Br. C. Davison — The Stafford Earthquakes of 1916. 305 



It is remarkable that all these earthquakes, with two exceptions 

 (the Inverness and Carnarvon earthquakes) belong to tlie class of 

 twin-earthquakes. 



Nature of the Shock. 



The following accounts may be regarded as typical of the nature 

 of the shock in different parts of the disturbed area (see Fig. 2) : — 



(1) Chebsey : the shock consisted of one series of vibrations 

 accompanied by sound which increased in strength to a maximum 

 and then died away. 



(2) Coton Hill (near Stone) : the shock consisted of one continuous 

 series of vibrations, lasting about six seconds, increasing in strength 

 from the beginning and dying away towards the end; in the middle, 

 there were two maxima separated by about half a second. 



(3) Brereton : the shock was in two distinct parts, the first being 

 the stronger, and the interval between the two series being two or 

 three seconds. 



(4) Kinnersley: the shock was again in two distinct parts, 

 separated by an interval of two seconds, the second part being the 

 stronger. 



Over the greater part of the disturbed area, and in some directions 

 even as far as the isoseismal 3, the shock consisted of two distinct 

 parts. Thus, the wide distribution of the places at which the 

 double shock was felt precludes the supposition that the earthquake 

 was caused, by a single impulse, the resulting waves of which were 

 duplicated by reflection or refraction. Nor can the two parts of the 

 shock represent direct and transverse vibrations resulting from 

 a single impulse ; for, as will be seen later, the duration of the 

 interval between the two parts does not increase with the distance 

 from the origin. Nor, lastly, could the two parts be caused by 

 successive impulses in the same focus; for, if this were the case, 

 there could be no part of the disturbed area in, which the two series 

 of vibrations coalesce. We are thus led to conclude that the two 

 parts of the shock were derived from distinct impulses originating in 

 different foci. 



"When the places are mapped at which the double and single 

 shocks were felt, we find that the former are absent from a band 

 which crosses the isoseismal 7 at right angles to its longer axis 

 at a short distance to the west of the centre. The band is very 

 slightly curved, the concavity facing the west. Within this band 

 (as at Chebsey), the shock was either single or consisted of one 

 series of vibrations with two maxima. Close to the band (as at 

 Coton Hill, one mile to the east), and on either side of it, the 

 shock was single with two maxima. At many places the observers 

 differed, some regarding the shock as single, others as double. For 

 instance, at Hanley and Stoke-on-Trent, which lie one mile to the 

 west of the band, fifteen observers regarded the shock as single and 

 five as double. At places farther from the band, such as Brereton 

 (13 miles) and Kinnersley (4 miles), the two parts were entirely 

 distinct. 



DECADE VI. — VOL. VI. — NO. VII. 20 



