282 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 39 



an average. These earthquakes have been, in general, more severe 

 than the one which, in 1933, caused 50 million dollars damage at 

 Long Beach, Calif. It has just so happened that structures liable 

 to damage were not situated near the origins or epicenters of these 

 shocks. Canada is building up rapidly; the earthquakes will con- 

 tinue. You may draw the obvious and inevitable conclusion. 



It was an attempt to ameliorate in some measure the damage 

 caused by earthquakes which led initially to their being studied by 

 scientifically trained men. Progress was slow at first, but there has 

 gradually evolved a branch of the subject which may be called 

 "engineering seismology." Considerable progress has been made of 

 late years, particularly in Japan, in Italy, and in California. It is 

 now possible to design houses which, except in the most extreme 

 cases, will not be destroyed by an earthquake, will not be set on fire 

 thereby, and will not kill their inmates or passers-by with falling 

 debris. Moreover, the specifications for such structures are now 

 written into some building codes, for example those in parts of 

 California. 



But, as someone once remarked that Kipling said — "That is another 

 story." It is well to remember that this presentation is limited to 

 a very sketchy resume of but one phase of seismology. Much will 

 be left unsaid. These "other stories" are not unimportant but at 

 this time we must pass them over. They form part of the answer 

 to the question, "Why study earthquakes?" but not part of the sub- 

 ject, "The Structure of the Earth as Revealed by Seismology." 



Taking for granted part of that which we shall later find to have 

 been revealed by seismology, we may consider the earth to be a sphere 

 consisting of three main divisions: A central core, with a radius of 

 about 2,200 miles; surrounded by the mantle, a concentric sphere a 

 little less than 1,800 miles thick; which is, in turn, surrounded by a 

 crust of a varying thiclviiess which may be set down roughly as 25 

 miles. We take in our stride the responsibility for the approxima- 

 tions as to the sphericity of the earth and the various dimensions 

 given. We shall discuss the theory and the teclinique of determining 

 earth structure under four different conditions and in the order 

 named: the crust, the mantle, the core, and, finally, returning again 

 to the upper part of the crust — that region which is within reach 

 of the drill. 



One of the first things that men learned about the crust of the 

 earth is that it does not have uniform stability. Frequent earth- 

 quakes occur only in certain seismic zones. One of the routine activi- 

 ties of seismological investigation is that of locating as accurately 

 as possible all recorded earthquakes. To do this, international coopera- 

 tion is necessary. At the present time more than 250 seismic stations 



