MODERN" SEISMOLOGY HODGSON 351 



at the depth of more than 300 km (186 miles), while the shallow 

 ones at about 40 km (25 miles)," Gutenberg, in a publication 

 which has just issued from the press,^^ tabulates the values of depth 

 of focus as determined by nine different seismologists for 16 differ- 

 ent earthquakes. With the single exception of Wadati's deep earth- 

 quakes, the determinations all lie at depths of 28 miles or less. 

 Seismologists generally agree that the data for determining the 

 depth of focus are not as precise as could be desired, but that earth- 

 quakes probably originate, in general, at depths of 25 miles or less. 



The uncertainty with regard to the depth of focus is due largely 

 to our uncertainty as to the velocity of propagation of the seismic 

 waves in the uppermost layer of the earth's crust. To determine 

 this velocity we must have an earthquake of which we know, ac- 

 curately, the time of occurrence and the depth of focus. We can 

 be sure of this last requirement only where the depth is zero, i. e., 

 where the. focus is at the surface. On February 18, 1911, a dis- 

 turbance was registered which was traced to the Pamirs, in central 

 Turkestan. Investigation showed that a great slide had occurred in 

 which from Y to 10 billion metric tons of rock had fallen a dis- 

 tance of from 400 to 800 yards. The tremors, registered on the 

 seismographs at Ottawa and generally throughout the world, were 

 believed to have been the result of the rock fall.^* The exact time 

 of the fall could not be determined. Moreover it is now questioned 

 whether the fall was the cause or the result of the earthquake.^^ 

 Such a question could be raised in the case of practically any such 

 earthquake. We fall back upon the velocity of earth tremors gen- 

 erated by an explosion. 



The velocity of seismic waves has been studied in the case of a 

 great explosion at Oppau, in the works of the Badische Analin und 

 Sodafabrik in the Bavarian Palatinate, on September 21, 1921.^'^ 

 The tremors were registered at five seismograph stations ranging in 

 distance from 68 miles to 227 miles. The chord from Oppau to Do 

 Bilt — the farthest station — dips only about 2 miles below the sur- 

 face at the middle of the arc. We may thus consider the waves 

 as being well within the upper layer of the earth. The exact time 

 of the explosion is loiown within one second. The records give the 

 velocity of propagation of the most rapid tremors as 5.4 km (3.35 

 miles) per second. 



"Gutenberg, B., Theorie der Erdbobenwelleu, Gebriider Borntraeger, Handbueh dcr 

 Geophysik, vol. 4, Lief. 1, 298 pp. (see p. 234), Berlin, 1929. 



" Klotz, otto, The earthquake of Feb. 18, 1911 — A discussion of this earthquake by 

 Prince Galitzin with comments thereon — Journ. Roy. Astron. Soc. Canada, vol. 9, pp. 428- 

 437, November, 1915. (See also footnote 16.) 



^* Macelwane, James B., S. J., Are important earthquakes ever caused by impact? 

 Bull. Seism. Soc. America, vol. 16, No. 1, pp. 15-18, March, 1926. 



i» Jeffreys, Harold, The earth, 278 pp. (see pp. 169-170), Cambridge University Press, 

 1924. 



