The magnitude 7.7 earthquake on October 25th west of South Pagai, a small island off the west coast of Sumatra, was a large aftershock of the magnitude 8.4 Sumatran earthquake of September 12, 2007. Based upon analysis by the US Geological Survey, it is likely that the aftershock was caused by rupture of a small patch of the Sunda megathrust (see map below), next to a bigger undersea fault line running parallel to Sumatra, that scientists have said is due to produce a more severe quake within the next few decades.

The 2007 8.4 earthquake also had several aftershocks in the hours and months following. These included a magnitude 7.9 earthquake 12 hours after, a 7.0 earthquake 15 hours after, and a 7.0 earthquake 5 months later.  Each of these aftershocks occurred around the perimeter of the large patch on the megathrust that broke during the 8.4 earthquake (see map).

The Sumatran GPS Array, a network of GPS instruments run collaboratively by the Earth Observatory of Singapore (EOS) and the Indonesian Institute of Science (LIPI) in Bandung continuously monitors tectonic movement along the west Sumatran coast and the Mentawai islands offshore. “Over the next few months, we will be analyzing the data from these instruments to gain a better understanding of yesterday’s earthquake,” said Prof Kerry Sieh, EOS Director.

According to LIPI earthquake geologist Danny Hilman Natawidjaja, “yesterday’s earthquake was clearly part of an ongoing healing process following the September 2007 magnitude 8.4.”  “What is less clear is whether or not it is part of a wounding process that will lead up to the rupture of the remaining large patch of the Sunda megathrust farther north,” he added.
 
Several sections of the megathrust have ruptured sequentially over the past decade, causing a series of earthquakes along the western coast of Sumatra. Based on patterns of large earthquakes over the past seven centuries, scientists believe the current sequence of earthquakes will eventually culminate in a much larger quake, approaching the size of the one that caused the Indian Ocean tsunami in 2004. Exactly when that might happen isn’t clear. As they’ve said previously, it could occur “in 30 minutes or in 30 years.”

Yesterday’s earthquake did nothing to alleviate the possibility of this larger anticipated magnitude 8.8, but might indeed bring it closer to failure.

Scientific research does not allow us to predict whether an earthquake will happen in Sumatra on a specific day, month, or year.  Any such predictions are fake and do not come from reputable scientists.

Our research does show that a great earthquake and its tsunami are very likely sometime in the next few decades in the area between the Batu and Pagai Islands and along the neighbouring mainland West Sumatran coast.  However, we cannot say exactly when within the next few decades this might occur.  This means that young people living in this region today are very likely to experience this great earthquake and its tsunami within their lifetimes.  There are many actions that can be taken in the time between now and then to lessen the loss of life, livelihood and property.

Penelitian ilmiah tidak atau belum memungkinkan kita untuk mem-prediksi akan terjadinya gempabumi di Sumatra pada hari, bulan dan tahun  tertentu. Apabila mendengar ada ramalan atau prediksi tentang akan terjadinya gempabumi pada waktu tertentu, hal tersebut adalah tidak benar, dan pasti tidak berasal dari ahli yang berkompeten.

Penelitian kita menunjukkan bahwa gempabumi and tsunami-nya akan terjadi di suatu saat dalam kurun waktu beberapa dekade ke depan di wilayah antara kepulauan Batu dan Pagai dan sepanjang pantai di pinggiran Sumatra-Barat yang ada di dekatnya. Namun demikian, kita tidak dapat menentukan kapan waktu tepatnya di beberapa dekade ke depan tersebut gempa akan terjadi. Hal ini bermakna bahwa generasi muda yang ada saat ini di wilayah tersebut berkemungkinan besar untuk mengalami gempabumi tersebut di masa kehidupannya. Banyak tindakan yang dapat dilakukan di waktu ini sampai yang akan datang untuk mengurangi kerugian jiwa, dan harta benda.

Additional Media Coverage can be found here.