Besides the subduction zone (where the Indian plate dives under the Burmese plate) that lies to the west of Indonesia, the Indian Ocean has a diffused seismicity zone. This zone lies to the west of the subduction zone.

The east-west trending zone extends from 80 degree East longitude to around 93 degree East longitude (where the April 11, 2012 quake occurred). The length of the zone is around 1,300 km.

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There have been seven major earthquakes between 6 and 7.5 magnitude in this region during the last 25 years. The 8.6 magnitude quake of April 11 and the aftershock of 8.2 magnitude should now be added to this list.

Intra-plate quake

The April 11 quake was not in the subduction zone but to the west of it. “It is an intra-plate earthquake,” R.K. Chadha, Chief Scientist at the National Geophysical Research Institute (NGRI), Hyderabad, told The Hindu. An earthquake is called intra-plate when it occurs within the plate and not at the edges of two plates.

Since it is an intra-plate earthquake, no subduction takes place and the fracturing causes a strike-slip fault — where one edge of the plate grinds past the other horizontally. The diffused seismicity zone has an east-west direction. This zone could have formed as a result of westward transfer of stress from the subduction zone.

“The April 11 earthquake was about 100 km west of the subduction zone,” he said. “It is within the Indo-Australian plate.” According to him, the movement of this plate (at a rate of 52 mm per year in a NNE direction) in the diffused zone of seismicity is governed by mid-ocean ridges in the Indian Ocean. There are two ridges there — southeast Indian Ocean ridge and the South Indian Ocean ridge.

New boundary

“There is a belief that a new boundary is developing in the Indo-Australian plate along the diffused zone of seismicity,” he said. “Some scientists speculate that this may become the new boundary between the Indian plate and the Australian plate.”

“The April 11 earthquake appears to have originated near the subduction zone,” he said. “Looks to be near the junction where the strike-slip fault meets the subduction zone, causing a triple junction.”

In the last 100 years of earthquake data, the 15-year-period between 1950 and 1965 witnessed a clustering of large quakes of magnitude greater than 8.5. “A similar clustering of large-magnitude earthquakes is now being observed since the December 2004 Sumatra earthquake,” Dr. Chadha said. “Starting from December 2004, there have been about six quakes of magnitude greater than 8.5 globally. It is following the same pattern of 1950-1965.”

The chances of another high-magnitude quake occurring at some place cannot be ruled out if this clustering is indeed happening. “Earthquakes occur randomly, but we are still able to see some kind of a pattern now emerging,” the NGRI scientist said.

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