Some seven million people in the United States live in areas that are vulnerable to earthquakes caused by wastewater disposal, according to a report published by the U.S. Geological Survey.

The states with areas at the highest risk of such disasters include Oklahoma, Kansas, Texas, Colorado, New Mexico, Ohio and Alabama.

Some states have experienced an increasing number of man-induced earthquakes, many of which have been attributed to fracking (also known as hydraulic fracturing). Oklahoma, for instance, has experienced hundreds since 2009 after this practice became common in the area. Prior to this, the state had an average of two earthquakes per year.

In the fracking process, water and other fluids are injected into shale rock below the surface to free oil and gas trapped within it. However, this yields a high volume of chemical wastewater, which is disposed by pumping it into deep wells for storage.

The water often seeps into gaps or cracks within the crust, known as faults, and acts as a wedge between opposite sides of the crack.

While most earthquakes caused by wastewater disposal are relatively small—in the range of magnitude 3—some have reached magnitude 5. Also, there is uncertainty as to how powerful they can become.

According to The Washington Post, scientists “do not know if there is an upper limit on the magnitude of induced earthquakes.”