(Reuters) - A series of earthquakes near a northern Oklahoma City suburb struck along a known fault line and damaged two power substations, resulting in about 5,000 residents temporarily losing electricity, officials said on Thursday.

The quakes on Wednesday night near Edmond included one with a magnitude of 4.2, and came after the state imposed guidelines to reduce the risk of quakes caused by man-made activity related to hydraulic fracturing, or fracking, in its oil-rich shale formations.

The number of quakes rattling Oklahoma has fallen after the state guidelines went into effect late last year. New fracking activity has also declined, officials said.

In 2016, there were nearly 2,200 earthquakes with a magnitude of 2.5 or above in Oklahoma, against slightly more than 600 as of the end of July this year, according to the Oklahoma Geological Survey.

The Edmond quake was the state’s fourth this year with a magnitude of 4 and above, while last year, there were 15.

“We are optimistic that the seismicity rate has gone down but we still believe that the seismic hazard is still significant in Oklahoma,” said Jake Walter, state seismologist for the Oklahoma Geological Survey.

He said the Edmond quakes took place on a mapped fault and included one of the strongest recorded quakes to hit the area.

Seismologists and state officials have said an increase in the frequency of quakes over the past few years in Oklahoma has been tied to the disposal of wastewater from fracking.

The Oklahoma Corporation Commission, a state regulatory agency for the energy industry, said it was investigating the Edmond quake series.

It added that the quakes took place in an area where the commission has instituted volume reduction in disposal well operations.