Can humans start an earthquake?

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.

­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.