27 January 2017
Cromwell Gorge: earthquake-induced groundwater changes in very deep-seated landslides
Cromwell Gorge in New Zealand is the site of a series of spectacular, extremely deep-seated landslides in schist. These landslides became famous during the construction of the Clyde Dam between 1976 and 1988, when a huge programme of mitigation was enacted to ensure that they remained stable during the filling of the reservoir between 1992 and 1993. There are 17 very large landslides, all of which now have extremely low movement rates as a result of these engineering works, which include extensive drainage to draw down the groundwater level, toe buttresses and in one case a drainage blanket. The image below shows the banks of Cromwell Gorge, taken from a helicopter hovering near to the dam:-
These landslides are extremely well-monitored in every respect, providing a fantastic opportunity to understand the nature of processes occurring within them in response to external forcings. In a paper recently published in the Journal of Geophysical Research, O’Brien et al. (2016) have looked at the groundwater response in these landslides to nearby earthquakes. This work provides two really interesting insights. The first is that there is a very clear short term hydrological response to the passage of the seismic waves through the landslide body. The graph below is an excerpt of one of the figures from the paper showing the groundwater response from two piezometers located in the Nine Mile Downstream landslide. The red bars are earthquakes analysed in this study:
The authors found that the earthquakes induced substantial changes in groundwater levels, with the response being variable within the landslides (note the image above shows one response in which groundwater increases and another in which it declines). These changes were more substantial than those generated by short term rainfall events, and in many cases the changes occurred over a period of a month or so. Increased flow was also observed in the v-notch weirs that are used to monitor flow from the drainage tunnels. It appears that the earthquakes are driving short term increases in the permeability of the landslides that allows water to move between different sections of the landslide mass. These changes appear to be temporary.
But in many ways it is the other finding that is more significant. O’Brien et al. (2016) have looked at the response of the groundwater in the Cromwell Gorge to different types of earthquake forcing. The authors found that greater changes occur in earthquakes that generate high-amplitude, long-duration, broad frequency bandwidth earthquake shaking. This differential sensitivity to the nature of the earthquake is intuitively unsurprising, but really important. We observe that similar earthquake magnitudes can generate very different landslide responses; understanding why is a key issue. This paper, and the very detailed data that it describes, provides a key insight.
O’Brien, G. A., S. C. Cox, and J. Townend, 2016, Spatially and temporally systematic hydrologic changes within large geoengineered landslides, Cromwell Gorge, New Zealand, induced by multiple regional earthquakes. Journal of Geophysical Research – Solid Earth, 121, 8750–8773, doi:10.1002/2016JB013418