At Ars Technica, Scott K. Johnson what has been learned about the devastating earthquake that struck Nepal earlier this year:
The mighty Himalayas have been driven up into the sky by the collision of Eurasia and India, which has migrated north like a tectonic rocket over the last 100 million years. The Indian plate is being crammed beneath the crumpled Himalayan rocks along a dangerous fault that ramps downward to the north.
Lots of GPS sensors and seismometers have been deployed in the area to help seismologists study earthquakes here. Combined with precise satellite measurements of surface elevation changes, researchers have the means to work out where the movement on the fault must have occurred.
The earthquake began about 80 kilometers northwest of Kathmandu and about 15 kilometers beneath the surface. Geologists like to talk about faults “unzipping,” which is a helpful way to visualize what’s going on. A small patch of the fault plane slips, and then expands outward along the fault. In this case, the patch unzipped about 140 kilometers to the east in under a minute, traveling horizontally along the fault plane. Within that patch, the rocks slipped as much as six meters past each other.
Although it’s the seismic energy released by that sudden motion that causes the damage, the surface changes are still eye-catching — some of the GPS stations ended up two meters south of where they had been before the earthquake.
As for that seismic shaking, the pattern of building damage in Kathmandu was partly the result of the geology beneath the city. It sits on a roughly 500-meter-thick stack of lake and river sediment filling a bedrock bowl. The reverberation of seismic waves in that bowl produced a resonance, building stronger waves with a period of 4 to 5 seconds. While fewer homes were actually damaged than expected, taller buildings — which can sway at about that same frequency — didn’t fare as well. (A similar thing happened in the 1985 Mexico City earthquake, when buildings between 6 and 15 stories bore the brunt.)
