Major Earthquakes, New Zealand Region

Three major earthquakes struck the New Zealand region today, all three generated tsunami waves.

Friday 5th March, 2021

The New Zealand region experienced three major earthquakes today, one centred near the North Island’s East Cape and two in the Kermadec Islands. The East Cape quake generated a local tsunami, and the largest of the Kermadec quakes generated a tsunami which reached a number of Pacific shores, including New Zealand.

East Cape, M7.3 at 02:27 NZDT
Tens of thousands of New Zealanders were woken by a magnitude 7.3 earthquake located near East Cape early this morning. The quake, which struck at 2:27 a.m. NZDT was centred 105 km east of Te Araroa at a depth of 90 km according to Geonet, which reports it as magnitude 7.1. The quake was felt from Doubtless Bay in the far north of the North Island to Invercargill and at the Chatham Islands. It was felt strongly in the Bay of Plenty; and in Wellington the shaking, though mild, continued for over a minute. In Tawa, Wellington, the shaking consisted of three distinct phases of increasing movement which declined before the next one commenced.

GeoNet Seismograph Network 5 March 2021

The quake showed strongly on GeoNet’s seismograph network.

The US Geological Survey reported this quake as magnitude 7.3 centred 174 km north-east of Gisborne at a depth of 21 km. In their report, the USGS finds that the earthquake probably occurred in the subducting Pacific Plate close to the oceanic trench where it slides under the Australian Plate. It is thought that a section of tectonic plate 85 km long and 25 km wide slipped during the earthquake.

The USGS report noted that four earthquakes of magnitude 7 or greater have occurred within 250 km of this morning’s event within the past century. The most recent quake had been a magnitude 7.0 earthquake in September 2016, 45 km north-west of this morning’s quake.

The Pacific Tsunami Warning Centre reported this earthquake as magnitude 7.3 at depth of 10 km. The earthquake generated a local tsunami wave which the PTWC reported as 26 cm at East Cape, arriving at 03:00 NZDT with a wave period of 6 minutes. Late today, GeoNet reported the tsunami as 35-40 cm at East Cape and their tsunami gauge clearly shows an initial surge.

GeoNet Tsunami Gauge Network

On GeoNet’s tsunami gauge plot the wave appears to be a surge of nearly a metre followed by several recessions of the sea level over the next few minutes.

New Zealand’s tsunami assessment panel was convened to examine the tsunami risk but there wasn’t sufficient time to issue an evacuation order that would have been effective in getting people to safety before the first waves came ashore. The problem with strong but close offshore earthquakes is that, if they generate a tsunami, it can come ashore very quickly in areas closest to the epicentre. This has been demonstrated in several events such as the Kaikoura earthquake of 2016 and near Gisborne in 1947 when a tsunami wave swept ashore seven minutes after a large off-shore quake.

The mantra is, ‘If it’s long and strong, get gone’ and some people in tsunami-risk areas wisely took this advice on board and self-evacuated after the early morning earthquake.

The Ministry of Civil Defence and Emergency Management website had a bland and bureaucratic advisory that the tsunami assessment panel was being convened to assess the earthquake. It should have been more clear, knowing the location and magnitude of the earthquake, in recommending that people living in tsunami-prone areas near the epicentre should evacuate as a precaution until a full assessment could be undertaken.

Behind the mantra are two sound rules of thumb. If an earthquake is strong and lasts for more than a minute, then it is prudent for people in at-risk coastal areas to move to higher ground in case the earthquake was centred close by. In addition, if it is difficult to stand because of the strength of an earthquake, this indicates that the epicentre of the quake could be close to the observer and again, it is prudent to move to higher ground away from at-risk areas.

MCDEM issued its first advice of a threat to beach, harbour and estuary areas between Cape Runaway to Tolaga Bay at 3:33 a.m. one minute before the first waves were expected to reach East Cape. In fact, the tsunami gauge at East Cape had recorded the first wave at 3 a.m., 33 minutes after the earthquake occurred.

Foreshock, Kermadec Islands, M7.4 at 06:41 NZDT
At 6:41 a.m. New Zealand Daylight Time a magnitude 7.4 earthquake struck near the Kermadec Islands. This quake was centred 995 km north-east of Whangarei, 1037 north-north-east of Whakatane, 1049 km north-north-east of Tauranga, at a depth of 53 km.

The U.S. Geological Survey analysis of the earthquake found that this quake occurred on or near the westward-dipping plate interface between the subducting Pacific Plate and the overriding Australia Plate when a section about 70 km long and 35 km wide slipped. The movement occurred in a 200 km long section of the plate boundary which has produced more than 50 earthquakes of magnitude 6 or larger during recorded history. The largest earthquake in this section of the plate boundary occurred 100 km north of today’s event in January 1976 when a magnitude 8 earthquake struck.

The report noted the earlier earthquake near Te Araraoa, 900 km south in the same subduction zone, but considered that the stress changes caused by the quake four hours earlier were unlikely to have triggered the magnitude 7.4 earthquake.

The Pacific Tsunami Warning Centre issued a warning for this earthquake, estimating tsunami waves of up to a metre for the Kermadec Islands and less than 0.3 metres for many Pacific locations including New Zealand. A wave of 17 cm was recorded at Raoul Island Boat Cove at 6:55 a.m. NZDT and a 31 cm tsunami wave was recorded at Fishing Rock on Raoul Island at 6:58 a.m.

Mainshock, Kermadec Islands, M8.1 at 08:28 NZDT
Events overtook evacuation planning for the magnitude 7 earthquake when a magnitude 8.1 earthquake, centred 1028 km north-east of Whangarei, 1055 km north-north-east of Whakatane, 1070 km north-north-east of Tauranga struck 107 minutes later at 8:28 NZDT. This shallow earthquake was finally determined to be 19.4 km deep.

This earthquake was about eleven times larger than the foreshock and centred about 50 km further east. The U.S. Geological Survey report noted that a section of westward-dipping plate interface about 175 km long and 75 km wide moved during this earthquake. This quake occurred in an even more active area of the plate interface which has hosted 215 earthquakes greater than magnitude 6 in the past century.

The Pacific Tsunami Warning Centre issued 20 bulletins for non-US territories for this event. Initially, it warned that widespread hazardous waves were possible for Pacific states from Tonga to New Zealand. This was quickly refined to waves of more than 3 metres for the Kermadec Islands, one to three metres for New Caledonia and Vanuatu, and up to a metre for South America, Antarctica and Pacific island states including New Zealand.

New Zealand’s National Emergency Management Agency (NEMA) aka the Ministry of Civil Defence and Emergency Management (MCDEM) and GNS Science had already convened their tsunami assessment panel of experts to consider the earlier Kermadec quake and NEMA issued an evacuation order for various coastal areas of New Zealand. People near the coast in areas from the Bay of Islands to Whangarei, Matata to Tolaga Bay and Great Barrier Island were instructed to immediately move to high ground or as far inland as possible.

With New Zealand at Covid-19 Alert Level 2 and the Auckland region at Alert Level 3 because of the global pandemic it was necessary to point out that the evacuation order overrode restrictions imposed by the Covid-19 orders. Under Alert Level 2 gatherings are limited to no more than a hundred people and social distancing of 2 metres is required, but this clearly would be impossible for evacuees in some places.

Compliance was high and even though some traffic congestion resulted, the expected arrival time between ten o’clock and half-past for threatened locations allowed most people to get themselves to safety. Low-lying coastal towns like Opotiki and Tolaga Bay closed completely and became ghost towns. Tsunami sirens sounded, emergency services spread the warning and volunteers offered lifts to friends and strangers. By all accounts there was good spirit at the gathering points with community-spirited people making refreshments available where they could.

The wait was long, and complicated by the fact that the magnitude 8 earthquake had severed communication with the instruments on Raoul Island. This left a gap in New Zealand’s tsunami monitoring with both tsunami gauges unavailable, and the cameras and seismic instruments on the island also inaccessible. Readings from the gauges on the island where the first tsunami wave was expected at 8:48 a.m., some twenty minutes after the earthquake, would have yielded valuable information.

The first useful readings of tsunami activity were reported by the Pacific Tsunami Warning Centre at 12:41 p.m. NZDT. A half-metre wave (56 cm) had been recorded at Norfolk Island at 11:39, a 30 cm wave at Great Barrier Island at 11:48 and smaller waves at North Cape, East Cape, Apia and Nukualofa (Tonga). With readings from six locations, three of them in New Zealand, it was then possible to begin to assess the ongoing risk, bearing in mind that the first tsunami wave is not necessarily the largest. As further readings came in NEMA was able to withdraw the evacuation advice not long after one o’clock and people were able to return to their homes.

Looting is always a concern when people have to leave their homes in such an emergency, but only one report of a house break, in Opotiki, has been mentioned on news media.

The Outlook
A rule of thumb is that large shallow earthquakes are followed by a sequence of aftershocks as the Earth’s crust responds to the change in stress caused by the initial rupture. Generally, but not always, one of the aftershocks will be about a magnitude lower than the main earthquake. Nothing is ever certain and sometimes the main shock will be accompanied by a large aftershock of about the same magnitude as occurred in Hawke’s Bay in 1931.

If a larger earthquake occurs during the aftershock sequence, this changes the focus and the initial main quake is then considered to be a foreshock, as occurred in the Kermadec earthquake sequence this morning.

Earthquake scientists use this knowledge to consider how these earthquake sequences play out. They express the likelihood of certain events occurring in percentage terms, based on huge databases of previous events. GeoNet has issued a computer model forecast of possible earthquake activity off East Cape and this will be refined as further activity occurs and is analysed.

[Complied from data supplied by the U.S. Geological Survey, the Pacific Tsunami Warning Centre and GeoNet.]

One Response to “Major Earthquakes, New Zealand Region”

  1. Ken, thank you for your summary of Friday’s events. Very clear and interesting.
    Although we have personally been through several of these events over nearly 70 years, Friday discombobulating activities inspired a deep sense of fear in myself and it was of comfort to read your concise report.

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