It is important to put the tremors recorded during fracking of the Preese Hall well in 2011 into context, writes Dr James Verdon.
Any human activity that alters the state of stress underground has the potential to trigger seismic activity.
This includes: mining for coal (or other minerals); extraction of geothermal energy; building dams for hydroelectricity, and/or pumped storage; conventional oil and gas extraction; disposal of waste fluids into deep aquifers; underground storage of natural gas; and, of course, hydraulic fracturing.
Despite its long history in conventional oil and gas reservoirs, and the recent increase in its usage associated with shale gas extraction, hydraulic fracturing has generated very few earth tremors in comparison to some of the above technologies. For example, coal mining in the UK is estimated to have induced hundreds of earthquakes in the past 40 years. Researchers in Durham have said recently that fracking “is not really in the premier league for causing felt seismicity. Fundamentally it is never going to be as important as mining or filling dams”.
With respect to induced earthquakes, the key parameter is the magnitude, or size of the event. Typically, humans are only able to notice events larger than about magnitude 2. It takes events of about magnitude 4 or above to cause any damage to nearby buildings or infrastructure. The largest event measured at Preese Hall was magnitude 2.3. The UK experiences about 50 earthquakes with magnitude of 2 or greater every year, so events similar to those at Preese Hall are in fact a weekly occurrence somewhere in the UK.
The Traffic Light System (TLS) imposed to regulate any seismicity induced by hydraulic fracturing in the UK is also discussed at length in your article. It should be made clear that the purpose of the Traffic Light System is not just to avoid earthquakes that could cause any damage to nearby buildings or infrastructure, but to avoid any tremors that could be felt by humans at all.
The TLS red light is set at magnitude 0.5. This is too small to even be felt by humans at the surface, let alone produce any damage whatsoever to nearby buildings or infrastructure. Such small events can only be detected with sensitive, high-tech monitoring systems and sophisticated mathematical processing techniques. This is because the levels of vibration caused by a magnitude 0.5 event are smaller (often much smaller) than the vibrations caused by everyday human activities such as cars driving along roads and farmers ploughing their fields. Therefore seismologists must use sophisticated methods to separate earthquake signals from this everyday background noise.
In fact, recent studies have estimated that the surface vibrations caused by a magnitude 0.5 event are smaller than the vibrations caused by your average human walking across a wooden floor. The statement in your recent article “The tripping of traffic lights will merely indicate that damage has already occurred” is therefore clearly and demonstrably incorrect.
Very rarely, in certain geothermal settings (absolutely not “all seismic events”, as stated in the recent article), events up to 1 magnitude unit larger have been observed after hydraulic fracturing was stopped. The TLS is designed with this post-injection rise in mind: by stopping at magnitude 0.5, a post-injection rise of 1 implies a maximum event magnitude of 1.5, which is still generally considered to be too small to be noticed by humans at the surface (let alone cause any damage).
With regards the “self-regulation” aspect of the TLS, it is indeed the case that the operator is required to monitor and report earthquakes themselves, though I note that DECC has previously said that it will have a representative on site during hydraulic fracturing and the HSE and Environment Agency have the power to make unscheduled site inspections should they wish to. This “self-regulation” is the norm for almost every type of industry in the UK, industries which we trust and use on a day-to-day basis. The airline industry springs to mind, for instance: we don’t expect the government to oversee every aspect of airplane construction, maintenance and operation. It is left to the operators to self-regulate, and yet the airline industry has an enviable safety record compared to most other modes of travel.
More importantly, earthquakes, by their very nature, are difficult if not impossible to hide. The British Geological Society has also deployed seismic monitoring networks in prospective shale gas areas (as have a number of UK universities), operating independently from the monitoring networks deployed by the operators. If future events are induced by fracking, there is simply no way the operators will be able to hide them.
In conclusion, the likelihood of fracking activities inducing earth tremors of sufficient magnitude to be felt is very small, and the likelihood of any damage to nearby buildings or infrastructure being caused by such events is negligible.
*Dr James Verdon is a BGS-Funded Research Fellow in the School of Earth Sciences at the University of Bristol. He holds a Ph.D. in geophysics, and is an expert in monitoring and modelling seismic activity caused by human industrial activities.
Next week: Anti-frackers have their say