Nepal’s quake could have been much worse

The magnitude 7.8 earthquake in Nepal in April 2015 triggered far fewer landslides and much less damage to glacial lakes than expected, a new study has found.

By contrast, quakes in in mediaeval times entirely recast the landscape, according to data from another paper.

Both papers published today in the journal Science.

One team used satellite imagery from more than 10 satellites and four countries to help identify areas where landslides had happened, particularly in remote areas.

To this data was added information from media reports, eyewitness photography and field assessments from helicopters.

The researchers limited their analysis to the period between the earthquake and the onset of the monsoon season in June, in order to distinguish between earthquake-related landslides and landslides triggered by rain.

The researchers found that the 4,312 landslides that happened within six weeks of the quake. That is far fewer than has occurred after similar-magnitude quakes in other mountainous areas. 

“It was a really bad earthquake – over 9,000 fatalities in four countries, primarily Nepal,” said team leader Jeffrey Kargel from the University of Arizona. “As horrific as this was, the situation could have been far worse for an earthquake of this magnitude.”

Co-author Eric Fielding of NASA’s Jet Propulsion Laboratory, Pasadena, California, used satellite radar imagery to create a map of the terrain that dropped and rose during the earthquake. The results reveal Earth’s surface dropped almost 1.4 metres in some places, and rose as much as 1.5 metres in others.

By overlaying Fielding’s map with the landslide map, the scientists could see if there was any correlation between the number of landslides and Earth’s displacement. They found that most of the documented landslides occurred in areas where the ground surface dropped down, rather than in areas where the ground was uplifted. That pattern was unexpected and hasn’t been observed before, Kargel said.

The research team is currently investigating why there were fewer landslides than expected and why they are distributed as they are. One possible explanation is that the quake caused much less shaking at the surface than other quakes of similar magnitude.  

In the second study, professor Oliver Korup of the University of Potsdam discussed the results of a geologic survey of Pokhara, Nepal’s second largest city.

The researchers found Pokhara is built upon debris generated by at least three mediaeval quakes of magnitude 8 or greater, occurring in approximately 1100, 1255 and 1344 A.D.

In a grim Quake debris from the Annapurna Massif buried approximately 148 square kilometres of the Pokhara Valley, invading river valleys for up to 7 kilometres upstream.

Nepal earthquake 2
The Annapurna Massif thrones above the city of Pokhara, and has shed several cubic miles of earthquake debris to form the city’s foundations.
University of Potsdam Germany
Pokhara deposits
Gravel layers deposited by catastrophic outburst floods have formed just after several large mediaeval earthquakes
University of Potsdam

Korup says Pokhara’s landscape shows that large Himalayan earthquakes can unleash mass movements far larger than those sustained in the 2015 quake.

“Pokhara’s recent geological foundations are a rare opportunity to grasp the landscape-changing impacts of large Himalayan earthquakes,” Korup said. “The way that rivers are still working their way through the quake debris many centuries later illustrates a serious long-term consequence of strong seismic ground shaking in mountains.”

In a grim piece of accurate forecasting, Cosmos magazine predicted the earthquake in Nepal by several months. We looked at ongoing research into past quakes that is helping identify where and when new ones might take place.

For more detail of how the geologists are working see Kathmandu’s earthquake nightmare.

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