The Sumatran-Andaman earthquake on Dec. 26, 2004, which caused a tsunami killing hundreds of thousands of people, was a surprisingly complex event that began gradually, exploded to life and then tapered off slowly, according a series of studies released Thursday.

In a special section in the May 19 online issue of journal Science, scientists issued six research papers to re-evaluate this earthquake.

The event was the second-largest earthquake on record and the third most fatal. The amount of energy that the earthquake released was equivalent to a 100-gigaton bomb, or about as much energy as the United States uses in six months. Although there have been multiple earthquakes, some with destructive tsunamis, in this region over the last 200 years, nobody could have foreseen the magnitude and complexity of last December's earthquake, scientists said.

If the movement along the northern portion of the fault had not been slow, the earthquake would likely have generated tsunamis along the entire 1,300-km fault, causing even more devastation, they said.

Two studies have reconstructed the Sumatran-Andaman earthquake rupture from one minute to the next. Scientists analyzed the P-waves, the fastest seismic waves that travel outward from an earthquake, recorded by seismometers around the world.

They found that the rupture initiated slowly, with small slip and slow rupture in the first 40 to 60 seconds. Instead of slowing,the rupture accelerated to 3 km per second for the next four minutes, and then maintained an average speed of about 2.5 km per second for six more minutes.

After this explosive period, the slip slowed at the northern end of the rupture, where it took approximately 30 minutes for 7 to 20 meters of displacement to occur.

This slow slip produced extremely long-period seismic waves, which were detectable for weeks. Researchers calculated that the slow slip released about one-third of the total energy in the earthquake.

The earthquake triggered a cluster of smaller earthquakes near the Mt. Wrangell volcano in Alaska, halfway around the world, a third study said. The Alaskan earthquakes occurred in phase with one of the types of seismic waves generated by the Sumatran-Andaman earthquake, at 20-to 30-second intervals for 11 minutes.

In the fourth study, researchers concluded that the earthquake was the results of crustal deformation that had occurred just seconds earlier rather than the results of stress that had accumulated more gradually.

Scientists analyzed seismic information from around the world and deduced that the fault slipped rapidly along the southern portion of the rupture zone near Banda Aceh, Sumatra, but more slowly to the north along the Andaman and Nicobar islands.

They also noted that seismometers listening to the approach of seismic waves in Russia heard the waves at a higher pitch than seismometers in Australia heard waves that were receding.

According to another study, the earthquake caused substantial movement at global satellite positioning system (GPS) stations located up to 4,500 km away from the epicenter, allowing researchers to use GPS measurements to estimate how the rupture progressed and the amount of energy released.

These results, independent from the seismic data, suggest that more than 5 meters of average slip occurred along the full length of the rupture and that considerable energy was released more than one hour after the earthquake started and after the full fault had ruptured.

The GPS stations to the east of the rupture moved to the west, and vice versa, which is consistent with the earthquake's thrust motion.

Source: Xinhua