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A pilot project highlighting the risks to lakeside towns of tsunamis caused by landslides has been presented to the Tekapo community.
According to the National Institute of Water and Atmospheric Research (NIWA), the project provides the most advanced mapping of a New Zealand lake ever.
NIWA geologist Joshu Mountjoy said a key message from the study is that, as with the coastal environment, if people feel a long or strong earthquake they should move to higher ground and encourage others to do the same.
The project, which was led by Mountjoy, mapped the floor of Lake Tekapo using multibeam sonar equipment to collect high-quality bathymetric maps. Seismic reflection survey equipment was then used to obtain images of the sediment under the lake floor.
Currently, there are large rivers moving sediment into Lake Tekapo and creating huge deposits known as deltas. These deltas and the steep sides of both the lake bed and the mountains next to it, he explained, are prone to collapsing.
Based on the evidence from the size and location of past landslides, the researchers modelled tsunamis from slope failures within and into the lake. The results show that waves could exceed 5m at many locations around the lake shoreline.
“A really interesting aspect of the results from this study is that there is clear evidence that many landslides have occurred at the same time and we believe this has happened during large earthquakes,” Mountjoy said. “We cannot be sure if this is an Alpine Fault earthquake or something local, but it does mean there may be a natural warning if such an event happens again.”
He noted that this is not a unique hazard to Lake Tekapo, for other large and popular lakes on the South and North Islands may experience a tsunami at some point.
“Most people think of tsunamis as ocean based, but they are just as capable of happening in lakes, although little work has been done on this worldwide,” he added. “Given New Zealand’s geological make-up, this is something New Zealand communities should be more aware of.”
The study was funded by the Natural Hazards Research Platform, and is a joint NIWA-GNS Science project. It was published by the Geological Society of London and presented as an opening keynote at an international conference on underwater landslides.
Researchers hope that the study can be used as a basis for research on tsunami hazards in other large New Zealand lakes such as Wakatipu, Wanaka and Taupo.
