Why was there was no tsunami this time?

By Sylvia Varnham O'Regan 12 April 2012
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Though the Indonesian earthquake on Wednesday was huge, it didn’t generate much of a tsunami. Why?

AFTER A MAGNITUDE 9.1 earthquake struck off the coat of Sumatra, Indonesia at 7:58am on December 26, 2004, locals thought the worst was over. Unlike the Pacific Ocean – where tsunamis were common and a warning system had long been in place – the Indian Ocean rarely experienced tsunamis and had no warning system. 

The series of towering waves that crashed onto the shores of Indonesia, Thailand, Sri Lanka and several other countries only hours after the earthquake were not only unexpected, but were fatal on a massive level –  about 230,000 people reportedly died. 

On Wednesday, 11 April, the coast of Sumatra was rocked again, this time by a magnitude 8.6 quake and equally large aftershocks. A tsunami warning was issued and locals scrambled to reach higher ground, fearing the worst. But the waves that followed only reached 80cm at the highest and the warning ceased, leaving many to wonder why this tsunami was so small compared to the one in 2004.  

The trouble with predicting tsunamis

Tsunami prediction is “very difficult” says James Goff, co-director of the Australian Tsunami Research Centre and Natural Hazards Research Centre at UNSW.

“A tsunami has to be caused by something. They are usually caused by earthquakes, but we can’t predict earthquakes. Then we can’t necessarily predict that an earthquake will generate a tsunami. So it’s a double-barrelled problem,” he says.

The reason this most recent Indonesian earthquake didn’t cause a devastating tsunami like that in 2004 was that it was generated by movement of a ‘strike-slip’ fault, where two tectonic plates grind past each other horizontally, releasing energy, but causing little displacement of water – and therefore only barely detectible tsunami waves. 

The 2004 earthquake, on the other hand, resulted from a thrust fault, where one tectonic plate is forced under the other, displacing the plate vertically.

Recent Indonesian earthquake in a different location

And the quakes’ differences do not end there. While the earthquake on Wednesday occurred off the coast of Banda Aceh, in Northern Sumatra, which was hit hard in 2004, it was located quite a distance away from the ‘subduction zone’ (where the tectonic plates meet) at the epicentre of the 2004 quake and on a different fault line, James says.

The 2004 Indonesian earthquake, while similarly shallow at 32km below the surface, displaced about a 1300km-long section at the boundary of two tectonic plates, by about 10m vertically – a huge mass that transferred its enormous energy into deadly waves. 

“This time, we were lucky. It didn’t happen on the main plate boundary on the Australia-India plate and the Asian plate, like the Boxing Day one did. It happened on a major crack in the Australia-India plate, well to the south-west of Sumatra,” seismologist Gary Gibson from the University of Melbourne told ABC Radio 702.

“This is the largest earthquake that hasn’t happened on one of the main plate boundaries ever. The previous largest was between Australia and New Zealand 15 years ago and back in 1957 in Mongolia,” he said.

Tsunamis largely unknown

Although much is known about earthquakes, there is a lot we don’t know about the tsunamis they generate. 

Clive Collins, senior seismologist at Geoscience Australia, says that there is no way of really detecting a tsnuami until it’s too late as they’re barely visible in the open ocean depths. 

Warning systems are based on the characteristics of the earthquake – gained from data from seismographs from around the world – and kick into motion when certain conditions are met – such as the magnitude of the earthquake, whether it’s under the ocean and how deep the epicentre is. But it takes time for the data to be analysed – about 15-20 minutes before a government is given the warning. 

They also draw on earthquake patterns and data, Clive says. “You can look at the historical records and see where earthquakes occurred and how often they occurred and make predictions from that.”

About 85 per cent of all historical tsunamis have occurred in the Pacific and the majority of those were caused by shallow earthquakes – at a depth of 70km or less – usually magnitude 6.5-6.8 and above.

They can also be caused by landslides, asteroids and volcanoes.