I appreciate waste water injection can come from many industrial processes but since it's the waste water from the Fracking process, then it is related to Fracking in this instance.
It needs to be differentiated however as they are not the same thing. You don't need to inject waste water into reservoirs to frack other reservoirs. It's another example of using US data and extrapolating it (incorrectly) to the UK.
IIRC there are specific wells that have been located too close to faults that have caused the majority of these events as well as the sheer volume of fluids being injected. It's not every single injection well. TBH it's not really my area of expertise so I can't really argue either way much more on the subject.
Also you may be interested to know waste water from wells has been injected back down wells in the UK for decades. Again, something that has been ignored or overlooked by many campaigners.
out of curiosity why do they do that?
As far as I know it's cheaper than getting it tanked away and processed.