Experts ponder huge Hikurangi trench earthquake and tsunami hitting Gisborne

Monday, 19 September 2022

The National Oceanic and Atmospheric Administration animation illustrates what happens when a major tsunami hits.

There’s no easy way of putting it. The impact of a magnitude 9 earthquake in the Hikurangi Trench and the tsunami that hits Gisborne a short time later does not bear thinking about.

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

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

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

Tairāwhiti/Gisborne has a significant chance of a major earthquake in the Hikurangi Subduction Zone.

* Scientists say sea shells should shed substantial light on seismic situation

* Scientists prepare for Hikurangi subduction zone faultline to rupture in future

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

There was much about the workshop that was alarming. It began 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.

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

“That’s a much less popular view than it used to be,” he added.

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

The question was around when such a quake might occur and being prepared for it if it hit, he said.

Tsunami scientist Jose Borrero reminded attendees, helpfully, that they were gathered in “one of the highest hazard areas in Aotearoa” due to its proximity to the subduction zone and having the least time to react.

Borrero discussed models and probabilities, but most tellingly, said there was simply no way of knowing when a quake might happen or how big it would be.

“To put it in perspective the 2004 Indian Ocean tsunami, which was one of the biggest earthquakes ever recorded on earth, happened on a subduction zone where there was thousands of years of recorded history, and they’d never had an earthquake that big,” he said.

Kate Clark, a paleoecologist/earthquake geologist at GNS, discussed the use of past records of earthquakes to forecast future earthquakes, and said there had been no large earthquakes recorded on the subduction zone in our “fairly short” historical record.

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

There was no strong evidence to date to suggest the Hikurangi Trench had ruptured in the Gisborne/Tairāwhiti area and how large any tsunami might have been, she said.

Preliminary tsunami evacuation modelling by senior geophysicist William Power revealed that many people in Gisborne city would be unable to evacuate within the 30 minutes between an earthquake striking in the trench and the subsequent tsunami.

There was still much discussion required to understand the local situation, he said.

Much of the talk was around the need to educate the population on the ‘Long or Strong, get Gone’ message and getting communities to practise for such an event was critical, said Marion Tan, a postdoctoral fellow at the Joint Centre for Disaster Research.

Gisborne District Council principal scientist Murry Cave spoke about the area’s geology, which he compared to “porridge”. Hillsides would crumble and liquefaction was likely to be much worse than that seen after the 2011 Christchurch earthquake, further compounding evacuation and recovery efforts, he said.

The workshop will run until Wednesday.

Tairāwhiti Emergency Management manager Ben Green said the workshop would look at a number of areas including the immediate post-disaster impact and recovery planning for the city, and outcomes from the gathering would set the conditions for the regional tsunami plan.