Lake tsunamis
Tsunamis only happen in the ocean, right? While it’s rare, tsunamis can happen on lakes too.
If you feel a long (more than a minute) or strong (hard to stand up) earthquake, move away from the lake shore to higher ground if possible.
A tsunami would cause lake levels to rise and fall rapidly. It may not flood land but could affect the steep lake beach or create strong currents. Staying out of the water and away from the shore can save lives and reduce the load of emergency responders after a big earthquake.
How lake tsunamis are created
There are four ways a tsunami could be generated on a lake.
- Movement on an earthquake fault under a lake, which could suddenly disrupt the lake bed. There are known earthquake faults under Lakes Ruataniwha, Alexandrina and Takapō in Te Manahuna/the Mackenzie Basin, and Te Hāpua Waikawa/Lake Lyndon near Porters Pass, which could offset the lake beds by 2-3 metres.
- Collapse of the underwater sands and gravels (the delta) at the heads of lakes, most likely caused by an earthquake. Lakes Takapō, Pūkaki and Ōhau all have large deltas at their northern ends.
- A landslide into a lake, again most likely caused by an earthquake. Many of our lakes have steep slopes near them.
- Shaking or tilting of the lake bed as earthquake waves pass through them from nearby or distant large earthquakes. This can cause water in lakes to ‘slosh’ back and forth, also known as a seiche (pronounced ‘saysh’). Seiches don’t usually cause large waves at the shore.
Lake Ōhau
Lake Ruataniwha
Collaborating for research and emergency preparedness
We work with researchers and hydro-electric power generator providers to better understand our lake tsunami risk, and help district councils use this information for emergency management planning and education.
Historic lake tsunamis
Tsunamis have occurred on lakes in Waitaha in the past. In 1992, two rock avalanches fell from Mt Fletcher at the head of Whimiahoa/Godley River that feeds Lake Takapō and travelled down the Maud Glacier. These created tsunamis in the lake at the end of the glacier that left icebergs stranded 20 metres above the lake level.
More recently, the 2011 Christchurch earthquake caused ice to fall off the end of Haupapa/Tasman Glacier and into Tasman Lake, creating a tsunami with waves up to 3.5 metres high.
Find more information on historic lake tsunamis in the Department of Conservation’s lake tsunami report and in the New Zealand Tsunami Database.
Lake Takapō tsunami investigation
In the summer of 2015/2016, researchers from NIWA, GNS Science and Otago University mapped the shape of the Takapō/Tekapo lake bed and the sediments under the lake bed.
The initial results showed underwater landslide deposits on the lake bed. Large landslides appear to have occurred approximately every 1,000 years over the last 12,000 years. There also appears to have been at least three events involving multiple landslides in the lake over the last 12,000 years, most likely triggered by a large, local earthquake.
The researchers used information on the size of the landslide deposits and where they were to model what size tsunamis they may have produced.
The modelled tsunami scenarios generated waves of 1-5 metres above lake level at the southern lakeshore, where Lake Tekapo village is located, within 20 minutes.
The impacts of a tsunami at Lake Tekapo village would depend not only on the size of the tsunami but the lake level at the time, which can vary by up to 9 metres.
Even if a tsunami in the lake does not flood land, it may still cause dangerous surges on the lake shore, so it is important to move away from the lake shore if you feel a long or strong earthquake.
Read the Southern Lakes Tsunami Hazard report by NIWA and GNS Science (PDF file, 1.68MB)
Lake tsunami reports
You can find more detailed information about the studies into lake tsunamis in the Te Manahuna/Mackenzie Basin in the following reports.