Earthquakes have far-reaching underground impact

A landslide monitoring data set, provided by Contact Energy Clyde Dam, has been analysed to determine the effects of earthquake shaking on sensitive groundwater systems. Photo / File

The impact earthquakes can have on our city structures has been made frighteningly clear over recent years: but what effects can big shakes have underground?

Now, new research shows how an earthquake even hundreds of kilometres away can damage groundwater systems.

The study from Victoria University and GNS Science, based in the Cromwell Gorge used a large hydrological data set that has recorded groundwater responses to a series of earthquakes in the region in the past few decades.

It was the first time the landslide monitoring data set, provided by Contact Energy Clyde Dam, has been analysed to determine the effects of shaking on sensitive groundwater systems.

Lead author Grant O'Brien said the data set used was of rare international size and scope, and unique in spanning multiple sites and multiple earthquakes in such a long period of time: 23 years.

"This means we have been able to construct a detailed picture of the behaviour of different groundwater systems in response to earthquakes, ongoing engineering activities and other factors such as intense rainfall."

Results found the sensitivity of groundwater systems to seismic waves stretched well beyond areas of obvious shaking and surface ground damage.

"Our research showed that groundwater systems within seven large landslides in the Cromwell Gorge are surprisingly susceptible to seismic shaking, with the pressures and flow rates responding systematically to large earthquakes at distances of several hundred kilometres," O'Brien said.

Differences in the type of earthquake shaking, in particular the duration, amount and frequency of shaking, resulted in different groundwater responses.

The Cromwell Gorge data showed that these changes were triggered by stresses associated with the passage of the seismic waves from the earthquake.

"We found that long-duration earthquakes producing seismic waves with a broad range frequencies cause larger groundwater responses that die away more quickly than those produced by short, sharp earthquakes."

Scientists have yet to examine the effect of November's 7.8 Kaikoura earthquake on the groundwater systems at the gorge, but O'Brien expected the monitoring equipment will have recorded a response.

The research further highlighted how components of New Zealand's engineering infrastructure that were dependent on groundwater, such as dams or irrigation schemes, were affected by earthquakes happening in other parts of the country.

"Although the groundwater changes did not affect the motion of Cromwell Gorge landslides, which are engineered to stop movement, the monitoring by Contact Energy provides an incredible record for application elsewhere.

"The different ways groundwater systems respond to the type of earthquake shaking we have observed is critical to understanding how landslides evolve in the short and long term."

The study was supervised by Dr Simon Cox from GNS Science and Professor John Townend from Victoria's School of Geography, Environment and Earth Sciences, and funded by the Earthquake Commission and the Marsden Fund managed by the Royal Society of New Zealand.

It has been published in the Journal of Geophysical Research.