EXPERT REACTION: NZ earthquake and tsunami warning

The Tsunami warning for the NZ earthquake shows the importance of clear, rapid and reliable emergency communications.

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.
 
Australia has a buoy system to detect tsunami in deep parts of oceans and indeed there are active stations in the east of Australia. A notable tsunami is not expected to hit the Australian east coastline as a result of this series of earthquakes since the epicentres are more than 3000km away from the Australian coastal lines.
 
It should be highlighted that these earthquakes in our neighbouring countries and regions are a warning for Australia, and let’s not forget about the 1968 Meckering earthquake with a magnitude 6.5 at a shallow depth of 15km that damaged structures, inclusive of a damage toll in excess of $2 million, with more than 20 people injured.

Australia is relatively lucky when it comes to our risk from tsunamis as we sit in the middle of a tectonic plate, some distance from the nearest subduction zones where tsunami causing earthquakes to occur.

The Kermadec Trench, where today’s earthquake occurred, is one of the closest subduction zones to the east coast of Australia and so it does pose a potential tsunami threat to our eastern coast.

The Australian Bureau of Meteorology reported tsunami waves of up to 64 cm at Norfolk Island this morning as a result of today’s earthquake. This might seem small, but tsunamis have very long wave periods (the time between successive wave peaks) and so even small waves like this can cause significant hazards to people swimming or boating and damage to infrastructure.

The Bureau of Meteorology and Geoscience Australia run Australia’s Joint Tsunami Warning Centre which monitors for earthquakes and provides warnings for tsunami including the type of impacts we might expect to see such as marine only impacts or inundation on land. This gives emergency managers time to warn communities of the potential threat.

In general, Australia has a relatively low risk of tsunami but there is always the potential for a much larger event so it’s critical that we continue to monitor for earthquakes and tsunami.

Two separate tsunami warnings were issued this morning in response to large earthquakes offshore of New Zealand. Warnings triggered evacuations of low-lying communities along coastlines. As observed in other events, tsunami can pose the risk of catastrophe. Evacuations would have been challenged by the short warning time available and that some people may have been asleep.

 
Tsunami, are caused by the displacement of the ocean as a result of large undersea earthquakes. Australia does have a risk of a tsunami. In 1960, a tsunami originating from a 9.6 magnitude earthquake resulted in strong currents across the east coast and sinking of some small boats after they slipped their moorings. 
 
Most tsunami that impact Australia will only result in strong currents and rips in coastal areas, however, there is a small probability of land inundation in some scenarios.
 
In Australia, tsunami warnings are issued by the Joint Australian Tsunami Warning Center. This Center was established in response to the 2004 Asian tsunami. However, you should not always rely on receiving a warning and if you feel a strong or long earthquake in a coastal area you should head to higher ground immediately.

This morning’s large earthquakes to the north of New Zealand occurred on the boundary of the Pacific and Australian tectonic plates. The first, a magnitude 7.3 at 1:27am NZDT, was only 100km off the NE coast of Gisborne and was felt widely across the country. About 4 hours later a magnitude 7.4 earthquake occurred over 1000km to the north on the same plate boundary in the Kermadec Islands region, followed two hours later by a massive magnitude 8.1 in the same area.

On average, a magnitude 8 or larger earthquake only occurs once a year anywhere in the world, so this is a significant earthquake and at a depth and magnitude to potentially generate a tsunami.

New Zealand is well-known as an earthquake hotspot. The recurrent earthquakes in Christchurch is a reminder of the significant seismic risk. It is important to note that the 7.1 magnitude 2010 earthquake in Christchurch was followed by another 6.2 magnitude earthquake about four months later, which is understood to have been an aftershock of the previous earthquake.

The recent 8.1 magnitude earthquake near the Kermadec Islands is of a very high measure, and in addition to the tsunami warning that has been issued by their National Emergency Management Agency (NEMA), there is potential for strong aftershocks to occur, even after an extended period. Therefore, preparedness measures need to take this in to account. New Zealand does have a Tsunami Gauge Network and therefore vigilance over the short to medium term would allow broadcasting early warnings in the event of a future tsunami due to an aftershock.

For Australia, it is important to note that a tsunami threat has been issued by the U.S. Tsunami Warning System for the east coast. Previous earthquakes, such as the 1989 Newcastle earthquake, indicate that this region is at seismic risk and even though a tsunami has not occurred yet, it is a real likelihood. There is limited awareness of this risk and there is a strong need for public awareness campaigns and also the development of a tsunami early warning network.

There have been a number of large (> magnitude 7) earthquakes in the Kermadec Bay region on the northeast 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.

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.