EARTHQUAKES, 



121 



The vorticose motion in sncli cases is probably not real, but only ap- 

 parent. 



But tliere are other cases of undoubtedly real vorticose motion ; as, 

 for example, straight rows of trees changed into interrupted zigzags by 

 fissures and displacement. All such cases of real hoisting are prob- 

 ably explicable on the principle of concurrence and interference of 

 loaves. If two systems of waves of any kind meet each other, there 

 will be points of concurrence where they re-enforce each other, and 

 points of interference where they destroy each other. Suppose, for 

 instance, a system of water-waves, represented by the double lines i, i 

 (Fig. 102), running in the direction o #, strike against a wall, w w : 

 the waves would be reflected in the direction c c, and are represented 

 by the single lines r, r. Then, if the lines represent crests, and the 

 intervening space the troughs, at the places marked with crosses and 

 dots there would be concurrence, and therefore higher crests and deeper 

 troughs, while at the points indicated by a dash there would be inter- 

 ference and mutual destruction, and therefore smooth water. The same 

 takes place in earth- waves. If two systems of earth- waves meet and 

 cross each other, we must have points of concurrence and interference 

 in close proximity. The ground, therefore, will be thrown into violent 

 agitation — points in close proximity moving in opposite directions 

 (twisting). If the motion be sufficient to rupture the earth, restoration 

 is not made by counter -twisting, and the earth is left in a disyjlaced 

 condition. 



The causes of interference may be various — sometimes by the nor- 

 mal and transverse waves combining differently so as to produce motion 

 in different directions in contiguous places ; sometimes by difference of 

 velocity of waves, already explained, by which some overrun others, 

 concurring and interfering; more often it is the result of reflection 

 from surfaces of different elasticity. For example, it is well known 

 that the most violent effects of earthquakes, especially twisting of the 

 ground, usually occur 

 near the junction of 

 the softer strata of the 

 plains with the harder 

 and more elastic strata 

 of the mountains. Xow, 

 suppose from a shock 

 at X(Fig. 103) a sys- 

 tem of earth - waves 

 should emerge at a, 



and run as a surface- wave toward the mountain m. The waves, strik- 

 ing the hard, elastic material «?, would be partly transmitted and 

 partly reflected. The reflected waves running in the direction of the 



Fig. 103.— Reflection of Earthquake-Waves— Section. 



