Asia marks fourth anniversary of 2004 Boxing Day tsunami:

Earthquake supercycle to trigger another tsunami

Four years after the December 26, 2004 Indian Ocean tsunami, scientists are predicting that another major earthquake along the same fault line that sparked it is likely in the next several decades.

This is indeed bad news as we commemorate the fourth anniversary of that devastating tsunami that destroyed most coastal areas of Indonesia, Sri Lanka, Thailand and India, killing an estimated 250,000 people, including nearly 40,000 in Sri Lanka. Over hundreds were killed in Africa.

The next tsunami could unleash as much or more destruction, new research suggests. The only consolation is that all countries in the region are much better prepared in terms of a tsunami early warning system and the population too is highly aware of the tsunami phenomenon. They are now prepared to vacate their homes and buildings as soon as a warning is given, unlike in 2004 when the phenomenon was totally 'new' and took everyone by surprise.

The 2004 tsunami was sparked by a magnitude 9.2 earthquake (or is it a Seaquake?) along the Sunda fault off the coast of Sumatra, Indonesia. A major 8.4 temblor and aftershocks along a southern section of that fault called the Mentawi patch shook up the region just last year. But just how did scientists arrive at this alarming conclusion?

Analysis of coral growth patterns near Indonesia suggests that the 2007 quake may have been just the first episode in an "earthquake supercycle," or set of large quakes that have occurred in the region roughly every 200 years for the past seven centuries.

Sections of the Earth's crust called tectonic plates are likely to rupture again under the Mentawi patch within several decades, possibly generating a magnitude 8.8 quake, according to research published in this week's Science. The study looked at ancient corals around the Mentawai islands off Sumatra, along a 434-milefault line called the Sundra megathrust.

"If previous cycles are a reliable guide, we can expect one or more very large west Sumatran earthquakes...within the next two decades," co-author Kerry Sieh, a professor at the California Institute of Technology's Tectonics Observatory, said.

The scientists studied coral because-like growth rings on trees-the reefs record the history of the sea level where they grow. When tectonic plates push the ocean bed up, the sea level goes down, and coral can no longer grow vertically. It can, however, grow outward, so those cross sections reveal the pattern of tectonic movement. They base their prediction on previous earthquake evidence going back 700 years.

The first occurred in the 1300s, the second took place in the late 1500s and the third occurred from 1797 to 1833.

They therefore believe that the latest earthquakes, including the one in September 2007, mark the beginning of another active period that could extend into the coming decades.

The September 2007 earthquake was the first in a series of large failures in the fault line, and the December 2004 quake was a rupture in the same fault further to the west.

Large sections of the fault in this eastern flank of the Indian Ocean have failed in the past eight years in an extraordinary sequence of powerful earthquakes, the scientists said.

"The largest of these failures of the Sundra megathrust, in 2004, caused the most devastating tsunami the world has seen in many generations," they said.

The Mentawai part of the fault had been dormant since the earthquakes of 1797 and 1833, until it ruptured in the 2007 earthquake.

"Over the past 700 years, three episodes of emergence lasted variously from a few decades to a little over a century. This past variability precludes a precise empirical forecast of the next great earthquake and tsunami," the scientists said.

Each supercycle comprised the build-up of tectonic strain over time that was released by two or more large earthquakes. Because a strain-relieving quake hasn't rocked the so-called Mentawai patch since 1833, an 8.4-magnitude earthquake that struck in September 2007 is probably the first of a new series, Sieh said. The next may be as big as 8.8-magnitude.

Because the length of time between each event in past series varied from a few decades to more than a century, predicting precisely when the next will occur is impossible, he said. Still, the quake will trigger a tsunami that may be as high as 5 meters (16 feet) at Padang, a city of about 750,000 people in western Sumatra, Sieh said. The 2004 tsunami was between 5 and 12 meters high when it struck Banda Aceh in northern Sumatra. About 30 percent of the people in Padang have been trained in how to respond to an earthquake and tsunami and when to evacuate.

But it is not only earthquakes that can trigger a tsunami. About 2,300 years ago, a giant tsunami crashed ashore where New York City stands today, according to latest research.

No one knows for sure what caused it, but new clues found in the Hudson River's silt suggest an asteroid slammed into the Atlantic Ocean nearby.

Katherine Cagan of Harvard University and a team of researchers found carbon spherules - perfectly round particles that form in the extreme pressures of an impact - and other grains of shocked minerals in the sediments as well, but the discovery remains controversial.

Meanwhile, it has been estimated that 97 per cent of houses planned for tsunami-affected people are finished or under construction in all the 2004 tsunami-hit countries.

Courtesy: Internet