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Test of New Zealand’s tsunami response in 5 March 2021

Test of New Zealand’s tsunami response in 5 March 2021

Paul Somerville and Andrew Gissing, Risk Frontiers

A sequence of three earthquakes occurred near New Zealand on 5 March 2021 (Risk Frontiers, 2021). The first of the three large earthquakes, a magnitude 7.3 off East Cape, was felt throughout New Zealand and forced many to start their day at 2.30am (left side of Figure 1). The second and third earthquakes, M7.4 at 6:41am and M8.1 at 8:28am, centred near the Kermadec Islands, were barely felt on the main islands of New Zealand. This unfolding sequence of three large earthquakes within six hours put emergency preparedness for tsunami to the test (Cochrane, 2021).

The approach to tsunami warning differs according to the proximity of the source of the tsunami (right side of Figure 1). A local tsunami is triggered nearby, and will arrive at the coast within one hour, which may leave insufficient time for an official warning to be made and heeded, but may be felt by residents in a “natural” warning. This gives rise to government messaging that “if a quake is long or strong, get gone.” A regional tsunami is triggered further away, and will arrive at the coast within one to three hours. It is not as far away as a distant tsunami, which offers more time to prepare, but far enough that it is necessary to rely on an official warning for evacuation because the triggering earthquake is unlikely to have been felt for very long or strongly, if at all. There will be no self-evacuation underway, creating a high-pressure situation for those involved in producing official tsunami warnings.

 Locations of the 5 March 2021 earthquakes.
Figure 1. Left. Locations of the 5 March 2021 earthquakes. Right: The three tsunami zones, local, regional, and distant, used in tsunami warning. Source: GNS Science.

This usually leads to reliance on data from the Global Seismic Network to locate the earthquake, and it commonly takes about 20 minutes to get a reliable magnitude and depth unless data from the source region are available.  Meanwhile the tsunami is on its way. As multiple events unfolded on March 5, decisions had to be made by different groups around the country with limited information to draw on.

The first information came from the United States Geological Survey (USGS, 2021), which estimated a magnitude of 7.3 and placed the East Cape earthquake at a depth of 20 km, which might warrant a tsunami warning, However, Geonet subsequently estimated a depth of about 100 km, which appears to be incorrect and which would not warrant a tsunami warning, and the estimated magnitude was 7.1.  The first tsunami warnings for this event were based on information about the earthquake from the USGS, which included the possibility of a tsunami. Tsunami alerts and warnings issued after this event are shown in Figures 2a and 2b. The depth of about 100 km estimated by Geonet may have been a topic in the assessment indicated in the two alerts at the top of Figure 2a.

Gisborne (Southeast of Auckland, south of East Cape)

A Civil Defence alert message this morning came to Gisborne residents three hours after a severe 7.1 magnitude earthquake shook them awake (Angeloni, 2021). The 5.20am alert said they no longer needed to evacuate for a tsunami threat, following an earthquake that struck at 2.27am off the North Island’s east coast, 105km east of Te Araroa. That was the first alert message Gisborne residents received.

The first alert message was only sent to those most at risk from the potential tsunami – from Cape Runaway down to Tolaga Bay, north of Gisborne. This left people who had felt the earthquake but not received a warning unsure as to whether they should evacuate. The emergency mobile alert went to the areas where there was a life risk because that’s what that channel is for. People around the Bay of Plenty or in areas for which there was a beach or marine threat did not receive an EMA [emergency management alert) because they didn’t have a life threat. However, Gisborne residents were included in follow-up alert messages that morning, warning people north of Tolaga Bay to evacuate after a magnitude 8 earthquake near the Kermadec Islands had the potential to cause a life-threatening tsunami. Residents called for the return of a Civil Defence Facebook page in Tairāwhiti (Gisborne region) that provided prompt updates that would immediately be on the Facebook page, and that there needed to be one prompt messaging system across local and central government, like there had been during Covid-19.

Gisborne residents complained that within minutes of feeling the earthquake, the US Geological Survey posted a possible tsunami warning, but it was more than half an hour before any mention appeared on any official New Zealand website.  An hour later Civil Defence was carrying a “tsunami threat being evaluated” message, and that did not change until later in the morning. The front page of the council’s website carried a “no current emergency” tag and the tsunami alert did not appear for some time. It was explained that council takes its advice from NEMA as the official source of information, and sometimes there is a delay as they do assessments and make those decisions, but we want people to move quickly.

Hawke’s Bay, Napier, Hastings (south of Gisborne)

Hawke’s Bay Civil Defence Emergency Management Group posted what appeared to be conflicting messages immediately after the first earthquake, magnitude 7.3, struck off the east coast of the North Island at 2.27am (Hawke’s Bay Today, 2021). There was widespread severe shaking that woke thousands of Hawke’s Bay residents, many of whom evacuated to higher ground. Hawke’s Bay’s Civil Defence Emergency Management Group initially noted in a Facebook post that the 2.27am quake was unlikely to pose a tsunami threat. Local residents pointed out that this was inconsistent with Government messaging that if a quake is long or strong, get gone. Up to 30 minutes later the post was edited to match the national Civil Defence advice to take refuge on higher ground if you were in coastal communities. Local residents pointed out that if there had been a tsunami most people in the red zone would have been exposed, and did not wait for public advice before heading for higher ground.

After the third large earthquake at 8.28am, an 8.1 earthquake near the Kermadec Islands, Civil Defence issued a beach and marine threat for Hawke’s Bay, and asked everyone in the tsunami red-zone to evacuate to higher ground. This then led to a flurry of people trying to check if they were in the red zone, which crashed Civil Defence’s interactive map. After it came back up, there needed to be a clearer message saying the red zone needs to evacuate, and clear time stamped should have been put in their posts.

Hawke’s Bay does not have a tsunami siren. The only siren system in Hawke’s Bay is in Napier and it is not a tsunami siren, it is just a mass public alerting system.  Hawke’s Bay Civil Defence Emergency Management Group controller made the questionable assertion that setting off sirens for this event would have been really thorny issue, because more people would get injured and killed in evacuations than they would in the event itself. Napier Port’s chief executive said a tsunami evacuation practice run a few months prior had paid off, and that people knew what they were doing, where to go and what to expect.

Northland (north of Auckland)

Northland is much more distant from the first earthquake that occurred off the East Cape, and coped fairly well with the tsunami warning after the large Kermadec earthquake (Northern Advocate, 2021). The main problem was with the gridlock that occurred in Whangarei, and Whangārei CBD was jammed with vehicles evacuating the city centre which is in a tsunami evacuation zone, indicating the need for more work on its evacuation plan. A tsunami warning was issued by Civil Defence at 8.46am, and the all-clear was given at 3.34pm. The warning said people in coastal areas must leave immediately from evacuation zones and move to high ground or as far inland as possible. At the same time Northland’s network of 200 tsunami sirens went off along the coast to give those that may not have received the text message a warning of the risk.  Four Far North tsunami sirens south of Cape Reinga experienced problems due to a Top Energy power outage during the tsunami evacuation.

References

Angeloni, Alice (2021). Tsunami alert: ‘Every time we have this communication confusion’. https://www.tvnz.co.nz/one-news/new-zealand/tsunami-alert-every-time-we-have-communication-confusion

Cochrane, Ursula (2021). Friday delivered a big test for NZ’s tsunami response. How did we do? The Spinoff, https://thespinoff.co.nz/science/08-03-2021/friday-delivered-a-big-test-for-nzs-tsunami-response-how-did-we-do/

Hawke’s Bay Today (2021). Hawke’s Bay: Civil Defence to review its communication of large quakes, tsunami threat. https://www.nzherald.co.nz/hawkes-bay-today/news/hawkes-bay-civil-defence-to-review-its-communication-of-large-quakes-tsunami-threat/5BBQHVOSH4WV5MGFXXGQKYCYS4/

Northern Advocate (2021). Northland tsunami alert: Region copes well in tsunami threat but more could be done. https://www.nzherald.co.nz/northern-advocate/news/northland-tsunami-alert-region-copes-well-in-tsunami-threat-but-more-could-be-done/LNPYYLJM7IW6R33GP47IXRMQ34/

Risk Frontiers (2021). Implications of the 5 March 2021 New Zealand and Kermadec earthquakes. Risk Frontiers Newsletter Volume 20 Issue 1, March 2021.

USGS (2021). Kermadec and New Zealand Earthquakes. https://www.usgs.gov/news/kermadec-and-new-zealand-earthquakes

Alerts and warnings issued after the Mw7.3 East Cape Earthquake
Figure 2a. Alerts and warnings issued after the Mw7.3 East Cape Earthquake
Figure 2b. Alerts and warnings issued after the Mw7.3 East Cape Earthquake
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