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Sect. VI 1. 1 



EAllTHQUAKE PHENOM 



^T 4. 



■jy 



from co-exlstent forces, such as those of volcanic eruption, 

 permanent elevation or depression of land, &c., winch, how- 

 ever closely they may be connected with the originating 

 impulse of the earthquake, form no true part of it— they 



merely c 



omplicate its phenomena 



of 



rally conceived to be a sudden volcanic outburst, or sudden 

 upheaval or depression of a limited area, or sudden frac- 

 ture of bent and strained strata. This origin should be 

 carefully sought for, as to its nature and position. 



An earthquake may have its oripn either inland or at 

 sea ; and as this may be, a different set of phenomena 



In the former case we may ex- 



ist, The Great Ea 



will present themselves, 

 pect, in the following order 

 or true shock, a real roll or undulation of the surface tra- 

 velling with immense velocity outwards in every direction 



dentFi 





from the centre of impulse. If tliis be at a 



below the surface, the shock will be felt principally hori- 



zontally ; but if the origin be profound, the shock which 



is propagated from it in every direction in spherical 



shells will be felt more or less vertically ; and in this case 



also we mai/ be able to notice two distinct waves, a greater 



and a less, following each other almost instantaneously ; 



the first due to the originating normal wave, the second 



to the wave viLrating at right angles to it. It we can 



find the point of the surface vertically over the origin, 



and the direction of emerirence of the shock at a distant 



point or at several, we can find the depth of the origin 



from the surface. 



There are, therefore, certain lincb at points in which the 

 shock. In passing outwards from the origin, is simulta 



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