The third strongest earthquake ever reported in Oklahoma struck Saturday morning. Registering a 5.1 on the Richter scale, the quake was followed by several aftershocks, including one with a magnitude 3.9, according to the U.S. Geological Survey. There has been an increase in seismic activity in the state in recent years, matched by a flurry of hydraulic fracturing and horizontal drilling. Seismologists warn that fracking could be the direct cause of the earthquakes.

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The USGS said there is no known link between this weekend’s earthquake and fracking activities, but the practice has been connected to similar quakes in other regions. A magnitude 4.6 quake in British Columbia in December 2015 was recently confirmed to be the result of hydraulic fracturing, and is the largest earthquake to date to have a proven link to the oil and gas industry.

Related: New interactive map reveals sites of fracking accidents across the U.S.

This is also not the first Oklahoma quake to get people talking about fracking as a possible cause. The increase in seismic activity in the state since 2009 has led local officials to suspect fracking, including the 2011 5.7 quake which was the largest ever recorded in Oklahoma. An ongoing investigation into the causes of this most recent quake will determine whether there is a link to fracking, but the evidence is mounting.

Via Reuters

Images via Shutterstock and USGS