Seismic risks of building a ferry terminal at Clifford Bay

The Boo Boo Stream is a pleasant little waterway which meanders into the sea just south of Clifford Bay on the Marlborough Coast. On a more menacing note, its name was given to the 70 kilometre long Boo Boo Faultline, which runs east to west between Cape Campbell at the southeastern tip of Clifford Bay, and Cape Palliser, the most southerly tip of the North Island.

The discovery in 2004 that the Boo Boo Fault is much longer than previously thought led scientists to increase their risk rating for a strong shake in Wellington and to assess an associated “subtle but significant increase” in the risk of tsunami for Coastal Marlborough.

The Boo Boo Fault is just one of many known submarine faultlines occurring throughout Cook Strait. Some of these are extensions of large land-based faults, which converge into the Cook Strait from both the North and South Islands.

The South Island land-based faultlines include a major set of faults known as the Marlborough Fault System. The most significant of these are the Wairau Fault, Awatere Fault and Clarence Faults, which reach out across Marlborough like parallel tram-lines. The Wairau Fault is the most northern, running a west to eastern line entering the Marlborough Coast not far to the north of Blenheim. Running on similar lines, the Awatere Fault enters the coast near the northern end of Clifford Bay and the eastern most trace of the Clarence Fault appears to tail out near the southern end of Clifford Bay.

Consistent with the presence of these faults, Marlborough’s Emergency Group Plan 2011 – 2016, states that Marlborough is one of the more seismically active areas of New Zealand.

As well as the many submarine faults in the Cook Strait, coastal Marlborough and Kaikoura are very close to the southern end of the “Hikurangi subduction zone”, which forms part of the well known “Pacific Ring of Fire” and is reported as having the potential to cause very significant near shore earthquakes (like the 2011 Japan Earthquake). As recently as January this year, a visiting seismologist said that the Hikurangi subduction zone, as well as two similar trenches off the coast of the USA and Alaska, would “form the focus of a major global study of megathrust earthquakes, which can often be of magnitude 9.0 or higher and cause large tsunamis.”

As you would expect given the presence of these near shore faults, the Marlborough Emergency Plan also deals with tsunami risk. The comments about the risk of near-field tsunamis (which are likely to be generated by local earthquakes and fault ruptures, submarine landslides and large coastal landslides) are particularly stark:

Near field tsunamis are most likely to cause severe damage to infrastructure, given the short warning time available and other than identifying prone areas. Very little can be done in practical terms to protect people and property to such hazards.

Simply put, people will have no time to flee.

The Marlborough Emergency Plan specifically notes that “the open coast of Marlborough (Cloudy and Clifford Bays) may be the most prone area to tsunami”. It also records:

A 10 metre high tsunami today (as occurred in 1855) could be quite devastating. A study carried out indicates that Cook Strait (or “Tsunami Alley” as Dr Goff calls it) is particularly dangerous given its funnelling effect, which can increase the size of a tsunami.

All this leads to the question - as Clifford Bay is clearly recorded as being among the country’s highest earthquake and tsunami risk areas, isn’t that a primary issue for Government’s investigation of the viability of the location as a site for critical national transport infrastructure?

Irrespective of whether the commercial case can be made - is there too great a risk in having thousands of people at any given time in an exposed, low lying coastal area that is recorded as having such high risk from both earthquakes and tsunamis? Especially when hazard warning records state that “very little can be done to mitigate the risk of near-field tsunami to people and property” in the area?

Those responsible for weighing Clifford Bay’s natural hazard risks would do well to consider that the risks around earthquakes and tsunamis here are not just theoretical.

In New Zealand’s relatively recent history, over just a seven year period the area was devastated by earthquake and tsunami on two occasions. Our settlers’ historical accounts make sober reading.

On 16 October 1848, the Awatere Fault ruptured not far from Clifford Bay and with an estimated magnitude of 7.5. The first earthquake hit at 1.40pm in a heavy storm and was reported to last for about two minutes. Many large aftershocks followed, with a report of more than 100 aftershocks over the early hours of the following day. After a few days of these tremors, the settlers in the Cloudy and Clifford Bay areas became sufficiently terrified to abandon the land, seeking refuge in Wellington. Despite a heavy storm, they fled with their families in open boats over Cook Strait. Some never returned.

Their decision to leave and stay away was prescient. Only seven years later, at 9.11pm on 23 January 1855, an 8.2 magnitude earthquake ruptured the Wairarapa Fault near Wellington. This was the most powerful earthquake ever recorded in New Zealand. It permanently altered the Wellington landscape and created what one historical account called “the day of the Tsunami”.

The Wellington settlers, no doubt including some who fled from Marlborough seven years previously, were shocked to see Wellington harbour ebb and flow “for eight hours – the tide approached and receded from the shore every 20 minutes, rising from eight to ten feet and receding four feet lower than at spring tide.”

This tsunami, estimated to be 10 metres high, went through Cook Strait and swept over the open coast at Cloudy and Clifford Bay.

Learning from the past is not a “should” – it is a “must”. With the presence of such significant natural hazards and coastal environment issues, any decision to commit to Clifford Bay will be a very big call indeed. Perceived economic significance should not always trump risks.

ENDS

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