At 7.6, Tuesday's quake biggest in Andamans since 2004

CHENNAI: The biggest earthquake in India in the last four years struck 260 km off Port Blair early on Tuesday morning triggering a tsunami alert across the Indian Ocean region and causing tremors across Chennai, West Bengal and Orissa.
The quake measuring 7.6 on the Richter scale occurred undersea at 1.25 am on Tuesday at a depth of 33.1 km. The Pacific Tsunami Warning Centre immediately issued a tsunami alert for India, Myanmar, Bangladesh, Indonesia and Thailand.Exactly 12 minutes later, another quake measuring 6.4 on the Richter scale struck off the southern coast of Honshu, Japan, at 1.37 am.
The Andamans quake is the largest in the region since the 2004 earthquake, measuring 9.1 on the Richter Scale in the Indo-Sumatran plate, which caused the devastating tsunami. The biggest quake to have struck the Andamans region before that was on June 26, 1941, about 20 km from Flat island, which is reported to have caused a tsunami killing about 3,000 persons on the east coast of India and Sri Lanka. On December 31, 1881 a temblor of magnitude 7.9 struck the Car Nicobar Islands.
On Monday night, residents of the islands rushed out of their homes as the quake rattled. "We were frightened and ran out of the houses with our children. We stayed out for about an hour and a half after the quake. Electricity was shut down and we ventured back into our homes only after it was restored," Venkatesh, a resident of Mayabunder, close to the epicenter in North Andamans, told The Times of India.
While the island administration insists that officials swung into action immediately, residents differ. "There was a tsunami alert given but we did not know about it. Government officials and policemen who are provided with mikes and speakers to alert us in such cases were not there," Venkatesh said.
"We came to know about the tsunami alert nearly two hours later when we went back into our houses and switched on the TV," another resident said.
Andamans chief secretary Vivek Rae told ToI that there were no casualties but a few buildings had developed cracks. "Everybody in the administration is on constant alert. All our SHOs and policemen were on the streets almost immediately and our control rooms function round the clock. Our officials ensured there was no panic," he said.
In Tamil Nadu, residents of several coastal hamlets rushed inland with their valuables and stayed awake until morning following the tsunami alert. In Nagapattinam and Thanjavur districts, fishermen did not venture into the sea following the tsunami alert. People moved to safer places as a precautionary measure.
An expert on earthquakes, Dr N Venkatanathan of Sastra University, said, "The earthquake did not cause a tsunami as it occurred at a depth of 33.1 km. Most tsunamis are caused by shallow earthquakes. At a depth of 33 km, more land needs to be displaced before the water above is displaced. That large a land mass will not move easily and the force will not come to the surface. However, if the quake happens at a depth of about 10 km, there is every possibility of a massive tsunami."
He also points out that these quakes are basically settling of faultlines caused by the earthquake that triggered the tsunami in 2004. "As a precursor to this massive quake, there was a series of earthquakes of light to moderate intensity in the Andamans region between July 24 and 26," he said.
As the quake hit the island, tremors were felt in Chennai and other places across India. " I had just gone to bed after a night shift and suddenly felt my bed shaking. A friend messaged me a few minutes later asking if I had felt the tremors," Meera S of T Nagar said.