It took just ten seconds to move Kathmandu five feet to the south with its people, houses, temples, trucks and trees. Ten thousand square kilometres of Nepal's mountains, rivers and agricultural land followed suit. The birds were the first to know of this great tectonic shift; they took to the air within seconds of the arrival of the earthquake waves. Then the weakest buildings began to crumble, tanks and pools sloshed water over their banks, and people lurched unsteadily. What the earthquake on 25 April 2015, and a months-long sequence of aftershocks, left behind is tragic and all too well known – a death toll exceeding 8500, an unimaginable disorder of damaged and destroyed buildings, over 3.5 million people homeless, and a 1000 severed tracks and paths in the mountains impeding the delivery of urgent relief supplies.
Two decades ago, for a Himal issue in 1994, this writer described the potential effects of 'the next great earthquake' to hit Kathmandu. In many ways, the April earthquake fulfilled those predictions. But the bad news is that it was not the anticipated 'big one'; a large future earthquake lurks in western Nepal. The good news, however, is that lessons learned from this earthquake could reduce both the economic consequences and loss of life in the next one.
The physics of the April earthquake
The mechanics of Himalayan earthquakes have fascinated physicists for decades. The fundamental reason behind these earthquakes is that it is the destiny of the Indian plate to descend beneath Tibet. India is just the surface expression of a vast, slowly overturning plastic chunk of the Earth's mantle, driven by radioactive heat deep beneath the surface. We do not need to worry about the enormity of this subterranean process because it is ponderous and steady – India has been heading north for more than 170 million years. But for it to descend beneath Tibet, it must overcome the frictional resistance of the rocks at the base of the Himalaya. It does so every few hundred years in great earthquakes, where chunks of Himalaya hundreds of kilometers long, slip southward over India.
The southern Himalaya acts like a battering ram moving with the Indian plate, gradually squeezing the rocks of northern Nepal that store elastic energy like a giant compression spring. Eventually, the energy stored in these compressed rocks will exceed the friction at the base of the Himalaya, driving central and southern Nepal southward. In the last 200 years, the Indian plate has moved toward Tibet by about four metres. This is roughly the distance the Himalaya slipped southward in the Nepal earthquake.