EARTHQUAKES 



BY J. C. BRANNER 



President, Stanford University 



ALL parts of the earth are subject to earth- 

 quakes, but they are notably more common 

 in some regions than in others. Some earth- 

 quakes are felt all over the entire globe, others are 

 confined to areas of only a few square miles. It 

 is a matter of common knowledge that the Pacific 

 Coast is a region in which earthquakes are more 

 frequent than they are in other parts of North 

 America. 



It is not uncommon for those who have had 

 no experience of earthquakes or whose imagina- 

 tions have been impressed by stories about them 

 to suppose that earthquakes are, one and all, mys- 

 terious and disastrous cataclysms, while exaggera- 

 tions of the most extravagant kinds often add to 

 their fancied terrors. To suppose that earthquakes 

 are necessarily disastrous, however, is as far from 

 the truth as it would be to suppose that all rains 

 produced disastrous floods or that all winds were 

 destructive tornadoes. 



As a matter of fact, earthquakes are natural 

 phenomena, and there is no more reason for being 

 alarmed by them than there is for being alarmed 

 by wind or rain or snow. 



Earthquakes are directly related to the geology 

 of the regions in which thev are felt. The shocks 

 are simply vibrations, jars, or elastic waves, propa- 

 gated through the rocks of the earth's surface. The 

 vibrations are caused directly by the breaking of 

 the rocks or by the slipping of them against each 

 other. This breaking is due to uneven strain or 

 pressure within the rocks themselves, and of suf- 

 ficient energy to cause the displacement or slipping. 



Faults. — When a fracture is produced and the 

 beds or rock masses are dislocated these breaks 

 are known technically as faults. The dislocations 

 of the rocks along faults may be small or great, and 

 the faults may be long or short, shallow or deep, 

 and the movements may be vertical or horizontal 

 or at any angle or in any direction whatever. 



The depth of faults is probably limited to the 

 outer ten or eleven miles of the crust of the earth 

 for the reason that below that depth the rocks yield 

 to the pressure of the overlying rocks and act 

 somewhat like plastic bodies. Faults are not to 

 be thought of as open gaping cracks, however, for, 

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