28/8/2020

Researchers in geophysics from Victoria University have returned to surveying Wellington central after COVID forced a pause on their data collection. Master’s student Alistair Stronach has been mapping the rock base and sediment beneath the city to better understand the effect of earthquakes on the area.

When the magnitude 7.8 Kaikoura earthquake struck on November 14, 2016 the effect on Wellington City was inconsistent. Some parts of the city experienced an unusual amplification of shaking while other areas seemed more stable.

“The 2016 earthquake had particularly bad shaking in Thorndon and around the waterfront and that’s mostly due to the depth of sediment there. The deeper it is the worse the shaking is,” said Stronach.

Stronach likened the sediment above the rock base in Wellington to jelly in a bowl.

“When you shake that bowl, the jelly wobbles all over the place, much more than the bowl did,” he said.

The shape of that bowl or basin is also a part of Wellington’s unique recipe for earthquake damage. The presence of two fault lines either side of a low base created a basin-like shape that is filled with the softer sediment. Models suggest that waves of earthquake energy reflected off the edges of the bowl and created areas of resonance and the phenomenon of standing waves. This meant certain areas were likely to experience a greater degree of shaking.

“People think [the sediment] is about 200 metres thick but that’s about plus or minus 50 metres which makes a huge difference in terms of how bad an earthquake actually is at that spot,” said Stronach.

Professor Tim Stern, who instigated the project, pointed out that awareness of the depth and shape of the city’s base rock is derived from a small handful of boreholes scattered about the area.

“I’ve always been aware that the only gravity survey in Wellington city was done in the 1960s. The previous model for the basin is not quite right. We could try to understand the shape of the basin underneath the city and better understand the amplification of the earthquake due to the sedimentary layer,” said Stern.

In comparison, Stronach has made over 200 measurements within the central Wellington basin and plans to add another 150 before he would begin his modelling of the city.

The 2016 earthquake resulted in significant damage to almost 40 buildings in the CBD. The Statistics NZ building required demolition after only being completed in 2005. The Asteron Centre that housed the Inland Revenue and The Civil Aviation Authority suffered stair failure despite the building being only six years old.

Apartment dwellers in the city reported experiencing shaking so violent they expected to emerge into a city of flattened buildings and mass casualties.

Covid-19 has paused what could be a landmark study into Wellington’s earthquake vulnerability. The data and modelling generated from this research would contribute to wider study by GNS Science to better predict damage and ensure more resilient building standards for future earthquakes.