The Hindu : Front Page : `Aftershocks will continue for six more months in the Andamans'

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Tuesday, Jan 25, 2005

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`Aftershocks will continue for six more months in the Andamans'

By R. Prasad

CHENNAI, JAN. 24. A month after the high-intensity earthquake off the coast of Sumatra causing tsunami that killed thousands, the Katchall island today recorded another earthquake of 6.3 intensity on the Richter scale. This, according to R.K. Chadha, scientist at the Hyderabad-based National Geophysical Research Institute, is an aftershock. According to the Indian Meteorological Department, it is the 27th aftershock in the Andaman region with an intensity of six and above.

Aftershocks of reducing intensity can be expected to rattle the Andaman and Nicobar islands as they fall in the subduction zone. [A subduction zone is the place where one plate dives below another. In this case, the India plate dived below the Burma microplate. The trench is the surface expression of this phenomenon. The subduction zone in this case is nearly 100 km in width and stretches from Sumatra to the Andaman group of islands].

"The earthquake of 8.7 that happened in Arunachal Pradesh in 1950 (Mishmi thrust) recorded aftershocks for one-and-half years," Dr. Chadha said. "So we can expect aftershocks in the Andaman region for at least six more months. And these will be of lesser intensity." Aftershocks, as the name indicates, will generally be of lesser intensity than the main earthquake.

Though the intensity would decrease, some sporadic aftershocks with a higher intensity could happen. "This is not an unusual phenomenon," Dr. Chadha said. In due course the intensity would reduce and come down to normal background seismic activity level. The background seismic activity in the Andaman region was around 2-3 on the Richter scale. "So an earthquake will be called an aftershock only if its intensity is greater than four and anything below three will be background seismic activity," Dr. Chadha said.

Aftershocks can occur only in the same fault region. In this case, it will be restricted to the 100 km wide region extending from Sumatra to the Andaman group of islands. "Aftershocks will never strike India," he stressed. "But if an earthquake strikes the peninsular region within one year then it will be one triggered by the larger December 26 event." Again to be called an earthquake triggered by a larger earthquake, the intensity should be less than what had been recorded as the capacity of that particular region.

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