More than a dozen areas in the United States have been shaken in recent years by small earthquakes triggered by oil and gas drilling, a government report released Thursday found.

The man-made quakes jolted once stable regions in eight states, including parts of Alabama, Arkansas, Colorado, Kansas, New Mexico, Ohio, Oklahoma and Texas, according to researchers at the U.S. Geological Survey.

Experts said the spike in seismic activity is mainly caused by the oil and gas industry injecting wastewater deep underground, which can activate dormant faults. A few instances stem from hydraulic fracturing, in which large volumes of water, sand and chemicals are pumped into rock formations to free oil or gas.

Many studies have linked the rise in small quakes to the injection of wastewater into disposal wells, but the Geological Survey's report takes the first comprehensive look at where the man-made quakes are occurring.

"The hazard is high in these areas," said Mark Petersen, who leads the agency's national mapping project.

Oklahoma lately has been rocked by more magnitude-3 quakes than California, the most seismically active of the Lower 48 states, Petersen said.

Oklahoma was not on scientists' radar until recently when the state experienced a spate of quakes, the largest registering a magnitude-5.6 in 2011. Earlier this week, the Oklahoma Geological Survey acknowledged that it is very likely most of the recent shaking is from wastewater disposal.

Many faults awakened by drilling have not moved in millions of years, Geological Survey geophysicist William Ellsworth said.

"They're ancient faults," Ellsworth said. "We don't always know where they are."

A message to the American Petroleum Institute was not immediately returned. The industry group has said efforts are made to map fault lines where drilling occurs.

A group of experts met last year in Oklahoma to pinpoint seismic hotspots around the country caused by induced quakes. Scientists initially identified 14 regions affected by quakes linked to drilling. They later added three other high-risk areas — northern Oklahoma-southern Kansas; Greeley, Colorado; and Azle, Texas.

The findings were released at a Seismological Society of America meeting in Pasadena, California.

Seismic hazard maps produced by the Geological Survey and used for building codes and insurance purposes don't include quakes caused by the oil and gas industry. Scientists said it's difficult to know what jobs will trigger shaking.

Researchers study man-made quakes in the affected areas to determine how often they are expected to occur in the next year and how much shaking they would produce.