What are the chances of Christchurch getting smashed by a tsunami?

UPDATE: November 14, 2016 – As most of the world knows by now the north Canterbury region (near Hanmer Springs/Kaikoura) of New Zealand was hit by a massive 7.5 earthquake at 12:02 this morning. It was felt by us here in Christchurch – in fact, it was felt all over New Zealand, even as far north as Auckland. Due to the distance from Christchurch, it was a long, rolling quake that lasted for well over a minute (which feels like an eternity!). Very little damage here, if any, since our infrastructure, homes, bridges and office buildings are all built out of rubber making them basically quake-proof however other parts of the country have not been so lucky, particularly around the top of the South Island.

However, after the quake and getting back to sleep, we were rudely awakened at 2:14am by a tsunami siren (since we live near the beach in Christchurch). So the first thing you do is verify that it is a genuine evacuation, particularly since directly after the quake it was declared there was no tsunami threat. So it seems to be the real thing so we make the decision to leave (besides there’s no getting back to sleep with that scary airhorn of a siren going off – honestly having that thing go off at two in the morning is enough to send the sanest person into a frenzied panic).

So now it’s all a question of what to take, where are we going (you think living in a disaster zone we’d be better prepared). There’s precious little official information available online – of course all the informal lines of communication are going nuts and that doesn’t always help. So we throw some clothes on (phew, lucky we had the presence of mind for that at least), grab the dog, the go bag and some alcohol (definitely a must in any disaster situation).

Driving out of New Brighton there’s not a whole lot of traffic but we do start to get stuck near the Bridge St roundabout (a massive bottleneck around these parts). For some reason, the sirens have stopped – is there no longer a threat? We think maybe so, and u-turn back home. At the same time there’s a cop car speeding around blaring something out – moving way too fast to really catch what they’re saying but we wonder if the warning is still in place. I talk to one of my neighbours, who’s up on his roof with another local – they plan to stick it out there, which apparently is on the list of recommended actions for tsunamis, if you can’t get out of the area that is.

We think some more then decide to evacuate anyway, but go a different way. Pages Rd seems like a good route until we hit Aranui where the congestion really kicks in. Traffic is crawling. Nice to see no ones leaving the furkids at home. Saw one family walking it, struggling with a carseat and other essentials – hopefully they weren’t going too far. There was a little kid with a suitcase walking in the wrong direction. It seemed like most people were just plain confused about what to do.

We take a side road and get on to Wainoni Rd where it’s free and clear. Good move. We get into town relatively quickly and setup a temporary base at my work, where we have relaxing cups of tea and wifi to monitor the situation.

Eventually, at around 8:30am we’re given the all-clear to return home. There was no tsunami wave or surges this time but I guess it was best not taking any chances.

Hopefully, Civil Defence, the police and other agencies that exist for times like this make some improvements for next time and develop a coordinated line of communication, a clear plan and safe passage for residents to easily get out of the way of an incoming tsunami – should we ever get one.

I guess ever since the earthquakes started here in Christchurch we’ve all become a little too sensitive to natural disasters. We talk about them like we’re talking about the weather. And now we’ve even become bored with earthquakes so move on to the next thing – tsunamis.

So is a tsunami a real threat for Christchurch residents (particularly those in the seaside suburbs of New Brighton and Sumner)? Should we be running for the hills every time we get a decent shake?

Recently the Christchurch City Council published some reassuring information on the subject, as well as a factsheet, the contents of which are shown below. Hopefully it answers any questions you had on the subject, and will equip you well to become the resident tsunami ‘know it all’, which sounds like a put down but come big wave time they’ll come running for you … or possibly they’re just running for higher ground.

Why didn’t the September 2010 and February 2011 earthquakes cause tsunamis?

The September 2010 and February 2011 earthquakes did not cause tsunamis because they happened on land and did not displace the sea floor. The aftershocks from these earthquakes are also too small to cause a tsunami, even if the earthquake happens offshore. An earthquake out at sea
usually needs to be greater than magnitude 6.5 to cause a tsunami.

Local and distant tsunamis

A tsunami caused by an earthquake at sea but relatively close to Christchurch is called a local tsunami. A tsunami caused by an earthquake in the Pacific Islands or further across the Pacific Ocean is called a distant tsunami.

What is the risk of a local tsunami?

The likelihood of a tsunami generated close to the Christchurch coast is very low. Based on the information we have, there are no faults large enough to cause a significant tsunami in Pegasus Bay.
NIWA scientists have just completed a seismic survey of the sea floor in southern Pegasus Bay to better understand the geology and to find out if there are any faults we are not currently aware of. The results of this survey will be available in the coming weeks.

Would an offshore earthquake as large as the September one cause a tsunami?

The faults that caused the September 2010 and February 2011 earthquakes mostly involved sideways motion, with only a small amount of up-and-down motion. Should another earthquake of September’s magnitude occur offshore, we expect any resulting tsunami would be relatively small – less than 1.5 metres at the Christchurch shore. Only beaches, estuaries and very low lying coastal land along Pegasus Bay and northern Banks Peninsula would be affected, however currents would be created that could affect boats.

What would happen in Christchurch if a tsunami occurred in Kaikoura?

Should a tsunami occur in the Kaikoura Canyon just south of Kaikoura, it might affect the South Kaikoura coastline, but would be too small by the time it got to Pegasus Bay to affect Christchurch.

What should we do if we are near the sea and there is a strong earthquake?

It is very unlikely the earthquake will cause a tsunami but like anywhere in New Zealand, if you are at the beach or in a low lying coastal area and feel strong ground shaking for more than 20 seconds, or notice a sudden rise or drop in sea level, immediately move to higher ground or inland from the evacuation zone for distant tsunamis (see map overleaf).

Why did the recent Japanese tsunami cause so much devastation?

The Japanese tsunami was caused by a massive earthquake from a known large earthquake fault not far off the Japanese coast. We do not have a known fault like this off the Canterbury coast, so a large tsunami like the one in Japan would not happen here.

What is the risk of a distant tsunami?

The likelihood of a distant tsunami causing damage to Christchurch is low. Such a tsunami would most likely originate from across the Pacific Ocean (e.g. South America) and would take around 12-15 hours to cross the Pacific Ocean. This would allow time to issue warnings and evacuate coastal areas if land is likely to be flooded.

What will happen if a distant tsunami threatens Christchurch?

Most distant tsunamis that reach New Zealand are too small to flood coastal land, although they can create strong currents in water that can affect ports, boats and people in the sea and at river mouths. However, sometimes a distant tsunami can be big enough to flood coastal land. If the Ministry of Civil Defence & Emergency Management issues a tsunami warning that includes a threat to land, the Police, Fire Service and Civil Defence will start evacuating Christchurch, the area indicated on the map below. They will door-knock to make sure people evacuate and will also use vehicle sirens and public address systems to alert people to leave. Evacuation messages will also be provided on the radio, television and the internet.

Why were we not evacuated during the February 2010 Chilean tsunami?

Coastal areas were not evacuated during the February 2010 Chilean tsunami because the tsunami was not large enough to flood land in Christchurch. However, it did create strong currents in the sea and in the estuary, and that is why people were warned to stay off beaches and out of the sea.

When will warning sirens be installed?

Sirens are being installed along coastal areas of Christchurch to warn of distant tsunamis. They were due to be operational by July 2011, however this timeframe may be delayed by a few months because of the February 2011 earthquake. A series of community meetings will be held once the sirens are operational to explain what to do if the sirens are activated. It is very unlikely the sirens would be used to warn of a local tsunami because the tsunami would reach the coast before the sirens could be activated.

Things to know and do if you need to evacuate

• Be prepared by having a household evacuation plan and storing important personal items and documents where you can quickly gather them.
• Share any evacuation messages you get with neighbours.
• Check to see if your neighbours need assistance evacuating.
• Secure your home and leave a telephone book outside your door to let emergency services know you have left.
• Move to higher ground or inland from the areas shown on the map. You may need to stay with friends or family outside the evacuated area, or go to a designated emergency welfare centre which will be set up by Christchurch City Council.
• Do not return home until the tsunami warning has been cancelled and Civil Defence gives the all clear. The first surge in a tsunami may not be the largest and surges may continue for many hours. The extent of any flooding will also depend on the tides at the time the surges arrive.

More information

You can find information on the Coastal Evacuation Plan for Christchurch at http://bit.yl/eeCTyX. The pamphlet will soon be updated once the coastal sirens are installed. You can also find more information on Canterbury’s tsunami hazard in the booklet Q Files: Tsunamis on the Environment Canterbury website at www.ecan.govt.nz/qfiles.