Nepal earthquake offers hints of worse to come

On April 25, a major earthquake devastated Nepal. The shaking collapsed buildings, triggered avalanches and killed at least 5,000 people.

Unfortunately, the magnitude-7.8 quake won’t be the last to hit this region of southern Asia. Even larger quakes could strike to the west, scientists warn. The same goes for east of Nepal, in nearby Bhutan.

Earthquakes happen along fractures in Earth’s crust, called faults. Some faults mark the boundaries between giant slabs, called tectonic plates. These plates make up Earth’s uppermost layer. Nepal sits atop the boundary between the Indian and Eurasian plates.

The coming together of these two plates also uplifts the Himalayan mountain range. This range’s soaring peaks include the world’s tallest mountain, Mount Everest. But as plates slide and push against each other, some stress will develop. And it keeps building until parts of Earth’s crust slide or slip. This triggers a quake.

The recent Nepal earthquake relieved some stress along just one stretch of that fault. In time, other segments along that boundary between the plates could produce even larger quakes, scientists warn.

The size and location of future quakes in the region will depend in part on the shape of the fault responsible for the Nepal quake. Right now, researchers don’t fully understand that shape. However, new research has provided some glimpses. And it suggests that some areas could be even more at risk of strong quakes than previously thought.

“The hazard isn’t gone,” says Kristin Morell. She’s a geologist of the University of Victoria in Canada. “The Himalayas are a very long mountain belt,” she notes. “And strain is still building up in all the other regions from Pakistan all the way to eastern Tibet.”

Slipping slowly, until wham!

For around 50 million years, the Indian tectonic plate has been slipping under the Eurasian Plate. The Indian plate slips at a rate of about 15 to 20 millimeters (0.6 to 0.8 inch) each year. This slow slide doesn’t always go smoothly. The Indian plate dives downward at different angles along various parts of the boundary between the plates. In some places, the plate is almost level. At others, it plunges at an angle of more than 30 degrees.

Steeper angles increase friction between the plates. And with friction, stress builds up. The energy can accumulate over centuries — only to be released in seconds during an earthquake. The spot where the Nepal quake began, called its epicenter, was along one of these strongly sloping sections.

When an earthquake hits, it doesn’t strike along the full stretch of a fault. Scientists believe this is because of physical barriers along the fault that can halt the movement. These barriers can include locations where the angle between two plates changes quickly. In general, the larger an uninterrupted fault segment, the more powerful the earthquakes it can produce.

At places, the Himalayan fault hides tens of kilometers (miles) below ground. Experts know little about large sections of this fault. Luckily researchers have indirect ways to “eye” the fault’s layout. The buried fault can cause changes on the surface above. For example, where the Indian plate dips steeply, it pushes the overlying ground upward. This uplift steepens the terrain. And that causes erosion. Streams here cut deeply into the ground too.

By looking at the surface, Morell and her colleagues have been able to make out the structure of part of the fault. This segment lies west of where the recent earthquake struck. The area includes a spot where the angle between the plates rapidly steepens. A magnitude 8 or greater quake could happen on either side of this transition, the researchers reported online March 12 in the journal Lithosphere. (The earthquake scale is not linear, so a magnitude 8 quake would actually be at least 1.5 times more powerful than a 7.8 quake.)

A separate research team used a similar technique to look at a fault farther east. That fault is in the nation of Bhutan. There, the experts uncovered a wide segment of fault that appears to have no boundaries. That means this large segment of uninterrupted fault has the potential to produce even larger earthquakes than previously expected. Details on this fault will appear in a future issue of Geophysical Research Letters.

“If there is a big earthquake in Bhutan, it could be larger than the recent earthquake in Nepal,” says coauthor Rodolphe Cattin. He is a geophysicist at the University of Montpellier in France. (Geophysics is the study of matter and energy on Earth and how they interact.)

A fault to the west of the Nepal quake’s epicenter could be prepped for a big quake as well, says Simon Klemperer. He’s a geophysicist at Stanford University in Palo Alto, Calif. In fact, that fault segment is at least twice as long as the one that recently rattled the country’s eastern side. This western segment has been building stress since its last major quake in 1505.

“The earthquake I worry about is not the one that happened” on April 25, says Klemperer. “It’s the one that could be a magnitude 8.6 to the west.”

Power Words

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angle  The space (usually measured in degrees) between two intersecting lines or surfaces at or close to the point where they meet.

earthquake    A sudden and sometimes violent shaking of the ground, sometimes causing great destruction, as a result of movements within Earth’s crust or of volcanic action.

crust    (in geology) Earth’s outermost surface, usually made from dense, solid rock.

epicenter   The underground location along a fault where an earthquake starts.

erosion  The process that removes rock and soil from one spot on Earth’s surface and then deposits the material elsewhere. Erosion can be exceptionally fast or exceedingly slow. Causes of erosion include wind, water (including rainfall and floods), the scouring action of glaciers, and the repeated cycles of freezing and thawing that often occur in some areas of the world.

fault  In geology, a fracture along which there is movement of part of Earth’s rocky, outermost shell, or lithosphere. 

fracture    (noun) A break. (verb) To break something and induce cracks or a splitting apart of something.

friction  The resistance that one surface or object encounters when moving over or through another material (such as a fluid or a gas). Friction generally causes a heating, which can damage the surface of the materials rubbing against one another.

geology  The study of Earth’s physical structure and substance, its history and the processes that act on it. People who work in this field are known as geologists. Planetary geology is the science of studying the same things about other planets.

geophysics    The study of matter and energy on Earth and how they interact.

plate tectonics  The study of massive moving pieces that make up Earth’s outer layer, which is called the lithosphere, and the processes that cause those rock masses to rise from inside Earth, travel along its surface, and sink back down.

range  The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists. (in math or for measurements) The extent to which variation in values is possible.

strain (in physics) The forces or stresses that seek to twist or otherwise deform a rigid or semi-rigid object.

stress (in physics) Pressure or tension exerted on a material object.

tectonic plates  The gigantic slabs — some spanning thousands of kilometers (or miles) across — that make up Earth’s outer layer.