On April 25, 2015 at 11:56 local time, part of the tectonic boundary between India and the rest of Eurasia began to slip, sending powerful earthquake waves to Nepal’s capital city of Kathmandu just 50 miles away. While this earthquake spread damage throughout the region beyond Nepal, it did not destroy the capital. USGS scientists were surprised at how well the city and the surrounding valley fared -- in contrast to the devastation in rural areas -- and are looking for lessons that could help reduce future earthquake losses in Nepal and elsewhere.

A Worst Case

Like many cities around the world, Kathmandu sits in a valley that, over time, was filled with sediments eroded from the adjacent uplands. This setting tends to amplify the shaking and damage of an earthquake.

The Kathmandu valley also lies on the edge of the Eurasian tectonic plate atop one of Earth’s major subduction zones. This is one of the world’s most seismically hazardous regions. A M7.5-8.0 event in 1943 severely damaged Kathmandu and killed about 10,600 people. In recent years, the valley’s population has grown rapidly to about 2.5 million people.

USGS earthquake scientist Sue Hough, lives in California and first learned about the earthquake while getting up that morning. 

“My heart just about stopped. I had visions of a far worse humanitarian disaster than the one that came into view -- which, of course, was plenty bad enough.”

The location and other data dispatched from the USGS National Earthquake Information Center placed the focus of the M 7.8 main shock at a depth of only 8 miles with a 75-mile-long rupture and substantial seismic energy propagating east -- very close to the city. The motion on the fault shifted Mt. Everest 1.2 inches to the southwest.

Shaking from the April 25 earthquake was felt in much of Nepal, northern India, parts of Bangladesh, Bhutan, China, and as far away as Pakistan. Overall, the losses included about 9,000 fatalities and 23,000 injuries, with more than 700,000 damaged or destroyed houses in Nepal. In Nepal, the fatalities and injuries were mostly caused by homes made of rock, brick, or concrete that were not properly reinforced, and by landslides.

In India, more than 78 people were killed and 560 were injured. In China, at least 25 people were killed and 383 injured. Twenty major hydroelectric power plants in the region were damaged.

Landslides

About 200 people were killed in the Langtang landslide, 20 people were killed and 120 injured from an avalanche at the Mt. Everest base camp.

USGS scientists went to Nepal following the deadly earthquake and estimate they witnessed tens of thousands of landslides during their field work.

The Driving Force of the Unexpected

When Hough arrived in Nepal, about one month after the main shock, she was struck by how well the Kathmandu valley had fared. 

“In many places you wouldn't know an earthquake had happened, much less a magnitude 7.8 on a shallow fault directly beneath the valley,” said Hough.

The earthquake originated at a depth of about 8 miles, yet it didn’t rupture the surface. There was little evidence of liquefaction, and aside from monuments, most of the valley’s buildings were not seriously damaged.

With local data from instruments installed at only four sites in the Kathmandu area prior to the mainshock by the USGS and Nepal’s National Society for Earthquake Technology, and a recording from the Department of Mines and Geology in Kathmandu, scientists began a major effort to collect shaking data by all means possible to explain why the valley’s damage wasn’t worse. They ended up with the largest conventional intensity data set compiled to date for any earthquake. It confirmed that mainshock intensities were mostly moderate and that the ancient sediments absorbed rather than amplified the shaking.

In contrast, the rural villages and mountainous areas had pockets of severe damage and were marred by landslides. As bad as that situation was, it could have been much worse.

Science on an International Scale

Learning why Kathmandu held up as well as it did could help cities elsewhere in the world to reduce their hazards. In Nepal, Hindu tradition teaches that destruction begets progress. This tragic and devastating event opened an opportunity to continue the training and capacity development work that was ongoing but difficult to support before the earthquake. 

Hough and another USGS scientist Dan McNamara are in Nepal to participate in an international lessons-learned workshop in Kathmandu on the anniversary of the M7.8 earthquake. They will lead a training workshop for the National Society for Earthquake Technology - Nepal, the USGS’s lead partner agency since before last year's earthquake. They will also assess damage, improve earthquake monitoring, and assist the U.S. Agency for International Development and the U.S. Embassy.

The USGS efforts in Nepal are done with support from the USAID and in collaboration with organizations in the United States and Nepal.

Sources/Usage: Public Domain.

Hough recounts a moment from her last trip. When walking down a road in remote hard-hit area, wearing a USGS cap, she passed two people who were speaking Nepali, when one said "USGS." The Nepali colleague from the embassy who was with Hough said, "You're famous. Everybody knows the USGS."  

“It really brought home the wide reach of the work we do,” said Hough.

Learn more:

Nepal M7.8 Event Page

M7.3 Aftershock Page

USGS News Release - Landslides Triggered by Nepal Earthquakes

Seismological Research Letters Issue with Nepal Focus Section

The M7.8 Nepal Earthquake, 2015 – A Small Push to Mt. Everest

The April-May Nepal Earthquake Sequence Educational Slides