More than a dozen areas in the U.S. 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 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. 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. 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.