Source study of local coalfield events using the modal synthesis of shear and surface waves

Abstract

SUMMARY Coalfield events account for a large fraction of the short period seismic recordings made on the British Geological Survey's UK Network. These data provide an ideal opportunity to investigate the source of such seismic disturbances in more detail. This is achieved by analysing vertical component seismograms, generated by local coal-mining tremors in the Midlothian Coalfield in Scotland, using a waveform matching technique. The basis for this technique is the modal summation method for constructing synthetic seismograms. This computation of synthetic seismograms facilitates a full comparison between the effects of different earth structures, source depths, source orientation and types of event. The results of these analyses are applied to the S- and surface wave portion of the seismogram (for two stations the P-waves also) to yield well resolved details of the depth (to within ± 100 m) and nature of the seismic source. The final solutions constrain the range of physical mechanisms which could have caused the tremors and hence the degree to which the events resemble natural earthquakes.