The Swansea Earthquake of 1906. 787 



The shock consisted of two distinct parts— the first part being 

 much weaker than the second, except at places within an oval 

 area lying some miles to the east of the Swansea epicentre. The 

 existence of a secondary focus beneath this area is also indicated 

 by the relative positions of the isoseismal lines, the isoseismal 8 

 being much nearer the isoseismal 7 at the western than at the eastern 

 end. In twin earthquakes, it is difficult to ascertain the position 

 of the focus in which the weaker impulse originated, but, in the 

 Swansea earthquake, observations in mines offer an unexpected 

 help. The shock was felt severely iu mines within an area 8 or 9 

 miles in diameter, and as a tremor outside. The centre of this 

 area lies about 1 mile west of Llwynypia, 22| miles east of 

 the Swansea epicentre, and is close to the centre of the area over 

 which the weaker part of the shock was felt. 



Observations, 53 in number, were obtained from 39 pits, dis- 

 tributed over an area 49 miles in length, from near Kidwelly to 

 near Pontypool. The shock was, as usual, less strongly felt in 

 pits than on the surface ; and the sound was more uniform and 

 monotonous underground. Both shock and sound were observed 

 in pits over about the same area. 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, it seemed as a rule to 

 pass overhead or to travel along the workings. There is some, 

 though not decisive, evidence for supposing that the fault was felt 

 more severely in the lower than in the upper workings of a pit. 



The originating fault in the neighbourhood of Swansea must 

 run from E 5° N. toW. 5° S., hading to the south, and passing not 

 far from the line joining Llanelly to Neath, which is 5 or 6 miles 

 to the north of the great east-and-west fault under Swansea Bay. 

 The first movement occurred in the eastern focus near Llwynypia, 

 and this was followed after a few seconds by a much stronger 

 movement in the western or Swansea focus. The interval between 

 the parts was such that the earlier impulse was felt first all over 

 its disturbed area ; but, as the foci were quite detached, the earth- 

 quake was no doubt a true twin-earthquake. 



6. 'The Ochil Earthquakes of September 1900 to April 1907/ 

 By Charles Davison, Sc.D., F.G.S. 



During this interval, a series of slight shocks was felt chiefly in 

 the villages of Blairlogie, Menstrie, Alva, and Tillicoultry, lying 

 between the Ochil Hills and the river Eorth. There were four shocks 

 in 1900, one in 1903, ten in 1905, nineteen in 1906, and eight up 

 to the end of April 1907. The strongest shock of the series 

 occurred on September 21st, 1905 ; its intensity was 6, and it 

 disturbed an area of about 1000 square miles. The originating 

 fault must be directed from about E. 27° ]S r . and W. 27° S., hading 

 to the north, and passing not far from the villages mentioned above. 

 It cannot therefore be identified with the great Ochil Fault, which 

 in the district referred to runs from about E. 13° 1ST. to W. 13° S. 

 and near Dollar hades to the south ; although it is possible that 

 some or many of the slighter shocks may have been due to slips 

 along this fault. 



