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Three Big Earthquakes In One Morning - Expert Reaction

Coastal communities are returning home after a morning spent on alert for tsunamis triggered by significant marine earthquakes.

The three significant earthquakes that took place over the course of this morning triggered tsunami alerts across the Pacific, with the final earthquake off the coast of the Kermadec Islands measuring up to 8.1 in magnitude.

The SMC asked experts to comment in response to this morning’s events.

 

Professor Andy Nicol, School of Earth and Environment, University of Canterbury, comments:

“The evidence for the earthquake hazard is primarily coming from the seismologists interpreting information from seismographs. Initial information on the potential tsunami was derived from modelling using information from seismology.

“What we know about the earthquakes so far (as of 3pm Friday):

  1. “We have had 3 large magnitude earthquakes (>M7) and many smaller earthquakes.
  2. “The first two large earthquakes at 2.27am (M7.3) and 6.41am (M7.4) seem to be deep (~90km and 55km) and were most likely in the subducting Pacific Plate. My initial interpretation is that they were caused by plate bending stresses.
  3. “The largest earthquake (M8.1) was much shallower (~19 km deep) and was at least partly on the subduction thrust fault.
  4. “Earthquakes 2 (M7.4) and 3 (M8.1) were only 50 km apart and it is possible that earthquake 2 triggered earthquake 3 (earthquake 2 was a foreshock for the bigger event).
  5. “We will continue to experience M5-7 earthquakes over the coming weeks. We do not know how today’s earthquakes have changed stresses in the crust and we cannot completely discount the possibility of more M8 events in the next few weeks.”

No conflicts declared.

 

Dr Jennifer Eccles, School of Environment, University of Auckland, comments:

“Today we have collected a lot of data from seismometers and tide gauges and buoys that will be subject to increasingly sophisticated analysis beyond what can currently be done in real time. Instrumentation is however sparse and, particularly regarding the tsunami, people will want to collect as much information as possible in different places to better calibrate future tsunami models and predictions.

“Many of the processes for characterising an earthquake are automated and we quickly get initial estimates of earthquake location, depth and, for large earthquakes, their ‘focal mechanisms’ that tell us about the type of fault rupture that occurred.

“From there, modelling of the rupture can occur with increasing sophistication through time. The quality of this analysis is dependent on the data input into the process and today’s two distinct clusters demonstrated this. GeoNet is great for earthquakes on or near the New Zealand landmass but, as it only uses its own data, is not as good for events, even though in NZ waters, far from our array of instruments.

“For the Kermadec events, the USGS – which utilises a much sparser international array of instruments – provided the better data for consideration. In terms of an initial estimate of risk, the size, location and depth are critical. The larger the event, the more concern. However, its location relative to population or critical infrastructure matters, and if it is deep, the shaking will be less intense as the energy can be partially attenuated within the earth before it reaches the surface.

“For offshore earthquakes, the size, depth and type of earthquake (and ideally initial modelling as to whether it has ruptured the seafloor) are important in terms of assessing whether it can generate a tsunami. A lot of the message ‘long and strong get gone’ and so on is about assuming the worst. It is better to be inconvenienced then dead. Accurate real-time tsumami modelling still has space to improved, but the input data is initially very sparse with just seismological information to work from until other sorts of data from DART buoys in the ocean start coming in to refine models.

“But even then, the ocean is a difficult/expensive place to instrument and data is sparse, although new instrumentation in the process of coming online. A new MBIE funded Endeavour programme aims to work in this space.

“Today was unusually complex because of the two different clusters of events. The first east of Gisborne was just getting resolved in terms of response when the second near the Kermadec Islands struck. This caused extremely rare stress on the scientific and Civil Defence response and overlapping messaging at play. As mentioned above, the science of accurate real-time tsunami modelling is evolving and the worst case scenario possible on the data available is assumed until it is proven this is not the case. That means often the reality will not live up to worst scenario but it ‘could’ and, beyond the science, the comfort of society with risk is an ongoing discussion.

“Certainly the Civil Defence announcements pushed out to phones could be more timely as I certainly received my ‘all clear’ sometime after the same information was available elsewhere. I will note the local modelling behind the 1-3m tsunami did not appear to be publicly available on any scientific site (but obviously went straight to emergency response and the media) with the only information I could independently access being from internationally, which put the estimate in the 0.3-1 m range.”

Conflict of interest statement: “My role at a university means my employment is independent from explicit responsibility within natural hazard monitoring. I am however involved in a newly funded MBIE Endeavour Programme (PI Bill Fry, GNS Science) that seeks to improve the speed and quality of scientific information coming through the wake of an earthquake, particularly re characterising the more distant offshore ones and their potential for tsunami generation.”

 

Professor Mark Stirling, Chair of Earthquake Science, University of Otago, comments:

“A trio of major earthquakes occurred during the morning of 5 March 2021, and all were produced by the major subduction zone between the Pacific and Australian plates. The zone runs from Tonga in the north to Cook Strait in the south, and is locally named the Hikurangi subduction zone where it sits alongside and beneath New Zealand. All three earthquakes were produced by the process of subduction of the Pacific plate beneath the Australian plate.

“The first of the earthquakes occurred to the east of East Cape, and was magnitude (M) 7.3. Geonet analysts showed that the earthquake had a largely ‘reverse’ sense of slip (slip on a fault when two blocks are pushed together), and occurred within the subducting Pacific plate. This is known as an ‘intraslab’ earthquake, and is produced by the internal stresses and strains produced by the plate being pushed down underneath the overlying plate.

“The magnitude and offshore location of the earthquake was not big enough to cause major tsunamis in East Cape. Had the event been M8 or more, the tsunami would have been much larger. Other earthquakes of similar magnitude have been observed in the northeast coast of the North Island in the 180-year historical record of New Zealand. The 1940s saw several tsunami earthquakes close to the settlement of Tatapouri, and a significant event occurred further offshore on Waitangi Day, 1995.

“The second and third earthquakes occurred well to the north of New Zealand, and in the vicinity of the Kermedec Islands. Both appear to have occurred on the actual boundary (interface) of the Pacific and Australian plates, so are referred to as ‘interslab’ events. The earlier of the two events was M7.4 and was a foreshock of the later M8.1 earthquake. Both earthquakes were ‘thrust’ earthquakes, meaning that the earthquakes were produced by the Pacific plate pushing underneath the Australian plate, releasing a large amount of energy. Although the M8.1 was big enough to produce a major tsunami, the location far from New Zealand meant that such a tsunami was not experienced at our coasts.

“The two areas of earthquake activity were too far apart to have been part of the same earthquake sequence. They just happened to occur at a similar time, and were only associated through being part of the same overall subduction zone. Both areas have produced aftershocks that are all lesser in size than the respective mainshocks, and more may occur in the following days.

“The geologic record of the East Coast and Marlborough shows that much larger earthquakes than the M7.3 have happened in pre-historic (pre-180 years) time on the Hikurangi subduction zone. The accumulating data, analysis and modelling of the subduction zone is being used to inform hazard maps, the building code, and emergency management response. Three generations of seismic hazard maps for New Zealand and two generations of tsunami hazard maps have incorporated ‘state-of-the-art-of-the-time’ knowledge of the subduction zone, so subduction zone earthquakes are not a new thing for us. On the contrary, they are an integral part of life on the ‘Shaky Isles’.”

No conflict of interest

 

Kasper van Wijk, Associate Professor in Physics, University of Auckland, comments:

“The earthquakes were recorded on seismic stations in New Zealand and around the globe. With earthquakes this large, the recordings would have been very distinct, everywhere around the world. We even recorded these on our school seismometers [such as at Rangi Ruru school].

“I can only speak for the seismologists [in terms of what we’re looking for], but we are looking for large aftershocks that can sometimes even end up bigger than the main shock.”

“Given the complexity of the situation with multiple very large earthquakes in a span of hours, I think the level of precautions that were taken were very appropriate.”

No conflicts of interest.

 

Associate Professor Caroline Orchiston, Centre for Sustainability, University of Otago, comments:

“The events of today are a timely reminder earthquakes and other hazard events can happen with no warning anywhere in Aotearoa, and as individuals and communities – and as a society – we need to prepare ourselves. Look after your whānau and friends, help those around you, and make some preparations if you can. Get more information about what your community’s plan is, talk to your neighbours and your whānau and friends.

“Today many New Zealanders took the right action after the tsunami by following the ‘Long Strong, Get Gone’ message, and moved inland and uphill as soon as the shaking stopped. That is good news. People who choose not to evacuate could be putting others at risk, since family and community members could return to find them.

“When large earthquakes happen offshore and we don’t feel shaking on land, the risk of tsunami still needs to be taken seriously and people should take advice from NEMA. Tsunami warnings are issued online, on radios and on TV. Don’t return to the coast until given the all clear.

“Others things people can do, include:

  • If you live by the coast, practice your ‘tsunami hikoi’ and time your walk to high ground. There is a community hikoi being planned on the East Coast during the week of March 8-14th – join in! Check out East Coast LAB (Life at the Boundary), who specialise in community preparedness for hazard events.
  • Prepare a ‘go-bag’, with essential items in it so you don’t have to delay your evacuation to search for things to take. Include medicines, warm clothing, food, water, and any specific items your whānau might need (e.g. nappies). Copies of personal ID are also very useful.
  • There are potential hazards that can effect most of New Zealand. Get to know your local hazards and try and get prepared for them. Talk to your local Civil Defence Emergency Management group for more information.”

No conflict of interest.

 

Dr Julia Becker, Senior Lecturer, Joint Centre for Disaster Research, Massey University, comments:

“I imagine that people being evacuated would have felt uncertain and anxious about the situation. However, it is important to know they did the right thing, as evacuating kept them safe from potential tsunami. Given that tsunami can be a series of waves, it’s important to wait for the official ‘all clear’ message before returning home, as I’m sure many did.

“Also by evacuating for this warning, people will have had good practice for future evacuations – they can evaluate what worked well for them, and what they need to improve for next time. That might include mode of evacuation (with walking and cycling the best options, if possible), speed of evacuation (did they evacuate immediately on feeling a ‘long or strong’ earthquake, or on receipt of an official warning?), evacuation route (is there a faster or better route to take), and what they might need to bring with them (did they have a grab and go bag? Were the right items in it?).

“It was important for people to wait the length of time they did for the ‘all clear’. The tsunami generated from an earthquake may not be just one wave, it may be a series of waves, and the first wave might not be the largest. It may be that subsequent waves are larger. Therefore, it is important to wait until scientists can identify that all potential waves have passed, before emergency management can give an all clear, and people can return home safely. In this case it took several hours. While it’s possible that people might have tired of the long wait, we know that if people understand the reasoning behind having to wait for an ‘all clear’ (i.e. it will keep them safe from a series of waves), then they are more likely to be happy to do it again in future. So in our communication, it’s important to let people know why they need to wait, and highlight that those who did wait took the correct action.”

No conflicts declared.

 

Dr Sarb Johal, psychologist, author of “Steady: Keeping Calm in a World Gone Viral”, comments:

“The two earthquakes overnight offshore are likely to have woken many people up, who then had trouble getting back to sleep, or who had to evacuate because they lived in a potential tsunami-risk area. Or perhaps they had kids who were frightened and couldn’t get back to sleep. Most recently, a third large earthquake triggered tsunami warnings and evacuations in areas of the North Island coast, such as Northland, Great Barrier Island, and the East Coast.

“It’s a sharp reminder that in New Zealand, we live in a land filled with multiple hazards, with multiple overlapping uncertainties. And this can take its toll when it feels like everything is happening at once.

“Some of these are natural hazards, like the earthquakes last night and all we have experienced over the last decade and longer. Others have a large natural component too, but with contributions through human activity to a greater or lesser extent, e.g. climate change and arguably the coronavirus pandemic. And we also face other human activity which pose a real and present danger, e.g. the recent arrests connected with activity designed to cause terror on the anniversary of the Christchurch mosque attacks later this month.

“As well as this tsunami threat, later today Cabinet meets to decide the Alert Level settings for New Zealand as we deal with the latest pandemic cluster.

“It’s worth knowing that when your brain is constantly responding as if you are in imminent danger, it’s very hard to do anything else. Your brain is focused on staying alive and you simply don’t have the mental space or resources necessary for creative problem-solving and strategic thinking. These activities that are so crucial in a crisis get demoted to ‘nice-to-have-once-I-survive-this-threat’ status. Your brain doesn’t particularly care where you’re headed, it just wants to deal with the threat – and it’s exhausting.

“So, don’t be surprised that you might be feeling exhausted, especially as the adrenaline of the immediate threat starts to wear off. Simple breathing exercises can help with worries and anxiety. Slow, normal breaths, from your tummy – like a balloon gently inflating and deflating will start to calm your body down and this tip works with kids too.”

No conflict of interest.

 

Dr Emily Lane, hydrodynamics scientist, NIWA has previously commented on how tsunami evacuation warnings and Covid-19 Alert Levels interact.

Our colleagues at the Australian SMC have compiled the following expert reactions to this morning’s news. You can read the full list of expert comments online here.

 

Associate Professor Behzad Fatahi, Head of the Discipline of Geotechnical and Transportation Engineering and Associate Professor in Geotechnical and Earthquake Engineering, University of Technology Sydney (UTS), comments:

“At 13:27:36 (UTC) on 4 March 2021, a magnitude 7.3 earthquake at a very shallow depth of 20.8 km occurred about 174 km northeast of Gisborne (population of 37,000) in northeastern New Zealand.
“Another earthquake with magnitude of 8.1 at a rather similar depth also occurred six hours later near Kermadec Islands about 1000km northeast of the earlier Gisborne earthquake.

“Since then, 21 other earthquakes have hit the New Zealand and Kermadec Islands area (magnitudes between 4.6 and 7.4) plus five other earthquakes in Vanuatu, Solomon Islands and Loyalty Islands regions (magnitudes between 4.8 and 6.1).

“These series of earthquakes were the results of very complex tectonic activities between Australian and Pacific Plates colliding under New Zealand (approximately 30-60 mm/year of relative movement between tectonic plates). The complexity of fault lines and tectonic activities is due to the fact that tectonic plates in some locations such as east of the North Island move toward each (creating compressive stresses), and in some other regions such as South Island slide past each other (generating shear stresses), both creating shallow earthquakes.

“Let’s remember the series of devastating earthquakes in the last 10 years in the same region including the Christchurch earthquakes in September 2010 and February 2011 directly beneath the City of Christchurch.

“It should be noted that due to the complexity of plate boundaries in this area, particularly the different orientation of fault lines, it is very likely that several fault lines are activated in one earthquake.

“Since the epicentre of the earthquake was offshore in the south Pacific ocean, a tsunami with a height of up to 1.2m could be expected that would take hours to hit some of the shorelines on neighbouring cities and countries including New Zealand and the east coast of Australia.

“It is foreseen that due to the pre-shocks, main earthquake and follow-up after-shocks, as a result of underwater landslides and liquefaction of sea bed, damage to coastal and offshore pipelines in the vicinity of North Island of New Zealand may happen.

“Indeed, although liquefaction of loose materials may happen well below the ground surface, it can be manifested as the surface subsidence. Furthermore, considering the terrain in the North Island of New Zealand, serious rockfalls and landslides may happen within the radius of 150km away from the epicentre of earthquakes (even coastal cliffs) and authorities should make an assessment of slopes in the affected areas. Indeed, aftershocks can further degrade the weakened slopes and cause landslides, deteriorating the condition.

“Furthermore, Seiche (standing wave in an enclosed or partially enclosed body of water; different to Tsunami) may happen at a great distance from the earthquake source in lakes such as lake Waikareiti and Lake Waikaremoana, or even much further away from the epicentre, such as Lake Taupo, due to these series of earthquakes and the aftershocks. People should be warned about this and let’s not forget the 1964 Good Friday earthquake in Alaska that produced damaging waves up to 2m high in lakes.

“Since the earthquake occurred at a shallow depth, light damage to buildings such as cracks in the masonry walls in the regions near Gisborne, Toaga Bay and Manutuke, may be observed. Temporary excavations, particularly near existing structures and vibration-sensitive structures, must be checked to ensure excessive movements have not occurred.”

Behzad has not declared any conflicts of interest.

 

Dr Brendan Duffy, Fellow in Structural Geology and Tectonics in the School of Earth Sciences, The University of Melbourne, comments:

“USGS earthquake data indicate that the largest event, with a magnitude of 8.1, occurred on a shallow dipping fault between the Pacific and Australian plates at a depth of around 20km. Similar size events occurred within a radius of 200km in 1917 and 1976 and are a normal part of the hazards on this subduction zone.

“The 8.1 earthquake occurred about two hours after a deeper, smaller foreshock. A foreshock can only be identified as such in hindsight. The foreshock in turn occurred a little under 4 hours after an earlier M7.1-7.3 earthquake on New Zealand’s Hikurangi subduction zone that shook much of New Zealand’s north island.

“Scientists will evaluate whether and how stress may have been transferred in the coming days but given the intervening 1000km distance, and the observation that the aftershocks of the New Zealand and Kermadec earthquakes form two distinct clusters, there is nothing presently to suggest they are connected.

“These earthquakes are generating significant but expected aftershock sequences that appear to be following reasonably normal patterns.\

“The occurrence of several earthquakes with magnitudes greater than 7 within a single day in a given region is unusual but not unprecedented in areas with high rates of subduction like we see on the Tonga-Kermadec trench.

“High rates of subduction results in high rates of seismicity. Dr Steve Hicks has pointed out that some previous examples are associated with major earthquakes like Chile 1960 and Japan 2011, but smaller examples are also known, such as the 2010 Mindanao earthquakes in the Philippines. Note again that today’s events appear to be two separate clusters separated by 1000km.

“All affected New Zealanders should closely follow instructions from the relevant emergency services.”

Brendan has not declared any conflicts of interest.

 

Dr Jane Cunneen, Adjunct Research Fellow at the School of Earth and Planetary Sciences, Curtin University, comments:

“There have been a number of large (> magnitude 7) earthquakes in the Kermadec Bay region on the north east side of New Zealand’s north island, as well as many smaller earthquakes of magnitude 4-6. The quakes are occurring in the Tonga-Kermadec subduction zone, where one tectonic plate is pushing over the top of another; this kind of movement means earthquakes are common. The largest two quakes so far, with magnitudes 7.4 followed by one of magnitude 8.1 less than two hours later, are large enough to generate a significant tsunami wave capable of crossing the Pacific Ocean.

“Tsunami warnings have been issued for countries around the Pacific Ocean rim, and small waves have already been recorded in New Zealand and nearby Pacific islands. The size of the waves suggests there is no risk to the remainder of the Pacific Ocean, and tsunami warnings for Hawaii and the west coast of the USA have been cancelled. The main risks for New Zealand and the Pacific islands are unusual currents and tidal activity, especially in ports and other coastal areas where the wave activity can be amplified by the shape of the coast.”

Jane has not declared any conflicts of interest.

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