STRUCTURE OF THE EARTH — HODGSON 291 



to the depths and distances required. Recourse must be had to 

 earthquakes — not severe, but well located. Such small shocks usually 

 follow a severe earthquake and are known as aftershocks. Imme- 

 diately after a major earthquake, portable seismographs may be 

 rushed to the epicentral region, a timing system arranged, and con- 

 tinuous records made. The aftershocks will originate not far from 

 the original source. Several hundred, sharply defined, will often 

 be experienced. The data obtained jdeld much information as to 

 the structure and thickness of the earth's crust in that particular 

 vicinity. 



Studies of this kind have been made in Japan, in central Europe, 

 and in California. The crustal structure determined for Japan 

 seems to be the simplest, that in California the most complex. No 

 such studies have been made in Canada, but the data obtamed from 

 the records of the Timiskaming earthquake of 1935 show that the 

 velocity in the outer edge of the mantle immediately under the crust 

 is higher in Canada than in the outer edge of the mantle under 

 Japan, central Europe, or California. However, we are here con- 

 sidering the methods of determining earth structure, not the results 

 obtained, so we shall pass on to the methods of probing the mantle 

 of the earth: that spherical shell about 1,800 miles thick below the 

 crust and above the central core. 



Before doing so it is necessary to note one further very necessary 

 contribution from crustal analysis. In speaking of the methods used 

 for locating earthquakes care was taken to say, "given a dependable 

 time-distance graph." Science gives nothing gratis and yields her 

 most valued and most valuable data only to those who seek ; though 

 sometimes, it is true, they do not know exactly for what they are 

 looking or why they require it. One of the difficulties of obtaining 

 a time-distance graph has been mentioned — the great areas on 

 both land and sea unserved by seismograph stations at which 

 to record the exact time of arrival of the elastic waves from an 

 earthquake for which is accurately known the epicenter, focal depth, 

 and time of occurrence. Even had we the optimum distribution of 

 stations, it would still be necessary to have, for each of a series of 

 selected earthquakes, the position of the epicenter, the focal depth, 

 and the focal time. In other words, to locate an earthquake we must 

 have accurate time-distance graphs, and to construct such gi-aphs we 

 must accurately locate an earthquake or earthquakes. It appears to 

 be an impasse ; but seismology has advanced, nevertheless, by a series 

 of approximations — applications of the method of "cut and try." 



Progress has been slow but steady over the years, as seismologists 

 have gradually arranged for more stations, devised more efficient 

 seismographs, and perfected timing arrangements. We may not 



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