Tsunami watch: Evaluating the threat

Sketchy data in the early stages of last night's earthquake in Chile made it difficult for Geonet scientists to gauge the tsunami threat to New Zealand. Photo / Michael Cunningham

Scientists last night were working with minimal data as they tried to evaluate any potential tsunami threat to New Zealand after an 8.8 quake in Chile.

For hours last night, Ken Gledhill, Geonet project director says, the initial advice was that there was none.

"The perception of the earthquake moved around a bit before it settled down. That's why at the start we said there was no threat to New Zealand. It was at 8.5 (on the Richter Scale), then it dropped to 8.3, then it went up to 8.8, and in the meantime the estimates of its depth were getting shallower, and then it was confirmed as offshore so it was going to lift the sea floor, and that's the key thing.

"For that period at the beginning we were saying this earthquake wasn't big enough. That was the best information we had and that is not unusual with trans-pacific tsunamis. They generate very long waves in the Earth and you have to sum those up for a long time to get a really good look at the size and that process is not straightforward.

"So the magnitude will often start small and then go up. What fooled us this time was that it went down and then went up again."

Between 9pm and 10pm last night GNS decided that the tsunami could pose a serious threat.

"Once the size of the force, which is really the big unknown, once that was known, and that it was a thrust event which pushes the seabed up, we had enough information to put a handle on it and say, 'this is probably what is going to happen'.

"This was a very big earthquake, probably the fifth largest since 1900. It filled a gap between earthquakes in 1960 and 1877. This was the stress that was building up in this part of the subduction zone. It sits right in that gap, if you like.

"It will relieve the pressure, but the problem is it could put pressure somewhere else."

When asked if that could pose a more serious threat to New Zealand, he said: "We are too far away. The effect is the tsunami, it's on the [Pacific] Ring of Fire, which we are a part of, but it's not frequently going to affect us."

He said the GNS team have to make very broad estimates from very little data at first.

"It's under sea and it lifts the seabed, and we model that, and then model as the wave travels all the way across the pacific as it arrives here. There are ways of short-cutting that process, and if you do it as high resolution right from the word go, that will take a day or so

"We have pre-calculated models, we could try and find an event close to this one that we've already modelled, but there is nothing like the hard yards.

"Ideally you have the seismic information and the sea level information near where the earthquake happened. We have things called dart buoys in the deep ocean. They don't have a big signal, 20cms to 30cms or even less, but we use that to give us an idea to callobrate the models. That process - you can do it with a broad brush quickly, or more accurately and slowly, and it's that tradeoff. You'd usually do a more broad brush look, and then refine it with time.

"We were using estimates based on the models. But there's very little land mass between here and South America. There are two dart buoys, there's de-motion buoys and there is Easter Island, and then you're almost to the Chatams before you have any other information. Most of the energy was actually heading towards Japan. We've been side-swiped basically, and you have to estimate what that side-swipe is going to be, which is harder than being right down the barrel, but it's also kind of better [in terms of danger]."

He said as soon as the event takes place, GNS will discuss with Civil Defence about the seriousness of the threat.

"Last night it was complex because it started off being a smaller event, potentially on land rather than offshore. It was not clear that it was going to be a threat to New Zealand for some time.

"If the wave is at the 1m level it's serious if you're at the beach. It's probably not serious if you're in most places unless you're in a place where there is some kind of channel where that energy runs up further and causes some kind of damage. Three metres is quite serious."

- NZ HERALD STAFF