METHODS OF STUDYING EARTHQUAKES 305 
while the inner one separates those places where the sound 
was loud from those where it was distinctly fainter. As the 
more prominent sound-vibrations appear to come from the upper 
margin of the seismic focus’, the northwesterly shift of the 
sound-curves with respect to the isoseismal lines implies that the 
fault hades in the direction opposite to them. 
One of the most interesting features of British earthquakes, 
though it is by no means confined to them, is the double nature 
of the shock. At 
many places there 
are two distinct 
series of vibrations 
separated by a brief 
interval of absolute 
rest and with a large 
number of observa- 
tions—they are asa 
rule quiet. This was 
the case during the 
Hereford earth- 
quake of 1896 near- 
ly all over the dis- 
Gor 
iUGweG saneay | e-NS) a 
rule, however, a weak tremor and a faint rumbling noise are 
observed during the interval at places near the epicenter ; while 
at considerable distances from the origin these become imper- 
ceptible, and the shock appears to consist of two detached por- 
tions. The double shock is chiefly characteristic of strong or 
severe earthquakes but there are several slight ones in which it 
has been observed. Attempts have been made to explain it by 
the reflection or refraction of the earth-waves at the bounding 
surfaces of strata, or by the existence of longitudinal and trans- 
versal vibrations. But the wide distribution of the places where 
the double shock is observed and the fact that the relative nature 
of the two parts of the shock is not constant all over the 
* Phil. Mag., Jan. 1900, pp. 66-70. 
