The magnitude 7.8 Nepal earthquake the killed more than 1,100 people Saturday occurred in a seismically dangerous region with a history of catastrophic temblors made all the more destructive by weak building codes.
The U.S. Geological Survey issued a summary of Saturday's quake, explaining the seismic history of the area and why the destruction was so great. Studies had warned that many buildings in the region could not withstand heavy shaking.
Why did the quake cause so much damage?
For one thing, the fault system involved in the quake ran around heavily populated areas.
As the USGS wrote in its summary, the quake "occurred as the result of thrust faulting on or near the main frontal thrust between the subducting India plate and the overriding Eurasia plate to the north. At the location of this earthquake, approximately 80 km to the northeast of the Nepalese capital of Katmandu, the India plate is converging with Eurasia at a rate of 45 mm/yr towards the north-northeast, driving the uplift of the Himalayan mountain range. The preliminary location, size and focal mechanism of the April 25 earthquake are consistent with its occurrence on the main subduction thrust interface between the India and Eurasia plates."
How frequent are quakes in the area? Have there been others with massive casualties?
According to the USGS, a quake in the region in 1934 is believed to have killed roughly 10,600 people. Another in 1988 killed 1,500.
But the experts said big quakes in the area are not necessarily common.
"Although a major plate boundary with a history of large-to-great sized earthquakes, large earthquakes on the Himalayan thrust are rare in the documented historical era," the USGS wrote. "Just four events of magnitude 6 or larger have occurred ... over the past century."
What are the seismic building standards in Nepal?
As in earthquake-ravaged places such as Haiti, the loss of life is likely going to be tied to weak building standards, compared with Western countries. Outside groups have been working with the government in Nepal to develop better seismic safety rules, according to a report by USAid.
The report said consultants were working with government officials to build more quake-resistant buildings, increase public awareness and train engineers.
The report said the need for better seismic regulations was great, including in the Katmandu area.
"Based on an estimated current population of three million inhabitants in Katmandu, experts believe that a large earthquake would result in at least 100,000 deaths, 300,000 injuries, and 1.6 million displaced in the capital city alone." the report said.
A study by Geohazard International found that two-thirds of the structures built in the area did not meet seismic code standards.
That study "found that rapid urbanization, with its resulting unplanned growth and inadequate enforcement of regulations, has led to substandard and unsafe housing patterns."
Geohazard said in a report that it partnered with the World Health Organization to study how to make hospitals in Nepal safer. The group said it submitted a report to government officials on steps that could be taken to make three hospitals safer.
What is the geology of the area?
According to the USGS: "Seismicity in the Himalaya dominantly results from the continental collision of the India and Eurasia plates .... Northward underthrusting of India beneath Eurasia generates numerous earthquakes and consequently makes this area one of the most seismically hazardous regions on Earth."