Page last updated at 12:59 GMT, Thursday, 1 October 2009 13:59 UK

Padang lives with quake stress

By Jonathan Amos
Science reporter, BBC News

Map showing major ruptures

The large quakes that hit the Sumatran region on Wednesday and Thursday were expected by scientists.

They were not able to say precisely where and when these tremors would strike, but their understanding of what triggers such events has increased immeasurably over time.

This has allowed researchers to make informed statements about the probable evolution of fault systems - to say with some confidence that certain locations are more likely to experience a major seismic occurrence.

And so it proved with Sumatra. Scientists have been expecting large quakes to occur south of the region affected by the great 26 December 2004 tremor.

That Magnitude 9.3 quake stemmed from a rupture along the line where the Indian/Australian and Eurasian tectonic plates grind over each other.

The associated tsunami wrought destruction throughout the Bay of Bengal, from Northern Sumatra to Thailand, Sri Lanka, and India.

It was then followed by a Magnitude 8.7 in March 2005 - with the rupture occurring further south along the plate boundary.

What was clear to scientists at the time was that there was still huge strain bound up in the fault; and that if it let go, the at-risk areas were further south still.

This was evident, for example, at GPS stations in the region which trace the millimetric movement of island coastlines.

The data demonstrated that some of the shorelines were becoming submerged. The base of trees at the water's edge were being lapped in a way they were not in previous years.

This is a tell-tale sign that the leading edge of an overriding plate is sticking to, and being pulled down slightly by, the colliding plate which is diving (subducting) into the Earth.

The sudden release of this stickiness is what triggers earthquakes; and if these are shallow enough to occur close to the sea-bed and deform it, they can initiate a tsunami.

It was no surprise to scientists then when a major quake occurred in southern Sumatra in 2007. A Magnitude 8.4 event was closely followed by a 7.9 and a 7.1.

What was interesting, though, was that this rupture was a good deal further south, offshore of the city of Bengkulu. It left perhaps a 300km segment largely untouched - a gap, if you like, near Padang.

This raises the question of whether Wednesday's 7.6 quake (and the series of aftershocks) near the city will have in any way released some of the energy that is pent up in this gap. Scientists will need to do their analysis but experts who know the area well suspect it will not have made a major difference.

"We know a lot about earthquakes in this area," said Professor John McCloskey, of University of Ulster, at Coleraine, UK.

"We know for sure that the area under the island Siberut which is in the middle of this gap hasn't slipped in a big earthquake for over 200 years. The last earthquake was in 1797.

"We know how quickly the plates are converging so we can calculate how much stress has been accumulated there; and we know from those calculations that the amount of stress that is there now is more than was there when the last earthquake happened."

A Siberut earthquake of historic size, of M8.5 say, would be capable of sending a tsunami towards Padang with wave heights in excess of five metres.

The short distance between the city and Siberut means the waves would take just 30 minutes to arrive from the onset of the quake.

None of this, of course, gives any comfort to the people of Padang who are having to deal with a tragedy today. Quite the opposite.

But work continues, undertaken by NGOs such as Kogami, to raise awareness and preparedness for earthquakes and tsunami in the region.

Jonathan.Amos-INTERNET@bbc.co.uk



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