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Quake may hit trench, says expert

By Patrick O' Sullivan


Less than 100km off the coast of Hawke's Bay is a deep-water trench that could be the site of a potential megathrust earthquake similar to the 2011 Japan earthquake, says seismologist Kevin Furlong.

Despite the Hikurangi Trench's potential, he said very little was known about the underwater valley, where the Pacific plate was dragged underneath the Australian plate.

Professor Furlong, of Pennsylvania State University, said the worst-case scenario for the East Coast was not yet known.

"Many, if not most, scientists working on these megathrust earthquake plate boundaries would argue that, although it is very, very unlikely, until we can demonstrate otherwise we should expect that major segments of these boundaries could rupture simultaneously.

"Most of the time, as was the case in Japan for the past several hundred years at least, segments rupture individually and so maximum earthquakes are in the mid-to high magnitude 7 range. But on rare occasions, such as in 2011 in Japan, bigger ruptures can occur.

"We need to decide how best to manage that potential and uncertainty."

The trench will soon be part of a global study into megathrust earthquakes.

"Although we understand the general concept and general physics of megathrusts - the big subduction zone earthquakes - we are finding in our data from recent major events such as in Sumatra [Boxing Day 2004], Chile in 2010, and most recently in Japan, that they each have characteristics that differ from each other, and our existing models of how we might think they should behave during the actual earthquake rupture are incomplete," he said.

"So we need to improve our understanding, to understand what is causing this variability and whether we can anticipate it in advance. Part of the reason for this is that, during the main era over which our understanding of subduction zone earthquakes was developed from the mid-1960s until the 1990s, as the theories of plate tectonics were developed, we didn't have any earthquakes of the size of these recent big ones. So although our understanding worked well for the slightly smaller events, it isn't adequate for the really big ones."

He said regular quakes felt on the East Coast were related to the Hikurangi subduction zone and plate boundary.

"Some are in the upper plate - the North Island - and some are on the boundary between the upper plate and the subducting Pacific plate. They all provide information about how the plate boundary is locked and how the different components are deforming. But unfortunately these earthquakes don't help release the stress that is building up. That will only be released by big earthquakes but perhaps not for hundreds of years.

"In our research, one of the things we do investigate is what spatial patterns there are to these smaller earthquakes, as we can use that information to estimate the way in which the plate boundary is locked."

The 2011 Japan megathrust earthquake measured a magnitude 9 and triggered a tsunami that reached heights of up to 40.5 metres in constricted places, travelling up to 10km inland. There were 15,878 deaths and 2713 people reported missing.

- HAWKES BAY TODAY

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