Human activities can cause earthquakes. It sounds a little crazy to say, but it's something we've known about for a while. For instance, seismologists say that a 6.3 magnitude quake that struck India's Maharashtra state in 1967 was directly caused by the 1963 construction of a major dam and reservoir project in that region.
Basically, fault lines exist. When we start messing with them—applying very heavy weights, taking very heavy weights away, or lubricating the fault line with various liquids—we can trigger movement. Usually, these are not large earthquakes. But they can be felt. And they are something we want to avoid.
Now, a study done by the Ohio State Department of Natural Resources has concluded that a series of small quakes in that state were directly caused by improper disposal of wastewater from a natural gas fracking operation.
Fracking, as a reminder, is a process of freeing up trapped natural gas by injecting liquid into the Earth. The force of the water cracks rocks so the gas can flow through. This is not the part of the process that's been implicated in the Ohio earthquakes, however. Instead, it's about what happens to that liquid once the fracking is done.
Fracking liquid is called "brine" and it's often referred to as being water, but it's actually water mixed with a lot of other stuff, some of it toxic. Wastewater treatment plants aren't set up to deal with this kind of contamination, so the standard way of disposing of this liquid is to pump it into the ground. In Ohio, regulators say, the site chosen for wastewater disposal wasn't vetted carefully enough. Instead of being geologically inert, it turned out to be the site of a fault line. The liquid lubricated the fault line and helped it move. The result: Earthquakes.
Now, according to the Associated Press, fracking operations disposing of wastewater in Ohio are going to have to follow much more stringent rules and provide a lot more geologic data to the regulators before they'll be allowed to pump any more liquid into the ground. The report states that this process can be done safely. It just wasn't being done that way.