Fracking was halted early tonight after a tremor at Cuadrilla's site near Blackpool.
A seismic event with a magnitude of 0.4 on the Richter Scale was detected in Lancashire today close to where the energy firm is fracking for gas in shale rock.
Because it coincided with pumping operations at the site, and appears to have occurred within Cuadrilla's operational boundary, this is considered an Amber event in accordance with the Traffic Light System of safety monitoring.
Cuadrilla said it had "adopted extra caution" and stopped fracking for the day.
A spokesman for Lancashire For Shale said: "A 0.4 magnitude event like this is so small it is only possible to detect it using very sensitive instruments like those deployed locally by Cuadrilla and the British Geological Survey.
"The very open manner in which this event has been reported, and Cuadrilla's reaction to it, demonstrates just how robust the new controls are and that the Traffic Light System is working effectively."
Cuadrilla began fracking only days ago at its site in Preston New Road, Little Plumpton.
Opponents of fracking claim it can cause earth tremors and water pollution - and at least five tremors have already been reported in the area .
The Traffic Light System ensures that work has to be scaled back or stopped depending on the scale of seismic activity.
Cuadrilla said tonight: "This is an extremely low level of seismicity, far below what could possibly be felt at the surface but classed as an amber event as part of the Traffic Light System (TLS) in place for monitoring operational activity.
"As such we are required to reduce the rate we are pumping fracturing fluid once it has been detected.
"In fact we have adopted extra caution and have stopped pumping for the day."
"Seismicity will, as always, continue to be monitored closely around the clock by ourselves and others and we plan to continue hydraulic fracturing again in the morning.
"Local residents should be reassured that the monitoring systems in place are working as they should.
"These are tiny seismic events that are being detected by our monitors as we fracture the shale rock and are not capable of being felt much less cause damage or harm."
A spokesman for Frack Free Lancashire said: “Frack Free Lancashire is greatly concerned about these seismic events. It is most alarming for residents. Considering this is only the exploratory phase and only one frack a day imagine how much worse it could be.
“Who knows what is happening under our feet? This is the earth giving out a warning, as predicted by geologists following the previous failures of fracking in Lancashire.
"The increased risk of larger magnitude quakes is serious. Cuadrilla must stop now, for all our sakes.”