The Hikurangi Trench: New Zealand’s biggest tsunami hazard

Last night on the Seven Sharp programme, the hosts had a flat of young people in Napier City conduct and earthquake and tsunami evacuation drill. The earthquake, a fictional magnitude 8.9 on the Hikurangi Trench has triggered a tsunami and the occupants have 20 minutes to reach ground, with 60 seconds to get what they need and get out of the house. The objective immediately post earthquake was to get to Hospital Hill in a set period of time before a tsunami arrived.

The participants in the Seven Sharp drill did not quite make it. 2 young ladies and a young man had 60 seconds to decide what to take and get out of the house. They had to reach Hospital Hill about 2 kilometres away in the estimated 20 minutes they would have. They were out the door in a minute and were pretty sensible about what they took with them – something to stay warm, water and so forth and they had to carry it. Speed was of the essence, and at times despite the foot wear they were wearing, the ladies were running where they could.

One day there will be a substantial earthquake on the Hikurangi subduction zone. It will be anywhere between magnitude 8.5 and 8.9. It will last between 4-5 minutes and be felt the length and breadth of New Zealand. The above drill was aimed at Hawkes Bay, which will feel the full effect of the earthquake and be one of the first places to be hit by the tsunami – Gisborne and Wairarapa being next.

The biggest problem will not necessarily be the earthquake, although that will in itself be a massive event. Rather it is the tsunami risk that should cause people the most concern. From the subduction zone boundary to coastal Hawkes Bay is 110 kilometres. In the travel time of a tsunami that is about 20 minutes, possibly less.

One should not rely on the authorities to issue a warning in time. The warning system might well have been damaged in the earthquake. The people manning it might have injuries or other more immediate concerns such as making sure their headquarters is safe to occupy and getting the Emergency Operations Centre established in accordance with the Civil Defence and Emergency Management Act, 2002. During that time a tsunami heading for nearby coast lines would have a significant head start.

In a real event though, things will be about 10x tougher. We will assume that it is indeed at night time. When the shaking stops different people will react differently to the situation. A lot will be in shock and not thinking coherently. Before one can leave the house or building they are in to go inland they need to be able to safely clear a route and make sure all of their fellow occupants are safe. 1 minute would easily pass in that time – 4-5 minutes to get out might be more realistic. The power will most certainly be out in large parts of the city if not across the region so there will not be any working street lights, traffic lights or other lighting to guide them. There will be constant aftershocks and some might be substantial events in themselves. In a city built on marine sediments, liquefaction will have flooded many roads, which will also be cracked. Downed power lines, foot paths blocked with debris from buildings and trees pointing in crazy directions will also hinder progress. And if all that is not enough, there will most certainly be people who are – semi-understandably – panicking even though that is the worst thing one can do and trying to drive out despite the obstacles and potentially blocking others.

The tsunami will behave differently depending on several physical factors. For example the sea floor topography will help to determine the size and shape of the waves as they approach. Will they slow down gradually as they run up a fairly open and gently grading beach, where the classic waves that most associate with tsunami’s will form? Will they be coming up a narrowing bay that forces the waves to converge and become closely packed with short distances between them?

So, there you have it. Could those of you in a tsunami inundation zone find your way to high ground following a big earthquake?

Rejection of application to assess Franz Josef shows lack of understanding

The Minister for Regional Development Shane Jones has rejected an application to assess the suitability of Franz Josef for relocation. The rejection of the application, which came in the wake of destructive floods in March and growing concerns about its safety in an Alpine Fault earthquake, indicates a lack of understanding about the dangers posed to the popular West Coast township.

My main concern is that this will unnecessarily increase the risk to residents and visitors to the township. All residents and visitors have a right to be safe in the township during an emergency event, that structures and natural features have been subject to an E.I.M. assessment and matching appropriate steps have been taken:

  • E is ELIMINATE – ELIMINATE the hazard, whether it securing the parapets, chimneys of buildings to make sure they do not collapse onto the street or into adjacent buildigns
  • I is ISOLATE – ISOLATE is what gets done when a hazard cannot be eliminated, requires the separation of the hazardous feature and possibly a buffer zone around it to contain it if still dangerous
  • M is MINIMISE – MINIMISE the hazard if it cannot be ELIMINATED or ISOLATED, by diverting, removing or stopping all non essential functions/features/activities in the vicinity of the hazard

We cannot ELIMINATE the Alpine Fault and the Waiho River. We cannot ISOLATE their reach in Franz Josef’s current location. MINIMISING the risk by relocation off the probable area of the fault scarp and the active Waiho River bed is the best way of reducing the likely damage. The town would be immediately and violently subject to any Alpine Fault earthquake with massive disruption of power, water and sewerage, road and telecommunication links. A fault scarp possibly 2-5 metres high would rupture right through the township with 8 metres or more lateral displacement. The only thing therefore to do is either move all of the non-essential infrastructure of the township away from its current location and establish somewhere else or move the whole town. Neither option is going to be cheap and will probably result in having to buy up land somewhere.

One has to accept that there will still be considerable damage in Franz Josef even if it does get moved. However infrastructure will be far more quickly repaired. It will be spared the likely landslide dam burst emergency that would occur in the Callery River catchment immediately upstream of the Waiho River bridge. And most importantly it would be spared the massive ground damage that would be caused by the surface rupture and which alone would take months to begin to repair – if at all.

The ugly reality facing Franz Josef

Franz Josef is a picturesque town in Westland District. It is nestled against the lower flank of the Southern Alps, with the Waiho River immediately to the south of the township. The town and the nearby Franz Josef Glacier are named after the Austrian Emperor Franz Joseph, by explorer and geologist Julius von Haast.

But for all of its mighty charm nestled in temperate rainforest, Franz Josef is caught between a rock and a hard place. In terms of geological and geomorphological hazards it is in a location that in the long term, and increasingly likely in the short term, untenable. This article takes a look at the danger facing Franz Josef.


New Zealand straddles the boundary of two tectonic plates. To the west is the Indo-Australian Plate and to the east is the Pacific Plate. The onshore boundary is denoted by the Alpine Fault, a large fault line with a repose period of 300-350 years and a tendency to only move in magnitude 8.0+ earthquakes. The last one was about 1717AD.This onshore boundary is where about 25-30mm of tectonic uplift occur per annum as well as a similar amount of erosion, which means there is a continuous supply of sediment waiting to enter the catchments of the West Coast and east coast rivers.

This relentless uplift creates a lovely mountain range with steep hydrology – from the summit of Mt Cook to the Tasman Sea is about 45 kilometres. Being in the prevailing westerly belt of winds that sailors call the “Roaring Forties” because of the latitude, moist westerly air comes off the Tasman Sea and empties its moisture content – often over 200 millimetres and up to 500 millimetres in a day – on the West Coast side. Unsurprisingly flooding becomes a major problem.

How does this affect Franz Josef?

Immediately after one crosses the Waiho River heading south, the road takes a hard right turn. In the corner is a hotel that sits behind a substantial stop bank. On the other side of that stop bank is a riverbed that is rising at a rate of about 300mm each year. The rise is because a large volume of sediment is continually entering the Waiho catchment. This poses an increasing flood risk on a river where water levels start responding to heavy rain in less than an hour.

Franz Josef straddles the Alpine Fault, which runs right through the middle of the township. It crosses the Waiho River in the immediate vicinity of the Milton Hotel, which was flooded and suffered severe damage in a 2016 outbreak. Westland District Council published Plan Change 7 (P.C.7), which was meant to identify a zone through central Franz Josef, where there is high confidence of the Alpine Fault’s exact location, with a view to moving essential services and businesses out of the zone. However after considerable public opposition, P.C.7 was scrapped.


Time is running out. The stop bank is about as tall as it can realistically get without massive supporting earth works. When the river tops it, it will start eroding away the stop bank and try to reclaim the riverbed that the stop bank was originally built over. This may claim several farms when it happens.

But there is a bigger problem. The Alpine Fault is now due for another earthquake. Should it rupture whilst Franz Josef is in its current location, the town will be subject to immediate and unmistakably violent shaking lasting up to 3 minutes. There will be between 8-10 metres lateral displacement to the right and up to 3 metres vertical displacement. Only the newest structures would probably be still standing.

Before then though, there may be another rain storm of similar magnitude to the one that occurred between 25-27 March. Should that happen, similar damage to what happened as a result of that storm should be expected. This has a high probability of including the bridge over the Waiho River, which was destroyed on Tuesday 26 March. The next rain fall event might not even need to be that big.

What is the solution?

In the absence of P.C. 7 existing, one option is to give up on the stop bank and let the Waiho River reclaim the riverbed. The problem here is that several farms and the air field would have to move. It also does not address the long term problem of the Alpine Fault. Perhaps the most feasible option is progressively relocate Franz Josef township’s population and amenities to neighbouring towns. Westland District Council and West Coast Regional Council have a duty of care to the residents and the tourists and other visitors to their District/Region to make sure that they are in no undue danger.

Where could the people go?

There are several nearby townships where the people of Franz Josef could be moved to. Ross and Whataroa are two, though these are quite near the Alpine Fault. Harihari is a third. All are on the same road, State Highway 6, as Franz Josef. It would be likely that West Coast Regional Council and Westland District Council would need to prepare a joint request for Government assistance purchasing land and working out appropriate resource management issues.

For their part the Government would most likely need to provide assistance. The West Coast is economically one of the poorer parts of New Zealand. It has a small rate payer base and this has a good chance, even if well planned and executed, of blowing whatever budget is set. And if the plan went ahead, it might have to be applied to Fox Glacier as well, as it too is very near the Alpine Fault.



Getting ready for the Alpine Fault

It is New Zealand’s biggest seismic hazard, short of a Hikurangi Trench subduction zone rupture. The Alpine Fault earthquake that is expected to occur in the next 50-100 years has been well publicized. But how much are communities close to the fault doing to prepare for a magnitude 8.0+ earthquake?

The Alpine Fault, New Zealand’s answer to the well known San Andreas Fault in California, is the tectonic plate boundary between the Pacific Plate and the Australian Plate where they intersect in the South Island. Every 300 years or so this fault line ruptures in a magnitude 8 earthquake. A sequence of 24 events over the last 8,000 years points to earthquakes in 1100AD, 1450AD, 1620AD and finally around 1717AD,

No part of the South Island will be spared prolonged shaking. Many people, especially i in the lower North Island, will also notice the earthquake. Shaking intensities along the rupturing segment of fault are likely to be up to MMX, which is strong enough to heavily  damage all structures, with many failing and large objects such as televisions and microwaves being moved about. Liquefaction, lateral spreading, landslides and seiching of lake bodies will occur as well.

Alpine Fault Magnitude 8 is a collaborative and ongoing project to improve the readiness of councils across the South Island in terms of their ability to respond to such an event. It has buy in from emergency services, Civil Defence, social groups, the N.Z.D.F., agencies working with lifeline infrastructure and others. The aim is to improve modelling of the potential hazard, engage emergency management and planning experts and use the knowledge gleaned to fill gaps about how to respond.

I anticipate that much of the work that has been done will have been brought into sharp focus by the Kaikoura earthquake in 2016. This was the largest onshore earthquake to hit New Zealand since Murchison in 1929. It caused widespread damage across the northern South Island and lower North Island. The quake exposed weaknesses in transport arrangements with both the railway line and State Highway 1 closed – traffic had to be rerouted through the Lewis Pass in order to reach Picton.

Despite the Kaikoura earthquake and lingering shadow of the Christchurch earthquake, not all councils appear keen to progress their disaster planning. Westland District Council found itself in hot water in 2016 for rejecting Plan Change 7, which sought to address the planning issues that Franz Josef township finds itself confronting. The township straddles the Alpine Fault, which is clearly visible from the air as a crude gash in the landscape. Critics pointed out that the council has a duty of care to all in the District and that by failing to address the risks posed, it leaves itself open to court action by anyone in the District at the time of such an earthquake.

Yet the risk remains. Other councils are pressing ahead with their own plans individually, to be fed into the overall A.F.8. planning framework. It is a proactive council that stands the best chance of success, for no one knows when 300 years of seismic stress on the Alpine Fault will give up the ghost. The only certainty is that with the same confidence that darkness will come into a room when the light goes out, one can conclude it is inevitable.

New Zealand’s megathrust earthquake problem

Scientists are drilling into the tectonic plate boundary off the coast of the North Island. They are trying to find out how close it is to rupturing and looking for clues to indicate levels of tectonic stress.

This is not something anyone should be surprised about. As a nation straddling an active tectonic plate boundary with both strike slip and dipping tectonic plate interface, we are subject to a range of future seismic hazards that at the moment are low risk, high consequence. Those unfortunately are drifting gradually towards high probability/high consequence – the longer it takes for one to happen the worse it will be when it happens.

This is why much research is underway both on land and at sea to understand the hazard posed by the tectonic plate boundary. Not only that, but to also understand how the tectonic interface works, whether there are geophysical or geochemical changes happening in the rock strata that might indicate how much time we have left.

A megathrust earthquake is one involving a large segment of dipping tectonic plate boundary that ruptures at once. Examples include the Tohoku magnitude 9.0 off the coast of Japan in 2011, the 2004 Sunda earthquake that unleashed a magnitude 9.3 earthquake. New Zealand has not had a megathrust earthquake in recent centuries.

This would be a devastating event if it happened in New Zealand. The energy released in the event would be immense – possibly 2000 times more than the It would involve the Hikurangi Trench rupturing from off the east coast of the South Island up past East Cape. The potential magnitude of the earthquake could be anywhere between M8.5 and M9.0. It would involve probably 5 minutes of sustained shaking – 3 minutes longer than the 2016 Kaikoura earthquake and comparable in length to the 2010 Chilean M8.8 event.

The earthquake would very likely trigger a tsunami. The major megathrust earthquakes of the last century have all triggered destructive tsunami – the worst in terms of casualties and overall damage without doubt being the 2004 Sunda event and the 2011 Tohoku event.

The message for coastal areas of New Zealand since the Kaikoura earthquake has not changed: LONG (lasting more than a minute) AND STRONG (can’t stand up)? GET GONE.¬†And for all people caught in an earthquake that is not over in a few seconds and especially if at that point it seems to intensify, the message is “DROP (to your knees). COVER (under a doorway or desk). HOLD (on)”.