So, we've established that fracking won't cause volcanoes. But what about earthquakes? It's well known now that fracking activities appear to have produced two earthquakes at the Cuadrilla site near Blackpool. These earthquakes - in April and May 2011 - had magnitudes of 1.7 and 2.5. Earthquakes of this size have never been experienced during fracking before, so it's really interesting to have a look at them.
Firstly, how do their magnitudes correspond to what has gone before, and what earthquakes are like in nature. All fracking produces 'microearthquakes' - these are just like earthquakes except, as you'd guess from the name, much smaller. Typical earthquakes during fracking have magnitudes of -3 to 0. These are too small to be felt at the surface, even by a seismogram. However, geophysicists like myself will place geophones down boreholes near to the fracture stimulation to detect these microearthquakes, and use their locations to map the course of the fracture as it propagates out from the well. Earthquake magnitudes are on a logarithmic scale, so a magnitude -3 event releases 10^11 Dcm (that's dyne-cm, don't ask) of energy. A mag 0 releases 10^16 Dcm, so that's 10^5, or 100000 times larger. A mag 3 event releases 10^20 Dcm, so 10000 times larger than the largest event seen during fracking so far.
Why did this event happen then? Well, the official report claims that a critically stressed fault, near to the fracking site, was reactivated. As the fluid from the frack impinges on the fault, it increases the pore pressure, reducing effective normal stress and triggering failure. Seems plausible enough to me. A pre-existing fault is pretty much required to get a mag 3 quake. If the fault was close to critical stress, it implies this earthquake would have happened soon anyway - the fracking didn't produce a new earthquake, it just hurried it along.
A magnitude 3 earthquake is classed and 'small', and not damaging. If we lived somewhere like California or Japan, a mag 3 wouldn't even be noticed. It once you get up to mag 5 or 6, 1000s of times larger again than a mag 3, that people need to start really worrying. We have about 1 or 2 mag 3 events occurring naturally in this country every year. It'd feel something like a large truck going past your house at speed, rattling the window frames and such, but no damage caused.
So what does this mean going forward? Is a mag 3 quake worth stopping fracking and shale gas exploration for? Unfortunately, this is where scientists provide the facts (see above) and then hand over to the policy makers. The current system put in place by Cuadrilla is that if they trigger anything above a mag 1.7 they'll stop fracking and drain down the pressures. So it remains to be seen if they'll trigger more earthquakes in future wells. The report seems pretty confident that they won't, but then they would say that wouldn't they. They say it's an unfortunate accident of unique geology. However, the Bowland shale is pretty deformed and faulted, so it'll be really interesting to see if they can miss all the faults in the future.