Devastating quakes are priming the Himalaya for a mega-disaster

Sunrise strikes the Himalayan peaks on November 8, 2018, as seen from Manang, Nepal. PHOTOGRAPH BY DONALD MIRALLE/ GETTY LUMIX

Sunrise strikes the Himalayan peaks on November 8, 2018, as seen from Manang, Nepal. PHOTOGRAPH BY DONALD MIRALLE/ GETTY LUMIX

The sun poked through small breaks in Nepal's overcast skies on the brisk spring morning of April 25, 2015. The day started like any other, but at 11:56 a.m. local time, it turned deadly.

powerful 7.8-magnitude earthquake gripped the region, sending residents scattering as buildings swayed and crumbled with the ground's convulsions. Numerous landslides careened down the rugged terrain—and a deadly avalanche swept down Mount Everest.

The human toll was staggering. The events killed nearly 9,000 people and injured thousands more. The Gorkha earthquake, as it came to be called, left nearby towns and cities in ruins, destroying more than 600,000 houses. Nearly four years out, with billions of dollars already spent, recovery is ongoing.

But a number of studies since then have raised a concerning point: The earthquake likely wasn't the worst the region has in store.

A study published January 3 in Nature Communications provides new evidence that, rather than releasing seismic tensions in the crust, the 2015 quake likely loaded the surrounding region for an even more destructive mega-earthquake, which could clock in at magnitudes of 8.5 or higher. The study’s numerical simulations probe the conditions behind how and why moderate earthquakes trigger massive ones, helping scientists understand the accumulation of stress along faults.

“This result has disconcerting implications for the seismic risk of the Indo-Gangetic plain that extends to the north of the Indian peninsula and is populated by over 400 million people,” geophysicist Luca Dal Zilio, the lead author of the study, says via email.

It's impossible to say exactly when this megaquake will actually happen, whether years, decades, or centuries from now. But understanding the risks to the region is vital for getting protections in place for the many populations nestled atop this restless geologic giant.

Loading up an earthquake

Nepal is no stranger to earthquakes. The Himalaya are among the most seismically active regions in the world, the result of an ongoing collision between two continental plates: the Indian and the Eurasian.

The Indian plate continually marches northward a few centimeters each year, shoving its way under the Tibetan plateau in fits and starts. Each jerky advance causes earthquakes of varying intensity. Think of it like shooting a rubber band, explains Rebecca Bendick, a geophysicist at the University of Montana who was not involved in the new study. Tension in the crust builds like stretching the band back. At some point, you have to release it, turning all the stored potential energy into kinetic energy as the projectile flies through the air. That's essentially an earthquake.

In the Himalaya, this release or geologic shift usually occurs along the boundary between the two plates—what's known as the Main Himalayan Thrust. And it can have visible effects on the surface, elevating the landscape feet at a time.

Super-cycle builds super-sized quakes

Earthquakes in the Himalaya, however, present a curious puzzle: “When you look at the historical seismicity, it looks like you don't have enough earthquakes to balance the energy that is accumulating because of this loading,” says Caltech's Jean-Philippe Avouac, coauthor of the new study.

“It has to be released at some point,” he says. “So the question is: How is it released?”

Read the full story on National Geographic’s website