Oklahoma has suddenly become a seismic state over the last decade, as an abundance of small earthquakes has accompanied the expanded use of deep injection wells. The wells are used to dispose of wastewater that would be expensive and difficult to treat. Instead, it gets pumped into salty aquifers that are already unsuitable sources of drinking water. Most of the wastewater comes from oil and gas wells in the region—some of which involve fracking, but many of which are older, “conventional” wells.

Fluid pressure in the saline aquifers seems to be transmitting into the underlying igneous and metamorphic basement rock, where it “lubricate” ancient faults and allow them to move.