Four mini-earthquakes have been recorded close to the Preston New Road fracking site, sparking concern among protesters.
The tremors were recorded close to the Little Plumpton site on Thursday and Friday by the British Geological Society, but were too small to be felt on the surface.
Fracking firm Cuadrilla said their monitoring system was “working to the highest standard.”
The first tremor took place at 3.48pm on Thursday, with a depth of 3km and a magnitude of -0.2.
The second and third tremors took place at 10.54pm and 11.44pm that same day, with magnitudes of -0.8 and -0.3.
The final tremor took place on Friday at 1.20pm, with a depth of 3km and a slightly increased magnitude of 0.3.
All four quakes took place close to the Cuadrilla fracking site, where work to extract gas began on Monday.
A spokesman for Frack Free Lancashire said: “We will continue to monitor these events closely.
“Considering the relevant past history with the Fylde Coast’s geology and having two significant seismic events which effectively shut down the fracking industry, all eyes are currently on Cuadrilla.
“These events may be dismissed as being minor and “can’t be felt” but what occurs under the earth is beyond anyone’s control.”
A spokesman for Cuadrilla said: “Three microseismic events were detected by the highly sensitive Traffic Light System Cuadrilla has installed around the Preston New Road site and were very far below levels that could be felt at surface.
“The recordings from these seismometers are shared with the British Geological Survey to help them reassure the public that operations are continuing safely as planned.
“We take the monitoring and regulation of seismicity seriously, with daily reports sent to the regulators.
“The microseismic events recorded were extremely low and well within the Green Light threshold and confirm that the monitoring system is working to the highest standard.”
They added: “We’ve been monitoring baseline seismicity for some time in the run up to starting hydraulic fracturing operations, and we have noted fluctuations at this very low level of microseismicty during that time.”
University of Glasgow Professor of Geophysics David Smythe, who opposes fracking, said: “Recent research by Stanford University shows that these tiny tremors can be indicators of bigger quakes to follow - like canaries in a coal-mine.
“The problem for Cuadrilla is that if it carries on regardless, bigger earthquakes may well be triggered.
“To quote Cole Porter; ‘there may be trouble ahead’. Cuadrilla’s only safe option is to cease fracking.”