Drilling at Preston New Road site in Lancashire has triggered 37 minor quakes in three weeks

A senior executive at the fracking company Cuadrilla privately said this summer it did not expect to cause earthquakes that would be serious enough to force it to halt operations.

But despite that confidence, the company has triggered 37 minor quakessince it started fracking for gas at its Preston New Road site in Lancashire three weeks ago.

Two of those have been powerful enough to exceed a regulatory threshold that requires fracking to stop, and on a third occasion the company voluntarily ceased operations when it neared the limit.

Cuadrilla has said such tremors are to be expected from fracking, which involves pumping water, sand and chemicals 2km underground at high pressure. Under government regulations, fracking must stop if a 0.5-magnitude earthquake is registered.

But during a tour of the site in June, Matthew Lambert, the government and public affairs director at Cuadrilla, said: “Because we are managing that risk I don’t really accept that we are likely to cause seismicity above that level [an apparent reference to 0.5-magnitude] and we will not be causing seismicity which will damage property.”

He described the system of monitoring seismicity as “highly regulated” and said the company had to “manage that risk” of causing tremors, a recording of the discussions shows.

The Green party said the comments, made months before earthquakes that breached the regulatory limit, showed Cuadrilla was “obviously in way over their heads”.

Jonathan Bartley, the co-leader of the Green party, said: “They were unaware they would cause tremors anywhere near this strong, and they evidently don’t know how to stop them.” He said he felt it was unambiguous that Lambert was referring to the 0.5 limit with his reference to “above that level”.

Francis Egan, the chief executive of Cuadrilla, last week urged the government to relax the regulatory threshold or risk stifling shale gas exploration. The energy minister, Claire Perry, rejected such calls, saying only a “very foolish politician” would do so at this point.

The Green party accused Cuadrilla of trying to “strong arm” the government into changing the limit.

Bartley said any relaxation of the rules would be “unacceptable for residents who are concerned about the security of their homes”.

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Cuadrilla has not triggered any tremors since Sunday 4 November, when a 0.7-magnitude quake was registered. As it happened at a time when fracking was not under way, it did not register as a “red light” on the traffic light system of regulation.

That has not reassured Labour and Conservative MPs in north-west England, who wrote to the business secretary, Greg Clark, last week over their concerns about the tremors.

Lambert said: “Cuadrilla is operating within the traffic light system managed by the Oil and Gas Authority. The micro seismicity that has been detected at our exploration site in Preston New Road is way below anything that can be felt at surface and a very, very long way from anything that would cause damage or harm.

“This is what my quote refers to. In line with regulations, seismicity will continue to be closely monitored by Cuadrilla and the relevant regulators.”