Update: Saturday's earthquake was initially estimated at a magnitude 5.6, tying it with a 2011 earthquake near Prague for the largest seen in Oklahoma in recent years. After further analysis to compare the two events, the USGS has upped the estimated magnitude for Saturday's earthquake from 5.6 to 5.8. They also ended up revising the estimate for the 2011 earthquake upward to 5.7. In a press release, USGS geophysicist Gavin Hayes explained, “While the difference in size between the two events is less than 0.1 magnitude units, rounding magnitudes to one decimal place means that the magnitude of the Prague earthquake is Mw 5.7, and the Pawnee earthquake is Mw 5.8.”

So by a slim margin, the Pawnee earthquake sets a new record for Oklahoma—although the limited data recorded during earthquakes in 1952 and 1882 prevents precise estimates of their magnitude, which are believed to have been between 4.8 and 5.7.

Original story: 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 is separated from oil and gas produced from 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 “lubricates” ancient faults and allows them to move.

At about 7:00am Saturday morning (local time), things moved again: a magnitude 5.6 earthquake struck near Pawnee, Oklahoma, northeast of Oklahoma City. Unless the magnitude estimate changes after further analysis, this will tie the strongest earthquake the region has seen during this strange period—a magnitude 5.6 near Prague in 2011. In February of this year, Fairview, Oklahoma, experienced a 5.1.

At least one injury was reported in Pawnee, according to the Tulsa World. A handful of buildings saw some damage, including an historic downtown building that lost some of the sandstone blocks from the veneer on its exterior wall, which fell to the sidewalk. A local grocery store saw lots of stock end up on the floor and damage to its suspended ceiling.

Further Reading New Oklahoma rule aims to reduce earthquakes

The Oklahoma Corporation Commission, which regulates the disposal wells in the state, responded by ordering a precautionary shutdown of 37 wells in a 700-square-mile area around Pawnee over the coming week and a half. Oklahoma has been working to get a handle on ways to limit injections and thereby reduce the risk of earthquakes.

This is certainly nothing like a magnitude 7 earthquake in heavily populated coastal California, but it’s significant for a state still getting used to the possibility of seismic activity—a state where buildings and infrastructure have not been designed with seismic activity in mind. Analysis is necessary to link this specific earthquake to disposal well activity, but it fits with the recent trend in the region that researchers believe clearly points to earthquakes being induced by injections.