Tsunami in Devonport? What Are The Chances?

Posted by on Mar 15, 2011 | 3 Comments

Having been shocked by the images from the Boxing Day tsunami and more recently the extraordinary raging floodwaters that struck Queensland, The Speculator has been overwhelmed by the horrifying images of the Japan tsunami sweeping across farmland, towns and villages.

The monstrosity doesn’t deserve to be referred to as a wave, or even as water, for these are things with which we are familiar. It is neither. It is a rare manifestation of Nature’s gigantic kinetic power, transmitted through the medium of water and unleashed upon our modest notions of civilization; concrete, timber, metal and plastic, which it has spread like thorny excrement across hundreds of kilometres of inhabited land thousands of people once considered home.

The inevitable question we will ask is; could it happen to us? As a village surrounded by water and open to the Pacific Ocean, it’s a fair question. The Speculator has done some research on the subject.

Tsunami is a Japanese word meaning ‘harbour wave’, and is commonly used to describe the series of waves formed when the sea floor is disturbed; usually due to an earthquake.  Sudden uplift of the sea floor or large underwater landslides cause waves to move away from the source like ripples in a pond into which a stone is tossed.  Tsunami can also be caused by volcanic eruptions and meteorite impacts.

Tsunami can travel as fast as 700 km/hr across the deeper parts of the ocean and can travel for thousands of kilometres.  They are barely noticed by anyone at sea as the wavelengths are long – up to several hundred metres.  As the tsunami enters shallower water, the front of the wave slows down and the back catches up, piling up into larger waves.  They can be up to tens of metres high in shallow water, however most are considerably smaller than this.

Of the 32 recorded tsunami to hit New Zealand between 1840 and 1995, 12 have been detected within Auckland waters.  Although information about many of these is sketchy, all were small (less than 2 metres) and if any damage occurred it was very minor – in fact most would not even have been noticed.

However, 150 years is not a very long time and more extreme events may have happened in the past. Scientists calculate there is about a 50% chance within the next 50 years that Auckland will be hit by a tsunami originating from a large earthquake off the west coast of South America.  Wave heights of up to 4 metres could occur in the outer Hauraki Gulf. Excluding those caused by an asteroid impact, this represents the most likely damaging tsunami for Auckland.

The severity of the tsunami is largely dependent upon the event that causes it. There are three main possible sources of a tsunami striking Devonport.

Possible Tsunami Origin Locations

Possible Tsunami Origin Locations

1. As mentioned, a tsunami event from an earthquake off the West Coast of South America is the most likely source. Historically this region has produced the largest tsunami in the Hauraki Gulf.

Of the South American tsunami sources, it is those lying between the Peru-Chile border (19S) and the 8S line of latitude, which are most effective at directing tsunami towards New Zealand. The tsunami of 1868, which was the worst distant source tsunami of historical times in this country, originated from the southern half of this region (about 17.7S).

The last large tsunami from the northern half of this region (about 12.5S) was in 1746, too early to appear in written records in NZ, but modelling suggests that such tsunami are likely to also have a strong impact here. Locations on the east coast of New Zealand tend to be the most vulnerable to South American tsunami, but the ability of tsunami to bend around corners in the coastline, means that they can still pose a hazard to locations which are out of the line-of-sight.

A tsunami triggered by a massive South American earthquake would swamp Auckland motorways, coastal roads and low-lying bays, a recent study found. The Niwa report for the Auckland Regional Council used computer modelling to predict the impact of the wave along vulnerable coastal areas.

A South American tsunami was chosen as the region’s most likely risk, expected to reach our shores every 50 to 100 years. The study looked at the worst-case scenario of a wave striking land at high tide.

Findings showed “significant inundation” to key roads including the Northwestern Motorway between Pt Chevalier and Te Atatu, Tamaki Drive at Hobson Bay, and the harbour bridge approaches. But few people would be forced to flee their homes.”

“Our risk is relatively minimal with regards to devastating damage to developed areas,” said regional council hazards manager Greg Holland. This sounds more like wishful thinking.

“What we’ve seen is infrastructure, like the approaches to the harbour bridge and the Northwestern Motorway, gets into a little bit of trouble, so that’s the areas we’d need to keep away from.” I’ve watched a fair bit of the tsunami coverage. There is no such thing as a “little bit of trouble.”

Currents would sweep through the harbour at up to 3m per second. The wave would appear low, but could have enough power to surge 2m to 3m beyond the high tide mark, Holland said.

“You wouldn’t want to be down on the beach when the tsunami came in.”

On the North Shore, flooding would be mostly confined to the coast, except streams at Long Bay, Winstones Cove and Torbay, and low-lying areas of Browns Bay and Mairangi Bay although previous presentations to the council showed Browns Bay could be wiped out and Devonport cut off from other land if a tsunami hit the Shore. Milford would also be badly affected. Westhaven Marina and parts of Freemans Bay, Mission Bay and Glendowie would be swamped. Waiheke Island would be inundated at Blackpool and Surfdale.

East of the city, parts of Cockle Bay, Beachlands Marina, Kellys Beach, Te Puru Stream and Maraetai Beach would be affected. Experts haven’t yet determined how many homes would flood, but “but if you live on the waterfront, you should be careful and get to higher ground,” Holland said.

Auckland Civil Defence Emergency Management Group controller Harry O’Rourke said the modelling was a valuable tool. “We can now plan and move forward with options for public warning systems.”

Work is under way to assess the impact of a tsunami from a closer source, such as the Tonga-Kermadec trench. Such a wave would arrive faster and be more devastating, but is far less likely to occur.

Tsunamis could also originate from locations in the Northern hemisphere, such as Cascadia, the Aleutians, and also from areas of the southwest Pacific north of New Zealand, which would tend to have their greatest impact on Northland, the Coromandel, and the Bay of Plenty.

Tsunami Time Delays

Tsunami Time Delays

2. A tsunami generated by a local earthquake along the Kerepehi Fault. This fault bisects the Gulf, has been active during the last century at the southern inland end, and is overlain by a considerable thickness of soft sediment that may amplify the seismic waves.

The active Kerepehi Fault probably extends into the Hauraki Gulf about 40 km east of Auckland, and is the only offshore active fault known in the Auckland region. The fault can produce earthquakes up to about M 7, similar to those in the Bay of Plenty. At 40 km distance, it is considered unlikely that the fault poses a tsunami hazard to Auckland. Modelling of a tsunami generated by the Kerepehi Fault found it would not produce run-up of more than 2m in Auckland.

Local tsunami generated by submarine landslides and thrust faults could also have a large local impact on the east coast of New Zealand from Kaikoura northwards to Northland, but there are few situations where these are likely to occur.

3. A tsunami generated by a volcanic eruption within the Auckland Volcanic Field. This field has involved a series of mainly monogenetic basaltic eruptions over the last 140,000 years. Many of these eruptions have involved eruptions around the coastal margins, or within the shallow waters close to Auckland. Volcanic eruptions from the Auckland Volcanic Field could also produce tsunami though wave heights are unlikely to be much larger than 1m.

Earthquakes on land can also cause tsunami in protected harbours, lakes or reservoirs. Called seiches, these waves move backwards and forwards, sloshing water from side to side and can cause flooding along the shore for many hours after the earthquake.

Whatever the source of the tsunami, people living on land which is less than 15 metres (vertically) above low tide mark are encouraged to register with the Council telephone tsunami alert system.  Workplaces on low ground are also advised to register.

High ground - above 15 metres

High ground - 15 metres above sea level

In the event of a tsunami evacuation warning, people registered on the system would receive an automated phone warning. People who want to register should contact council Actionline, phone (09) 486 8600 or email actionline@northshorecity.govt.nz. Each person who registers is asked to specify a phone number, which can be either a mobile or landline. The link to the page is here. While all these addresses and URLs still relate to the old North Shore City Council, they ARE the ones provided on the new Auckland Council site. There is also more information available on tsunami alerts here.

When a tsunami alert is in force, the Civil Defence and Emergency Management team keep in close contact with the Civil Defence and Emergency Management authorities in Auckland and Wellington. Council advice is based on the information they distribute. The automated tsunami warning phone system is activated only when there is genuine concern that a tsunami large enough to be a threat is likely to reach the North Shore.

If the tsunami originates some distance from New Zealand, the wave height monitoring systems in the Far North of New Zealand and in the Pacific Islands mean that the authorities can assess the risk of a tsunami with considerable accuracy.

However, the authorities under the new council structure appear to have made some bad calls. Somewhat incredibly, a key base for Auckland’s new civil defence emergency response is being set up just above sea level in an area identified as a tsunami risk – Orewa.

Civil Defence is handled by territorial local authorities and has been restructured as part of the amalgamation of Auckland’s seven councils. Agency spokesman Colin Dale confirms the headquarters for emergency response operations to the north of the region will be run from the current Rodney District Council offices in Orewa.

Asked if it made sense to station a base on a flood plain at Orewa, he says: “Orewa , quite rightly, has issues with tsunami, but it’s not critical. You only really need one main centre to run an emergency response from and that is in Pitt St [Auckland CBD]. If the main one wasn’t available for any reason then any one of the other facilities can step in.”

“In an emergency, operations can be run out of a school hall or a fire station. They will set up wherever they need to be.”

As if this nonsensical reply from Mr Dale wasn’t enough, a perfectly good purpose built Civil Defence centre already exists – on high ground – at Mairangi Bay. However, the new council has seen fit to close this facility, instead using it for training purposes. Go figure.

So what does it all mean? In a nutshell;

– The most likely threat comes from a long way away, which means a decent period of warning for a tsunami originating in Latin America.

– There is a less likely but potentially more devastating threat from a tsunami generated by an earthquake in the Tonga-Kermadec trench. Scientists are currently investigating this area, but it does mean you may not receive any warning if a tsunami originates from here.

– The official view appears to be designed to minimise fear while establishing response systems. Some of these are in place. Some of these are in the worst possible place.

– Check our map, and identify where your house or place of work sits in relation to the 15 metre zone. Make a note of where the closest safe zone is, and the fastest way to get there on foot and by car. It only takes one panicked person to abandon their car in the middle of the road to prevent you from driving past.

– If you are below the 15 metre line, sign up to the tsunami alert system; details are in this article.

– Be cool. The chances are we’ll be fine.

3 comments

  1. JL says:

    I was interested to note that, even though I’m on the register, no tsuname warning call came in.
    Could the fact that it wasn’t activated be due to the fact that the new supercity council has closed all civil defence offices, including north shore’s, with the sole exception of one in Pitt St?…..Possibly a tad short sighted of them, under current circumstances?

    That definitely sounds like it needs following up on JL; would be interested in the outcome of your investigation if you were to conduct one.

    As mentioned in the article, there is of course one other civil defence centre – the one snugly located in the tsunami zone at Orewa. But as the official quoted in the article said, that doesn’t really matter, ‘cos they only need one anyway. – Ed

  2. Chris Werry says:

    Note the council only allows you to register one phone number per household. So if there’s more than one of you, you either have to choose who’s mobile gets the warning or register your land line instead. If you use your land line you’d better hope you’re home when the alert is raised.

  3. F says:

    Let no-one tell you the tsunami had no effect here on Saturday. At Bayswater Marina there were strong tidal surges for quite a time round about mid-day. The effect on the pontoons made quite an impression on those who saw what was happening.

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