If we could put our planetary clock on fast forward, Earth’s surface would writhe with activity. Continents would scurry across the globe, oceans would open and close, and new mountains would shoot up toward the sky.
Yet even as mountains rise, they also periodically sink back down when the stress from tectonic collisions triggers earthquakes. These events happen in a cycle, like the chest of a rocky behemoth drawing uneven breaths, explains Luca Dal Zilio, a geophysicist at the California Institute of Technology.
The forces driving this cycle are incredibly complex, and nowhere is that more evident than in the 1,400 miles of jagged peaks that make up the Himalaya. Nailing down the fundamentals underlying this range is vital to understanding the local risk of earthquakes, which threaten hundreds of millions of people living in its shadow.
In a new paper published in Nature Reviews, Dal Zilio and his colleagues have stitched together results from more than 200 past studies of Himalayan geology to lay out the intricate mechanisms behind these geologic respirations—as well as the many challenges that remain.
Since similar geologic “breaths” have been documented around the world, the new work is key to grasping the processes that sculpt many of Earth’s mountain ranges—and figuring out the risks those ranges might also pose. The Himalaya’s lengthy expanse and geologic complexity make it a terrific natural laboratory, says study co-author Judith Hubbard, a structural geologist at Nanyang Technological University in Singapore.
“It’s almost like the Earth is running the experiment for us,” she says.
The birth of a behemoth
The planet’s tectonic plates are in constant motion, reshaping the surface as they pull apart and collide. The Himalaya are the dramatic result of one such tectonic pileup some 50 million years ago, when the Indian continental plate rammed into the Eurasian plate. Both land masses are thick and buoyant, so as the continents compressed and India began to shove beneath Eurasia, the landscape crumpled and the crust thickened, raising the majestic peaks. (Learn more about the mysteries behind this continental smashup.)
To this day India continues marching northward at a rate of nearly two inches each year. But the land doesn’t slide smoothly beneath Eurasia, and as India presses on, the Eurasian plate bunches and bulges. This process drives the mountains slightly higher into the sky in a lengthy inhalation. Eventually, the stress hits a breaking point, and the landmasses shift in a ground-rattling quake—the geologic version of an exhale, or cough.
Different zones in a string of mountains may produce different types or intensities of exhalations. While some violently cough, others might generate a series of hiccups, Hubbard says. And just because a section of mountains exhaled one way at one time doesn’t mean it’ll do so again.
“Even the same patch can have different behaviors at different times,” says Rebecca Bendick, a geophysicist at the University of Montana who was not involved in the new paper. “And pretty much nobody has the foggiest clue why.”
To understand these complexities, scientists have to tie together mountain-building processes that happen on drastically different time scales—from the slow-speed inching of tectonic plates to the near-instantaneous shifts of an earthquake. This is no easy feat: Different measurements are needed to understand each phenomenon, which often enlists researchers from varying geologic specialties. (The latter comes with its own unique challenges, Hubbard says, “Sometimes the same word means two different things to two different people.”)
The new paper attempts to make some of these time-spanning connections in the Himalaya, examining how each may influence the way future earthquakes unfold.
Crossing through time
One major way to bridge the gaps between time scales is to look at the shape of the fracture between the two tectonic plates. In the Himalaya, this fault spans some 1,400 miles, and it has several kinks and bends—remnants from the ancient smashup that raised the mountains in the first place. These features have slowly evolved in the millennia since, and they can influence the progression of a quake today.
In one paper included in the new review, Hubbard and her colleagues found that subsurface bends surround the section of the fault that shifted during the 2015 temblor. This hints that the structures imposed limits on the extent of the break, and thus the magnitude of the quake.
Other structures formed through the ages might be present across the length of the range, and they could similarly limit how far a quake might propagate near the surface, Dal Zilio says. The review paper highlights how gathering the range of available information can help scientists develop a more complete understanding—and more robust computer models—of not only how the range grows, but also its deadly potential.
“The ultimate goal is to know what kinds of earthquakes we can expect and what kinds of damage they will produce,” Hubbard says, adding that doing so requires significant detective work. “If we’re trying to learn about that exhale or cough process but the earth isn’t exhaling or coughing, it’s really hard to learn about it.”
To help fill in the gaps, some researchers are studying scars left from historical earthquakes, she notes. Other researchers are working to craft a more complete map of the fault’s twists and turns and the thickness of sediments near the surface—both of which could affect the location and intensity of future seismic shaking. But it’s quite a challenge, explains Shashank Narayan, a graduate student at the Indian Institute of Technology, Kanpur, who is not an author of the new paper.
He’s learned this from personal experience mapping the structure of the Himalayan fault along a roughly mile-long cross-section in the Central Himalaya using a process akin to sonar. Geologists often rely on seismic waves to decipher what lies deep underground, since their speed and scatter can reveal different rock types or subsurface structures. Narayan and his team created their own waves by thumping the ground with a heavy weight and then “listening” at a distance with instruments known as geophones.
But the terrain in many other regions poses a major obstacle for such surveys, Narayan says. Just to the west of their site, for example, the mountains present drastic changes in elevation over short distances. “You cannot lay down a single sensor in this region,” he says.
Spinning things forward
As a mountain’s cycles of inhalation and exhalation continue, the system itself will also change, further complicating the picture. Some of the accumulated stress from every inhale deforms the rock permanently, which sticks around even after the next geologic exhale. If all of the stress got released with each cough, then there wouldn't be any mountains still standing, Hubbard notes.
And over time, as India continues to inch northward under Eurasia, other landscape features will morph. For one, the position of the active fault will jump, gradually shifting southward as it finds fresh paths to the surface.
Also, Bendick says, “at some point, Nepal will cease to exist.” The Indian plate’s progression over tens of thousands of years causes its southern border to creep ever northward, slowly squeezing Nepal.
“At that very long time scale, nothing is very fixed,” she says. “‘Set in stone’ is not the right phrase.”
For all the lingering uncertainties, though, Bendick says she is impressed by the review paper because of the vast range of data the team was able to bring together, linking each available measurement to the mountain-building process.
“I was struck not by any one piece,” she says, “but just how important it is that all these things work together to shape the world out my window, basically, and the risk that human communities are exposed to.”