Tuesday, September 6 2005 12:14 Hrs (IST) - World Time -

Mumbai: Earth scientists and volcanologists are examining whether the series of earthquakes that rocked the Sumatra in Indonesia for the last one year would lead to volcanic eruption in the region and Government preparedness in such eventuality.

History cannot forget the eruption of Krakatau, Indonesia, in 1883, which was preceded by an earthquake of magnitude 8.7. Already, during the last eight months, the volcanoes like Telang in Sumatra, Barren Island in Andaman region have been activated after the series of earthquakes.

More than a dozen of earthquakes preceded the major earthquake of Sumatra (magnitude 9) in December, 2004 that triggered tsunami in Indian Ocean killing thousands of people, over 20 earthquakes that preceded March, 2005 (M 8.7) of Sumatra and already over 200 earthquakes of over M 5.5 and 6.5 have been recorded in the Sumatra region, says IIT volcanologist Prof D Chandrasekharam.

He said in various data published after December 2004, the Indian plate was constantly being drawn below the Sumatra plate.

"What is happening to the material that is being subducted below Sumatra?," he said adding "the answer could be that the subducting material must be melting thus promoting continued subduction. This process of melting would result in seismic activity."

This phenomenon is true in all subduction zones, the scientists said. The rate of subduction during pre-December 2004 earthquake was estimated at 6.7 cm per year while it was estimated more than 6.7 cm per year after December, 2004 earthquake, Chandrasekharama said.

Chandrasekharam says the accumulation of melted material cannot stay below the earth's crust for a long period. As the volcanoes in this region have already been activated, close monitoring is essential.

"These are relatively small volcanoes compared to those like Krakatau, Toba and Sorikmarapi. Eruption of Krakatau in 1883 was preceded by an earthquake of 8.7 magnitude," he said. Toba and Sorikmarapi lie just below the area where all the present earthquake loci are located (Nias and Northern Sumatra).

More than 130 active volcanoes are present in Indonesia, more than any other country on earth. They comprise axis of the Indonesian island arc system, which is generated by northeastward subduction of the Indo-Australian plate.

Toba is a Stratovolcano, which erupted about 74,000 years ago ejecting 2800 cubic metres of tuff into the sky, which created a major disaster over the entire Asian continent. Now this is a dormant volcano. Michael Rampino, of New York University, warned a massive volcanic eruption capable of causing as much devastation as the cosmic bodies occurs every 50,000 years. The Young Toba Tuff has an estimated volume of 2,800 cubic kilometers (km) and erupted about 74,000 years ago, he said.

There have been no eruptions at Toba in historical time but the area is seismically active with major earthquakes in 1892, 1916, 1920-1922, and 1987.