Fracking will continue at Cuadrilla’s site in Lancashire after tremors were detected underground on Wednesday.
The company called a temporary halt to the work after a “tiny” tremor was detected on Tuesday afternoon at the shale exploration site at Preston New Road while fracking operations were taking place.
But following more work on Wednesday morning, the British Geological Survey (BGS) reported more small tremors in the region, including one of 0.5 local magnitude at about 1pm and another of 0.4ML about 25 minutes later.
Cuadrilla confirmed tremors had been detected, but stressed the “tiny seismic events” were within operating expectations and had not occurred during the pumping.
It said the BGS had rounded the magnitude up to 0.5ML from 0.48ML, so the quake was still below the level of a red event which would require the firm to halt the work for at least 18 hours.
Hydraulic fracturing operations will begin again on Thursday morning and the company said both tremors were well below the magnitude capable of being felt on the surface.
A spokesman added: “Following on from hydraulic fracturing operations today Cuadrilla has detected micro seismic activity of 0.48ML.
“This is within operating expectations and the sophisticated system of monitoring in place is working as it should.
“These are tiny seismic events that are being detected by our monitors as we fracture the shale rock 2km underground and are many hundreds of orders of magnitude below what is capable of being felt much less cause damage or harm at surface.”
The 0.4ML tremor on Tuesday was classed as an amber event as part of the traffic light system in place for monitoring seismic events during operations.
Under Oil and Gas Authority regulations, a tremor between 0ML and 0.5ML requires workers to reduce the rate at which it was pumping fracturing fluid once the seismic event had been detected, but Cuadrilla said it had “adopted extra caution” and had stopped pumping for the day in response.
Local magnitude measures the strength given off by an earthquake by measuring the ground motion, with tremors below 2ML rarely felt above ground.
Work got under way again on Wednesday morning, however fracking had stopped before further tremors were detected.
The BGS said seismic activity is not unexpected because fracking is accompanied by microseismicity or very small earthquakes.
But it said it has deployed additional sensors across the region to provide an independent assessment of earthquake activity.
It added: “This dense network of sensors allows us to detect smaller earthquakes than we are typically able to do in other parts of the UK.
“The BGS is not a regulatory body. Our role is to provide impartial data that is available for everyone to view and analyse.”
The controversial fracking process began earlier this month after an environmental campaigner failed in a High Court bid to block operations.
The exploration taking place at Preston New Road is the first fracking in the UK for seven years, after work by Cuadrilla was halted in 2011 following two tremors near Blackpool.