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ABSTRACT Induced seismicity, associated with longwall coal extraction in the North Staffordshire coalfield, England, has been investigated in order to establish the relationship between mining and sefsmicity. A seismometer network, operated during the mining of two longwall panels, has been used to obtain tremor hypocentre locations and magnitudes. The largest earth tremors had local magnitude values of approximately 3. While 54 were felt at the surface by local inhabitants, none were felt underground by coalworkers. The earth tremors are shown to be located adjacent to the active mine workings and move in unison with the advancing faces. Seismicity does not appear to be associated with faults in the area, nor is the area seismically active naturally. The majority of 'larger tremors have a shear source mechanism and occur in pillars of previous workings of other seams, when the active face passes under or above those pillars. The smaller magnitude tremors have an implosional source mechanism and are interpreted as being generated by strata collapse within waste adjacent to the advancing faces. Seismic activity did not start until the faces had each advanced approximately 25Om.
INTRODUCTION Whilst mining induced seismicity is known to occur during coal, gold and ore mining, the vast majority of mine excavations are non seismic. There is little documented research into tremors or seismic emissions in British coal mines, with the exception of the descriptions and analysis of rock bursts by Phillips (1945) and of minor tremors associated with coal mining in Scotland (Mushkour, 1976) and in North Staffordshire (Davison, 1905), indicating that such stress related phenomena, in the weak rock of the Coal Measures cyclothem, have not generally been considered important.
Seismicity associated with coal mining has occurred intermittently throughout the last century in the North Staffordshire coalfield. However in the years 1975 to 1977 a series of earth tremors occurred which were widely felt at the surface and caused considerable public alarm. The area affected lies in the southern part of the Stoke-on-Trent conurbation which is extensively undermined. The felt events, of which there were 55, had magnitudes (ML) ranging from 2.5 to 3.5 and the most damaging event, a modified Mercalli scale intensity of VI. Westbrook (1977) and Kusznir et al. (1980) have shown that a causal relationship existed between active coal mining and seismicity. The important question which must be answered is why mining in this particular case generated large magnitude tremors felt by local residents, whilst other mining in the North Staffordshire coalfield or in other British coalfields, in supposedly similar conditions, does not. An understanding of the mechanism of earth tremor generation is an important step in the answering of this question. MINE LAYOUT AND GEOLOGY IN THE SEISMIC AREA
The coal workings, in the area experiencing seismic activity, are shown in Figure 1. Panels 204, 205 and 206 of the Ten Feet seam were being mined when seismic activity occurred. Also shown in Figure 1 is the fault distribution adjacent to the Ten Feet panels and previous workings in the Ten
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