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The risk of stronger shaking in parts of Wellington’s CBD after an earthquake may be higher than previously thought, new research suggests.
Research published this month in the New Zealand Journal of Geology and Geophysics indicates the soft sediment basin beneath the city is up to twice as deep andhas a steeper edge than thought.
With depths of an estimated 500 metres close to the regional stadium, 200m in much of Thorndon, and 150–200 metres beneath Waitangi Park, next to Te Papa, there’s an increased likelihood earthquake waves will be trapped and amplified – even if the quake is far away, the study found.
Victoria University of Wellington (VUW) researchers worked with colleagues from the United States on the study, which used new gravity and seismic reflection methods to determine the basin’s thickness and geometry.
“It just really highlights the importance of how the Wellington Basin can trap waves, and they can resonate and cause damage,” said lead author Professor Tim Stern from Victoria University.
This image of Wellington city shows the position of the harbour with respect to the Wellington fault.
Researchers discovered the western edge of the basin is defined by a steep, fault-like structure extending from Lambton Quay beneath the railway station, continuing west of the stadium and reaching toward the ferry terminal.
“The surprising thing was that this edge of the basin was in the place that it is,” Stern said.
While many had assumed the edge of the basin fell on the Wellington faultline by Tinakori Hill, it actually fell on the Lambton fault, he said.
The report predicted shaking could be amplified by about three times in a narrow corridor just east of the Lambton fault, rather than uniformly across the CBD.
“Much of the amplification is due to earthquake waves becoming trapped in the basin and resonating, much like waves caught in a swimming pool or bathtub,” said Stern.
Stern said the research had a “historical precedent”, with basin resonance thought to have contributed to sustained shaking in the CBD after the 2016 Kaikōura earthquake, which also caused extensive damage in Wellington city.
“Very distant earthquakes could also produce the same effect of trapping low-frequency waves in the Wellington area and into the Wellington Basin,” Stern told the Herald.
Sediment thickness estimates include a maximum of about 500m close to Sky Stadium.
Two earthquakes in 1942 near Masterton also caused severe damage in Wellington, leaving buildings unrepaired and at least 10,000 chimneys damaged.
“What our study highlights is ... that distant earthquakes are really important to consider, not just earthquakes on the local faults around Wellington,” Stern said.
A figure showing the contours of depth to basement in the Wellington CBD.
The Wellington City Council told the Herald that it, with other organisations, was aware of the sediment basin beneath the central city.
“We will be interested to talk to the VUW team to know more about their findings,” a spokesperson said.
Sammy Carter is a journalist for the New Zealand Herald covering news in the Wellington region. She has previously worked at the Rotorua Daily Post.
