September 2007 Sumatran earthquakes research findings

Three great earthquakes and destructive tsunamis over the past four years is not enough to spare the region of another large earthquake, warns an international group of earthquake researchers in their paper published in the Dec. 4 issue of the journal Nature. The first of the recent great earthquakes, a magnitude 9.2 in December 2004, produced the most widespread and destructive tsunami in living memory. The cause of that calamity was the rupture of a 1600-km-long piece of the Sunda megathrust, a 6,000-km-long active fault that dives gently landward from the seafloor a couple hundred kilometers offshore of Myanmar, Sumatra, Java, and Bali. The section that broke in 2004 is a northern portion, between southernmost Myanmar and Aceh province in Sumatra.

Only three months later, the next section to the south, offshore Aceh and North Sumatra, also ruptured suddenly. That 350-km portion, just north of the Equator, produced a magnitude 8.7 earthquake and a more modest tsunami.

Says Professor Kerry Sieh, one of the paper's authors and Director of the Earth Observatory of Singapore, Nanyang Technological University, "Before the 2007 earthquakes, we knew that a big section still farther south had not broken for the last two centuries. There was great concern that this patch would break. What we learned in this study is that only part of it did during the 2007 earthquakes."

After the 2004 and 2005 earthquakes, the only large, unbroken section of the megathrust off the coast of Sumatra was a 700-km portion south of the Equator, beneath the Mentawai Islands, a chain of large islands offshore from the major cities of Bengkulu and Padang.

"The great fault that runs along the eastern flank of the Indian Ocean has been the world's most energetic fault zone lately," says the paper's co- author, Danny Hilman Natawidjaja, of the Indonesian Institute of Science. "Nonetheless," he says, "one large section offshore Sumatra has still not broken."

This was known because Natawidjaja and his colleagues had used old corals littering the Mentawai islands' coastlines to determine that this long section had not suffered large earthquakes since an 8.8 in 1797 and a 9.0 in 1833. They had also shown from growth patterns in modern corals and a network of GPS instruments called the Sumatran GPS Array (SuGAr) that this section was continuing to accumulate strains that would eventually produce a large earthquake.

Thus it came as no surprise when on 12 September 2007 the Mentawai section produced two big earthquakes just twelve hours apart: first a magnitude 8.4 quake and then a magnitude 7.9. Both earthquakes were felt widely in the region; even many Singaporeans, hundreds of kilometers away from western Sumatra, felt them, albeit mildly. Reports from the west coast of Sumatra told of very heavy shaking and frightening tsunamis.

Understanding the details of the 2007 events was very important: Did the entire Mentawai section break, thus relieving most or all of the strains that had been accumulating since the early 1800s? Or did only a portion go? Prof Sieh, who was visiting Singapore at the time of the earthquakes worked with Natawidjaja to organize a whirlwind trip to the Mentawai islands, to measure anticipated uplift of the coral reefs and to download GPS data from SuGAr stations whose telemetry had malfunctioned. Meanwhile others retrieved additional SuGAr data from the region by telemetry.

In the work described in Nature, the scientists analysed the coral and GPS data together with seismological records and remote sensing (inSAR) data to determine the details of the 2007 breaks.

What they found is that only one 200-km-long portion and another much smaller portion had broken. Not nearly enough. "In fact," says Ozgun Konca, a Caltech graduate student and the paper's first author, "we saw release of only a quarter of the moment needed to make up for the accumulated deficit over the past two centuries." (Moment is a measure of earthquake size that takes into account how much the fault slips, and over how much area.)

"The 2007 quakes occurred in the right place," adds Jean-Philippe Avouac, one of the paper's authors and Director of Caltech's Tectonics Observatory. "They were not a surprise. What was a surprise was that those earthquakes were way smaller than we expected."

A long stretch of the Mentawai section remains unbroken since 1797. "What we are now trying to understand," says Prof Sieh, "is how close it is to failing."

Source: Nanyang Technological University