both with the potential to affect the North Island.

“What we can say for sure is that right now, the sequence is behaving as we would expect it to.”
At the time of posting, the National Geohazards Monitoring Centre has recorded more than 140 earthquakes of M4 or higher since the M7.2 earthquake. The NGMC is actively monitoring earthquakes across New Zealand 24-7.
GNS Science experts are already researching these earthquake sequences to understand the relationship between the Kermadec earthquake sequence and the East Cape sequence. They are also looking in more detail at the M7.2 earthquake, which looks like a very complex rupture.

“There are already questions forming about whether this event was a single rupture – or something more complicated,” Dr Gerstenberger says.

GNS Science seismologists have developed three scenarios for what earthquakes may happen over the next 30 days. These are overall scenarios that include both sequences.

In summary, in the aftermath of a large earthquake, we expect to see a lot of earthquakes of varying size – over time, they become fewer and smaller. But there is still an increased probability of a bigger event and we need to stay vigilant. Every earthquake is 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.

Read more about aftershock forecasting following the M7.2 East Cape Earthquake.

In the event of an earthquake, Drop, Cover and Hold during the shaking. Protect yourself from the earthquake first.

As soon as the shaking stops, move immediately to the nearest high ground, out of all tsunami evacuation zones, or as far inland as possible.

NEMA (National Emergency Management Agency) have a great website with information on what to do before, during and after a tsunami. There you can also find tsunami evacuation zone maps for around New Zealand, and what supplies you need in an emergency.

### M7.1 East Cape Earthquake - Aftershocks

While no one can scientifically predict earthquakes, we can provide forecasts of future earthquakes using computer models that are updated as an earthquake sequence continues. Forecasts based on the 5 March 2021 East Cape Earthquake are in the table below.

This forecast only considers the M7.1 East Cape earthquake. For scenarios that include the M8.1 Kermadec event, please see here.

Given that the earthquake occurred some distance offshore, we cannot detect the smaller earthquakes that we usually use in our computer models. Therefore, we base our forecast on an average of other New Zealand sequences and a global model for subduction zones.

These two models provide the spread of the average number and the probabilities in the table below. The range covers the 95% confidence bounds including both models. The forecast applies to the region around the mainshock as indicated by the box on the map below. Earthquakes can and do happen outside this region, but the box represents the most likely area for aftershocks in this sequence.

\*Issued on 5 March 2021, 6 pm. This table shows a forecast for future earthquakes in the given time intervals from the time of issuing for a preliminary forecast area from 178.50-181.5 degrees longitude and -36.5 to -38.5 degrees latitude. *95% confidence bounds*
For example, this table says that:

• Within the next 30 days, there is a 69-85% probability (likely to very likely) of one or more earthquakes of magnitude 6.0 to 6.9 occurring in the area shown in the box in the map. It is expected that there will be between 0 and 5 events of this magnitude within the next year. • It is very unlikely (3-5%) that there will be a magnitude 7.0 or larger earthquake in the next day.

What you can do about this:

As described in this article about our forecasts, the best thing we can do is be prepared for earthquakes. You can follow the National Emergency Management Agency on Twitter and Facebook for the latest earthquake and tsunami preparedness information. EQC also have a great guide to Quake Safe your home and you can follow your local Civil Defence Emergency Management Group.

### A morning of large offshore earthquakes - what happened and future scenarios

Three large earthquakes have struck offshore New Zealand this morning all triggering tsunami alerts. Our scientists have developed three scenarios for what earthquakes may happen over the next 30 days.

## What's Happened:

Early Friday morning, 5 March 2021, three large earthquakes occurred offshore New Zealand beginning with a M7.1 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 Kermadecs.

We received over 52,000 felt reports for the M7.1 earthquake easily breaking our last record holder, a M5.8 earthquake offshore Levin in 2020, which had 36,867.

## Tsunami:

NEMA (National Emergency Management Agency) issued tsunami alerts to the public following these events and so far, we have recorded tsunami arrivals from the East Cape earthquake as ~35-40cm at East Cape, and ~15-20cm at Great Barrier Island.

## Future Scenarios:

We recognize these events are dramatic and unsettling for many people. Our experiences In Aotearoa-New Zealand with earthquakes over the last 15 years have been challenging, and at times, tragic and traumatic. While we do not know for certain what will happen next, we do know that there will be more earthquakes. How large these will be or what time they will happen is uncertain, but we do have some statistical models that help us know what could happen next.

Based on our forecast models we 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. The region considered for these scenarios extends from the central Kermadec region north of the March 5th M8.1 earthquake to offshore East Coast Aotearoa-New Zealand within the next 30 days, because this area of activity is closest to the land and people.

1. Scenario One – Very likely (up to 90 percent within the next 30 days) The most likely scenario is that further earthquakes of a smaller magnitude than occurred on 5 March will occur but will decrease in frequency over the next 30 days. This includes the potential for earthquakes in the M7.0-M7.9 range (more than 80% 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.

2. Scenario Two – Very unlikely (15 percent or less within the next 30 days) 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. 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.

3. Scenario Three – Extremely unlikely (1 percent or less 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, including the Hikurangi Subduction Zone. 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.

Remember, Long or Strong, Get Gone.

Drop, Cover and Hold during the shaking. Protect yourself from the earthquake first.

As soon as the shaking stops, move immediately to the nearest high ground, out of all tsunami evacuation zones, or as far inland as possible.

### M8.1 Kermadec Earthquake and Tsunami (Warning cancelled)

The Tsunami Warning for New Zealand coastal areas following the magnitude 8.1 earthquake near the Kermadec Islands Region has been cancelled.

NEMA (National Emergency Management Agency) issue official tsunami warnings for New Zealand.

The National Advisory issued at 2:48pm following this morning's earthquakes near the KERMADEC ISLANDS REGION is cancelled.

The advice from GNS Science, based on ocean observations, is that the Beach and Marine threat has now passed for all areas.

Strong and unusual currents and unpredictable surges will continue for up to another 24 hours. People should remain vigilant and take extra precautions with regards to beach and ocean activities.

Strong currents and surges can injure and drown people. There is a danger to swimmers, surfers, people fishing, small boats and anyone in or near the water close to shore.

For more information on the M8.1 earthquake, go here

### M7.3 Earthquake East of the North Island - Tsunami Threat Cancelled

The National Advisory of tsunami activity is no longer in effect following a M7.3 earthquake east of the North Island.

NEMA (National Emergency Management Agency) issue official tsunami warnings for New Zealand.

NEMA [National Emergency Management Agency] and GNS Science have assessed that there is no longer a threat of tsunami activity following the magnitude 7.3 earthquake east of the North Island. People who have evacuated may now return to their homes.

The first detection of waves on East Cape occurred at about 3am, and at Great Barrier Island at about 3:40am.

Details: M7.3 Earthquake - East of the North Island

The earthquake was felt throughout New Zealand and GeoNet received over 52,000 felt reports.

Our network of seismographs clearly show the shaking from the mainshock, and aftershocks throughout New Zealand.

The earthquake is in a similar area to the M7.1 East Cape earthquake in 2016, an area familiar with larger earthquakes with 40 events over M5.0 in the past 10 years.

Last Friday three large earthquakes occurred offshore New Zealand –a M7.3 off East Cape at 2:30am, followed by two Kermadec quakes M7.4 and M8.1 several hours later. They all produced tsunami that overlapped and were recorded all around New Zealand.

### What Happened?

Tsunamis can cause waves which can continue to arrive for many hours to days. Our instruments recorded the tsunami from the M7.3 East Cape earthquake reaching a maximum amplitude of ~30-35cm at Lottin Point (East Cape) and ~10-20cm at Great Barrier Island.

This was closely followed by tsunamis generated by the M7.4 and M8.1 Raoul Island earthquakes.

These waves overlapped to produce a tsunami between 35 and 40 cm in amplitude at the Great Barrier Island tsunami gauge and recorded at many other gauges around New Zealand.

Below are three tide gauge recordings from the North Island. They show the largest tsunami waves occurred just after the M8.1 Kermadec earthquake. Interestingly, we can also see the activity lasted several days. It’s worth noting that tide gauges just show the tsunami heights at specific locations. Tsunami heights vary significantly along the coast so other locations may have experienced larger tsunamis.

### We need your help!

There were a few photos and videos taken by members of the public of unusual wave activity following the tsunami like this image that Gina Pewhairangi took of surges arriving in Tokomaru Bay on the East Cape.

If you have noticed the sea behaving in an unusual way, or have photos or videos of this happening, we would love to see them. Please follow the link below to share them with us.

Click to access our tsunami survey

### So, what’s the big deal over some little waves?

Although waves of a few cms may not sound like much, tsunami are nothing like normal wind-generated waves. In a tsunami the whole water column is moving with the tip of the iceberg being the wave on top, often surging faster than you can run (10-30km/h at the coast and travelling 400-800km/h in the open ocean). Surges come in non-stop for around 5 minutes or more, and then recede just as fast for a similar period of time, acting more like a very rapid tide than a wave. This type of back-and-forth pattern (and sometimes weird eddies) continues for several hours (if the tsunami was generated close by) to days (for tsunami that has travelled across the Pacific). For someone fishing on the rocks, that relentless surge of water might be enough to knock them off their feet and not let them back up.

As our Seismic Duty Officer John explained: "50-60cm is like trying to stand in a fast-flowing river that is about knee height, and really hard to stand in."

Aotearoa New Zealand has a long history of devastating tsunami. We are glad this was relatively small as it allowed another practice run. It is crucial that everyone who lives, works or plays in an evacuation zone is ready. Remember, Long or strong, get gone!

What happened and future scenarios

M7.1 East Cape Earthquake – Aftershock forecasts

How do you know a tsunami is coming? And what should you do?

History of tsunami in New Zealand and what causes them.

Remember, Long or Strong, Get Gone.

Drop, Cover and Hold during the shaking. Protect yourself from the earthquake first.

As soon as the shaking stops, move immediately to the nearest high ground, out of all tsunami evacuation zones, or as far inland as possible.

## Tectonic Summary

The March 4, 2021 M 7.3 earthquake northeast of Gisborne, New Zealand occurred as the result of oblique reverse faulting at approximately 20 km depth near the plate boundary between the Pacific and Australia plates. Preliminary moment tensor solutions indicate that the earthquake likely occurred on either a north-east dipping reverse fault with oblique right-lateral strike slip motion, or on a south-east dipping reverse fault with oblique left-lateral strike slip motion. The Pacific plate begins its westward subduction beneath the Australia plate at the Kermadec and Hikurangi subduction zone trenches with a velocity of about 47 mm/yr. The location, depth, and faulting mechanisms associated with this earthquake indicate that the earthquake likely occurred within the subducted Pacific lithosphere very close to the oceanic trench between these two tectonic plates.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Oblique faulting events of the size of the March 4 earthquake are typically about 85x25km in size (length x width).

The Australia-Pacific plate boundary region east of the North Island of New Zealand has a history of large earthquakes both along the plate boundary proper, within the subducting Pacific Plate, and in the overlying Australia plate. Seismic activity is especially high in the Kermadec Islands region to the north. Within 250 km of the March 4, 2021 event, four earthquakes of M 7 or greater have occurred in the past century, with the most recent being an M7.0 oblique normal faulting earthquake in September 2016. The March 4, 2021 earthquake occurred ~45 km east-southeast of the September 2016 normal faulting earthquake. There have been 29 earthquakes of M 6 or greater within 250 km of the March 4 earthquake over the same time period. Despite the subduction zone environment, previous earthquake mechanisms in the vicinity of the March 4 earthquake exhibit a diverse range of faulting styles including predominantly thrust motion, normal motion, and oblique strike slip motion.

M 7.4 earthquake near Kermadec Islands, New Zealand occurred as the result of reverse faulting in the Tonga-Kermadec subduction zone, which extends north-northeast from the North Island of New Zealand for more than 2,500 km through Tonga to within 100 km of Samoa. Focal mechanism solutions for the event indicate rupture occurred as reverse faulting on a moderately dipping plane striking either SSW or NNE.

At the location of the earthquake, the Pacific plate moves westward relative to the Australia plate at a velocity of about 60 mm/yr, subducting to the west beneath the Australia plate at the Tonga and Kermadec Trenches. The eastern edge of the Australia plate may itself be viewed as a collection of microplates whose relative motions help to accommodate the overall Pacific-Australia convergence and associated back-arc spreading. The location and focal mechanism solutions of the earthquake are consistent with it occurring on or near the westward-dipping interface between the subducting Pacific plate and the overriding Australia plate (in detail, the overriding Kermadec microplate).

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Reverse events of the size of the March 4 earthquake are typically about 70 km long x 35 km wide (fault length x fault width).

The interaction between the Pacific and Australia plates creates one of the most seismically active tectonic environments in the world. Since 1976, a 200-km-long section of the Tonga-Kermadec subduction zone, which includes the epicenter of the March 4th earthquake, has produced more than 50 earthquakes of M 6 or larger. The largest event was a M 8 earthquake in January 1976, more than 100 km to the north, with no associated casualties or damage. The March 4, 2021 M7.4 earthquake occurred ~4 hours after and ~900 km north of a M 7.3 earthquake on March 4 in the same subduction zone. While the two earthquakes occurred in relatively close temporal and spatial proximity to each other, the ~900 km distance between them likely indicates that static stress changes induced by the earlier earthquake did not directly cause the M 7.4 earthquake.

And the M 8.1 earthquake near Kermadec Islands, New Zealand occurred as the result of reverse faulting in the Tonga-Kermadec subduction zone at a depth of ~22 km. The Tonga-Kermadec subduction zone extends north-northeast from the North Island of New Zealand for more than 2,500 km through Tonga to within 100 km of Samoa. Focal mechanism solutions for the event indicate rupture occurred as a result of reverse faulting, either by low angle thrust motion on a west-dipping fault or by high angle reverse motion on an east-dipping fault. The location, depth, and style of faulting are consistent with the earthquake having occurred on or near the west-dipping subducting plate interface between the Pacific and Australia plates.

The March 4 2021 M 8.1 earthquake was preceded ~107 minutes by an M 7.4 thrust earthquake located ~50 km west of the M 8.1. The proximity and timing of the two events indicates that the M 7.4 was likely a foreshock of the M 8.1 earthquake. In terms of seismic moment, the M 8.1 was ~11 x larger than the M 7.4 foreshock. Additionally, a M 7.3 oblique reverse faulting earthquake occurred ~6 hours prior to the M 8.1 and ~900 km to the south; however, the spatial and temporal gap between these earthquakes likely indicates that static stress changes induced by the earlier M 7.3 did not directly cause the M 7.4 or M 8.1 earthquakes.

At the location of the earthquake, the Pacific plate moves westward relative to the Australia plate at a velocity of about 60 mm/yr, subducting to the west beneath the Australia plate at the Tonga and Kermadec Trenches. The eastern edge of the Australia plate may itself be viewed as a collection of microplates whose relative motions help to accommodate the overall Pacific-Australia convergence and associated back-arc spreading. The location and focal mechanism solutions of the earthquake are consistent with it occurring on or near the westward-dipping interface between the subducting Pacific plate and the overriding Australia plate.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Reverse events of the size of the March 4 earthquake are typically about 175 km long x 75 km wide (fault length x fault width).

The interaction between the Pacific and Australia plates creates one of the most seismically active tectonic environments in the world. In the past century, 215 earthquakes >M 6 have occurred within 250 km of the M 8.1 earthquake, including the M 7.4 that occurred ~107 minutes prior. The largest previous event was a M 8 earthquake in January 1976, more than 100 km to the north, with no associated casualties or damage.

See also the 2021 Loyalty Island Earthquake and Tsunami