The field team (L) Matt Gerstenberger, Mark Stirling, Erin Todd, Calum Chamberlain, Ting Wang, Andrea Perez Silva and Jess Allen. Photo: Supplied/Illuminate Science
Researchers are working to improve earthquake forecasting along New Zealand's biggest fault - the Hikurangi subduction zone - where a massive quake and tsunami could result in tens of thousands of casualties.
Off the North Island's East Coast the Pacific tectonic plate dives underneath the Australian plate.
Under a 9.1 Hikurangi earthquake and tsunami scenario, the National Emergency Management Agency said tens of thousands of people could die, hundreds of thousands could be displaced and the cost of building damage could top $100 billion.
Researchers from Otago, Victoria and Massey universities, along with GNS Science, were now trying to establish the relationship between earthquake swarms and the potential for sudden large earthquakes where strong shaking lasted seconds to minutes, causing widespread damage.
Swarms were clusters of smaller tremors - many too minor to be felt - while slow slip quakes happened "silently" over weeks to months without any shaking.
Researchers systematically analysed whether earthquake swarms and slow slip quakes were linked to large Hikurangi subduction zone earthquakes, and whether they provided early indicators of big earthquakes.
They developed a catalogue of earthquakes along the subduction zone, capturing the larger quakes scientists already knew about, as well as many smaller lower-magnitude earthquakes that usually went unnoticed.
The researchers found a link between swarms and slow-slip earthquakes along the Hikurangi subdication zone, but often with a time delay.
An earthquake map of the North Island's East Coast. Photo: Supplied/East Coast Lab
Understanding differences in the delay could help pin-point what drove the connection between swarms, slow-slip earthquakes and large quakes, the researchers said.
They said there were 92 slow-slip earthquakes in the last 20 years, mostly along the North Island's East Coast, at a frequency that suggested stress was released in some areas while potentially building up in others.
Despite these findings, there had not been any large earthquakes along the coast since modern records began, making it challenging to prove a direct link between slow-slip earthquakes and large quakes.
The 2016 Kaikōura earthquake blocked State Highway 1. Photo: Tess McClure
University of Otago Professor Ting Wang said some studies suggested a 26 percent chance of an earthquake over 8 magnitude in the lower North Island in the next 50 years.
"We are trying to shorten the time frame because currently we are providing data for the next 50 years, but we are trying to, hopefully, predict for next year," she said.
Under NEMA's 9.1 Hikurangi quake scenario, more than 22,000 casualties were estimated - mostly in the tsunami - and nearly 26,000 more could be injured, assuming 70 percent of people were able to evacuate.
About 400,000 people could be displaced and 30,000 homes destroyed or damaged from the tsunami alone.
There was estimated to be about $144 billion worth of damage to buildings from the earthquake and tsunami.
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