A 5.1 magnitude earthquake shook northwestern Oklahoma Saturday afternoon, an event the U.S. Geological Survey said could relate to wastewater injection in the area.

The U.S. Geological Survey said the earthquake could have been caused by wastewater injection, which can involve taking the water used in fracking and injecting it into underground wells to not contaminate drinking water. If the quake is confirmed to be related to fracking, it would be one of the largest earthquakes caused by hydraulic fracturing.