Experts reflect on massive Hikurangi earthquake and tsunami hitting Gisborne

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There’s no easy way to tell. The impact of a magnitude 9 earthquake in the Hikurangi Trench and the tsunami that hit Gisborne soon after is not worth thinking about.

But thinking about it is precisely what needs to be done and it is what led a group of international and national experts to present to a gathered crowd the first of a three-day workshop on Monday.

Off the coast of the Gisborne/Tairāwhiti region is the Hikurangi Trench, a subduction zone where the Pacific plate is moving under the Australian plate, creating pressure that could trigger a major earthquake and a subsequent tsunami.

There’s no way of knowing if or when that might happen.

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There were many things in the workshop that were alarming. It started with a map presented by GNS geophysicist David Burbridge in which he showed the locations of large destructive earthquakes on subduction zones around the Pacific. The notable gap in this circle was around the Hikurangi trench.

Tairāwhiti/Gisborne has a significant risk of a major earthquake in the Hikurangi subduction zone.

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Tairāwhiti/Gisborne has a significant risk of a major earthquake in the Hikurangi subduction zone.

The “very optimistic view,” Burbridge said, was that there was something different about the Hikurangi subduction zone that either limited the size of earthquakes or reduced them entirely.

“It’s a much less popular point of view than before,” he added.

The question was when such an earthquake might occur and to prepare for it if it did, he said.

Tsunami scientist Jose Borrero helpfully reminded attendees that they were gathered in “one of the most dangerous areas of Aotearoa” due to its proximity to the subduction zone and the fact that they had less time to react.

The Hikurangi subduction zone could produce a devastating tsunami and earthquake.

The Hikurangi subduction zone could produce a devastating tsunami and earthquake.

Borrero discussed patterns and probabilities, but most tellingly there was simply no way of knowing when an earthquake might occur or how big it would be.

“To put things into perspective, the 2004 Indian Ocean tsunami, which was one of the largest earthquakes ever recorded on earth, occurred in a subduction zone where there were thousands years of recorded history, and they had never had such a large earthquake,” he said.

Kate Clark, paleoecologist/earthquake geologist at GNS, discussed using past earthquake records to predict future earthquakes, and said there have been no large earthquakes. land recorded on the subduction zone in our “rather short” historical record.

The record was “too short to understand the danger these large subduction earthquakes pose to us,” which is why geologic records of things like tsunami deposits were used, Clark said.

There was no solid evidence to date to suggest that the Hikurangi trench had failed in the Gisborne/Tairāwhiti area and what the magnitude of a tsunami might have been, she said.

An underwater ROV has been brought in to help research into the tsunami and earthquake risk posed by the Hikurangi subduction zone off the east coast of the North Island.

Preliminary tsunami evacuation modeling by lead geophysicist William Power revealed that many people in the town of Gisborne would be unable to evacuate in the 30 minutes between an earthquake hitting the trench and the subsequent tsunami .

There was still a lot of discussion needed to understand the local situation, he said.

Much of the discussion focused on the need to educate people about the “Long or Strong, get Gone” message and getting communities to train for such an event was essential, said Marion Tan, postdoctoral fellow at the Joint Center for Disaster Research.

Gisborne District Council’s lead scientist, Murry Cave, spoke about the geology of the area, which he likened to ‘porridge’. The hills would collapse and the liquefaction would likely be far worse than that seen after the 2011 Christchurch earthquake, further aggravating evacuation and recovery efforts, he said.

The workshop will last until Wednesday.

Tairāwhiti’s emergency management officer, Ben Green, said the workshop would look at a number of areas, including the immediate impact of the disaster and recovery planning for the town, and that the outcomes of the meeting define the terms of the regional tsunami plan.


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