MINING-INDUCED EARTHQUAKES MONITORED DURING PIT CLOSURE IN THE MIDLOTHIAN COALFIELD

The British Geological Survey installed a seismometer network to monit or earthquakes around Rosslyn Chapel in the Midlothian Coalfield from November 1987 until January 1990. Accurate locations were obtained for 247 events and a close spatial and temporal association with concurre nt coal mining, with a rapid decay of earthquake activity following pi t closure, was demonstrated, indicating a mining-induced cause. Residu al stress from past mining appears to have been an important factor in generating seismicity, and observations indicate that limiting the wi dth of the workings or rate of extraction may significantly reduce or eliminate mining-induced earthquake activity, an extremely desirable p rospect which warrants further investigation. A frequency-magnitude an alysis indicates a relatively high abundance of small events in this c oalfield area. The maximum magnitude of a mining-induced earthquake li kely to have been experienced during the life of the coalfield (maximu m credible magnitude) was 3.0 M-L, although an extreme event (maximum possible magnitude) as large as 3.4 M-L was remotely possible. Signifi cant seismic amplification was observed at Rosslyn Chapel, which is fo unded on sand and gravel, compared with a nearby bedrock site. As a co nsequence, relatively small magnitude events caused high, and occasion ally damaging, seismic intensities at the chapel. This is likely to be an important effect at similar sites elsewhere.