EARTHQUAKES 



as a rule, the two sides of a fault are pressed closely 

 against each other, and the stories of yawning 

 chasms, and the swallowing up of people and build- 

 ings are the veriest fabrications. Indeed, faults 

 do not necessarily appear at the surface of the 

 ground at all, or, if they are visible, they are not 

 necessarily conspicuous. The dislocation along the 

 main fault on which the movement took place at 

 the time of the 1906 earthquake in California was 

 about eight feet, but in many places this movement 

 was taken up in the crushing of the surface soil, 

 while at others it appeared as a mere furrow. Nor 

 is it to be supposed that all faults are liable to slip 

 and to cause earthquakes. Some faults have long 

 been inactive, and the rock faces have been re- 

 cemented, and the rocks are now practically solid. 

 Others, however, may be regarded as active faults 

 and it is along them that the strains in the earth 

 and in the immediate vicinity are relieved. 



EPICENTER. Earthquake shocks must necessarily 

 originate well below the surface of the ground, 

 and spread as elastic waves from the centers or 

 planes of disturbance in all directions. From the 

 points where they reach the surface (called the 

 epicenters) they spread out on all sides, gradually 

 dying out or becoming less severe as they get far- 

 ther and farther from the epicenters. 



Once the rocks are broken or faulted the strains 

 within the earth and within a limited adjacent 

 area, are relieved along the same faults, for the 

 reason that on account of their being broken, move- 

 ments are easiest and strains are most readily re- 

 lieved and adjusted along those fractures. Within 

 a given region the earthquakes, therefore, tend to 

 repeat themselves in the same places. 



INTENSITY. The intensity or destructiveness of 

 earthquakes is determined to some extent also by 

 the nature of the ground. Loose materials contain- 

 ing water are more violently disturbed than are the 

 solid rocks. This violence or high intensity is due 

 to the fact that loose wet materials do not move 

 as promptly as the solid rocks, but are tossed about 

 like loose objects upon a table when the latter is 

 jerked beneath them. 



The earthquakes of California are almost in- 

 variably felt over limited areas only, and they are 

 rarely severe enough to cause damage. And when 

 damage is done it is quite as often due to faulty 

 construction as to the violence of the shocks. 



DISPLACEMENT. The amount of actual move- 

 ment of a point on the surface of the earth at the 

 time of an earthquake is not as great as is generally 



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