the man-made quakes jolted once stable regions in eight states , including parts of alabama , arkansas , colorado , kansas , new mexico , ohio , oklahoma and texas .
earthquakes , they said , were sometimes caused by fracking , in which large volumes of water , sand and chemicals are pumped into rock formations to free oil or gas industry injecting wastewater deep underground .
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 .