Oklahoma saw another bout of earthquakes over the weekend, just the latest example of the Sooner State experiencing historic levels of seismic activity this year.

Consider this: Between 1978 and 2008, an average of two earthquakes with a magnitude of 3.0 occurred each year in the state. Since 2009, the state has seen more than 20 quakes of at least 4.0 magnitude, including the 5.6 magnitude earthquake in November 2011 that was the largest in Oklahoma’s history.

Recent days have continued the trend. On Saturday and Sunday, Oklahoma saw four quakes with at least a 3.0 magnitude. Over the past week, there have been eight quakes of at least a 3.0 magnitude, according to the Oklahoma Geological Survey. The two strongest quakes both topped 4.0: A quake registering at 4.3 after noon on Saturday and another registering at 4.1 in the early hours of Monday morning.

These earthquakes add to a record-setting number that continues to climb. The previous record for most quakes of at least a 3.0 magnitude or greater was set last year with 109 earthquakes; Oklahoma surpassed that months ago, experiencing 145 such quakes by May and continuing to add to that total.

The U.S. Geological Survey and the Oklahoma Geological Survey said in May that the alarming number of earthquakes in the state made it more likely that a damaging earthquake could occur. They said that the recent increase was due to wastewater (a byproduct of oil and gas production) getting injected into deep disposal wells.

study published earlier this month in the journal Science found that while fracking — which creates millions of gallons of wastewater — could not be directly linked to the increase in quakes, the disposal of wastewater creates the pressure that can trigger such earthquakes. This followed a statement from the geological surveys saying that the recent increase in quakes did “not seem to be due to typical, random fluctuations,” but rather were due to the wastewater injections.

The surge in quakes has been particularly acute in the last few years in central Oklahoma and the area around Oklahoma City, the most populous city in the state.

This map shows where quakes occurred between 1970 and the end of April 2014, with blue dots showing quakes between 1970 and 2008 and red dots showing quakes between February 2009 and April 2014:

(U.S. Geological Survey)

In response to the quakes, researchers with the Oklahoma Geological Survey have increased the number of monitoring stations in the state.