Kansas officials have for the first time publicly linked fracking activity—specifically, the injection of saltwater into wastewater wells—with a dramatic uptick in earthquakes across the state.

The number of earthquakes in Kansas went from none in 2012 to more than 120 in 2014, the Kansas Geological Survey has said. Early Monday, a spate of four earthquakes sent tremors through southern Kansas and northern Oklahoma, registering between 2.7 and 4.1 on the Richter scale.

“We can say there is a strong correlation between the disposal of saltwater and the earthquakes,” Rick Miller, geophysicist and senior scientist for the Kansas Geological Survey, told the Lawrence Journal-World. During fracking operations, a mixture of saltwater and chemicals is used to break apart tight underground rock formations to release oil and gas, after which the fluids are injected deep into disposal wells.

Within the past year, scientists in Ohio, Texas, and Oklahoma have all linked increased seismic activity with fracking operations.