Quake-prone Nepal: key questions and answers
Here are answers to some key questions that have emerged since the Saturday quake that killed over 2,500 people, sowing terror and reducing buildings to rubble.
Q: What caused the April 25 earthquake?
A: The massive 7.8 magnitude earthquake — the worst disaster to hit the Himalayan nation in more than 80 years — occurred when a major fault broke, generating powerful seismic waves for about 100 seconds.
The colossal fault jerked after decades of pressure pushed shifting tectonic plates into a collision.
The rupture began northwest of Kathmandu and spread eastwards over a distance of some 100 kilometres (60 miles).
The quake resulted from a collision between the Indian and Eurasian seismic plates, according to the United States Geological Survey (USGS).
Jerome Vergne, a seismologist at the School and Observatory of Earth Sciences in Strasbourg, agreed.
“The Indian plate rises at a speed of some two centimetres (less than an inch) a year… and it is constantly trying to climb under the Tibetan plateau,” he said.
“But that shift isn’t smooth: it is very irregular. What happened here was a major jolt, a brutal rupture in the interface of the fault that separates the two plates,” Vergne added.
Q: What should we expect in the days and months to come?
A: Aftershocks are expected in the months, even years ahead. The rupture was not homogenous, and new jolts, however small, may still bring new quakes.
“To start with, the surface (of the fault) south of Kathmandu did not break,” said Pascal Bernard of the Institute of Earth Physics of Paris.
The partial break may mean the area has become even more vulnerable to new quakes in coming years, he said.
Aftershocks will continue, decreasing in number and intensity over time.
“We have observed aftershocks so strong their magnitude nearly matched the initial quake’s,” said Vergne, referring to Sunday’s mornings 6.7-magnitude tremor.
The impact of aftershocks, he added, has been worsened by the fact that structures were already weakened by the first quake.
The jolts will eventually slow down, but the plates will never stop shifting, causing new dangers as they move.
Q: Should we fear an even bigger quake?
A: This convergence of continents — which created the Himalayas to begin with — has made the area one of the world’s most quake-prone, with its colossal faults jammed close together.
“We know there will be new major earthquakes in the Himalayan area, which could have an even bigger magnitude than this one,” Bernard said.
“Imagine yourself stretching an elastic band: it will snap in the end,” he added.
A magnitude 8.1 quake killed 10,700 people in Nepal and India in 1934. A previous mega-quake in the area dates back to 1255.
Bernard said the next quake could have a magnitude of up to 9, but that it is hard to tell whether it will strike in a few years’ time, or two centuries from now.
“Yes, the worst is yet to come, but it may be in a few centuries’ time,” he said. – AFP