On Saturday 25 April 2015, a massive, 7.8-magnitude earthquake hit Nepal, the worst Nepal has seen in 80 years. At the time of Critic going to print, the death toll had passed 5,000, but continues to climb; time will tell what the full extent of the damage is. Kathmandu Valley is densely populated at about 2.5 million and, due to poor infrastructure, has been absolutely devastated by the quake.
Why was Nepal rocked by such a big quake, so out of the blue? The same process that created the staggeringly beautiful Himalaya mountain range — the tallest in the world and Nepal’s primary source of income via tourism — has just nearly wrecked the country built so precariously on the slopes it formed. The area around Kathmandu Valley is one of the most seismically hazardous regions on the planet: Himalayan earthquakes in 1905, 1934 and 2005 killed tens of thousands in Nepal, India and Pakistan. A major quake in the Kathmandu area seems to hit every 75 years or so.
Earthquakes occur when tectonic plates come in contact and get stuck due to friction, building up stress that eventually ruptures. The Nepalese earthquake occurred on a major boundary between the Indian and Eurasian plates. Hundreds of millions of years ago, these plates were on opposite sides of the globe, but over geologic time the Indian plate drifted north towards the Eurasian one and began slamming into it, pushing up and squeezing rock between the plates like an accordion to create the Himalayas. They’re still converging and uplifting at a rate of about 2 inches per year, which is pretty fast according to geologic time, and creates a hefty amount of strain on the rocks that make up the mountains.
The Nepalese are well aware of the risks of living in the Himalayas, and of the instability of their infrastructure. But the reality is, it’s extremely difficult to accurately predict the timing of the next quake. All seismologists have to work with is an incomplete historical record of quakes — reconstructed from imprints of deformation on rocks and human records — analysis of the rock type and its resistance to strain, and the geometry and movement of tectonic plates. Today, GPS-based deformation models are key to understanding earthquake-potential deformation and strain.
Seismologists such as Susan Hough from the U.S. Geological Survey are busy installing more seismic monitoring networks throughout the Himalayan region, in the hope that soon scientists and locals will have a better grasp on Himalayan seismic activity and risk.