CHAPTER VIII 

 EARTHQUAKES 



Earthquakes or tremblings of the earth's surface, when severe, 

 are the most terrifying phenomena of nature, with the possible ex- 

 ception of violent volcanic eruptions. The tremblings of the earth 

 vary greatly in their intensity, from those which cause great destruc- 

 tion of life and property to those which can be detected only by deli- 

 cate instruments. Although severe earthquakes occur only at 

 irregular intervals, specially constructed instruments called seismo- 

 graphs (Greek, seismos, earthquake, and graphein, to write) show that 

 the earth is never free from minor vibrations. Such minor trem- 

 blings are produced by water waves, by changes in atmospheric pres- 

 sure, by readjustments due to the lightening of the earth's surface 

 by erosion and its weighting by sedimentation, by the strains pro- 

 duced by the attraction of the moon and sun, and in other ways. 

 Destructive earthquakes are, however, of only occasional occur- 

 rence and arise from disturbances within the earth's crust (p. 281). 



The San Francisco Earthquake. — The earthquake which in 1906 

 shook California and wrought such havoc in San Francisco was the 

 most disastrous to property of any in North America within historic 

 times, although the loss of life was slight. Much of the destruction, 

 however, was due to the fires which were started as a result of the 

 shocks, and to the breaking of the water mains of the city, which made 

 it impossible to extinguish the flames. 



The shock came without warning, as is usually true of great earth- 

 quakes, and lasted less than one minute. It was followed by others 

 of less intensity during the day and for several weeks afterwards. 

 Where the shock was severe trees were injured, — some being broken 

 off, some overturned, and some split from the ground upward ; build- 

 ings were shifted horizontally and often badly broken ; animals were 

 thrown from their feet and persons from their beds. It was found 

 that the greatest intensity of the shock was along a fault line (p. 267), 

 and that in general the violence diminished with distance from the 



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