By Yeganeh Torbati
(Reuters) - Oklahoma is expanding restrictions on oil and gas drilling activities to stem a sharp increase in earthquakes that has worried residents, damaged homes and raised concerns about the future of the energy industry in the state.
But the action, announced on Friday, fell short of more drastic measures expected after a further increase in the rate of significant quakes last month prompted the state's regulator to say the situation was a "game changer."
Oklahoma and several other states in the central United States have experienced a sharp increase in earthquakes since 2009, a phenomenon scientists generally believe is linked to underground injection of briny wastewater, a byproduct of booming oil and gas production.
The earthquakes are not related to hydraulic fracturing or "fracking," a widely used drilling technique in Oklahoma. Instead, researchers say, the quakes stem from the re-injection of wastewater that occurs naturally in oil and gas formations.
Noticeable quakes - above magnitude 3.0 - now strike the state at a rate of two per day or more, compared with two or so per year prior to 2009. Residents in the affected areas say the state has been slow to address the problem and reluctant to impose restrictions on the lucrative and powerful energy industry.
On Friday, the Oklahoma Corporation Commission, the state's oil and gas regulator, released a directive that expands "Areas of Interest" - parts of the state that have been worst-hit by the quakes - and adds restrictions for 211 disposal wells.
Operators of wells in the affected areas, which now include 21 of Oklahoma's 77 counties, must show regulators they are not injecting water below the state's deepest rock formation, a practice scientists say is particularly likely to cause quakes. Operators of other wells that had previously reduced injection volumes must now also inject at a shallower depth.
A previous directive by the commission in March imposed similar restrictions on 347 wells to diminish the number of the tremors.
But that directive has not had the hoped-for impact. Since July 10, ten quakes of magnitude 3.0 or higher have occurred in the state, according to the Oklahoma Geological Survey.
(Reporting by Yeganeh Torbati; Editing by Steve Orlofsky)