TULSA, Okla. (AP) — Oklahoma’s 4.0 earthquakes are up significantly this year, but the overall rate of earthquakes is declining.

Oklahoma has had six quakes of at least magnitude 4.0 halfway through this year, which is one more than all of last year. But the overall rate of earthquakes has declined, with 96 quakes of magnitude 3.0 or greater through June 30, compared with 144 at this point last year and 302 by the end of 2017, the Tulsa World reported. A magnitude 4.6 in April near Perry was the 12th largest in state history.

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Scientists are largely seeing earthquakes on unmapped faults that were activated in 2014 by wastewater injection, said state seismologist Jake Walter. Scientists are researching specific mechanisms by which the state’s ongoing seismicity is triggered, he said. Wastewater can trigger the initial earthquakes, but quakes themselves can lead to more quakes.

“So in some ways the wastewater injection has created a new paradigm that defies how we would categorize main shocks and aftershocks if this were a fault that had slipped in a more natural setting,” he said.

Walter said that Oklahoma’s seismic risk appears to be similar to the latest hazard forecast put out by the U.S. Geological Survey in March. The agency calculated Oklahoma’s short-term hazard levels to be similar to active regions in California. The chance of earthquake damage in high-hazard areas of Oklahoma this year ranges from 1 percent to 14 percent, “much higher” than most parts of the U.S.

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Information from: Tulsa World, http://www.tulsaworld.com