Why earthquake did not cause a tsunami

Glenda Kwek April 12, 2012

Panic ... Acehnese women hug each other and pray shortly after the earthquake struck.

Panic ... Acehnese women hug each other and pray shortly after the earthquake struck. Photo: AFP

At a magnitude of 8.6, yesterday's earthquake off the Indonesian province of Aceh was one of the largest ever recorded.

Yet the massive tremor, which was followed by an 8.2-magnitude aftershock, did not cause a severe tsunami such as the one on December 26, 2004, which devastated countries around the Indian Ocean and killed more than 200,000.

So what is the difference?

Seismologists said it was all about the horizontal and vertical movements of the sea floors and where the quake took place.

Earthquakes usually encompass three types of motions - normal faulting, reverse faulting and strike strip faulting, Wayne Peck, a senior seismologist at the Seismology Research Centre in Melbourne, said.

Yesterday's quake was a strike strip, caused by a horizontal movement of the sea bed at a fault in the Indo-Australian plate, while the 2004 tsunami, sparked by a 9.1-magnitude tremor, was generated by a vertical movement that displaced water, he said.

"If you imagine you are standing in a perfectly circular pool with a flat floor, to generate a really large earthquake the best way is to have one half of it pop up vertically by six inches while the other half remains stationary. That would generate a large wave in the pool," Mr Peck said.

"If you have a floor that tears right up in the middle and your right foot is on one side of the tear and your left foot is on the other side of the tear, and your feet move so that you end up with your right foot in front of your left foot with no vertical difference, then there's not going to be any wave generated by that motion."

The quake also took place within a plate, rather than at the boundary of two plates, senior research seismologist Gary Gibson of the University of Melbourne said.

"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," Dr Gibson told ABC Radio 702.

But its size took seismologists by surprise, he added.

"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."

While a recent spate of strong tremors - the 8.8-magnitude quake in Chile and the 7.0-magnitude quake in Haiti in 2010, and the 9.0-magnitude Japan quake and the 6.3-magnitude Christchurch quake last year - appeared to suggest that there have been an increase in earthquake activity, researchers said data showed there was no change.

A study by Peter Shearer and Philip Stark of the University of California last year found there was "little evidence that the global threat of earthquake occurrence has changed in areas far from recent activity".

But they added that the current threat of large earthquakes was "above its long-term average in regions like Sumatra, Chile, and Japan".

Mr Peck said other factors, such as the exponential growth of the human population and the improved recording of tremors, also played a part.

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