Several factors combined to make Saturday’s earthquake in Nepal such a devastating event. The first was its basic magnitude. At 7.8 on the Richter scale, this was one of the most powerful earthquakes to strike the region in the past 80 years. In addition, it was a shallow event with a source that was only 11km below ground.
That has special consequences, according to David Rothery, professor of planetary geosciences at the Open University. “The shallowness of the source made the ground-shaking at the surface worse than it would have been for a deeper earthquake,” he explained. “I’ve seen pictures of poorly constructed old buildings destroyed in Kathmandu, and I’m concerned that in this mountainous region there could have been landslides that might have destroyed or cut off various remote villages.”
However, most areas touched by the earthquake lie on solid bedrock, said Rothery. This limited the amount of shaking that occurred except on the north Indian plans, near the Nepalese border, where the surface sands and silts shook more than the solid rock elsewhere.
As to the specific cause of the earthquake, it was triggered by the India tectonic plate, which is moving northwards at the rate of 5cm a year into central Asia. This results in thrust-faulting and has thrown up the Himalayan mountain range. It has triggered several other significant earthquakes in this region, including the 1934 quake at Bihar, which reached a magnitude of 8.2; the 7.5 event at Kangra in 1905; and the 2005 Kashmir earthquake, which reached 7.6. The latter two resulted in the highest death tolls for Himalaya earthquakes seen to date, together killing more than 100,000 people and leaving millions homeless.
