A second tremor in a 24-hour period has been recorded at the UK’s only active fracking site near Blackpool.

Cuadrilla was forced to halt operations for 18 hours on Friday after a 0.8-magnitude tremor. Fracking restarted on Saturday morning before a second tremor was detected.

Saturday’s tremor is the 18th in the area since fracking recommenced 12 days ago. It was too small to be felt above ground and was not categorised as a “red” event by the Oil and Gas Authority, unlike the one on Friday, because it occurred after operations had finished at 1pm. The company only has permission to frack until that time on Saturdays.

The 0.8-magnitude event was about 200 times smaller than the 2.3-magnitude tremor recorded in 2011 that led to fracking being suspended.

After much controversy, the practice resumed at Little Plumpton, the only fracking site in the UK, on 15 October.

If a tremor takes place during fracking operations, anything above 0.5 magnitude is considered a “red event” and requires the firm to stop injecting water and monitor the area for further seismic activity.

Q&A What is fracking? Show Fracking, or hydraulic fracturing, is a way of extracting natural gas from shale rock formations that are often deep underground. It involves pumping water, chemicals and usually sand underground at high pressure to fracture shale – hence the name – and release the gas trapped within to be collected back at the surface. The technology has transformed the US energy landscape in the last decade, owing to the combination of high-volume fracking – 1.5m gallons of water per well, on average – and the relatively modern ability to drill horizontally into shale after a vertical well has been drilled. In England, the government placed a moratorium on fracking in November 2019 after protests, legal challenges and planning rejections. A year earlier, the energy company Cuadrilla was forced to stop work at its Preston New Road site in Lancashire twice in four days due to minor earthquakes occurring while it was fracking. The tremors breached a seismic threshold imposed after fracking caused minor earthquakes at a nearby Cuadrilla site in 2011. In March 2019 the high court ruled that the government's fracking guidelines were unlawful because they had failed to sufficiently consider scientific evidence against fracking.

A spokeswoman for Cuadrilla said: “This is not a ‘red’ incident under the traffic light system operated by the Oil and Gas Authority as we were not pumping fracturing liquid as part of our hydraulic fracturing operations at the time. However, we will, as always, continue to monitor the seismic activity closely and plan to resume hydraulic fracturing on Monday.”

All relevant regulators had been informed, she said.

Three protesters jailed for blocking access to the site walked free after the court of appeal quashed their sentences as last week, calling them “manifestly excessive”.

The Guardian revealed earlier this month that the energy minister, Claire Perry, had proposed raising the regulatory threshold for tremors caused by fracking as the industry begins to mature.

The government subsequently denied it has any plans to relax the rules. Oliver Eden, a minister at the Department for Business, Energy and Industrial Strategy, said this week: “There are no plans to make changes to the traffic light system for monitoring induced seismicity.”