Yol. 63.] SWANSEA EARTHQUAKE OE 1906. 359 



49 miles long, from Carway (near Kidwelly) on the west to Yarteg 

 (near Pontypool) on the east, but are chiefly concentrated within 

 and near the Hhondda Yalleys. 



The observations in this district throw an unexpected light on 

 the position of the eastern or secondary focus. In eight pits, lying 

 within the area bounded by the dotted line in the map (p. 354), 

 the heaving of the floor was distinctly perceived ; while, in those 

 situated in the surrounding district, the shock was a mere tremor. 

 The area in question is 9 miles long from east to west, 8 miles 

 wide, and contains about 56 square miles. Its centre is situated in 

 lat. 51° 37-9' N., long. 3° 28*8' W., or 1 mile west of Llwynypia ; 

 and 22\ miles due east of the centre of the isoseismal 8. As this 

 point is close to the centre of the area within which the twin- 

 shock was perceptible, and also lies within, and near the western 

 margin of, the area in which the first part of the shock was stronger 

 than the second, it must coincide nearly with the epicentre of the 

 eastern focus. 



Even in collieries close to one of the epicentres, the shock was 

 not uniformly felt by all the men underground. It was most 

 sensible to those who were sitting at the time, or working at the 

 coal-face. It seems to have had but little effect on the roof and 

 floor of the workings. According to most of my informants, no falls 

 whatever took place in the mines under their charge, although a few 

 fragments fell from the roof at Ynysybwl and Ystrad Ehondda, and 

 dust rose from the ground at Llwynypia and Ynysybwl. 1 



Though the earthquake was observed in mines over an area 

 49 miles in length, no mines were more than 19 miles from the 

 nearest epicentre ; and, even at this distance, the rumbling sound 

 was accompanied by a distinct tremor. 



The sound observed in the mines differed slightly from that 

 heard on the surface. It was compared in 54 per cent, of the 

 records with moving vehicles (generally ' a journey of trams run 

 wild ') ; in 12 per cent., with thunder ; in 2, with wind ; in 5, with 

 falls of the roof in adjoining workings ; in 5, with the fall of a 

 heavy body ; in 17, with explosions or the firing of a blast ; and 

 in 5 per cent., with miscellaneous sounds. The sound was evidently 

 more uniform and monotonous underground than on the surface, 

 although the loud explosive crashes which accompany the strongest 

 vibrations were occasionally heard. 



In reply to the question whether the sound appeared to travel 

 overhead or below, I have received 25 answers, several of which 

 are the result of a large number of enquiries. In 6 cases the 

 sound seemed to pass overhead, in 13 below, and in 6 to travel 

 both above and below or along the workings. The observations 

 come from 18 collieries. In 8 of these, within a distance of 5 miles 

 from the nearest epicentre, the sound appeared to come from below, 

 with one exception when it seemed to travel along the workings. 



1 The heavy rock-falls reported in newspapers cannot, I believe, be traced 

 directly to the earthquake, some having occurred on the day before the earth- 

 quake and others on the day after. 



