There is overwhelming evidence that fracking causes earthquakes. Wastewater injection can create larger earthquakes because of the greater fluid volumes. The largest earthquake induced by fluid injection that has been documented in the scientific literature was the September 23, 2016 earthquake in central Oklahoma. It had a magnitude of 5.8.
Microseismic events have been monitored on certain production platforms that are caused by extraction and later injection operations. The 2.9-magnitude quake, recorded near Cuadrilla's site near Blackpool in august, is believed to be the biggest fracking-related tremor seen in Britain.
The science to determine likelihood and severity of fracking induced seismic eventsis very embryonic as yet. What is known is that all rocks are stressed to varying degrees, fractured to varying degrees and shales are highly anisotropic.
I would suspect that the shales in the uk are more tectonically stressed than the classic shale plays in the US (eg Marcellus) and hence react more forcibly when injection occurs.Alternatively the injection may have reactivated an existing fault with no surface expression. The behaviour of a wellbore during drilling can give an indication of stress states and modern borehole imaging tools can give hard data. It would make an interesting research project. The legacy data available in the UK is abysmal compared to what a suite of modern geophysical logs can obtain and there are few modern oil and gas wells in the uk that would have generated such logs. Modern imaging logs are less than 30 years old.
http://nora.nerc.ac.uk/id/eprint/517414/1/OR17048.pdf
(for anyone wading thru the paper LOT = pressure up formation until it fractures, normally run just after drilling out of casing,)
I hate to quote the Guardian but one expert at least agrees with me(sort of)
https://www.theguardian.com/science/2019/sep/03/terrawatch-warnings-fail-to-prevent-uk-fracking-quakes