Mackenzie Basin lake tsunamis

Researchers from NIWA, GNS Science and Otago University mapped the shape of the Tekapo lake bed, and the lake sediments under the lake bed, over the 2015/16 summer. This is the first time this sort of detailed survey has been done in a New Zealand lake.

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 (PDF file, 1.68 MB).

Like anywhere in New Zealand at risk from tsunami - if you feel a long (more than a minute) or strong (hard to stand up) earthquake, stay away from the lake shore and move to higher ground if possible.

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.

A tsunami could be generated on the Mackenzie Basin lakes in four different ways:

• 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.

Mackenzie District Council wanted to know more about the potential effects of a tsunami on Lake Tekapo for emergency management planning.

We widened the scope of the study to include the other Mackenzie Basin lakes, although most work is focussing on Lake Tekapo because it has the largest lakeside population.

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.

We undertake and support research on possible earthquake sources for Canterbury including the Alpine Fault, which is only one of many earthquake faults that could affect the Mackenzie Basin.

Instead of focussing research on one particular fault, we tend to focus on effects such as earthquake shaking, liquefaction and tsunamis, that could happen from a number of scenarios.

One of the potential effects of a large earthquake - from the Alpine Fault or any other big fault in the area - is a tsunami or seiche in the Mackenzie Basin lakes.

We wanted to know more, so that this possibility could be included in local emergency response plans for a big earthquake.

Yes, tsunamis have happened on lakes in New Zealand and overseas. It is possible that tsunamis have happened on the Mackenzie Basin lakes since they were formed almost 20,000 years ago, but none have been reported since European settlement.

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.

The potential impact of hazard information on the value of lakeside property is a legitimate concern.

However, research shows that other factors such as house size, age, location and amenity have a much greater influence on property values than hazard information. This is especially so for unique and scenic areas like the Mackenzie Basin.