Tsunami unravelled

New studies provide crucial lessons for predicting quakes

 
By Ritu Gupta
Last Updated: Saturday 04 July 2015

-- A NEW understanding of future earthquakes, especially in South Asia, has emerged from studies of the tremor that caused the devastating December 26, 2004 tsunami in the region. The findings of three studies on the Sumatra-Andaman quake have been published in the latest edition of the journal Science (May 20, Vol 308, No 5725).

Scientists remark that it was the first major earthquake to be observed with modern instruments, satellites, digital seismometers and global positioning systems. "We've never had such comprehensive data for a great earthquake," says Thorne Lay, a geologist at the University of California, usa, who worked on two of the studies.

Besides the December quake, which was the most powerful in the past 40 years, the scientists also studied the one that struck almost the same area just three months later on March 28, 2005. With an 8.7 seismic moment magnitude (a measure of total energy released by an earthquake), it became the second largest earthquake over the same period.

While one study provides an overview of the two earthquakes, another concentrates on the processes involved in the rupture of the fault. The third study illustrates how the earthquakes caused the whole planet to vibrate with free oscillations, similar to what happens when a bell rings.

What happened According to the first study, the Sumatra-Andaman quake lasted almost 10 minutes, unlike most large earthquakes that last only a few seconds. It measured 9.3. The ground shook more than 100 times harder than it did during the 1989 California Loma Prieta earthquake -- a magnitude 6.9 (on the Richter scale) event that caused major destruction from Santa Cruz to the San Francisco Bay, and which seismologists call "a monster earthquake," says Lay. Both the 2004 and 2005 earthquakes ruptured a 1,600 kilometre stretch of the fault between the Indo-Australian and Eurasian tectonic plates (see diagram: Seismic slip).
As per the other study, led by Charles J Ammon of the department of geosciences, Pennsylvania State University, usa, the earthquake occurred where the Indo-Australian plate plunges beneath the south-eastern Eurasian plate. Before the quake, the edge of the Eurasian plate was being dragged downward by the descending Indo-Australian plate. The tremor released the edge of the former plate that sprang back up, setting off the deadly tsunami.

The quake was centred near the northwestern tip of Sumatra, from where the rupture expanded along the fault at a speed of about 2.5 km per second toward India and Sri Lanka. The fault slipped by as much as 15 metres in places, averaging about 10 metres of displacement. But the initial movement of the fault was much less along the northern segment than in the south, which is why Indonesia suffered the most.

Eventually the northern part of the fault slipped about as much as the southern part, uplifting and tilting the Andaman Islands. But the tilting of the islands occurred gradually, without generating seismic waves. "We think that the slip was occurring in the northern part for about an hour, well after the 10 minutes of rapid motions were over," Lay said. Then there were the aftershocks, including the most energetic earthquake swarm ever observed: more than 150 earthquakes of magnitude 5 and greater occurred over a four-day period in late January.

The March 28, 2005, earthquake was not an aftershock, but a new rupture of an adjacent segment of the fault. "The occurring of this earthquake shows there is potential for future major earthquakes not only on the fault along the Indo-Australian plate boundary but also on a related fault system beneath Sumatra," says Lay.

The Sumatra-Andaman earthquake also produced the best documentation of Earth's free oscillations ever, says geologist Jeffrey Park of Yale University, lead author of one of the reports. "Just like grocery shoppers thump a watermelon to find out whether it's ripe, seismologists measure the natural tones (free oscillations) from earthquakes to detect properties of Earth's deep mantle and core," said Park.

These findings are crucial, as seismic hazards on numerous plate boundaries, such as the nearby Himalaya, have usually been assessed in terms of recent history, without the benefit of records that contain details of extreme events like these earthquakes. Seismlogists feel that these earthquakes and their analysis should serve as a wake-up call that conservative seismic forecasts may not serve society well.

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