A city in Texas has been hit with a string of earthquakes including at least 12 over a single 24-hour period, an occurrence that may be linked to the drilling practice of fracking. The U.S. Geological Survey has confirmed two more earthquakes shook the North Texas city of Irving Thursday following a series earlier last week. Previous studies have linked fracking to earthquakes in New Mexico, Colorado and Ohio through the injection of high-pressure water and chemicals to crack shale rock underground.