Last updated 07:45 UTC 15 April 2014
Since mid-December 2013, BGS have recorded 91 small earthquakes in and around New Ollerton, Nottinghamshire. This is an area with a history of seismic activity related to coal mining and location and characteristics of the recent activity are consistent with this. The events are broadly clustered to the north and west of New Ollerton. However, the errors in the locations are around 1-3 km, so it is possible the events all originate from the same location. This is supported by the similarity of the ground motions recorded at one of the closest stations.
The earthquakes have all been small, with magnitudes of less than 2.0 ML. The largest have had magnitudes of 1.8 ML. Many of the tremors have been felt by local residents. However, typically the strength of shaking experienced has been weak and some of the events were only just perceptible. Earthquakes of this size are not likely to cause any damage and the hazard from mining induced seismicity in the UK is typically low. There have been no reports of structural damage from mining-induced earthquakes in the UK in the past forty years and the maximum observed magnitude is ~3 ML. As a result, the seismic hazard and risk of damage is low. However, an event with a magnitude of 3 ML at a shallow depth could be strongly felt and could cause some alarm to local residents.
During the week commencing 3 February a temporary network of 6 seismometers was deployed north of the New Ollerton area and data began to be recorded at these sites from 5 February. A seventh seismometer was added to the temporary network on 2 April. The first batch of data was collected on 18 February and is currently being analysed. Many more smaller earthquakes have been detected.
The coalfields of Britain are frequently the source areas of small to moderate earthquakes and tremors in these areas have been reported for at least the last hundred years, for example the Stafford earthquake of 1916 (Davison, 1919). With the growth of instrumental seismic monitoring in the UK in the 1970s many more tremors were recorded in mining areas across the UK (Redmayne et al, 1988) and a number of temporary networks of sensors were deployed to study these events in more detail. This led to the conclusion that these events were related to ongoing mining activity and that these were quite distinct from the natural background seismic activity of the UK. Tremors around Stoke-on-Trent, Staffordshire in the period 1975-1977 were shown to originate from a region above active mineworkings (Westbrook et al, 1980). A network deployed around Rosslyn Chapel in the Midlothian coalfield recorded over 250 earthquakes between 1987-1990 were shown to have a close spatial and temporal association with mining activity (Redmayne et al, 1998). Between July 1989 and August 1990 over 130 tremors were felt and reported by people in the Edinstowe district of Nottinghamshire. A temporary network of sensors detected a further 785 microseismic events in the following 11 months helping to establish a causal relationship between the local microseismicity and coal production.
In the 1980s and 1990s mining events accounted for approximately 25% of all the earthquakes recorded in the UK (Browitt et al, 1985). Since the rapid decline of mining activity in the UK there has been a general decrease in the number of these events, however, events may still occur in these areas years after all mining activity has ceased.
Earthquakes generally result from slip along a pre-existing fault in the Earth. The slip is triggered when the stress acting along the fault exceeds the frictional resistance to sliding. The pre-existing state of stress on a fault determines how close it is to failure. Faults that are critically stressed may require only a small stress perturbation to cause them to fail. Human activities such as mining can change the state of stress on nearby faults by either the removal of sub-surface rock or by the collapse of old mine workings, which can move the faults closer to failure and result in induced earthquakes. This process is only likely to affect faults within relatively small distances of the stress perturbation and mining induced earthquakes generally within of a few kilometres of the mining activity. Similarly, sometimes the induced events occur shortly after the mining activity begins, but in other cases they happen long after it has ceased.
The hazard from mining induced earthquakes is likely to be rather low in terms of the probability of damaging ground motions and the spatial extent of the areas affected. In the UK, the maximum observed magnitudes from coal mining induced seismicity (Bishop et al., 1994; Redmayne et al., 1998) is around 3 ML. The maximum observed magnitude for tectonic events exceeds 6 ML (Main et al., 1999). This difference may be partly explained by the fact that coal-mining events occur at shallow depths in Carboniferous geological formations, where the Earth's crust is relatively weak, whereas the larger tectonic events tend to nucleate at much greater depths where the Earth's crust is significantly stronger.
An event of with a magnitude of 3 ML, even at a shallow depth, is unlikely to cause structural damage, and there have been no reports of structural damage from mining-induced earthquakes in the UK in the past forty years. However, such an event could be strongly felt and could cause some alarm to local residents. There are also a number of examples of mining-induced earthquakes of similar magnitudes in the UK that caused superficial damage (Westbrook et al., 1980; Redmayne, 1998), such as cracks in plaster.
Bishop, I., Styles, P. and Allen, M. (1994), Mining Induced Seismicity in the Nottinghamshire Coalfield, Quarterly Journal of Engineering Geology, 26 (4), 253-279.
Browitt, C.W.A., Turbitt, T. and Morgan, S. (1985), Investigation of British earthquakes using the national monitoring network of the British Geological Survey, Earthquake Engineering in Britain, Thomas Telford, London, 33-47.
Davison, C. (1919), The Stafford earthquakes of January 14-15, 1916 and the relations between twin earthquakes of the Midland counties, Geological Magazine, Decade VI, 6, 302-312
Main I., Irving D., Musson R. and Reading A. (1999), Constraints on the frequency-magnitude relation and maximum magnitudes in the UK from observed seismicity and glacio-isostatic recovery rates, Geophys. J. Int., 137, 2, 535-550
Redmayne D.W. (1988), Mining-induced seismicity in UK coalfields identified on the BGS National Seismograph Network, in Engineering Geology of Underground Movements, Geological Society Engineering Geology Special Publication, 5, 405-413
Redmayne, D.W., Richards, J.A. and Wild, P.W. (1998), Mining-induced earthquakes monitored during pit closure in the Midlothian Coalfield", Quarterly Journal of Engineering Geology, 31, 21-36
Westbrook, G.K., Kusznir, N.J., Browitt, C.W.A. and Holdsworth, B.K. (1980), Seismicity induced by coal mining in Stoke-on-Trent (U.K.), Engineering Geology, 16, 225-241