THE STKUCTUEE OF THE EARTH AS REVEALED BY 

 SEISMOLOGY ^ 



By Ernest A. Hodgson 

 Department of Mines and Resources, Ottawa, Canada 



[With 3 plates] 



You remember what Doc said of Dopey ,^ "He don't know if he 

 can talk or not ; he ain't never tried." For the same reason I do not 

 know whether I can succeed or not — having never tried to deal with 

 this phase of seismology before a general audience — but I hope it will 

 be possible for me, without recourse to technical details, to present a 

 clear and convincing picture of the means by which we have learned 

 something of "The Structure of the Earth as Revealed by Seismology." 



As usual, "The Greeks had a word for it." Their word for earth- 

 quake was seismos; hence, seismology, the study of earthquakes. 

 There is no reason, however, why seismology should be "all Greek" 

 to the layman. 



Why study earthquakes? Coming closer home, let us ask the 

 very pertinent, perhaps sometimes disconcerting, question, "Wliy 

 study earthquakes in Canada?" Throughout this presentation as 

 opportune moments present themselves we shall note phases of the 

 answer — in a series of brief asides. 



Earthquakes merit study. According to good authority approxi- 

 mately 800 earthquakes, strong enough to destroy cities and towns, 

 have occurred on land since the beginning of the Christian era. 

 Historj' shows that an average of 30,000 persons have been killed 

 each year during the past 2 centuries by these phenomena. More 

 than 20,000 earthquakes occur each year; a widely recorded shock 

 each 14 hours ; a destructive earthquake each 6^/^ days — on the aver- 

 age, of course. 



Perhaps, then, it may be assumed without argument that major 

 earthquakes should be studied. Why be prepared to study them in 

 Canada? Since Cartier's first voyage in 1534, an earthquake of 

 major proportions has occurred in Canada every 50 or 60 years — on 



^ Reprinted by permission from the Journal of the Royal Astronomical Society of 

 Canada, vol. 32, No. 6, July-August 1938. 



' Walt Disney's Snow White and the Seven Dwarfs. 



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