The major earthquake that shattered the western Indian state of Gujarat last week came as no surprise to seismologists. Nor was the toll: more than 10,000 people killed, villages at the epicenter leveled, and the nearby city of Bhuj almost completely destroyed. And worse may be on the way: Tectonics studies suggest that even larger quakes are bound to hit the subcontinent. "This is a wake-up call," says tectonophysicist Peter Molnar of the University of Colorado in Boulder, "but it isn't as big as the one that will come."
Last week's disaster was a so-called intraplate earthquake. Most earthquakes rupture faults where two tectonic plates slide by each other. On the plates themselves, the crust is usually more stable. But the Indian subcontinent has been torn, stretched, and strained by eons of plate tectonic jostling, and that has created weak spots in the crust. This weakened crustal block is under strain today: India is driving northward into Asia, pushing up the Himalayas and squeezing the whole subcontinent. Where that strain finds weak spots, earthquakes can strike, and they often do so completely haphazardly.
Since 1965, India had suffered five intraplate earthquakes that have killed as few as 26 and as many as 9748, according to a compilation published last November in Current Science by Roger Bilham of the University of Colorado in Boulder and Vinod Gaur of the Center for Mathematical Modeling and Computer Simulation in Bangalore. The Bhuj quake--the largest of late--seems to have occurred within a buried ancient rift that originated with India's separation from Antarctica and Africa 150 million years ago.
More intraplate earthquakes will undoubtedly strike India, Bilham says. But the biggest threat looms at the plate's edge in the northeast, where the subcontinent plows under the Himalayan plate. There, strain is building far faster than within the subcontinent; so fast that great earthquakes are expected every few decades on average. Bilham's particular concern is the massive river-side cities such as New Delhi where millions of people live on ground that an earthquake could turn to mush. "It looks pretty grim to me," he says.
With reporting by Pallava Bagla in Ahmedabad.