James Verdon, John-Michael Kendall, Antony Butcher, Richard Luckett, Brian Baptie

The U.K. has a long history of deep coal mining, and numerous cases of mining-induced seismicity have been recorded over the past 50 years. In this study we examine seismicity induced by longwall mining at one of the U.K.’s last deep coal mines, the Thoresby Colliery, Nottinghamshire. After public reports of felt seismicity in late 2013 a local seismic monitoring network was installed at this site, which provided monitoring from February to October 2014. This array recorded 305 seismic events, which form the basis of our analysis.

Event locations were found to closely track the position of the mining face within the Deep Soft Seam, with most events occurring up to 300 m ahead of the face position. This indicates that the seismicity is being directly induced by the mining, as opposed to being caused by activation of pre-existing tectonic features by stress transfer. However, we do not observe correlation between the rate of excavation and the rate of seismicity, and only a small portion of the overall deformation is being released as seismic energy.

Event magnitudes do not follow the expected Gutenberg-Richter distribution. Instead, the observed magnitude distributions can be reproduced if a Truncated Power Law distribution is used to simulate the rupture areas. The best-fit maximum rupture areas correspond to the distances between the Deep Soft Seam and the seams that over- and underlie it, which have both previously been excavated. Our inference is that the presence of a rubble-filled void (or goaf) where these seams have been removed is preventing the growth of larger rupture areas.

Source mechanism analysis reveals that most events consist of dip-slip motion along near-vertical planes that strike parallel to the orientation of the mining face. These mechanisms are consistent with the expected deformation that would occur as a longwall panel advances, with the under- and over-burdens moving upwards and downwards respectively to fill the void created by mining. This further reinforces our conclusion that the events are directly induced by the mining process. Similar mechanisms have been observed during longwall mining at other sites.