Evidence found of Hamilton jolts
Clear picture of past quakes may be pointer to future
Hamilton might be better known for its bridges and foggy mornings than big earthquakes but freshly unearthed evidence of shakes under the city has prompted a rethink.
In a new investigation, Waikato University’s Professor David Lowe and colleagues aim to get a clearer picture of past quakes around New Zealand’s fourth-biggest city, to better understand what might unfold in the future.
For decades, the basin that Hamilton sits in was thought to be free of faults, save for some older ones around its margins, and the distant Kerepehi Fault near Matamata.
But more recent work has upended that notion: surveys have revealed at least 25 faults in and near the city.
It’s also explained mysterious features preserved in ash layers that Lowe came upon in the 1980s when analysing sediment core samples recovered from beneath local lakes.
“In some cores, it was evident that some ash layers had holes in them and what looked like cracks beneath them infilled with volcanic ash projecting downwards into the lake sediment,” he explained. “I realised that the holes and ash-infilled cracks were likely caused by shaking.”
After further analysis, his colleague Dr Vicki Moon suggested there was also evidence for liquefaction — the quake-shaken slurry that left whole suburbs in eastern Christchurch covered in mud.
“We knew therefore that there had been at least one violent shaking episode in the Hamilton area in the last 20,000 years, because the lakes I had cored back in the 1980s are about 20,000 years old,” Lowe said.
I realised that the holes and ashinfilled cracks were likely caused by shaking.
Professor David Lowe
Considering the newly found faults and the ancient quake evidence together, it became obvious Hamilton wasn’t immune to big quakes.
But, with few signs on the surface, the scientists needed a way to assess how these faults behaved. They opted for a clever method that drew back on the markers, or “tephra seismites”, Lowe uncovered in the old lake sediment.
“There are around 30 lakes in the Hamilton lowlands scattered amid the newlydiscovered fault. We think the relationships between the faults and lakes provide the key to mapping and dating earthquakes in the lowlands since 20,000 years ago.”
His team will take samples from many of those 30 lakes and use CT imaging technology to characterise them.
“Some lakes are close to faults and others are distant — and so by examining which ash layers, if any, are liquefied in the sediments, we can potentially work out which faults were active and when.”
The Hamilton lowlands were still classified under New Zealand’s National Seismic Hazard Model as being of “lowto-moderate seismic risk”, but that was based on the limited historic record in the region — about 100 years.
He expected the new insights would help improve hazard planning in the region and offer up new approaches that could be applied to other areas.
Hamilton has about 170,000 people and crucial infrastructure such as a multibillion-dollar inland port.
Lowe’s study was being supported through grants from the Government’s Endeavour and Marsden funds.
A total of 125 projects under the Royal Society Te Apa¯rangimanaged Marsden Fund, announced yesterday, have been awarded more than $83 million.