So, I’ve been hearing that hydro-fracking to extract shale gas can cause earthquakes. Is that true?
Yes. A report released just last week concluded that some moderate earthquakes off the northwest coast of England in 2010 were caused by fracking, finding a strong correlation between seismic activity and injection of fluid down a nearby gas well (see this excellent write up by Eric at the Dynamic Earth). Another recent report cautiously suggested a similar association between hydrofracking and an increase in seismic activity in southern Oklahoma.
Oklahoma, you say? Where there were those big earthquakes last weekend?
There were indeed a couple of fairly big earthquakes in central Oklahoma last weekend. However, whilst earthquakes of this size are relatively uncommon in the interior of the US compared to, say, California, they are not unusual from a geological perspective. There is nothing to suggest a link to any shale gas drilling.
They’re certainly a lot bigger…
Yes, the earthquakes associated with fracking in the studies mentioned above were mostly magnitude 1.5-2.5, and all were less than magnitude 3. They therefore released many thousands of times less energy than the magnitude 4.6-5.6 earthquakes that made up the main shocks in Oklahoma. Basically, if we had the ability to pump that much energy into the ground, we’d probably be beyond the need to drill for shale gas in the first place.
The other argument that I’ve heard against these earthquakes being caused by fracking is that they’re also deeper.
This is potentially a little misleading. The earthquakes had a depth of around 5-7 km, which is not ‘too deep’ in the sense that we can’t drill wells that deep; we certainly could if we needed to. The key point is that the horizon being targeted by the gas drilling in Oklahoma is apparently mainly found at 1-3 km depth (see p49 of this pdf). Perhaps even more importantly, the majority of the drilling is in counties to the southeast. It is geography rather than any inherent properties of the earthquakes themselves that definitively rules out drilling as a suspect: it appears there is no gas drilling going on near the Wilzetta Fault, and the seismic activity that fracking can trigger seems limited to within a few kilometres of a well.
So fracking only causes small, localised earthquakes around the wells?
Directly, yes. There is, however, potentially a way that fracking could indirectly trigger a larger earthquake; if the drilling was close to an active, loaded fault that was close to rupturing, pumping fluids at high pressure into the subsurface could weaken the fault enough to enable it to rupture.
As I just said, that is clearly not a possibility here. And as far as I am aware there are no recorded instances of a significant (i.e., damaging) earthquake being directly linked to fracking, or even a large earthquake where a link has even been proposed. Given how many hundreds – thousands – of shale gas wells have been drilled in the US alone in the past few years, it seems clear that there is very little real-world risk in most cases. Or alternatively, the maximum size of earthquake that can be triggered is limited due to the small areas affected by fluid injection, and the kind of event listed in the studies I mentioned earlier – magnitude 2-3 or less – is the most we need to worry about except in very specific cases.
So I can stop worrying about fracking then?
If all you’re worried about is earthquakes, yes. However, there are still valid concerns over contamination of groundwater with fluids of unknown composition (although this seems to be more of an issue with poor well sealing and storage after use than the fracking process itself). Not to mention the fact that shale gas, sold as the answer to all our energy problems (even if not a particularly carbon neutral one), may have been a little oversold, and turn out to be a temporary stopgap at best.
So, to sum up: while there are plenty of important reasons to scrutinise this fracking business, the risk of triggering earthquakes is not one of them?
So say we all.