British shale gas company Cuadrilla has again paused hydraulic fracturing at its Preston New Road site in Lancashire, northwest England, after tremors were detected, the company said.

This marks the third time operations have been halted at the site following seismic activity under Britain’s so-called traffic light regulation system, since they began in October.

“A series of micro seismic events in Blackpool have been recorded on the British Geological Survey website this morning following hydraulic fracturing at our shale gas exploration site in Preston New Road, Lancashire,” Cuadrilla said in a statement.

The largest tremor, of 1.5 magnitude, took place after hydraulic frracturing activities had already stopped, it said.

“According to recent research by the University of Liverpool the impact would be like dropping a melon,” Cuadrilla said.

The company, which is 47.4% owned by Australia’s AJ Lucas and 45.2% owned by a fund managed by Riverstone, first attempted to frack gas near the coastal town of Blackpool in northwestern England in 2011, but the practice led to a 2.3 magnitude earth tremor.

It said then that the quakes at that site were caused by an unusual combination of geological features, but they led to an 18-month nationwide ban on hydraulic fracturing while further research was carried out.

The government has since introduced a traffic-light system that immediately suspends work if seismic activity of magnitude 0.5 or above is detected.

“Cuadrilla will pause and continue to monitor micro seismicity for at least the next 18 hours... Well integrity has been checked and verified,” it said.

Cuadrilla said last month it had extracted its first shale gas from the site. Gas flows were small but coming at such an early stage of the project were evidence of the site’s potential, it said.