Revised Tsunami Modelling

Environment Canterbury (ECan) recently commissioned GNS Science to model the impacts of various tsunami scenarios for specific parts of Canterbury. The results show that in worse-case scenarios, flooding could travel further inland than previously thought.

While modelling was focussed on Christchurch, it included the Waimakariri River mouth and has implications for coastal areas in Waimakariri including Kaiapoi.

As a result, ECan is commissioning multi-scenario modelling for the entire Pegasus Bay, which will help local authorities such as Waimakariri District Council update their tsunami preparations and evacuation zones.  The results are expected to be available later next year.

It’s thought that the largest tsunami risk to the Waimakariri coastline, in terms of the areas that could be flooded, would be generated by a very large earthquake off the coast of South America, which would take approximately 12 to 15 hours to reach Canterbury.  Such large events are rare, with estimated return periods of at least 500 years – possible in someone’s lifetime but unlikely.

Smaller tsunamis that could flood low-lying coastal land, or create strong currents in the sea and river mouths, are more likely but still uncommon. They include tsunamis from across the Pacific Ocean as well as those created close to our shores.

People who work or spend time in Christchurch should familiarise themselves with the new tsunami evacuation zones produced for the city.  More information can be found on the Christchurch City Council website.

The Council will update residents once the modelling for Pegasus Bay is completed and the tsunami evacuation zones are reviewed late 2020.  More information about the recent report can be found on ECan’s website. Frequently asked questions about the modelling can be found at the bottom of this page.

Tsunami Safety:

  • Long or strong, get gone – if you feel a rolling-motion earthquake for longer than a minute or a strong earthquake that makes it hard to stand up, you need to leave. When the shaking stops, head immediately to the nearest high ground or as far inland as you can.
  • Stay Informed – keep checking official Civil Defence channels for up-to-date information. These include the Waimakariri District Council website and Facebook page, and radio stations such Compass 104.9 FM and Radio NZ.
  • Be prepared – emergencies can happen anytime, anywhere. Check out getready.govt.nz for tips to make sure you’re prepared.
  • Have a plan – have an evacuation plan and possible route for your household, including pets. Think about where you would go and, if possible, make a plan with family or friends who live outside the tsunami zone to stay with them if you need to evacuate.  Share any official warnings you hear with family and friends near the coast.

When will we know more?

The Waimakariri District Council will update residents once the modelling for the rest of our coastline is completed. This is currently expected to be late 2020 – early 2021.

For more information about the current modelling, please visit Ecan’s website.

Is Kaiapoi/Woodend/Pegasus at risk?

The coastal townships of Woodend Beach, Waikuku Beach, The Pines Beach and Kairaki are already known to be potentially at risk and are included within the current tsunami evacuation zones.  A network of coastal warning sirens also cover these areas.

The new modelling shows that in the worst-case scenarios, parts of Kaiapoi may be at risk of flooding.

The current modelling does not include areas north of Kaiapoi such as Pegasus and Woodend.

We won’t have a full picture of the risks to our coastal areas or how our tsunami evacuation zones may need to change until the multi-scenario modelling for Pegasus Bay is completed.

How likely is a large tsunami and how much warning would I have?

It’s impossible to predict the exact likelihood of a damaging tsunami.  Large tsunamis that flood land are rare, however they can (and do) happen sometimes.

The most likely tsunamis are those that won’t flood land, but can create strong currents in the ocean, river mouths and estuaries. They are expected to occur around once or twice a decade – maybe a handful of times in someone’s lifetime.

A tsunami capable of flooding very low lying coastal areas could be expected to occur perhaps once or twice in someone’s lifetime.

A very large tsunami that could flood areas further inland, such as Kaiapoi, is expected to occur maybe once every 500 years – so that is possible but unlikely in someone’s lifetime.

Distance-Source Tsunami

The most likely source for a tsunami, of any size, is expected from somewhere across the Pacific. All four significant historic tsunamis to affect Pegasus Bay have come from South America (1868, 1877, 1960 and 2010). In these events, we are likely to have 12 hours or more before it reaches us, which is enough time to evacuate people from coastal communities.

Local-Source Tsunami

There are faults in Pegasus Bay but the recurrence intervals are very long (thousands of years) and all known evidence suggests they are too small to create large tsunamis.  They may however flood very low lying areas of coast and could arrive within 30 minutes.

Other-Source Tsunami

A large earthquake on the Hikurangi or Kermadec subduction zone, off the north and east coasts of the North Island, could create a tsunami that could flood land and would reach Pegasus Bay in 1.5 to 3 hours. The recurrence intervals of these faults are not well known but are probably in the order of several hundred years.

Remember, if an earthquake is long or strong, get gone! Don’t wait for an official warning.

For more information about potential tsunami sources, please visit the ECan Website.

How accurate is the modelling?

This is the most accurate tsunami risk modelling completed for our coastline. It uses the best modelling available in the world and has been produced by GNS Science, one of New Zealand’s principal organizations for modelling tsunami risk. It has also been internationally peer reviewed.

You can find out more about the modelling on Ecan’s website.

If I have to evacuate, where do I go?

Following a long or strong earthquake or an official evacuation message - head immediately west of State Highway One if you can or to the nearest high ground.

If possible, make a plan now with family or friends who live outside the tsunami zone to stay with them if you need to evacuate.

In case of evacuations, Civil Defence Waimakariri will open Civil Defence Centres. Check the Waimakariri District Council website or Facebook page for their locations.

What about if I work in Christchurch?

Christchurch City Council has released new tsunami evacuation zones.  We encourage you to familiarise yourself with the evacuation zones and evacuation routes if you work or spend time in Christchurch. More information can be found on the Christchurch City Council website.

Will this affect my properties LIM?

A Land Information Memorandum (LIM) is a document containing information that the Council holds about a property.

All known Natural Hazards information in the Waimakariri District is publicly accessible and included with a LIM.  If the tsunami evacuation zones are expanded or changed following the modelling for Pegasus Bay, this will be reflected in the property’s LIM.

Where can I read the report?

You can find the full GNS Science modelling report on ECan’s website.