Consequences of a Large Earthquake

Image courtesy of GNS Science

The Alpine Fault runs the entire length of the South Island, and mostly close to the West Coast. North of a line running horizontal through Hanmer Springs several other (The Marlborough faults) run off the main fault, which becomes the Wairau Fault. In past events it is only part of the fault that has ruptured, but this has been up to 400km in length. This produces large (magnitude 7-8) or even great (magnitude >8) earthquakes that would result in more ground shaking than has been experienced in the South Island since it has been inhabited by Europeans. Because of the proximity to the West Coast, and especially Westland, the effects would be more dramatic here than anywhere else, and our abilities to manage the situation would be tested to the extreme.

The Civil Defence Emergency Management Act 2002 required councils to consider their lifelines (electricity, water, sewerage, communications, road network, etc) and devise a strategy to protect and repair them in the event of natural hazard events. The West Coast Regional Council and each of the three District Councils have all identified an Alpine Fault earthquake as the most serious threat to the region, not only because of the significance of the consequences, but also because of the high likelihood of it happening in the near future. The following descriptions are based on the lifelines studies. These may be downloaded in full at http://www.wcrc.govt.nz/plans/emergency/lifelines.htm

Buller District has suffered earthquake effects more recently than other districts and in the case of an Alpine Fault rupture it may experience shaking that was similar to that in the 1929 Murchison earthquake. The difference in an Alpine Fault earthquake is that access to the Buller District would be severed for several days, and most likely a minimum of a week because the roads into the district either cross the Alpine Fault or go very close to it and travel though terrain that is very prone to landslide. Road access to Nelson is likely to be the first state highway to open after a week. Lewis Pass may be open after 2-3 weeks. Punakaiki may well be isolated for over a week, and Karamea for longer still.

Grey District would suffer greater shaking than the Buller, and there may be more likelihood of structural damage to buildings, especially the larger commercial ones. The Coast Road to Westport would have numerous landslips that would isolate communities such as Barrytown for over a week. The Grey Valley road may well suffer damage to bridges, but the landslide risk is less significant until Victoria Saddle, meaning that access to Reefton may not be too difficult to reinstate. However, the road does run close to the Grey River at many points and may well slip. Local roads nearer to the fault line at Lake Brunner would be extensively damaged and would be unlikely to be a priority for repair.

This was the scene captured in Kobe in the large earthquake (M7.2) of 1995. It offered little hope of survival.

Westland District would be the hardest hit. All transport links would be likely to fail except possibly the airport, which is build on the most solid ground around. The road to Greymouth may be open to high clearance 4WD vehicles after a few days, but all other roads would be impassable for an extended period. Arthur’s Pass would remain closed for up to 7 months, with the road travelling along the fault line for 10km and then heading through extensive landslip terrain, making it difficult to clear, especially with a risk of aftershocks. Access further south would be complicated by the number of bridges that are likely to be damaged. Harihari may remain isolated by road for 2 months, and Franz-Josef Glacier for 3 months. Harihari may have air support if power poles are removed from the main street, but Franz-Josef will only be accessible by helicopter. The Alpine Fault crosses the road between Franz-Josef Glacier and Fox Glacier many times and the nature of the terrain could make it almost impossible to re-establish the road on its current course. This would be the last transport link to be re-established. Fox Glacier access would be from Haast and the Haast Pass, which may be open after several months, with delays likely to repair the road near Knight’s Point because heavy machinery will be needed elsewhere. Damage to buildings would be significant in Hokitika and increasing to severe in Franz-Josef, which would likely experience the most severe shaking of any significant settlement.

Christchurch could be badly effected because of the soft sandy soils that most of the city is build upon. Liquifaction (which will also occur on the West Coast), in which water escaping from the soil under the action of shaking allows things to sink into it, may create significant damage to buildings and roads. The problem for the West Coast is that this may well divert key resources because of the greater population need.

All districts are likely to be without power for at least 3 days, with our main power lines crossing the Alpine Fault. Some small, local generation facilities may allow lesser supplies in some areas reasonably quickly after a few days, but priorities for supply will rest with essential services. Some communities in South Westland may not see power restored for several months. This will impact on many sewerage schemes and on water drainage in the main towns, where ponding may occur. All telecommunications will cease to operate except for those with battery or generator operated HF and VHF transmitters, and these will be needed for emergency services. Some local telephone services may be restored after a few days. Water supplies will fail as ground shaking breaks the supply lines. As we head south through the region, the restoration times will generally increase.

This was the scene captured in Kobe in the large earthquake (M7.2) of 1995. It offered little hope of survival.

The consequences to the West Coast seem very dramatic and challenging, but we are an adaptive community with a history of overcoming adversity. Our climate reinforces the need for flexibility and the ability to cope. With an earthquake of the this magnitude, individual preparation will help to prevent injury and possibly death as well as allow some degree of comfort after the event. Remember that in most earthquakes, most people survive! Our building code is amongst the best in the world for coping with large earthquakes, and that is not accidental – we live on a very geologically active land.

A photo taken at the same time showing the collapsed freeway gives a different picture. Here it is clear that most buildings survived the earthquake. Many of the injuries that were sustained in the strong ground shaking were avoidable if people had prepared their homes properly. We have that opportunity… Prepare Now!

But wait, there is more! Not all the nasties come in the initial shaking; some are delivered weeks and even years later. Rivers get dammed by landslides, and some of the dams breach rapidly after some delay and cause disastrous floods. Landslides in the mountains may cause river beds to build up over years, causing floods more frequently, or drastically changing river courses. And, if the whole fault length does not rupture at once, another bit may rupture soon after.

An Alpine Fault earthquake might not occur for another hundred years, but it is much more likely to happen tomorrow. Make sure that it is a lasting experience, by preparing today to survive it, earthquakes much better savoured in retrospect, than during the event.