As hydrofracturing, also known as fracking, has become more widespread, it has also grown more controversial.

Beyond groundwater pollution and environmental contamination, the multi-step process of extracting natural gas from the Earth by injecting liquids deep underground has raised concerns that parts of the process may be triggering earthquakes.

In fact, found a new study, small earthquakes are more common than expected near wells that are used for injecting waste fluids from fracking, as well as from other processes, including oil extraction and geothermal energy production.

It’s not the drilling into the earth or the fracturing of rock that causes the quakes, emphasized lead author Cliff Frohlich, a geophysicist and seismologist at the University of Texas at Austin. Instead, it appears to be the disposal of large amounts of fluids by injection deep underground that can trigger certain types of faults.