A new study has found that the 7.8-magnitude earthquake that shook Nepal back in April caused fewer landslides than anticipated. There is still, however, some debate on the severity of the quake’s impacts.

Scientists typically expect up to hundreds of thousands of landslides in the aftermath of a large earthquake in mountainous terrain.

In the case of Nepal’s Gorkha quake, satellite images showed only 4,312 landslides, as reported by a team of researchers from University of Arizona and over 35 institutions worldwide on Dec. 16 in the journal Science. No massive floods caused by glacial lakes also took place after the April 25 quake.

Lead author and University of Arizona (UA) senior associate scientist Jeffrey Kargel called it "a really bad earthquake," with more than 9,000 deaths in Nepal and three other countries. "As horrific as this was, the situation could have been far worse for an earthquake of this magnitude," he said.

Thinking of a way to help, he employed his satellite imaging expertise to map out the site of landslides, particularly in remote mountainous areas. Together with Gregory Leonard, a UA geologist, Kargel worked with other scientists from the Global Land Ice Measurements from Space (GLIMS) consortium to identify the impact on Himalayan glaciers and search post-quake landslides.

Satellite Image Findings

The team, along with the NASA Applied Sciences Disaster group, obtained thousands of satellite images from governments and private entities, with volunteer analysts from nine countries accessing data from more than 10 satellites from four nations.

The scientists also used media reports, eyewitness accounts and photos and helicopter-based field assessments in their analysis from the day of the quake to June 10, the monsoon’s onset.

Furnished an official report a month later, the Nepalese government then mobilized a geohazard task force, which employed more geologists to assess current and potential dangers.

The findings showed that the 4,312 landslides that occurred within a six-week period after the quake were far fewer than those of quakes of similar magnitude in similar terrain. After a survey of 491 glacial lakes, scientists also found only nine affected by landslides, with no flooding revealed by images.

A Database Of Geohazards

The task, initially humanitarian in purpose, offered scientists a large database of geohazards from the Nepal earthquake and others of its kind.

Quakes in mountainous areas are followed by landslides minutes or years after onset, with susceptibility varying from one quake to another.

The data also offered a glimpse of an unexpected pattern of the landslides’ locations.

Co-author Eric Fielding of NASA’s Jet Propulsion Laboratory used satellite radar imagery in creating a terrain map, and found that the Earth’s surface dropped or rose by nearly 5 feet in different places.

The team overlaid Fielding’s map with the landslide map and found that most of the recorded landslides happened in areas where the ground dropped instead of lifted. Kargel dubbed it an unexpected and never-before-seen pattern.

At present, the scientists are still probing where the Gorkha earthquake had fewer landslides and why they occurred that way. One potential reason was that the quake caused less surface shaking than others similar to it, as seismologists recorded in Kathmandu as well as other sites.

The team’s analysis was discussed last Wednesday at the fall meeting of the American Geophysical Union in San Francisco.

Photo: Edward Dalmulder | Flickr

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