How earthquakes move mountains - and what comes next

On May 12, 2008, central China was hit by one of the deadliest earthquakes in recent memory. The magnitude 7.9 Wenchuan Earthquake struck along the edge of the Tibetan Plateau, triggering violent landslides across the steep slopes of the Longmen Shan – known as the Dragon’s Gate Mountains.

The disaster killed more than 69,000 people, but nearly a third of those deaths weren’t from shaking alone. They came from geohazards – including more than 60,000 landslides that came tumbling down the mountains in the hours and days that followed.

Now, after nearly two decades of research, scientists at UC Santa Barbara and the University of Southern California say the story of that earthquake still isn’t over. Their recent study shows that the debris shaken loose by the quake is still working its way through the region’s rivers.

The team tracked how sediment surged into the Min River and how it changed the river’s behavior – possibly for decades. And their work could help answer a deeper question: Do earthquakes build mountains, or wear them down?

A river loaded with debris

When the quake hit, millions of tons of rock and soil collapsed into nearby streams. Much of it ended up in the Min River, which runs through the Longmen Mountains. The river became a conveyor belt for that debris.

Geologists call this “sediment flux” – the amount of material a river carries. It shows up in two forms. The first is suspended sediment, fine particles carried in the water, easy to measure. The second is bedload – coarser stuff like gravel and boulders that roll or bounce along the bottom. That’s harder to track.

“Before our work, people mostly focused on the sediment of very fine size,” said Gen Li, an assistant professor in UC Santa Barbara’s Department of Earth Science.

Suspended sediment is routinely measured by agencies around the world. But bedload? That’s trickier. You can’t collect it with a bottle. And during an event like the Wenchuan Earthquake, there’s suddenly a lot of it.

Scientists have long suspected that this surge in bedload plays a big role in post-quake flooding. The idea is simple: if a riverbed fills with debris, the channel gets shallower. Then, when a flood comes, it spills over more easily. But proving this has been a challenge.

How a dam became a natural lab

Back in 2001, the Sichuan Provincial Electric Power Company started building the Zipingpu Dam. By 2006, the reservoir began filling, just 20 kilometers downstream from where the Wenchuan Earthquake would later strike. It was an unintentional stroke of luck for science.

That dam turned into the perfect sediment trap. Shortly after the quake, Li and his collaborators – including researchers from the Chinese Bureau of Hydrology – began surveying it.

Daily data already existed for suspended sediment. What the scientists needed was the rest of the picture. They set out, year after year, using sonar to map the floor of the reservoir.

Each expedition added a new layer to their data. Over time, they could measure how much total sediment had built up. From there, they did the math. Total sediment minus suspended sediment equals bedload.

When earthquake effects last a decade

After the quake, the amount of total sediment in the Min River grew sixfold. But the real shock came with the bedload, which had increased 20 times. That is not normal. Bedload usually makes up about 20% of sediment in a mountain river. Here, it made up 65 percent.

The result didn’t surprise co-author Josh West. He had suspected that sediment flux would spike after a major earthquake, especially in the form of increased bedload.

What did surprise the team was how long it lasted. The elevated sediment flow didn’t return to normal after a few years. It stayed high for at least a decade.

“In fact, from the data we’ve collected so far, there’s no evidence yet of the total sediment flux declining back to background levels,” West said.

“Usually, we think the influence from earthquakes may last, at most, a few years after the main shock,” said Professor Li. “But this data shows that this is not true.”

Earthquakes set off a chain reaction

The study adds weight to a growing idea among scientists: the danger from natural disasters doesn’t stop once the main event ends. Earthquakes can set off a chain reaction. One disaster leads to another.

“Earthquake-triggered landslides are a great example,” West said. “As we prepare for natural disasters, we often think of them as being discrete events. But what’s left out of that is the longer tail that follows.”

In other words, rebuilding too quickly – or rebuilding in the same way, in the same place – can be risky. The land has changed. Riverbeds are higher and flood zones are different. What was once a safe 10-year floodplain may now be a ticking time bomb.

Do earthquakes build mountains?

Earthquakes are thought to push up mountain ranges. But they also cause landslides that scrape them back down. So which effect wins?

Professor Li looked into this in an earlier study. He used satellite images to estimate that the Wenchuan Earthquake caused about three cubic kilometers of material to slide off the Longmen Mountains. “That is around half of the sediment flux of all the rivers in the world over one year,” he said.

Li also looked at how much rock the quake pushed upward. Turns out, it was about the same. That means the quake may have added and removed equal amounts of mass. Whether the mountain grows or shrinks depends on what happens next.

If the river can carry all that debris away quickly, the mountain stays tall. If the debris piles up and clogs the system, the mountain starts to erode. So far, the Min River has flushed out about 10% of the landslide debris over ten years.

“The fact that the pace was sustained for ten years was a surprise on its own,” West said. But predicting what happens in the next few decades is harder. Watersheds evolve and landscapes shift. The balance is fragile.

Many questions remain

Professor Li is now investigating why this particular river carried so much bedload. The 2015 Gorkha Earthquake in Nepal, for instance, didn’t trigger the same kind of sediment surge in Himalayan rivers. This suggests other factors are at play.

The rock itself might be the key. Its type, texture, and how it breaks apart all affect how much debris a quake generates. The size of the grains also matters. And how those grains move downstream is still being modeled.

Li’s team is now studying these patterns in more detail – using field data and computer models to understand how sediment flows over time.

“It’s very satisfying to have been able to quantify something that we’ve struggled to quantify before and that has a wide range of relevance, from hazards to long-term consequences for understanding the evolution of topography over long periods of time,” said West.

The full study was published in the journal Nature.

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