People try to reach higher ground after a powerful earthquake off Banda Aceh, Sumatra island, Indonesia, prompted an Indian-Ocean-wide tsunami alert. Photograph: Chaideer Mahyuddin/AFP/Getty Images
The earthquake off the coast of northern Sumatra in Indonesia began with a sudden shift in a strike-slip fault, a line of weakness in the sea floor where two huge bodies of rock can slide past one another.
Unlike the earthquake that triggered the devastating 2004 tsunami in the region, the sea floor moved sideways instead of vertically, meaning it displaced less water and did not send giant waves around the Indian Ocean.
Warning buoys, installed after the 2004 disaster, picked up a tsunami soon after the earthquake, but the highest waves that reached the nearby shoreline of Sumatra were only 80cm. The warning from the Pacific Tsunami Warning Centre was later lifted, at 12.36GMT on Wednesday.
Seismologists traced the earthquake at 2.38pm local time to a depth of 14 miles (22km) and 250 miles (400km) from the coast of Banda Aceh. The slippage occurred in a fracture in a slab of oceanic crust that is bending downwards and beneath Sumatra. The US geological survey said the main shock was magnitude 8.6, which is extraordinarily high for a strike-slip fault.
Two hours after the main shock was felt, the area was hit by a magnitude-8.2 aftershock, probably caused by slippage along the same 125-mile fault. The huge release of energy redistributed stress to other weak spots in the crust, making them more likely to cause future earthquakes.
"After an earthquake occurs along a fault, stress is released in parts. But then, part of this stress is redistributed to other parts of the fault. This means that they are now more likely to become unstable, with many subsequent earthquakes. Aftershocks can continue for weeks and months after the main shock, sometimes even years," said Bruce Malamud, an expert on natural hazards at King's College London.
Malamud said there had been an average of 17 large earthquakes (magnitude 7 or greater) a year around the world since 1900, and about 15 a year since 1990.