Earthquake damage risk in Wellington greater than initially thought

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New research shows that the ground structure below Wellington may put the city at increased risk of destructive shaking during an earthquake.

The research, led by MSc student Alistair Stronach and Professor Tim Stern from Te Heringa Waka-Victoria University of Wellington, shows that the thickness of soft sediments below Wellington is twice as large as previously thought.

Kevin Stent/Stuff

New research shows that the ground below Wellington may put the city at increased risk of destructive shaking during an earthquake. (file image)

When the earthquake waves passed through layers of sediment — as opposed to base rock — they increased in intensity and led to more shaking, Stern said.

“This can have a devastating effect on cities, even when earthquakes occur several hundred kilometers away.”

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The research will help predict the shaking that Wellington can expect in earthquakes and highlights why the city was so badly damaged by the 2016 earthquake near Kaikoura, when powerful waves “trapped” in the sediment basin below Wellington were produced and caused. Unexpected damage in the Pipitea and CentrePort area of ​​the city.

“Fortunately, no lives were lost, but many tall buildings had to be demolished and the CentrePort pier was so badly damaged that it was out of service for several months.”

Kevin Stent/Stuff

The Statistics House on Wellington’s waterfront was one of the buildings hardest hit during the November 2016 earthquake.

This area’s susceptibility to seismic waves stemmed from the depth of the sediment, and the fact that it was mostly gully (new land created by filling in the edge of the harbor).

The research data will be used in future computer simulations to predict the shaking that might be expected in different areas of the city.

“These simulations are essential in planning building design and identifying which parts of the city are most susceptible to severe shaking from local and distant earthquakes,” Stronach said.

The research, which was funded by the Seismological Commission and published in the New Zealand Journal of Geology and Geophysics, used high-resolution measurements of Earth’s gravitational field to make a map of the sedimentary thicknesses under the city of Wellington.

The measurements were made using the latest gravimeters, which can determine gravity differences of one part in 100 million.

“We took the measurements throughout Wellington’s central business district and along the city’s outer hills,” Stronach said. “We have identified a maximum thickness of approximately 540m near Wellington Regional Stadium [Sky Stadium], which is double the previous estimates.

The search also mapped an extension of the recently discovered Aotea Fault as it passed from the harbor near Clyde Quay Wharf down Waitangi Park, before turning south, roughly along the Kent Terrace Line.

“Based on our modeling, this rift has several fissures – or tips – across the lower slopes of Mount Victoria and appears as a steep step in basement rock below the Te Aro portion of downtown Wellington,” said Stronach.

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