New Delhi, April 11: The 8.6-magnitude earthquake off the west coast of Sumatra spared India from giant tsunami waves today but it may have added extra strain in the high-risk seismic zones along the Himalayas, geophysicists said.
The earthquake with an epicentre about 435km southwest of Banda Aceh, Sumatra, and 963km west of Kuala Lumpur, Malaysia, occurred at a depth of about 23km beneath the sea.
A preliminary analysis of the earthquake, they said, suggests it was caused by horizontal movement of a chunk of the Earth’s crust lying within the Indian plate rather than a vertical motion of the Indian plate slipping beneath Burma.
“A horizontal motion in which the Indian plate has lurched northward could have serious implications for our Himalayan region,” said Shyam Sunder Rai, a senior scientist at the National Geophysical Research Institute, Hyderabad.
The high-risk seismic zones along the Himalayan region mark an inter-plate boundary where the Indian plate, moving northward at nearly five centimetres a year, pushes against the Tibetan plate.
“The easiest stress transfer would be in the northern direction — and this may contribute incrementally to the strain that has already accumulated along some sections of the inter-plate boundary,” Rai said.
Many geophysicists believe a high-magnitude earthquake has been overdue in certain sections of the Himalayas such as the Kumaon zone because of the accumulated strain. While the location and timing of earthquakes cannot be predicted, scientists say any added strain will only push such high-risk seismic zones closer to the big quake.
But scientists caution that such a scenario would need to be verified. While the early analysis by the US Geological Survey does suggest a horizontal motion of the crust, scientists also point out that there are two possible directions for such horizontal motion — and only one of these could contribute to the strain build-up in the Himalayan region.
“This horizontal motion could be across a north-south fault or an east-west fault,” said Malay Mukul, associate professor of earth sciences at IIT, Mumbai.
“We don’t know yet which direction it happened,” he told The Telegraph. “A northward lurch by the Indian plate could add strain along the Himalayas,” Mukul said. “We need to quantify displacement which could in turn tell us how much additional strain has built up — if at all.”
Scientists said the horizontal crustal motion could explain the absence of significant tsunami waves but the location of the primary 8.6 magnitude earthquake as well as the 8.3 magnitude aftershock are unusual.
“Vertical crustal motion beneath the sea has far greater potential for causing a tsunami than an earthquake with horizontal motion,” said Kusala Rajendran, a professor of earth sciences at the Indian Institute of Science, Bangalore.
Both the 8.6 primary and the 8.3 aftershock occurred at a distance from the boundary between the Indian and the Burma plate. “Two earthquakes, both higher than 8, outside the boundary is quite unusual,” she said.
“We need to understand whatever that is going on down there better,” Rajendran said.
Many scientists had assumed that the 9.1 magnitude earthquake of December 26, 2004 would have released a significant amount of the accumulated tectonic strain along the Sumatra region.