On the Leicester Earthquake of August 4, 1893. 93 



sound near the north-west end of the major axis, while the sound was 

 more prominent than the shock near the south-east end. 



Origin of the Earthquake. 



Whatever may be the extent of the fault-surface over which a slip 

 takes place, there must, in the simplest case, be a certain central 

 region over which the slip is comparatively great, surrounded by a 

 marginal area over which it is very small, and gradually diminishes 

 in all directions until it disappears along the boundary. The minute 

 and exceedingly rapid vibrations proceeding chiefly from this 

 marginal area are, I believe, those which are perceptible as sound ; 

 the sound which precedes, accompanies and follows the shock being 

 produced by the vibrations which come from the nearer lateral 

 margin, the upper margin chiefly, and the further lateral margin, 

 with reference to the place of observation. 



In the case of the Leicester earthquake, the sound was observed 

 before the shock all over the disturbed area, because no part of that 

 area was at a great distance from the nearer lateral margin of the 

 seismic focus. But some places, especially near the end of the 

 major axis, were so far from the further lateral margin that no 

 sound following the shock could be heard. 



At a place (like Quarndon) close to one end of the major axis, the 

 sound- vibrations from the further edge of the nearer lateral margin 

 were nearly or quite imperceptible, and the sound entirely preceded the 

 shock. A little nearer the epicentrum (at Derby) the sound-vibrations 

 from the nearer part of the upper margin were perceptible, and conse- 

 quently the sound partly overlapped the shock, but the beginning, epoch 

 of maximum intensity, and end of the sound preceded respectively those 

 of the shock. Lastly, at places close to the epicentrum the sound- 

 vibrations from the further lateral margin could be perceived, and 

 thus the sound was heard both before, during and after the shock, 

 while the epochs of their maximum intensity were nearly or quite 

 coincident. 



In the neighbourhood of the minor axis the sound was heard at 

 the same time as the shock, and generally both before and after. No 

 interval was apparent between their epochs of maximum intensity, 

 for the places of observation were at nearly the same distance from 

 the upper lateral margin and central area of the seismic focus. 



The time-relations of the sound and shock are thus readily 

 explained on the supposition that the sound- vibrations come chiefly 

 from the marginal area of the seismic focus. But this theory of 

 their origin receives a stronger support from the observations sum- 

 marised above than its mere capability of offering a satisfactory 

 explanation. The sound was heard before the shock practically at 



