PART TWO 



GENERALIZATIONS CONCERNING 

 EARTHQUAKES 



This paper is not a treatise on the general subject of 

 earthquakes, and hence I discuss the subject only 

 briefly. Volumes have been written about them and 

 the reader who wishes to delve more deeply into the 

 subject can find many books to satisfy his thirst for 

 knowledge.^ 



Modern seismologists hold that an ordinary earth- 

 quake is a deep-seated jar to the earth's crust caused 

 when the rock suddenly moves, snaps back, or elas- 

 tically recoils as a result of release from strain. This 

 is called the rebound theory. In harmony with it, Dr. 

 Daly defines an earthquake as a "snap." Such re- 

 leases of strains are usually local occurrences, which 

 are associated with earth cracks shown as "faults" 

 or "fault lines," and occur at spots along their extent. 



The focus of movement is usually more or less deep- 

 seated and may be several thousand feet beneath 

 where the shock is felt at the surface (the epicentrum) . 

 According to Daly, the most trustworthy observations 

 indicate no depths of movement greater than twenty- 

 five miles, the average being not far from half of that. 



^A good summary of the subject and the associated facts of up-to-date 

 geology, and one which treats of a living world at work, instead of a dead 

 and fossil one, may be found in the recently published work of Dr. Reginald 

 Daly, Professor of Geology at Harvard. It is entitled, "Our Mobile Earth" 

 (Charles Scribner's Sons, Dec, 1926). This work gives in comprehensible 

 English a summary of most that is necessary for the reader to know about 

 earthquakes and seismology. 



