88 



EARTH FEATURES AND THEIR MEANING 





DOM I 





Fia. 83. Seismotectonic lines of the West Indies. 



instability are known as seismotectonic lines earthquake struc- 

 ture lines. 



The heavy shocks above loose foundations. It is character- 

 istic of faults that they soon bury themselves from sight under 

 loose materials, and are thus made difficult of inspection. The 

 escarpment which is the direct consequence of a vertical displace- 

 ment upon a fault tends to migrate from the place of its formation, 

 rounding the surface as it does so and burying the fault line beneath 

 its deposits (Fig. 43, p. 60). 



This is not, however, the sole reason why loose foundations 

 should be places of special danger at the time of earth shocks, for 

 the reason that earthquake waves are sent out in all directions 

 from the surfaces of displacement through the medium of the un- 

 derlying rock. These waves travel 

 within the firm rock for considerable 

 distances with only a gradual dissipa- 

 tion of their energy, but with their 

 entry into the loose surface deposits 

 their energy is quickly used up in 

 local vibrations of large amplitude, 

 and hence destructive to buildings. 



The essential difference between 

 firm rock and such loose materials as 

 are found upon a river bottom or in 

 the "made land" about our cities 

 may be illustrated by the simple 

 device which is represented in Fig. 84. Two similar metal pans 

 are suspended from a firm support by bands of steel and "elastic" 

 braid of similar size and shape, and carry each a small block of 

 wood standing upon its end. Similar light blows are now admin- 

 istered directly to the pans with the effect of upsetting that block 





FIG. 84. Device to illustrate the 

 different effects upon the trans- 

 mission and the character of 

 shocks which are produced by 

 firm rock and by loose materials. 



