Warning of bigger tremor to come

01-Oct-2009

Padang Earthquake Struck at Edge of Zone
Where Much Bigger Quake Is Expected

SINGAPORE -- The 7.6-magnitude earthquake that struck Padang the evening of Sept. 30 was caused by the rupture of a fault that lies close to a larger undersea fault, running parallel to Sumatra, that scientists have said is due to produce a more severe quake within the next few decades.

Despite this latest quake, scientists are not narrowing their forecast of that future large earthquake; as they have said previously, it could occur “in 30 minutes or in 30 years.”

What caused the Padang earthquake?
This and most other earthquakes in Sumatra occur as a result of collisions between undersea tectonic plates, or large slabs of the earth’s crust that move slowly about the surface. In that area, Indian and Australian plates are sliding northeastward (at about 7 cm per year) under the Sunda plate, which includes Sumatra and Singapore. This dipping of one slab beneath another is called “subduction,” and the gently sloping upward thrust that occurs where they join is called the Sunda megathrust.

Until the “subducting” oceanic plates drop to about 30 km beneath Sumatra, the megathrust resists slippage and the plates becomes coupled, or locked together. As a result, strain accumulates over many decades and even more than a century, until the megathrust gives way. When that happens, the oceanic plate lurches suddenly many meters northeastward beneath Sumatra, and a big earthquake occurs. That’s what happened in the earthquakes and related tsunamis of December 2004, March 2005 and Sept 2007.

In contrast, however, initial analyses by the U.S. Geological Survey suggests that the Sept. 30 earthquake was caused by a sudden rupture within the descending Indian and Australian plates, about 80 km beneath the earth’s surface. A follow-up earthquake on Oct. 1 (magnitude 6.6) also did not occur on the megathrust. Instead, it occurred on the Sumatran fault.

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 sea, about 80 km down. By comparison, the great megathrust ruptures that caused the great Sumatran earthquakes and tsunamis in 2004, 2005 and 2007 were only 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 GPS instruments and from growth markers on ancient corals along the coast of Sumatra indicate that a series of large megathrust earthquakes strikes western Sumatra about every two centuries. Such flurries occurred in the late 1300s, in about 1600 and again in the early 1800s. The cluster of Sumatran earthquakes over the past decade heralds the beginning of a new flurry. The Sept. 30 earthquake appears to be just one in this new series that will culminate in an earthquake that could be as large as magnitude 8.8.

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. The two earthquakes that occurred farther south in Sumatra in September 2007 were quite a bit larger, of magnitude 8.4 and 7.8. However, they were farther from Padang and caused less damage there. The great 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 magnitude 9.15 earthquake.

Is it reasonable to expect this quake to be followed by an even bigger one?
The Sept. 30 earthquake appears to be part of the ongoing sequence of failure of the Sunda megathrust offshore from Padang. Scientists have forecast that this sequence will culminate in an earthquake of about magnitude 8.8, sometime within the next few decades. The latest earthquakes are another indication that people need to plan ahead and be prepared. Scientists say one major concern, for example, is that this latest quake could trigger the rupture of the megathrust fault below Siberut Island, in the Mentawai Islands west of Sumatra.

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. It occurred on the Sumatran fault, a great “strike-slip” fault that runs the length of Sumatra, from near Krakatoa in the south to Banda Aceh in the north. 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.

Earth Observatory of Singapore, NTU