Mackenzie Basin lake tsunamis
Tsunamis only happen in the ocean, right? Actually, tsunamis can happen on lakes too. They are pretty rare, but if you are on a lakeshore and feel a strong earthquake, simply move to higher ground, away from the shore.
Lake level may rise quickly after an earthquake or landslide. If you feel a long or strong earthquake, move away from the lakeshore to higher ground.
Les eaux du lac peuvent monter rapidement après un séisme ou un glissement de terrain. En cas de séisme important ou prolongé, éloignez-vous du lac et réfugiez-vous en hauteur.
Der Seewasserspiegel kann nach einem Erdbeben oder Erdrutsch schnell ansteigen. Wenn Sie ein länger andauerndes oder starkes Erdbeben spüren, verlassen Sie das Seeufer und suchen Sie höhergelegenes Gelände auf.
El nivel del lago puede subir rápidamente después de un terremoto o un deslizamiento de tierra. Si siente un terremoto largo o fuerte, aléjese de la orilla del lago y diríjase a una zona más alta.
भूक (भूचाल) या भूस्खलन के बाद झील की सतह तेजी से बढ़ सकती है। अगर आप एक लंबे या मजबूत भूक को महसूस करते हैं, तो झील के िकनारे से दूर हट कर ऊँची जमीन की ओर चले जाएं।
地震や地滑りの発生後、 湖面の水位が急上昇する 場合があります。長時間の揺れや強震を感じたら、 湖岸から離れ、高台へ 移動してください。
If people have moved away from the lake shore, it’s one less thing for emergency services to worry about in the aftermath of an earthquake.
We are currently working with NIWA and GNS Science to learn more about how tsunamis could be created on the Mackenzie Basin lakes – Tekapo, Pukaki, Ohau, Alexandrina and Ruataniwha – and how big they could be. We’re also working with Mackenzie and Waitaki district councils to use this research in emergency response plans and information for residents and tourists.
The initial results of this work showed underwater landslide deposits on the bed of Lake Tekapo. The researchers then used information on the size of the landslide deposits, and where they were, to determine what size tsunamis they may have produced. The modelled tsunami scenarios generated waves at the southern lakeshore, where Lake Tekapo village is located, of 1-5 metres above lake level. The largest waves were generated by underwater landslides in the Cass River delta, halfway along the western shore of the lake.
Rare multiple landslide events could generate wave heights higher than five metres above lake level at the southern lakeshore, but the height of these waves is difficult to model as they depend on the exact timing of each landslide, which is impossible to predict in advance.
The researchers did not model a landslide into the lake off Mt John because they didn’t find any clear evidence of landslide deposits in the lake sediments beneath Mt John.
So, how often do these landslides happen? Individual large landslides appear to have occurred, on average, 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, which were most likely triggered by a large, local earthquake.
The impacts of any tsunami in Lake Tekapo would depend on the size of the tsunami and the lake level at the time, which has an operating range of almost 9 metres (702.10 to 710.9 metres above sea level).
Even if a tsunami in the lake does not flood land, it may still cause ‘sloshing’ in the lake, and surges on the lake beach, so people should stay away from the lake shore/beach after a strong earthquake – that’s an earthquake so strong it is hard to stand up.
Read the NIWA/GNS Science research report.
There is only a small chance that a tsunami could be created on the Mackenzie Basin lakes after an earthquake, but if one is, it will arrive on the lake shore within 20 minutes.
A tsunami would cause lake levels to rise and fall rapidly. It would not necessarily flood land - it may just affect the steep lake beach or create strong currents in the lake.
Play it safe and stay away from the lake shore after a long or strong earthquake - it could save lives and make a big difference to the emergency response after a big earthquake.
- Movement on an earthquake fault under a lake, which could suddenly disrupt the lake bed. There are known earthquake faults under Lakes Ruataniwha (Ostler Fault), Alexandrina (Irishman Creek Fault) and Tekapo (Irishman Creek Fault, Coal River Faults) which could offset the lake beds by up to 2-3 metres.
- A landslide into a lake, most likely caused by an earthquake. Lakes Tekapo and Ohau have steep slopes next to them. Landslides into lakes can create large waves, particularly near the landslide, but few of these slopes have recently active landslides on them, and the potential for large landslides on them is not well known.
- Collapse of the underwater sands and gravels (the delta) at the heads of the lakes, again most likely caused by an earthquake. Lakes Tekapo, Pukaki and Ohau all have large deltas at their northern ends.
- Shaking or tilting of the land during a nearby or distant earthquake. This can cause water in lakes to 'slosh' back and forward, also known as a seiche (pronounced "saysh"). Seiches generally don't cause big waves at the shore, and the Mackenzie Basin lakes don't appear to be very susceptible to seiches - none have been documented in past large earthquakes.
A scoping study in 2015 led to the current work mapping the Tekapo lakebed and assessing the potential for underwater and on-land landslides to create tsunamis in Lake Tekapo. More detailed work on Lake Ruataniwha is also planned.
Environment Canterbury, Mackenzie and Waitaki districts, and power generators will use the results in emergency management planning and infrastructure planning and maintenance. It is unlikely the results will be detailed enough to use for land use planning.
The intention of this work is not to scare people - everywhere has its risks, but most people want to make informed decisions and weigh up the risks of where they live, work and play.
Tsunamis have happened on other lakes in the Southern Alps in the recent past. On 22 February 2011, the Christchurch earthquake caused ice to fall off the end of the Tasman Glacier and into Tasman Lake, creating a tsunami with waves up to 3.5 metres high.
In 1992, two rock avalanches fell from Mt Fletcher at the head of the Godley River that feeds Lake Tekapo, and travelled down the Maud Glacier creating tsunamis in the lake at the end of the glacier. Icebergs were left stranded 20 metres above the normal lake level in the first tsunami.
There are many documented cases of landslides into lakes and fiords causing tsunamis worldwide. These include one into Landslide Lake on Vancouver Island in 1946, the Lituya Bay landslide in Alaska in 1958, and more locally the Charles Sound landslide in Fiordland during the 2003 Fiordland earthquake. These all created large tsunami waves that flooded land around the landslide site.
Delta collapses causing tsunamis or water disturbances have been recorded on lakes and fiords, including Lake Wakatipu in the summer of 1937/38, Resurrection Bay, Alaska in 1964, Lake Nahuel Huapi, Argentina in 1960, and Lake Brienz, Switzerland in 1996. Because the delta collapses happen underwater, they are difficult to study.
Very large earthquakes (greater than magnitude 8) can set up seiches in lakes hundreds or even thousands of kilometres away from the epicentre. Many seiches were recorded in lakes across North America after the magnitude 9.2 1964 Alaska earthquake.