June 14, 2009 -- The vast majority of earthquakes in the world occur naturally. But that doesn't mean some can't be man-made.

Scientists call this phenomenon "induced seismicity," a geological term for minor earthquakes or tremors caused by human activity.

"Human-induced seismicity usually occurs at low levels and rarely causes damage," noted Colorado attorney Darlene Cypser, who has researched liability issues related to man-made quakes and tremors.

"Some induced earthquakes ... have caused serious property damage, and even loss of life," she wrote in a review of several questionable quakes. "This potential for personal injury and property damage generates questions about liability."

Large man-made reservoirs, for example, have triggered earthquakes, according to the U.S. Geological Survey, which monitors earthquake activity around the globe.

Experts believe water pressure in a dam's reservoir can create cracks and faults at the bottom, causing seismic activity that would not otherwise occur.

Such tremors usually are detected shortly after a massive reservoir has been filled to capacity.

The first earthquake related to a man-made reservoir was observed in Algeria in 1932. Since then, the largest reservoir-related quake was believed to have occurred in West India in 1967. It measured 6.3 in magnitude and flattened a village, killing 180 people.

Other man-made activities linked to earth tremors and quakes include coal mining and oil and gas drilling.

"You can find past examples where the drilling and fracturing of rock have caused earthquakes," said Paul Caruso, a geophysicist with the geological service's National Earthquake Information Center in Golden, Colorado.

A 1998 State of Texas Hazards Analysis, by the Governor's Division of Emergency Management, noted that "some human activities are known to cause or trigger earthquakes," including petroleum production.

The analysis noted there "have been earthquakes associated with oil and gas production at a number of fields," including those in East Texas.

"None of the quakes have been very damaging or very large; the largest had a magnitude 4.7," the analysis noted.

However, it contained an important disclaimer:

"Usually petroleum production does not cause earthquakes. In Texas, there are more than 2,000 oil and gas fields but only about five seem to have generated earthquakes."