TULSA, Okla. (AP) — Recent reports show that disposal well operators placed about 23 percent less saltwater into Oklahoma's deepest geological formation within the earthquake zone in 2016 compared to the previous year.

The year-end data reports about the Arbuckle formation come from the Oklahoma Corporation Commission, according to the Tulsa World. Commission spokesman Matt Skinner said the 2016 volume numbers are mostly complete, but some companies haven't reported their latest data.

Oklahoma Geological Survey data also showed there were 623 quakes of 3.0 or greater in 2016, a 31 percent reduction from 2015.

State regulators are developing a new directive with more restrictive wastewater restrictions following a 5.0 magnitude earthquake near Cushing in early November.

"We expect to announce it before the end of the month," OCC spokesman Matt Skinner said, noting that the Oklahoma Geological Survey "is working on gathering the latest data we need to firm up everything."

According to OGS data, the main cause of the state's man-made seismicity is the pressure created by large volumes of saltwater from oil and natural gas production put into the Arbuckle formation.

The peak annual injection volumes occurred in 2014, which was followed by a record number of quakes in 2015. In a December 2015 study, OGS hydrologist Kyle Murray found a 181 percent rise in saltwater injections into the formation from 2009 to 2014.