Post 2004 tsunami, Andaman seabed unusually ‘shaky’

An earthquake with 10-minute-long waves is quite unusual. Many like it, in quick succession, even more so. But beneath

the Andaman Sea

near Nicobar, the 2004 earthquake and tsunami has set off a series of intermittent earthquake swarms that also indicate simmering

volcanic activity

.

Scientists at the

CSIR-National Institute of Oceanography

in Goa on the research ship

RV Sindhu Sankalp

conducted a passive ocean bottom seismometer experiment for the first time in Indian waters.

They found 141 high-frequency earthquakes and swarms (bursts of quakes with a short period of time, ranging from hours to days) in areas where three faults —

the Andaman Nicobar Fault

, the West Andaman Fault and strands of the

Great Sumatra Fault

— meet.

Major swarms had occurred in the off Nicobar region in the Andaman Sea over five different periods — in January 2005, March 2014, October 2014, November 2015 and March 2019 — their study, published in Nature’s Scientific Reports this month, said.

They were really long. “The March 2014 swarm, for instance, had lasted 48 hours,” lead author Aswini KK told TOI. The swarm in January 2005 was the strongest ever recorded globally. Some earthquakes in 2014 had long period signals, like the 600-second signal they got. “This is rare and suggests the origin of the seismic waves is deep seated, located at a depth in the subsurface, about 30km below the seafloor,” corresponding author Kamesh Raju said. “At that depth, we expect magma at work.”

So while the December 2004 “tsunamigenic megathrust earthquake” itself was a reason, the active volcanism in the area has also been causing some shifts.

Does it mean an eruption might be in the offing? “The studies over the submarine volcanic chain that extends from Barren Island in the north to the onshore volcanoes of Sumatra would provide insights to answer the above question,” Raju said.