Ryan W. Miller

USA TODAY

While the earth continues to shudder more frequently than seven years ago beneath Oklahomans feet, the rate of earthquakes in the state in 2016 is down from last year.

The state has been shaken by 448 magnitude-3.0 and greater quakes so far this year, down from the 558 it experienced in the same time frame in 2015, according to data from the U.S. Geological Survey.

Increased regulation on wastewater disposal related to oil and gas extraction could be one reason behind the decline, said Robert Williams, a geophysicist at the United States Geological Survey. Wastewater disposal is linked to quakes in Oklahoma and other states.

Regulators this year increased restrictions limiting wastewater disposal and expanded the area in the state those constraints cover, Williams said. At the same time, a decrease in oil and gas activity led to a smaller volume of water being disposed of, also potentially contributing to fewer quakes, he added.

The extraction of oil and gas in hydraulic fracturing — also known as fracking — likely does not contribute to a rise in earthquakes, but wastewater disposal associated with the activity does, said Jefferson Chang, a geophysicist for the Oklahoma Geological Survey at the University of Oklahoma.

That's because during the disposal process, briny wastewater is injected at high pressures into rock formations underground. And that process takes place at greater depths underground than fracking does, Chang said.

While the overall rate of earthquakes is down, the temblors are still happening regularly in Oklahoma. In the past 30 days, 26 earthquakes with a magnitude-3.0 or greater have hit the state, according to USGS data. A magnitude-4.0 rocked an area just outside Oklahoma City early Wednesday.

“The people around here have felt this before, but that was probably a pretty good shock,” Williams said of Wednesday's quake.

Much of the seismic activity also takes place in clusters in parts of the state. In January, 70 small earthquakes shook Oklahoma in just one week, mainly in northwestern Oklahoma.

At a magnitude-3.0, the ground is moving enough that people actually begin to feel it, Chang said.

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In March, a USGS report linked activities related to oil and gas extraction, notably wastewater disposal, to seismic activity. The report found that Oklahoma along with five other states — Kansas, Texas, Colorado, New Mexico and Arkansas — faced the highest potential for earthquake hazards.

Since 2009, more than 2,000 magnitude-3.0 or greater earthquake have shaken Oklahoma. That's far above the one or two per year the state experience before that year, a change Williams called unprecedented.

In 2008, only two magnitude-3.0 earthquakes were registered in Oklahoma, according to the USGS. By 2015, that number climbed to 890.

There is no one clear answer why Oklahoma experiences more earthquakes than other states, but Williams thinks the location of many of the wastewater disposal sites, which lay on top of ancient seismic fault lines, contributes to the trend.

'Reawakened' faults could trigger big Okla. earthquakes

While scientists cannot link all quakes to wastewater disposal, one of the largest Oklahoma experienced in recent years, magnitude-5.6 in 2011, was induced by that activity, Williams said.

Katie Brown, a spokesperson for Energy In Depth, told USA TODAY in March after the USGS report was released that only a small percentage of wells are associated with the quakes. Energy In Depth is a program of the Independent Petroleum Association of America, a trade group.

However, Chang said monitoring earthquakes in Oklahoma has been difficult, and no study has conclusively surveyed the state. As a result, he said, these days, “it’s really hard to quantify what is episodic and what is ‘normal.’ ”

Oklahoma hit with 70 quakes in a week