‘Breathing’ mountains could help us understand earthquake risks
As the Earth’s surface shifts over time, oceans have opened and closed while new mountains climb towards the sky.
But mountains can also sink back to Earth, usually due to stress caused by the same collisions of tectonic plates that triggers earthquakes.
These events happen in cycles, and you could imagine it like the chest of a rocky giant breathing unevenly, explains Luca Dal Zilio, a geophysicist at the California Institute of Technology.
The best example of this phenomenon? The 2,200 kilometres of peaks that make up the Himalaya.
By understanding the complex forces driving this cycle, the local risk of earthquakes that threaten millions of people living nearby can be more well-understood.
Since similar geological ‘breaths’ have been documented worldwide, the review Dal Zilio and colleagues recently published in Nature Reviews could be the key to understanding the processes behind many of Earth’s mountain ranges - and the risks they might pose.
Compiling more than 200 studies of Himalayan geology, the paper looks to lay out the intricate mechanisms behind this ‘breathing’.
The lengthy expanse and geologic complexity of the Himalaya make it a terrific natural laboratory, says study co-author Judith Hubbard, a structural geologist at Nanyang Technological University in Singapore.
Inhaling and exhaling over time
The Himalaya formed from a tectonic pileup about 50 million years ago, when the Indian continental plate crashed into the Eurasian plate.
To this day, India continues moving northward by almost two inches every year.
However, the land doesn’t slide under Eurasia, instead causing the Eurasian plate to bulge and bunch and drive the mountains slightly higher in a long inhale.
Eventually, this pressure will hit a breaking point, where the land masses will shift in an earthquake in a geologic exhale, or cough.
A deadly example of this exhalation came in 2015, when a 7.8-magniture earthquake caused a part of the Himalaya to sink by almost 600 centimetres.
Different parts of a mountain range can exhale at different intensities. While some cough violently, others might experience a series of hiccoughs. They might not exhale the exact same way each time either.
“Even the same patch can have different behaviours at different times,” says Rebecca Bendick, a geophysicist at the University of Montana. “And pretty much nobody has the foggiest clue why.”
Putting the pieces together
To understand how this process works, scientists have to piece together mountain-building processes that happen on varying scales of time - from slow moving tectonic plates to near-instantaneous shifts of earthquakes.
Since these phenomena are measured differently, looking at the shape of the fracture between the two plates can help scientists bridge the gap.
In the Himalaya, the fault spanning 2200 kilometres has several kinks and bends remaining from the original collision that formed the range. These features have continued to slowly evolve and can influence how an earthquake progresses today.
In the recent review, Hubbard found a paper suggesting that structures surrounding the fault - such as bends beneath the surface - limited the magnitude of the 2015 quake.
Dal Zilio says that other structures that might be present across the rest of the range could similarly limit how far a quake might spread.
“The ultimate goal is to know what kinds of earthquakes we can expect and what kinds of damage they will produce,” Hubbard says. “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 fill in the gaps, some researchers are looking at the scars left from past earthquakes.
But there are many challenges that present major obstacles, especially when the terrain is too difficult to measure using current technology.
Shifting forward
As the cycle of inhalation and exhalation continues, the system will also change and make understanding it even more difficult. Some of the accumulated stress from every inhale will permanently deform the rock, even after the next exhale, as the release of all of the stress would mean that no mountain would still be standing, Hubbard notes.
As India continues to inch closer under Eurasia, other landscape features will also change.
Bendick says, “at some point, Nepal will cease to exist”, as the Indian plate’s movement over the next tens of thousands of years will cause the southern border to move ever northward and slowly squeeze Nepal.
“At that very long time scale, nothing is fixed,” she says. “‘Set in stone’ is not the right phrase.”