Source: Earthquake Commission – EQC
When an earthquake hits, it doesn’t just affect what’s on the surface, it also affects the ground it travels through – and that, says Victoria University of Wellington Professor of Geophysics Martha Savage, could be the key to understanding how earthquakes affect the ground and buildings on it.
“The ground underneath a building has a big influence on how and how much the building moves in an earthquake,” says Professor Savage. “By understanding how earthquakes change the ground below the surface, we can get a much better idea of what effect future earthquakes here or elsewhere will have.”
But seeing changes under the ground is not easy.
With EQC funding, Professor Savage will be testing techniques that investigate how changes in the speed of seismic waves relate to previous earthquakes and to changes in groundwater levels, and how that can be used to “see” changes in the ground.
“What we’ve seen in previous earthquakes, particularly Kaikōura, is that the water level underneath the ground changes in a medium to large earthquake. Earthquakes, including the Kaikōura earthquake, also cause tiny changes to the speed with which seismic waves travel. This project is testing how groundwater changes are related to changes in seismic wave speeds. This in turn may help us understand the processes controlling ground damage and groundwater storage from future earthquakes, and how that damage may recover over time.
“What we saw was that conditions slowly healed, returning to pre-earthquake levels within nine months to a year, and we want to get a better understanding of how that healing occurs and therefore how other areas might respond to future earthquakes.”
Professor Savage says that the team will be trialling new technology, “ambient noise interferometry”, as part of the testing. “Whether you hear it or not, there is always ‘noise’ under the Earth’s surface as it responds to temperature changes, wind, ocean waves and small earthquakes that aren’t detectable except through instruments. New techniques allow us to use that noise to see waves traveling between seismic stations and layers in the ground that act as reflectors, to determine a picture of what is happening under the ground. We can then compare that with what is happening with the groundwater.
“Wellington is an ideal place for this as there are many water level monitoring stations that we can add the seismic testing to, and see if we are on the right track.”
Dr Jo Horrocks, EQC’s Chief Resilience and Research Officer says further earthquakes and aftershocks can sometimes cause even more damage than the original earthquake. “Professor Savage’s project will test new ways that could help us understand what ground might be strongest and therefore which buildings could be most at risk after an earthquake – useful information not just after an earthquake, but for land-use planning to avoid areas that would be more likely to have strong aftershocks,” she says.
EQC funds more than $17 million each year of scientific data and research to reduce the impact of natural disaster on people and property.