A company using hydraulic fracturing to release gas from shale rock confirms that the controversial technique probably did trigger earth tremors in northwestern England in April and May.

Cuadrilla Resources said Wednesday that independent experts concluded that the tremors were due to an unusual combination of geology and operations and were unlikely to happen again.

The company said local geology would limit any future seismic events to around magnitude 3 on the Richter scale.

The tremor on April 1 measured 2.3 on the Richter scale.

Local campaigners have mounted a "Frack Off" campaign to oppose the drilling technique that cracks open rock layers to free natural gas.