Rescue workers search for survivors in Beichuan County.

Rescue workers search for survivors among the debris of collapsed buildings in Beichuan County, in southwest China's Sichuan province. See more pictures of natural disasters.

­AP Photo/Andy Wong

Can humans start an earthquake?

­In May 2008, one of the deadliest earthquakes in history struck Sichuan Province in western China. It registered 7.9 on the Richter scale, leaving a 185-mile (298-kilometer) crack in the Earth's crust and almost 80,000 people dead [source: LaFraniere].

In the aftermath of the disaster, attention quickly turned to a nearby dam, just 3.4 miles (5.5 kilometers) from the epicenter of the earthquake. Many scientists believe the Zipingpu Dam reservoir may have helped trigger the massive quake.­

­But aren't earthquakes natural disasters?

The vibrations of the Earth's surface we call earthq­uakes are typically natural occurrences. They're most often caused by the shifting of plates of rock under the surface of the Earth. These plates move along fault lines, which are places where the otherwise solid rock of the Earth's crust has cracked. When the plates slide against each other or away from each other, the Earth vibrates violently.

Less often, earthquakes are caused by natural occurrences like volcanic eruptions. But vibrations felt at the surface of the Earth can also result from Earth-shaking, man-made events like underground bomb testing and mine collapses -- and the filling and emptying of dam reservoirs.

It's easy to see how an underground explosion can shake the Earth. But a dam? A dam is just holding back water. How can that cause an earthquake?

In this article, we'll see how a dam -- and more specifically its reservoir activity -- can trigger a quake, and we'll find out whether dams have caused earthquakes and other "natural" disasters in the past.

­Most of us accept that our actions can have great effects on our natural environment. In the case of triggering earthquakes, we're talking about man's actions reaching deep underground, all the way down to the crust.

A woman sits amid the rubble of what was once the village of Longarone.

A woman sits amid the rubble of what was once the village of Longarone, below the Vaiont Dam in Italy.

AP Photo

Taking the "Natural" out of "Natural Disaster"

Building a dam is a massive feat of engineering. They are huge structures, and their reservoirs hold vast amounts of water. The Zipingpu Dam, for instance, is 50 stories tall; its reservoir is capable of storing more than a billion cubic meters of water from the Minjiang River. The weight and lubricating characteristics of that water may have triggered the 2008 earthquake.

It's simple if you think about it in terms of the ground's natural state. The earth beneath a river is accustomed to supporting a certain amount of water. That water exerts a certain amount of stress on the ground, and it seeps into the soil to a certain depth, all depending on the natural state of a particular body of water.

­When people dam a river, and expand and contract its water volume with a reservoir, those natural characteristics change -- sometimes quickly and dramatically. We're more familiar with these ­changes being expressed through landslides, when water loosens the ground supporting mountainous rock, and the seasonal raising and lowering of reservoir water depth puts fluctuating strain on the earth. One of the deadliest landslides in history, in northern Italy in 1963, wiped out an entire village of 2,500 people. It occurred when 400 million cubic yards (300 million cubic meters) of mountain rock fell into Vaiont reservoir, and the resulting tidal wave dwarfed the 856-foot-tall (261-meter) dam, washing away the town downstream [source: IR].

Literally dozens of landslides in China have been attributed to the building of the Three Gorges Dam across the Yangtze River [source: Hvistendahl]. In 2003, a month after the initial filling of the reservoir, a landslide in the area killed 14 people. Dozens more landslides occurred in 2006 after the water level was raised again, and another in 2007 buried a bus.

­The mechanisms at work when damming a river triggers a landslide are similar to those that can cause an earthquake. But in the case of the earthquake, the effects are occurring well below the surface.

The Three Gorges Dam

The Three Gorges Dam has already been linked to environmental problems, if not earthquakes.

­AP Photo/Xinhua, Du Huaju

Reservoir-induced Seismicity: Making the Earth Move

­Until recently, the most powerful earthquake attributed to dam activity happened in western India in 1967. Three years after builders completed the Koyna Dam, a 6.5-magnitude earthquake hit the area, killing 180 people.

The phenomenon of dam-triggered earthquakes is known as reservoir-induced seismicity. It's not well-understood, but basically what happens is this:

When a dam is built and the reservoir filled with water, the amount of pressure exerted on the earth in that area changes dramatically. When the water level of a reservoir is raised, pressure on the underlying ground increases; when the water level is lowered, the pressure decreases. This fluctuation can stress the delicate balance between tectonic plates beneath the surface, possibly causing them to shift.

Another factor is the water itself. When the water pressure increases, more of it is forced into the ground, filling cracks and crevices. All of this water pressure can expand those cracks and even create new, tiny ones in the rock, causing greater instability below ground. What's more, as the water sinks deeper, it can act as sort of a lubricant for rock plates that are being held in place by friction alone. The lubrication can cause those plates to slip.

In the case an earthquake, it's difficult to prove beyond a doubt that the culprit was a dam. It's just so hard to know exactly what's going on beneath the surface, with so many factors at work. In the case of the 2008 Sichuan earthquake, for instance, the jury is still out. While 730 minor earthquakes were recorded within the first year of the Zipingpu reservoir's first filling in 2004, most scientists agree it'll take a lot more research before the Zipingpu Dam can be conclusively linked to the big one [source: LaFraniere].

One thing we do know, however, is that a dam cannot cause an earthquake all by itself. The risk factors, specifically unstable fault lines, have to be there already. With the right conditions in place, though, a damn can trigger the event earlier than would have happened naturally, and perhaps even increase its magnitude -- which is why it's so dangerous to build a dam over a known fault.

And it's why so many scientists are warning of terrible results of China's Three Gorges Dam, which is built over the Jiuwanxi and the Zigui-Badong fault lines. Some say it's only a matter of time before the dam triggers a major earthquake, possibly like the one in Sichuan Province in 2008.

For more information on dam-triggered earthquakes and related topics, look over the links on the next page.

Lots More Information

Related HowStuffWorks ArticlesMore Great LinksSources
  • "Dam-induced Seismicity." International Rivers.
  • "Earthquakes Caused by Dams: 'Reservoir-Triggered/Induced Seismicity.'" Probe International.
  • "Earthquakes Triggered By Dams." International Rivers.
  • Hvistendahl, Mara. "China's Three Gorges Dam: An Environmental Catastrophe?" Scientific American. March 25, 2008.
  • Kirby, Alex. "'Earthquake risk' from dams." BBC News. May 9, 2002.
  • LaFraniere, Sharon. "Possible Link Between Dam and China Quake." The New York Times. Feb. 5, 2009.