A series of earthquakes in Oklahoma occurring within hours of one another has added evidence to earlier scientific claims that the unusual seismic activity registered in the state recently is caused by fracking-related operations, according to independent news website MintPress News.

Last week, seven earthquakes with magnitudes between 2.6 and 4.3 were recorded in a period of 14 hours. Earlier this year, the U.S. Geological Survey and the Oklahoma Geological Survey stated that the increase in the number of earthquakes was likely linked to fracking wastewater "injected into deep geologic formations." According to the statement, such events have been recorded in the United States for almost 50 years and have recently occurred in other locations, including Colorado, Arkansas, Ohio and Texas.

Scientists believe that it is not drilling to extract oil or natural gas from shale rock formations that is directly linked to seismicity, but the common practice of disposing of wastewater in underground wells. Even though the link has not been definitely proved, the U.S. Geological Survey has already started researching the phenomenon by mapping what it refers to as induced, or man-made, earthquakes.