Did drilling cause Oklahoma quake?

When an earthquake struck Oklahoma on Saturday, one of the first steps state officials took was to shut down 37 of the state's 3,200 active disposal wells -- a move that drew national attention to the link between oil and gas drilling and earthquakes.

No one was seriously injured in the Oklahoma quake, and investigations and cleanup has begun. Here's what you need to know.

What's a disposal well?

Disposal wells are used by oil and gas producers to get rid of wastewater from the drilling process. The wells push the wastewater deep underground, even deeper than where oil and gas are found.

The wastewater mostly consists of a substance called brine -- a mix of water and chemicals that comes to the surface with oil and gas when they are pumped from the Earth.

A small portion is also the water that's pumped underground in the modern hydraulic fracturing process, a drilling technique often referred to as fracking.

The EPA says there are about 40,000 disposal wells nationwide.

Did the disposal wells cause the earthquake?

That is the concern. Oklahoma didn't have much of a history of earthquakes. But a big one struck in November 2011, causing injuries and leveling houses, and officials said in 2014 that earthquakes in the statehave increased 5,000%.

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