208 



MOVEMENTS AND DEFORMATIONS 



points reached by the wave at the same moment. The lines normal to these curves 

 represent the directions in which the wave is propagated, in its various parts. The 

 meeting point of these lines gives the true focus, F, which is much nearer the 

 surface than F'. 



Amplitude of vibrations. From the disastrous effects of earth- 

 quakes it might be inferred that the vibrations have large ampli- 



Fig. 206. Seismogram of earthquake in Punjab, India, April 4, 1905, showing 

 the actual amount of movement. (Montessus de Ballore.) 



tudes; but it is chiefly their suddenness that makes them effective. 

 Except at their points of origin, most of them are only a fraction of a 

 millimeter, and few exceed a few millimeters. It is the oscillation 

 of the rock particles transmitting the vibrations that is here meant, 



Fig. 207. A fissure on East Street, San Francisco, near the water front, in 

 "made ground." (Lindgren, U. S. Geol. Surv.) 



