360 DR. CHARLES DAVISON ON THE L^Ug. I 9°7> 



In the remainder, situated at 5 miles or more from the nearest 

 epicentre, the sound seemed to pass overhead in 5 pits, beneath 

 the observer in 3 pits between 8 and 10 miles from the Swansea 

 epicentre, and along the workings in 2 pits. The pit most distant 

 from either epicentre in which such observations were made is 

 Cwmbran, near Pontypool, 19 miles from the eastern epicentre. 

 At a depth of only 100 yards in this pit, the sound seemed to 

 pass over the head of the observer. 



The observations made at different depths in the same pit are 

 comparable in three cases only. At Ystrad Bhondda (1 mile from 

 the eastern epicentre), the shock was not felt in the upper seams, 

 but was strong in the lower seams at depths between 500 and 

 700 yards. Also, at Cross Hands (8 miles south of Llandeilo, 

 and 11 miles from the Swansea epicentre), the shock was felt only 

 in workings below a depth of 280 yards. On the other hand, at 

 Nantymoel (5 miles east of Maesteg), the shock is reported to have 

 been less severe in the lower seams at depths of 300 and 307 yards 

 than in the upper seams at depths of 160 and 232 yards. It is not 

 clear how the comparison was made in the last-mentioned case, 

 while in the others the inequality is obvious. 



The conclusions to which we are led by a study of the observa- 

 tions in mines are as follows : — (i) the shock and sound were 

 observed underground over the same area ; (ii) in pits not more 

 than 5 miles from the nearest epicentre, the sound seemed to pass 

 below the workings, in those at a greater distance, as a rule, to 

 pass overhead or to travel along the workings, showing, I think, 

 that the foci cannot have been more than a few miles below the 

 surface ; and (iii) there is some, though not decisive, evidence for 

 supposing that the shock was felt more severely in the lower than 

 in the upper workings of a pit. 



III. The Origin of the Earthquake. 



The elements of the originating fault, as determined by the 

 seismic evidence, are: — (i) the mean direction of the fault in 

 the neighbourhood of Swansea is E. 5° N. and "W". 5° S. ; (ii) its 

 hade in the same district is towards the south ; and (iii) the fault 

 must pass a short distance to the north of Swansea, and probably 

 not far from the line passing through Llanelly, Loughor, Morriston, 

 and Neath. 



The principal focus, with its centre below a point about 3 miles 

 west of Swansea, must have been about 10 or 12 miles in length, 

 and possibly extended from near Llanelly to near Neath. The 

 existence of a secondary focus is shown by the excentricity of the 

 isoseismal 8 with regard to the isoseismal 7 in the east-to-west 

 direction, the twin-character of the shock, the area over which 

 it was perceptible, the distribution of the relative intensity of the 

 two parts of the shock, and lastly by the observations in mines. 

 The position of the secondary focus is determined with the greatest 

 accuracy by those observations, as 22| miles due east of the other. 



