The sea-bed off Indonesia has rocked again with significant intensity, bringing back memories of the magnitude 9.0 mega-quake of 2004 which resulted in a killer tsunami.

In fact, the epicentre coordinates could not have been more identical, with the Banda Aceh coast that was battered in 2004 lying in close proximity.

ALERT WITHDRAWN

A US Geological Survey (USGS) report said that Tuesday midnight's event measured 7.3 on the Richter scale.

It occurred at a depth of only 29 km, triggering a tsunami wave alert, though withdrawn later.

The epicentre was identified as just 420 km south-west of Banda Aceh, Sumatra, Indonesia.

And USGS quoted people in the area seeing a retreating sea after the earthquake, just like it happened in 2004 during the tsunami that killed more than two lakh people.

STRING OF SHAKES

This is among the latest string of ‘shakes' to India's north-east and out into the sea to the south-east in recent times.

Starting from December 28, no less than seven light to moderate events have been reported in the North-eastern States (see table) and into the sea to the south-east, not counting Tuesday's event.

This apart, three other tremors have been reported to the immediate north-west of the country in the ‘usual' suspect areas of the Hindukush, mainland Pakistan and Kashmir-Xinjiang border – all of which count no less than 11 in a space of 14 days!

Rumbles along the Pacific ‘Line of Fire' that includes the Indonesia-Sumatra belt are being watched for ramifications for the country's south-east coast.

‘STRIKE-SLIP'

The Tuesday event occurred as a result of ‘strike-slip' faulting within the oceanic lithosphere of the Indo-Australia plate.

Strike-slip faults are vertical (or nearly vertical) fractures where the blocks have mostly moved horizontally.

This is approximately 100 km to the south-west of the major subduction zone that defines the plate boundary between the Indo-Australia and Sunda plates off Sumatra.

At the location of this earthquake, the Indo-Australia plate moves north-northeast with respect to the Sunda plate at a velocity of approximately 52 mm/yr.

While they are rare, large strike-slip earthquakes are not unprecedented in this region of the Indo-Australian plate.

DIFFUSE BOUNDARY

Since the massive M9.1 earthquake of 2004 that ruptured a 1,300-km long segment of the Sumatran mega-thrust plate boundary, two M6.2 ‘strike-slip' events have occurred within 50 km of the epicentre of Tuesday's event - on April 19, 2006 and October 4, 2007.

These events seem to align with fabric of the sea floor in the diffuse boundary zone between the Indian and Australian plates, the USGS said.

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