Mining-induced seismicity of the North Staffordshire coalfield

Abstract

Since the latter part of the last century several areas in North Staffordshire have experienced occasional series of earth tremors which are known locally as "goths" or "bumps". In early 1975 a series of earth tremors, culminating in a large tremor on the 15th July, shook the Trent Vale-Hanford area in the south of Stoke-on-Trent where coal was being extracted. The occurrence of earth tremors in areas where coal was being mined suggested that the incidence of these earth tremors was associated with mining operations in the coalfield.
To establish the causes and mechanisms of earth tremor generation, a seismic network consisting of five vertical and two horizontal seismometers was installed in the Stoke area in order to locate the hypocentres of the seismic events. The monitoring of seismic activity was carried out for two years (between September 1975 and September 1977).
Dr. N. J. Kusznir and Dr. G. K. Westbrook have studied the seismic activity in the Trent Vale-Hanford area,in the southern part of the North Staffordshire coalfield. In this thesis the seismic activity in the northern part of the coalfield is investigated.
During the seismic monitoring period, more than 300 seismic events were recorded and found to have occurred in the northern part of the coalfield. 98 of these were located. Only events recorded on four or five seismometers were located. They had local magnitude values, M L, ranging between 0.1 and 2.5. The cluster of these seismic events coincides with regions of active mine workings. The tremor hypocentres were found to cluster in five areas. In three of these areas (areas A, C and D) seismic activity occurred as a result.of the interaction between the stress field of the current workings and the stress field associated with the old mine workings. In the other two areas (areas B and E) the seismic activity appeared to be associated with sliding movements along pre-existing faults due to the effects of mining operations. The depths of the tremor foci were concentrated between 1,000 m below O.D. and the surface, i.e. within the depth range of the active mine workings. The accuracy of the location was estimated to be of about 500 m in plan and less than that in depth.
First motion analysis of seismic events enabled the mechanisms of these events to be determined. Two types of seismic events with different first motions were observed in the coalfield. The first group of tremors showed both anaseismic and kataseismic first motions on seismometers, type of tremor is interpreted as occurring either as a result of sliding movements along pre-existing faults, or due to failure in shear of in pillars and ahead of the active faces. The second group of tremo p
duced kataseismic first motions on all seismometers. This type of is interpreted as having either an implosional source mechanism, and oc as a result of rock failures in compression (pressure bursts), or as having a collapse source origin, and occurred mainly in goafs as a result of collapse and roof caving.