Discussion on Papers 13 and 14

DR C.D.V. WILSON (Liverpool University): Would Dr Westbrook comment on the type(s) of focal mechanism that he thinks might account for the

dilational first motions that he has found for

some of the tremors?

DR WESTBROOK (Paer 13): This is problematical, but currently I consider that the two most likely causes of focal mechanisms producing dilational first motions are collapse of strata into the goaf, probably with accompanying floor heave, and bed separation of competent strata above the goaf, just behind the working face.

DR B.O. SKIPP (Soil Mechanics Ltd, Bracknell): Was there any correlation between 'weights1 in the mine and recorded tremors ('weights' being a mining term for a sudden onset of load on to the supports at the coal face)?