Nepal is one of the most seismically active places on the earth. For the last 50 million years the tectonic plate on which the Indian Subcontinent sits has been in collision with and sliding under (subducting) the Asian plate. This is the force that caused the Himalayas, on the Asian plate, to rise; it also creates a fault line running around the edge of the Indian plate. That fault passes through southern Nepal, at the center of the arc.
India moves north about 8 inches per year, and the process creates great stress. Frequent small earthquakes throughout the region partially help to relieve the forces, but they are typically only effective in limited areas.
There have been larger earthquakes along the fault too: A magnitude 6.9 quake in 2011 in Sikkim was the most recent. But the last major quake to shake Kathmandu was in 1934, and statistically the city and central Nepal were “due.” (Despite great effort, scientists are yet unable to predict quakes reliably.)
On Saturday morning, April 25, 2015, a magnitude 7.8 quake struck northwest of Kathmandu and caused the fault to rupture across an area of about 100 miles west-to-east and about 50 miles north-to-south.
The disruption from the main quake caused scores of significant aftershocks and many smaller tremors through the area. The map below shows the main shock in red and the major aftershocks in yellow. The aftershocks clearly define the affected area.
In 80 seconds the affected area moved about 10 feet south and about three feet higher. That movement caused the shaking and the damage in Kathmandu and rural areas nearby. Nepal’s second largest city, Pokhara, is west of the epicenter and closer to it than Kathmandu, but it was only slightly affected: some damaged buildings and two fatalities. In the affected area there are about 7,500 fatalities so far identified, and the number is expected to rise to perhaps 10,000-12,000.
Although the extreme damage was localized, the quake was felt as far away as Chengdu and Kerala, over 1,000 miles distant. At Mount Everest Base Camp, well out of the affected area, tremors caused a massive avalanche, killing 18 climbers and support staff. (Other damage in the area was minor.)
While the earthquake has been a disaster for those killed and injured and a major problem for several hundred thousand people whose homes are destroyed or damaged, the figures are quite a bit lower than estimated by planners in advance. And preliminary estimates are that the major stresses along this part of the Himalayan fault have been relieved. Once the aftershocks settle down, Kathmandu residents can expect no major quake for decades.