Shale gas firm Cuadrilla says it has resumed fracking at its site in Lancashire after halting work several times last year due to minor tremors.
UK rules mean exploration must be suspended if seismic activity with a magnitude of 0.5 or above is detected - much lower than the threshold in the US.
The company said last month it would resume operations at the Preston New Road site near Blackpool.
Fracking, or hydraulic fracturing, involves injecting water and chemicals at high pressure to break up rock and extract gas.
However, it can cause tremors and is opposed by environmentalists who say it can also contaminate drinking water. Protests have been a regular fixture at the Lancashire site.
Cuadrilla said the fracking would be completed by the end of November and then followed by flow testing of the well, with the results expected early in 2020.
Laura Hughes, projects and operations director at Cuadrilla, said the site was "one of the most monitored oil and gas sites anywhere in the world".
She added: "We have proven it is a well-run, entirely safe and environmentally responsible operation.
"Whilst there may well be low levels of induced seismicity, local people should be reassured that any resulting ground motion will be far below anything that could cause harm or damage."
Cuadrilla hopes its exploratory tests will show the potential of shale gas for the UK - and has said around 1,300 trillion cubic feet of gas is estimated to be contained in the area.
It wants to see a review of what it calls the "exceedingly low limit" on seismic activity associated with fracking with the UK but the government has said it has no plans to change the rule.
The government says shale gas could be an important new domestic energy source reducing dependency on imports as well as delivering economic benefits.
However, Jamie Peters, campaigner at Friends of the Earth, said: "Fracking just isn't part of the future if we are serious about avoiding climate breakdown.
"Instead of backing climate-wrecking fracking the government should ban it and support renewable energy and green jobs instead."