The composition of the soil beneath Wellington could put the city at increased risk of destructive tremors during an earthquake, new research shows.
The article, written by MSc student Alistair Stronach and Professor Tim Stern of Te Herenga Waka – Victoria University of Wellington, shows that the thickness of the soft sediment under the city of Wellington is up to two times higher than previously thought.
Stern said when seismic waves passed through layers of sediment – as opposed to rock underground – they increased in intensity and caused more shaking.
“It can have a devastating effect on cities, even when earthquakes are several hundred kilometers away. “
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The research will help predict the tremors Wellington can expect to experience during earthquakes and shed light on why the city suffered so much damage in the 2016 earthquake near Kaikōura, when strong waves were produced which “trapped” in the sediment basin below Wellington and caused unexpected damage in the Pipitea and CentrePort districts of the city.
“Fortunately, no lives were lost, but several high-rise buildings had to be demolished and the CentrePort wharf was so badly damaged it was out of service for months. “
The vulnerability of this area to seismic waves arose both from the depth of the sediments and from the fact that it was mainly reclaimed land (new land created by backfilling the edge of the port).
The research data will be used in future computer simulations to predict the tremors that one might expect in different areas of the city.
“These simulations are vital for planning building designs and identifying the parts of the city most vulnerable to intense tremors from local and distant earthquakes,” Stronach said.
The research, funded by the Seismic Commission and published in the New Zealand Journal of Geology and Geophysics, used high-precision measurements of the Earth’s gravity field to map the sediment thickness below the city of Wellington.
The measurements were made with a state-of-the-art gravimeter, which could determine differences in gravity up to one part in 100 million.
“We took measurements throughout Wellington’s central business district and along the city’s outer hills,” Stronach said. “We have identified a maximum thickness of around 540m near the Wellington Regional Stadium [Sky Stadium], which is double the previous estimates.
The research also mapped an extension of the recently discovered Aotea Fault as it passes from the harbor near Clyde Quay Wharf below Waitangi Park, before heading south, roughly along the Kent Line. Terrace.
“Based on our modeling, this fault has several flares – or branches – across the lower slopes of Mount Victoria and appears as a steep step in the bedrock below the Te Aro portion of downtown Wellington,” Stronach said.