Vol. 63.] SWANSEA EARTHQUAKE OF 1906. 361 



There is no known fault in the position assigned by the seismic 

 evidence. The great east-and- west fault un der Swansea Ba3", though 

 nearly in the required direction, passes about 5 or 6 miles to the 

 south of the line indicated, and cannot, I think, be connected with 

 the earthquake. This aud the other parallel faults of the district 

 are chiefly, if not entirely, of pre-Triassic age. Small displacements 

 have, however, taken place in post-Triassic times, and it is probable 

 that the Swansea earthquake was due to a movement along a fault 

 which has not yet affected the structure of the superficial beds. 1 



How slowly the fault is growing is indicated by the rarity of 

 true tectonic earthquakes. For more than 70 years, no slip of any 

 consequence has taken place along any one of the system of faults. 

 There may have been a few creeps along the fault-surface during 

 the 24 hours preceding the recent displacement ; but the first im- 

 portant movement occurred in the eastern focus near Llwynypia, 

 followed after a few seconds by a much more marked slip within a 

 detached focus near Swansea. 



EXPLANATION OF PLATE XXV. 



Map illustrating the area affected by the earthquake of June 27th, 1906. on the 

 scale of 29^ miles to the inch. The date is erroneously indicated on the 

 map as June 26th. 



[For the Discussion, see p. 374.] 



1 The faults of the epicentral districts are mapped in Sheets 217 & 248 of 

 the Geological-Survey 1-inch map. It is worthy of notice that the coal-seams 

 are traversed by faults with throws ranging up to 100 yards, of which there 

 are presumably no traces at the surface. 



