October 24, 2018

expert reactions to Lancashire earth tremors 24/10

Reactions to reports that earth tremors were detected near Lancashire fracking site.

Dr Jessica Johnson, Lecturer in Geophysics at the University of East Anglia, said: 

“The process of hydraulic fracturing fundamentally creates extremely small earthquakes. The earthquakes that have been detected are within the conservative threshold. These earthquakes cannot be felt and would not have been detected were it not for the additional monitoring. These sorts of earthquakes could be naturally happening elsewhere but we don’t know about them because we cannot feel them and they are not monitored. Cuadrilla have stopped operations for the time being, which is even more conservative than the regulations. There is no evidence to suggest that these tiny earthquakes will lead to larger ones.” 

Prof Ernest Rutter, Professor of Structural Geology at the University of Manchester, said:

“These events at Preston New Road can only be detected because the site is (as a requirement of the regulations) incredibly heavily monitored by close-in seismographs. The normal BGS monitoring stations, about 100 km apart, cannot detect anything so small. Coal mining used to generate MUCH larger tremors on a daily basis when it was ongoing, and never bothered anybody, and local traffic will also generate larger seismic signals.  Thus it is important to keep in mind the natural background, and not to get excited by such tiny events. The protest movement is, of course, determined to get excited about such things, whether it makes sense or not.”

Dr Ben Edwards, engineering seismologist at the University of Liverpool, said:

 “In order to understand background and induced seismicity the University of Liverpool, along with the British Geological Survey (BGS), have been independently monitoring the Preston New Road site for any earthquake activity for over a year.

“We do this with a network of high-sensitivity seismometers that monitor vibrations in real time. We have recently detected several events corresponding with the onset of fracking operations by Cuadrilla Resources, including the ‘amber traffic light’ magnitude 0.4 and the ‘red light’ 0.5 events on 23rd and 24th October. These earthquakes are far too small to be felt by people at the surface and are not uncommon in the UK – around five hundred 0.5 events occur annually, but the vast majority of these are not detected even by the BGS’s permanent seismic monitoring network. In fact even activity far above the 0.5 ‘red light’ threshold (up to around magnitude 2.0) is difficult for people to perceive unless directly above a shallow epicentre.

“The University of Liverpool recently undertook experiments to put vibrations from micro-seismicity into context. If directly above a 0.4 earthquake (at 2.5km depth) you may experience vibrations similar to a passing car, 0.9 is like a small plane landing 500m away, and 1.2 is like a passing bus. Minor damage due to earthquakes may be experienced close to the epicentre for events with magnitude greater than 4.5 – 5, which is around 10,000 times stronger and a million times more energetic than those experienced to date at Preston New Road.”

Dr Stephen Hicks, Postdoctoral Research Fellow in Passive Source Seismology at the University of Southampton, said:

“Hydraulic fracturing (‘fracking’) involves injecting fluid in the ground to create many fractures at a few kilometres deep in the ground. This fracturing is like how natural earthquakes occur – but at a much smaller scale – they generate tiny vibrations that can we record using instruments at the surface. The detection of six “micro-earthquakes” by the British Geological Survey in Lancashire over the past seven days, are a testament to the high sensitivity of the recording network of sensors (called “seismometers”) that have been installed by the British Geological Survey, UK universities, and the drilling operator. Data from these sensors is being analysed in real-time.

“The ground motion produced by a magnitude 0.5 earthquake (the largest recorded in Blackpool over the last 7 days) is probably less than the equivalent vibrations we might measure from nearby road traffic. Since the earthquake magnitude scale is logarithmic, the magnitude 2.3 Blackpool earthquake induced by hydraulic fracturing in 2011 released over 500 times more energy than this week’s 0.5 event.

“In contrast to small fractures causing microseismic events, stronger earthquakes that some people may slightly feel at the surface (i.e. magnitude greater than 2) would need to occur on pre-existing geological faults, which give the area needed for larger rupture.

“Many hydraulic fracturing operations have occurred around the world without inducing earthquakes that people can feel at the surface. There are only a handful of examples of hydraulic fracturing causing larger earthquakes. One such example is the magnitude 2.3 earthquake that Cuadrilla’s first hydraulic fracturing attempt in 2011. Whether felt earthquakes are induced depends on many factors. Such factors include the amount and rate of fluid injected, whether nearby faults are present and what their characteristics are, pre-existing stress conditions, and the frictional properties of the rocks at depth (i.e. shale) and how it responds to increased fluid pressure. The drilling operator (Cuadrilla) together with the British Geological Survey, Environment Agency, and other parties, will monitor changes in these earthquakes over time, and will respond appropriately (such as adjusting fluid injection volumes/rates) to further reduce the small risk of inducing felt earthquakes.

“The magnitude 0.5 earthquake triggered on the afternoon of 24th October now means that fracking operations could be suspended according to UK regulations.”

Stuart Haszeldine, Professor of Geology, University of Edinburgh, said:

“Earthquake occurrence of these small magnitudes is entirely as expected at the Preston New Road site. Although maybe not as hoped for, so soon, by Cuadrilla. The practical significance is not whether these tremors are felt at the surface or not, but in the potential to damage the borehole, and the potential to create gas pathways from the shale towards larger faults, towards shallower aquifers, and to the surface.

“This borehole is drilled through an existing fault large enough to be mapped in the sub-surface, and the earthquake locations cluster around and close to that fault, and the size of the earthquakes is currently increasing. If the movements on these small fractures, or subsequent larger movements on the existing mapped fault, intersect the borehole, it is known from the Preese Hall site that just tens of millimetres movement can break the gas-tight seal in and around the borehole. That requires expensive remediation, which can be difficult in soft rock such as shale.

“Leakage of rogue methane from around the borehole is unwanted for UK greenhouse gas and clean air compliance for the Climate Act and for IPCC climate objectives “well below 2C warming”. Measurement to detect leakage in and around this borehole will now be required during future years to a very high standard, to ensure no leakage. The geology across Lancashire is similar, so this additional monitoring and expense can be anticipated at many or even all future fracking boreholes.”

Declared interests

None to declare.