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?

1 thought on “The Hikurangi Trench: New Zealand’s biggest tsunami hazard

  1. As you so rightly state in your post, after a 8.9 quake, it won’t be as simple as grabbing your go bag and heading for the hills. That size quake will flatten most buildings, so extricating ones self from the rubble will, even in the best instance, eat up a lot of the 20 minutes before the Tsunami hits. I think that unless they live very close to the hill, the people of Napier will become victim statistics.

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