Geology Colloquium: Roger Bilham
The disastrous Nepal 2015 earthquake: What happened and what's next
The Gorkha earthquake: an incomplete Himalayan rupture
By Roger Bilham
Although an earthquake near Kathmandu had been anticipated for more than two decades, the Mw7.8 Gorkha earthquake of 25 April 2015 was smaller than expected and in the wrong place. Within ten seconds of shaking its 150x60 km2 rupture had lifted Kathmandu 1 m, and shifted it bodily 1.8 m to the south (http://youtu.be/VS6WVz4V0ps). Yet local accelerations (?0.25g) were lower than in historical Himalayan earthquakes, thereby averting a much worse disaster. The official deathtoll exceeded 8700, half a million homes were damaged or destroyed, and 4.5 million rendered homeless. Had the earthquake occurred, not a midday on Saturday, but 12 hours earlier, or during school hours the deathtoll could have been 4-6 times larger. The main tectonic feature of the earthquake is that it failed to rupture the entire Himalayan décollement to the surface, a feature that can now be recognized as common to half a dozen Himalayan Mw8.4 earthquakes. A great earthquake is anticipated in western Nepal as the repeat of a Mw>8.6 earthquake in 1505 or earlier, which should it occur now could in principle catalyze the release of both the 1833 and 2015 up-dip slip deficits south of Kathmandu. The timing of this future great earthquake remains uncertain.