Bracing for the next killer wave

The Star, 22 Dec 2014

Geologists are striving to provide sound science so policy makers and people in vulnerable areas can make the right decisions.

A GEOLOGICAL time bomb is ticking in Indonesia, and the city of Padang on Sumatra’s western coast is sitting smack on top of it.

In the last decade, five major quakes including the one that triggered the 2004 tsunami, have struck the city of 1 million, giving it the dubious distinction of being the current earthquake capital of the world.

Indeed, the level of seismic activity in western Sumatra is second to none.

While they may quibble about the details, experts at the Earth Observatory of Singapore believe that the next big one, which has been building up since 2000, will come in mere decades, the blink of an eye in a field where scientists more commonly study patterns in land and rock formations spanning millions of years.

This forecast is based on the work of the Earth Observatory’s director, geologist Kerry Sieh, and was first made nearly six years ago.

“Corals on the reefs of west Sumatra record in their annual growth layers patterns of large earthquakes,” he says.

“These patterns repeat about every two centuries, which is about the time since the last set of great earthquakes and tsunamis there.”

A giant 8.8 magnitude earthquake and tsunami similar to, but perhaps a bit smaller than, the one that caused the waves of destruction 10 years ago will strike within a few decades, he believes.

The quake itself will damage or destroy many existing buildings and bridges, and the resulting tsunami will reach the shores of the Mentawai Islands within five to 10 minutes.

And in 20 to 30 minutes, it will hit the mainland of the west Sumatran coast, including Padang.

“Scientists can’t predict the exact day, month, or year, but it will happen in the lifetime of the young people living there,” he says.

Massive earthquakes like this occur only in subduction zones where two of the tectonic plates that comprise the earth’s surface are converging, with one plate diving beneath another.

The Dec 26, 2004 earthquake happened because of the rupture of the Sunda Megathrust, which is the fault plane along which the Indian and and Australian plates slide beneath the Sunda plate and Sumatra.

Uplift of the seafloor, caused by the rupture of the megathrust further north, displaced the seawater above to start the worst tsunami on record.

The water spreading out from the fault reached speeds of up to 800kph and heights of 15m, slamming into 13 countries, with Indonesia the closest and worst hit.

About 250,000 people died in the disaster, and two million lost their homes.

Because the fault line from the Indian Ocean tsunami ran generally north to south, waves radiated out in a mainly east and west direction, and places to the west and east were hit hardest.

Singapore was spared because it is sheltered by the surrounding land masses. In addition, the shallow waters in the Malacca Straits and South China Sea – unlike the deep waters in the Indian and Pacific oceans – dampened the fury of the waves, and dissipated their energy. Australia, lying south of the original fault line, was also spared major damage.

Prof Sieh and his colleagues from Nanyang Technological University and Indonesia are now eyeing a 400km section further south, beneath the Mentawai Islands west of Sumatra. It is part of the fault line that makes up the 5,500km Sunda megathrust.

Called the Mentawai patch, it has remained intact for nearly 200 years, and is under tremendous and increasing stress.

“Large earthquakes on long, locked fault zones commonly trigger one another, and hence cluster together in time,” Prof Sieh says.

Dr Sylvain Barbot, a principal investigator at the earth observatory who is working on the subject, adds: “In the Mentawai area, we have seen great earthquakes occur to the north and to the south.

“We don’t know if the fault will generate two great (earthquakes) or one giant earthquake, but the fault must move somehow to catch up with its neighbours.”

Another expert on Sumatra earthquakes, Dr John McCloskey, professor of geophysics at Britain’s University of Ulster, likens the current build-up to the drawing of a bow.

“Off western Sumatra the bow is drawn tight,” he said back in 2010.

“The last shock happened more than 200 years ago and the stresses are probably larger now than they were then; the earthquake must happen soon.”

Prof Sieh adds that the intensity of the quakes in the last few years has been unprecedented, and no one can tell when the current cluster will end, although the countdown to failure has begun.

To forecast earthquakes and tsunamis more reliably, Prof Sieh and fellow researchers at the earth observatory use a Global Positioning System to continuously collect, process, analyse and archive data on tectonic plate movements in the region.

They also examine sand deposits and corals to work out when the last big quakes occurred.

And in recent work in a cave in west Sumatra, strata of sand and other deposits swept in from tsunamis that occurred over the past 7,500 years have unveiled the longest and most detailed record yet of the natural disasters that have hit the Aceh coast.

Radiocarbon analysis of materials such as charcoal fragments, clamshells and remains of microscopic organisms, unearthed evidence of 10 tsunamis before the one in 2004, and the scientists now know that tsunamis may cluster in time.

Prof Sieh says his role is to provide sound science so that policy makers and people at risk can make the right decisions, and he and his team have been involved in public education and outreach efforts for years.

His advice to the residents of Padang and Western Sumatra: “In most areas, there is not enough time to wait for official warnings or to see receding water or the tsunami itself.

“People living near the coast should evacuate to high ground immediately after feeling an earthquake that is strong or lasts longer than one minute.” – The Straits Times, Singapore

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