LONDON (Reuters) – British shale gas exploration firm Cuadrilla said another earth tremor occurred on Monday near its fracking site in northwest England, bigger than a 2011 quake which suspended operations at the site for seven years.

It is the third earth tremor in a week at the Preston New Road site in Lancashire, where activities only resumed on Aug. 15 after Cuadrilla secured all required permits following several stoppages last year because of minor seismic events.

“We’re aware of a seismic event at 8.30am (0730 GMT). We can confirm that no hydraulic fracturing was being carried out at the time and none has been carried out over the weekend. We are investigating alongside regulators,” Cuadrilla said in a statement.

The British Geological Survey said that Monday’s induced seismicity event had a magnitude of 2.9 ML (local magnitude) on the Richter scale, bigger than the 2.3 ML quake in 2011 at Britain’s only active fracking site.

Under Britain’s traffic light regulation system, work is immediately suspended if seismic activity of magnitude 0.5 or above is detected.

Fracking, or hydraulically fracturing, involves extracting gas from rocks by breaking them up with water and chemicals at high pressure.

It is fiercely opposed by environmentalists who say extracting more fossil fuel is at odds with Britain’s commitment to reduce greenhouse gas emissions.

(Reporting by Alistair Smout; editing by Emelia Sithole-Matarise)