Why do some parts of Wellington city shake more severely and suffer more damage in a large earthquake than others?
Armed with a gravity meter and a $60,000 Earthquake Commission grant, Te Herenga Waka—Victoria University of Wellington’s Professor Tim Stern and Master’s student Alistair Stronach hope to find out how future quakes will affect the ground our capital city is built on.
They will model small changes in the earth’s gravity field across the Wellington city region to determine variations in the thickness of sedimentary rock.
Professor Stern, from the School of Geography, Environment and Earth Sciences, usually specialises in large-scale tectonics and the Earth’s upper mantle but says this project is a chance to do something more practical.
“I can see the importance of it. I live in Wellington and I’m keen to see if we can contribute to understanding some of the hazards of the city we live in. It’s big change for me to get into engineering geophysics and work at much shallower depths.”
The project stems largely from the shaking that occurred during the magnitude 7.8 Kaikoura earthquake early on the morning of November 14 2016.
A number of places experienced “unusual amplification” of the ground, particularly around Parliament and in Thorndon, says Professor Stern.
Gravity surveying and then computer modelling of the sediment in the Wellington Basin— “crunched” between the Wellington Fault and the postulated Aotea Fault—will show how thick the layers are and how far down the basement rocks are, and help establish a link between sediment and shaking.
There are currently only a few wells that give scientists a good idea of how deep the sediment is, along with boreholes that don’t reach the basement greywacke layer.
“This critical area here—along Pipitea Street, Wellington Girls’ College and where the New World is—that is where they had quite severe shaking that destroyed a couple of buildings and other buildings were badly damaged.
“The sense is that the sediment is getting quite thick here, and out to the port area, the container terminal, where they had severe shaking.
“What happens is the seismic surface waves from a distant earthquake get into this basin and it is like waves in a bathtub, sloshing back and forth from each side, and you get the next coming wave greeting that returning one and you get some constructive interference, and so you get a standing wave.
“It’s possible that happened in the Kaikoura quake,” says Professor Stern.
He and Mr Stronach have begun making observations, but until the University reopens after the COVID-19 lockdown they cannot access their equipment.
The researchers will focus first on trying to find the boundary where the basin meets the proposed Aotea Fault—down Kent Terrace and Cambridge Terrace, and up and down Elizabeth Street and Majoribanks Street across Mount Victoria.
“Both faults on either side are effectively like the sharp, steep edges of a swimming pool or a bath tub, with the basin in the middle.
“We will be measuring the variations in the Earth’s gravity field. It varies, from the poles to the Equator because of the spin of the Earth, so you make a correction for that, and then you make a correction for your elevation with respect to sea-level.
“Once you’ve done all that, the Earth’s gravity field can then vary by a small amount, due to the types of rock beneath you.
“We are talking very small variations in the Earth’s gravity field, about one part in 10 million of the total field strength. So you need a very precise instrument. The new electronic version of the gravity meter costs about $200,000; it is about the size of a 12-volt car battery and records everything on hard disk.”
Most of the $60,000 from the two-year grant will fund Mr Stronach’s work.
Professor Stern says they will work with GNS Science researchers with expertise in simulating the amplification from earthquakes from different directions and frequencies.