Does fracking trigger earth tremors?

Kate - So, while methane emissions that we just heard about might take a while to show their effects, Britain has already experienced one of the other concerns about fracking.  In 2011, 2 earthquakes in Blackpool were linked to local fracking operations.  We're joined on the phone by Brian Baptie from the British Geological Survey.  Hi, Brian.

Brian -   Hello.

Kate -   So, were those 2 earthquakes in Blackpool the responsibility of the fracking operations that were going on nearby?  Was it just a coincidence?

Brian -   No.  The earthquakes were directly linked to the fluid injection that was going on as part of the hydraulic fracturing operation at that time.  In fact, there weren't just two earthquakes.  It was 58 recorded in total and most of those were very, very small.  In fact, the largest one was also pretty small, even by UK standards, only had a magnitude of 2.3, but nevertheless, it was felt by 50 or so people in the Blackpool and it did cause some alarm.

Kate -   How can we know that they're directly responsible?  Can we link them up time wise exactly or...?

Brian -   Yes.  It's basically looking for a spatial and temporal correlation between the hydraulic fracturing operation and in this case, we were able to show that the earthquakes occurred very close to the injection well and at roughly the same depth where the injection was ongoing, so, around about 2.83 kilometres.  And also, in terms of time, the earthquakes all occurred during the time and just after the fluid was being injected as part of the fracking operation.  Another piece of evidence is that there is little background seismicity in this part of the UK, so if you start to see seismicity in an area where there is none, that's another piece of evidence that maybe something is going on.

Kate -   How would fracking cause these earthquakes?

Brian -   It's really all down to the fluid injections.  So, what happens when you get and slip on a fork during an out crank is, the slip occurs because the shear stress that acts along the fault's surface exceeds the frictional resistance to sliding of the rocks on either side of the fault.  What happens when you inject fluid is that it reduces the effect of stress on the fault system.  It increases the poor fluid pressure.  And that's a bit like just lubricating the fault and makes it easier for the rocks to slip past each other.  So, what happened in the case of Blackpool and in many other examples of injection induced earthquakes is that these fluids build up and they can build up over relatively short periods of time and allow the rocks to slip past each other more easily.

Kate -   These earthquakes, so they were 2.3 on the Richter scale I think was one of the biggest.  I mean, when we hear about big earthquakes, we're on 8 or 9 on the Richter scale.  Could we even feel them?

Brian -   Yeah.  I mean, people did feel the Blackpool earthquakes.  They're around about the bottom of the range that people might feel earthquake activity, but of course, they were quite shallow.  So, that makes it easier for people to feel them as well.  But yeah, in comparison to large tectonic earthquakes, these earthquakes were tiny - pretty trivial in fact.  And in fact, the largest known example that we have of an injection induced earthquake during hydraulic fracturing occurred in Canada in 2011, and it had a magnitude of 3.8.  Now again, that might cause some noticeably strong shaking, but still, it's well below the level of these large tectonic earthquakes and probably, unlikely to cause any significant damage.

Kate -   If we've got a sort of a number of them - you said there were quite a few going on at the same time, even though they're tiny, should that be something that we're worried about or is this something sort of geologically, we can just let pass and not worry about?

Brian -   Well, I think it's a good analogy to compare these earthquakes to is, what happened during the period of coal mining industry in this country throughout the '60s, '70s and '80s.  We recorded hundreds, thousands of mining induced earthquakes and people would feel these reasonably regularly, and really grew to kind of be familiar with them and to live with them.  The largest of those earthquakes didn't exceed more than a magnitude for about 3.  So again, of course, some relatively strong shaking and the largest, there are accounts of some minor damage, things like cracks to plaster and stuff like that, but some structural damage.  So, there are similar types of industrial activity in this country that have caused similar earthquake activity.  So yeah, we have experienced this kind of thing before.