01-Oct-2009

SINGAPORE - The 7.6-magnitude earthquake that struck Padang the evening of Sept. 30 originated near a long undersea fault line, running parallel to Sumatra, that scientists have said is due to produce a more severe quake within the next few decades.

Sections of that fault, called the Sunda megathrust, have ruptured a number of times over the past decade, causing a series of earthquakes in the Sumatra area. Based on historical geological patterns, scientists believe this sequence of earthquakes will eventually culminate in a much larger quake, possibly on the scale 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.”

The Padang earthquake has not changed that forecast. Initial seismic data indicate that it resulted not from a rupture on the Sunda megathrust, but on a fault deeper below. More details will become known once researchers have collected and analyzed additional data.

What caused the Padang earthquake?
The Padang earthquake, like most earthquakes in Sumatra, occurred as a result of movements by undersea tectonic plates, massive slabs of the earth’s rocky crust that collide and slip past one another. In the Sumatra area the plates meet in a “subduction” zone, where the boundaries of one plate are forced beneath the other plate. The Indian and Australian plates are sliding northeastward (at about 7 cm per year) and dipping under the Sunda plate, which encompasses Sumatra and Singapore. The Sunda megathrust is the fault line along which the plates join, with the upper rocks thrusting upward in a gentle slope above the lower rocks. 

Until the “subducting” oceanic plates drop deeper into the earth to about 30 km beneath Sumatra, the megathrust resists slippage. Edges of the plates become coupled, or locked together. Tremendous strain builds up, over many decades or even longer than a century, until a section of the megathrust gives way. This rupture causes the oceanic plates beneath Sumatra to lurch forward suddenly, by many meters, in a big earthquake. This is what happened in the earthquakes and related tsunamis of December 2004, March 2005 and Sept 2007.

In contrast, initial analysis by the U.S. Geological Survey suggests that the Sept. 30 earthquake offshore of Padang was caused by a sudden rupture within the descending Indian-Australian plate, about 80 km beneath the earth’s surface. A follow-up earthquake on Oct. 1 (magnitude 6.6) also did not occur on the Sunda megathrust; it occurred instead on the Sumatran fault, which extends from near Krakatoa in the south to Banda Aceh in the north.

Why was there no tsunami this time?
Tsunamis accompany earthquakes when a subsea fault is close enough to the sea floor that the seabed is suddenly deformed and the displaced water rushes onto land. The Padang earthquake did not trigger a tsunami because it originated too deep below the seabed, about 80 km down. By comparison, the megathrust ruptures that caused the Sumatran earthquakes and tsunamis in 2004, 2005 and 2007 were zero to 30 km below the seafloor.

Why have there been so many earthquakes in Sumatra recently?
The recent flurry of large earthquakes in Sumatra began in 2000. Evidence from growth patterns in corals along the Mentawai Islands (which record sea-level changes over hundreds of years) and GPS data indicates that over the past 700 years, Sumatra has been hit about every 200 years or so with a sequence of large earthquakes caused by ruptures in a long stretch of the Sunda megathrust. This happened in the late 1300s, in about 1600 and again in the early 1800s. The cluster of Sumatran earthquakes over the past decade is part of a new cycle that will probably be completed within the next several decades with the occurrence of a major earthquake, caused by the failure of an as yet unruptured section of the Sunda megathrust. 

How does the Padang earthquake compare to other recent Sumatran earthquakes?
Seismologists at the U.S. Geological Survey found that the Padang earthquake registered 7.6 in magnitude. (Note: Scientists now use a different measure than the Richter scale, because that scale was designed to measure smaller quakes, generally less than magnitude 7.) That was strong enough for the shocks to be felt as far away as Singapore and Malaysia. Two earthquakes that occurred farther south in Sumatra in September 2007 were quite a bit larger, of magnitude 8.4 and 7.8; they were farther from Padang, however, and caused less damage there. The tsunami that swept across the Indian Ocean in December 2004, killing about 230,000 people and leaving some 2.1 million homeless, was triggered by an undersea earthquake of magnitude 9.15.

Is it reasonable to expect this quake to be followed by an even bigger one?
While the Padang earthquake appears to be something of an anomaly in relation to the recent megathrust-related earthquakes in Sumatra, there’s still a strong likelihood that a much bigger quake will hit the area – though perhaps not for some time. The findings of a recent study concluded that an earthquake as great as 8.8 in magnitude can be expected within the next 30 years, caused by a rupture in a 400-km (250-mile) stretch of the Sunda megathrust that lies beneath the Mentawai Islands west of Sumatra. One concern about the Padang earthquake is whether it might weaken the megathrust fault below Siberut Island and contribute to a rupture. Overall, the steady stream of earthquake activity in the region is an indication that people need to plan ahead and be prepared.

Will aftershocks from this earthquake be a problem?
Many aftershocks already have occurred, and more can be expected in the days and months ahead. Most aftershocks are at least two degrees of magnitude smaller that the main shock, like the first reported aftershock, which was 5.5 in magnitude. The 6.6-magnitude earthquake on Oct. 1 appears to not be an aftershock, strictly speaking. It occurred more than 200 km away from the Sept. 30 earthquake rupture, on a section of the Sumatran fault. That fault produced many large (magnitude 7 to 7.5) earthquakes between 1892 and 1953, but has been largely quiet for the past half-century.

See also: More maps on the Sumatran Earthquakes