Scotland is characterised by low levels of earthquake activity. Historical observations of earthquake activity date back to the 16th century, and show that despite many accounts of earthquakes felt by people, damaging earthquakes are relatively rare. The largest recorded earthquake in Scotland had a magnitude of 5.2 ML and only two other earthquakes with a magnitude of 5.0 ML or greater have been observed in the last 400 years. As a result, the risk of damaging earthquakes is low.
Most earthquake activity in Scotland is north of the Highland Boundary Fault, on the west side of mainland Scotland, and there are fewer earthquakes in northern and eastern Scotland. It is rarely possible to associate these earthquakes with specific faults because of uncertainties both in the earthquake location estimates, which are typically several kilometres, and our limited knowledge of faulting below the surface. Earthquake activity in the Midland Valley of Scotland is lower than that north of the Highland Boundary Fault, and most of the recorded earthquakes in this area in the 1970's, 1980's and 1990's were induced by coal-mining. Since the decline of the coal-mining industry in the 1990's, very few mining-induced earthquakes have been recorded. Most of the mining induced earthquakes are small and the largest mining-induced earthquakes in Scotland had a magnitude of 2.6 ML.
Earthquake activity rates for Scotland determined from 1970 to present suggest that, on average, there are eight earthquakes with a magnitude of 2.0 or above, which is roughly the minimum magnitude felt by people, somewhere in Scotland every year. Activity rates calculated for the Midland Valley are lower, although the small number of observed earthquakes for this area means the values have large uncertainties. This suggests that earthquake hazard in the Midland Valley is lower than elsewhere in Scotland.
Existing catalogues of earthquake activity in Scotland are incomplete at magnitudes below 2 ML, from 1970 to present, and for higher magnitudes prior to this. This is due to the detection capability of the networks of seismometers that have operated in the study area over the last few decades. This, together with the low background activity rates, limits our ability to identify any areas that might present an elevated seismic hazard for any Unconventional Oil and Gas ( UOG) operations based on seismic data alone. Similarly, limited information about the state of stress in the Earth's Crust means that it is not possible to identify any particular parts of the study area where faults are more likely to be reactivated and that may present an elevated seismic hazard for any UOG operations.
The process of hydraulic fracturing in order to increase the permeability of reservoir formations and stimulate the recovery of hydrocarbons is generally accompanied by microseismicity, commonly defined as earthquakes with magnitudes of less than 2.0 that are too small to be felt. In the US and Canada, the large number of hydraulic fracturing operations that have been carried out and the small number of felt earthquakes directly linked to these operations, suggests that the probability of felt earthquakes caused by hydraulic fracturing for recovery of hydrocarbons is very small. Over 1.8 million hydraulic fracturing operations have been carried out in the US in ~1 million wells and there are only three documented cases of induced earthquakes conclusively linked to hydraulic fracturing for shale gas recovery. The largest of these earthquakes had a magnitude of 3.0. However, in western Canada, increases in the annual numbers of earthquakes over the last ten years correspond to increases in the number of hydraulically fractured wells, suggesting that hydraulic fracturing has induced earthquakes. There are also a number of documented examples of earthquakes with magnitudes larger than 3 in Canada that have been linked to hydraulic fracturing for shale gas recovery. The largest of these was a magnitude 4.4 earthquake, which is the largest known earthquake suspected to have been triggered by hydraulic fracture operations in a hydrocarbon field anywhere in the world. However, as in the US, the probability of induced earthquakes that can be felt appears small given the large number of hydraulically fractured wells (>12,000).
Studies of earthquake activity in the Raton Basin (United States), an area that has produced coal-bed methane since 1994, suggest that this activity is related to the subsequent disposal of wastewater from the coal-bed methane extraction process by injection into deep wells, rather than from the extraction process itself. Literature was not located concerning induced seismicity and coal-bed methane extraction in Canada, Australia or other parts of the USA, suggesting that this is not a major issue in those areas
Recent increases in earthquake rates and significant earthquakes in many areas of the Central and Eastern United States have been linked to the disposal of wastewater by injection in to deep wells rather than hydraulic fracturing, and provide a considerable body of evidence that this activity has a non-negligible contribution to the seismic hazard. Seismic hazard forecasts for the Central and Eastern United States now include contributions from both induced and natural earthquakes and show increases in earthquake hazard by a factor of 3 or more in some areas of induced earthquake activity. However, although many wastewater injection wells can be associated with earthquakes, the majority are not. Additionally, the nature of the wastewater injected into deep wells varies: while some comes from hydraulic fracturing used in unconventional oil and gas production, many wastewater injection wells are used to dispose of produced water from conventional hydrocarbon production.
Although the triggering process of natural and induced earthquakes may differ, there is no evidence to suggest that the expected maximum magnitude will not be similar.
The UK Department for Energy and Climate Change ( DECC, 2013) published a regulatory roadmap that outlines regulations for onshore oil and gas (shale gas) exploration in the UK. These regulations contain specific measures for the mitigation of induced seismicity including: avoiding faults during hydraulic fracturing; assessing baseline levels of earthquake activity; monitoring seismic activity during and after fracturing; and, using a 'traffic light' system that controls whether injection can proceed or not, based on that seismic activity. Regulatory measures to mitigate the risk of induced seismicity are also in place in the US and Canada. In the US, much of this regulation is aimed at induced seismicity related to wastewater disposal in deep wells, although this is also relevant to induced seismicity from hydraulic fracturing. These measures are broadly similar to those specified by DECC.
In the UK, the magnitude limit for the cessation of hydraulic fracturing operations (0.5 ML) is considerably less than the limits in California (2.7 ML) and Illinois, Alberta and British Columbia (4.0 ML), and may be considered a conservative threshold. Local monitoring systems that are capable of reliable measurement of earthquakes with very small magnitudes will be required to implement the UK limit successfully. A magnitude 4.0 ML earthquake in an area of high population density, such as the Midland Valley of Scotland, would be strongly felt by many people and may even cause some superficial damage.
British Standards BS 6472-2 and BS 7385-2 define limits for acceptable levels of ground vibrations caused by blasting and quarrying and the limits for vibrations caused by blasting, above which cosmetic damage could take place. A comparison of modelled ground motions for a range of earthquake magnitudes with these limits suggests that earthquakes with magnitudes of 3.0 or less are unlikely to exceed the limits above which cosmetic damage may occur, as set out in BS 7385-2, except at distances of less than a few kilometres. Smaller earthquakes may also exceed the limits for vibration set out in BS 6472-2, but again only at small distances of less than a few kilometres.
Improved understanding of the hazard from induced earthquakes and the successful implementation of regulatory measures to mitigate the risk of induced seismicity are likely to require additional data from a number of sources:
(1) Higher quality earthquake catalogues that can be used to determine reliable estimates of background activity rates and that allow the discrimination and forecasting of induced seismic activity. Without these, any changes in the rate of small magnitude events may be obscured by the uncertainties. This will require denser arrays of seismic instrumentation than at present. These dense arrays are also required to provide high-quality, real-time earthquake locations, which are required as part of any traffic light system for mitigating risk. It is important that the data from any such arrays are openly available to maintain public confidence.
(2) Geological and geophysical data that can be used to map sub-surface fault systems in high resolution, measure the orientation and magnitude of the stress field, and determine the hydrological properties of the sub-surface.
(3) Industrial data from hydraulic fracturing operations such as injection rates and volumes, along with downhole pressures.