:220 Dr. C. Davison — Earth-shakes in Mining Districts. 



(2) The intensity of the shock is great near the centre of the area, 

 And dies away rapidly towai'ds the boundary. 



(3) The shock and sound are of short duration, the sound in four 

 out of every five records being compared either to the fall of a heavy 

 body or to an explosion. 



We may infer, from these facts, that the centre of disturbance is 

 situated at a very slight depth belovr the surface, and also that it is, 

 as a rule, of small dimensions. Falls of rock in pit-workings would 

 satisfy these conditions and would produce shocks very similar to 

 those observed, but none was found on any of the above occasions. 

 Many rock-falls are caused or accompanied by disturbances known 

 in the Staffordshire Colliery districts as 'bumps' or * goths.' They 

 are probably due to " the sudden release by fracture of a state of 

 tension in the strata either pre-existeut or brought about by the 

 mine-workings." ^ 



Another probable cause of earth-shakes in mining districts is that 

 suggested in a recent paper in this Magazine.' It is noticeable that, 

 in all the above cases, the centres of the disturbed areas lay close 

 to faults ; and those of the Pendleton earth-shakes were on the 

 downthrow side of the Irwell Valley fault, the coal having been 

 ■worked away right up to the fault. By the withdrawal of the coal, 

 the rock above is deprived of most of its support, and tends to sink 

 down and close up the worked-out seam. Especially would this be 

 the case in the neighbourhood of a fault, where a series of small 

 fault-slips would take place, each giving rise to a shock, violent in 

 the district immediately above it but rapidly fading away in strength, 

 and as a rule of brief duration and disturbing a nearly circular area. 



Direct proof of the occurrence of such semi-artificial fault-slips 

 could no doubt be afforded only by careful and prolonged observation 

 and experiment ; but indirect evidence supporting the above ex- 

 planation is furnished by three recent earth-shakes, those of Blisland 

 (Cornwall) on August 25, 1895, of Camborne on June 4, 1902, and 

 of Barnsley on October 25, 1903. 



Blisland Earth-Shake : August 25, 1895. 



Time of occurrence, about 12.30 p.m. ; intensity, 4 ; epicentre, in lat. 50° 31*9' N., 

 long. 4° 40'9' W. : number of records, 20, from 19 places, and negative records from 

 2 places. 



The Blisland earth-shake has already been described in this 



' W. N. Atkinson, Report of H.M. Inspector of Mines for the Stafford District 

 for the year 1903, p. 15. Mr. Atkinson remarks that the terms ' bump,' ' goth,' etc., 

 refer to " a sort of explosion or eructation in the strata which in some cases is so 

 severe as to be comparable to local earthquakes. They occur chiefly in thick and 

 deep seams, and vary greatly in intensity both as to sound and effect on the strata. 

 The sound is often compared to that of a shot, and appears to occur sometimes in the 

 coal itself and sometimes in the strata overhead ; in heavy bumps the sound is more 

 like thunder. The effect of the bumps on the strata varies greatly ; in slight cases 

 there is no observable effect except the sound ; when more severe there is vibration, 

 and falls of roof may take place, or coal be burst off the face ; in severe cases timber 

 is broken or reeled out, and the mine appears to be about to collapse. In some cases 

 the floor bursts up instead of the roof falling." 



2 Geol. Mag., Voh VII, 1900, pp. 174-177; Vol. VIII, 1901, p. 361. 



