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Andaman quake confirms worst fears
Published on 4 Apr. 2010 11:37 PM IST
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The 6.6 magnitude earthquake that occurred north of the Andaman Islands on March 30 is yet another confirmation that this particular region will continue to be source of severe earthquakes, says a leading Indian geologist.
“This is what we had been worried about,” Vineet Gahalaut at the National Geophysical Research Institute (NGRI) in Hyderabad told IANS in an interview.
He and his colleagues had warned that a segment of the “plate boundary fault” left unruptured after the deadly 2004 Andaman earthquake would be a source of high magnitude earthquakes.
Gahalaut said the unruptured segment that became deformed after the 2004 earthquake had already produced two powerful aftershocks in 2008 and 2009, and the March 30 earthquake was the third. “Such intra-plate earthquakes occurring in the subducting India plate under the unruptured segment are a matter of concern,” Gahalaut said.
The concern was first expressed by Gahalaut and his colleagues only three months ago in an article they published in the journal Tectonophysics.
The NGRI team had issued the warning after analyzing the two major aftershocks in the Andaman region after the deadly 2004 quake that produced a tsunami.
First was the 6.6 magnitude quake on June 27, 2008, near Little Andaman (LA). The second was the 7.5 magnitude quake of Aug 11, 2009, near Coco Island north of the LA earthquake. Last week’s earthquake that took place south of the 2009 Coco earthquake is the latest one. “Our concern that the unruptured segment will be a source of worry is coming out to be true,” Gahalaut said.
The 9.2 magnitude earthquake that produced the tsunami occurred in the Andaman Sumatra Subduction zone formed by the India plate sliding beneath the Sunda Plate.
According to the NGRI team, the 2004 earthquake ruptured about 1,400 km-long frontal arc of the Sumatra-Andaman subduction zone between northwest of Sumatra and the Andaman region but left a segment of about 400-500 km unbroken.
In their paper, the researchers had shown that both the 2009 Coco and 2008 LA aftershocks occurred in the subducting Indian plate under the unruptured segment of plate boundary fault. Gahalaut said simulation studies suggested that both these aftershocks, that were “very similar to each other”, were strongly promoted by the deformation of the Indian plate that took place due to the 2004 earthquake.
The fact that the latest March 30 earthquake also occurred south of Coco implies that similar strong aftershocks may occur under the unruptured segment that lies between the epicenters of Coco and LA aftershocks, Gahalaut said.
According to the scientists, the unruptured region is loaded by stresses caused by deformation due to the 2004 earthquake. “My worry is that it can potentially produce more earthquakes in the 7 to 7.5 magnitude range capable of triggering local tsunami,” Gahalaut said.
The NGRI researchers claim that the Indian plate obliquely subducts under the Burmese plate, carrying along with it an elongated sea mount about 3-km high and 100-km wide. This “ridge” acted as a barrier in the Andaman region and retarded the rupture speed of the 2004 earthquake at its northern end, and for some reason a portion of the plate boundary segment remained unruptured, Gahalaut said.

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