350 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1931 



Our subject requires us to consider the earthquake as a release 

 of energy — the agent which despatches a signal to be registered on 

 the seismograph set up at each of a network of stations distributed 

 over the globe. The signals are there recorded after having trav- 

 ersed the earth for various distances and along various paths, after 

 having penetrated to various depths. The nature of the records 

 indicates the paths by which the signals reached the instrument, 

 the properties of the materials through which they passed. They 

 are thus worthy of the greatest care in our choice of the network 

 of stations, our design of instruments and vaults, our maintenance 

 of continuous recording and accurate timing, our thoughtful study. 

 These constitute the modern seismology, in the light of which we 

 are to view the earth beneath. 



Our first problem may be stated thus: At what depth do earth- 

 quakes release their signals? Some undoubtedly occur on the sur- 

 face, though, presumably, the causal movements extend deep into the 

 earth. Others leave no traces of permanent shift at the surface. 

 The added fact that they are sensibly felt over wide areas indicates 

 that they originate far below. We can arrive at a conclusion with 

 regard to depth of focus, as it called, along different lines of rea- 

 soning. Dutton ® used as a means of determining depth a considera- 

 tion of the rate at which the intensity falls off as the disturbance 

 spreads out in all directions from the focus and makes its effects 

 apparent at the surface. He concludes that the maximum depth 

 of focus is of the order of 20 miles. Walker,^ working with Galit- 

 zin's measures of the angle, at which the seismic rays emerge from 

 the earth at the different stations in the vicinity of the focus, de- 

 duces that the depth of focus is of the order of one-fifth the earth's 

 radius — 800 miles ! Omori,^° making use of the duration of the 

 preliminary tremors of earthquake records deduces that the mean 

 depth of earthquakes in the Kwanto province of Japan is of the 

 order of 21 miles. Gutenberg ^^ studied the curve showing the time 

 of arrival of the first tremors and determined for an earthquake in 

 the Schwabian Alps a depth of 34 miles. A recent paper by 

 Wadati " deals very thoroughly with the various methods. He finds 

 that the Japanese earthquakes fall into two groups, which he terms 

 respectively shallow and deep. " The deep earthquakes take place 



8 Dutton, Clarence Edward, Earthquakes, 314 pp. (see pp. 185-193), Putnam's Sons, 

 New York. 1904. 



• Walker, George W., The problem of finite focal depth revealed by seismometers, Philos. 

 Trans. Roy. Soc. London, ser. A, vol. 222, pp. 45-56, August 5, 1921. 



''■" Oniori, Fusakiclil, On the focal depth of the earthquakes originating in the Kwanto 

 Province, Journ. Geogr., vol. 34, pp. 237-241, Tokyo, 1922. 



" Gutenberg, B., None Methoden zur Bestimmung der Ilerdtiefe von Erdbeben aus Auf- 

 zeichnungen an Herdniihegelegenen Stationen, Zeitschr. Geophys., vol. 1, Heft 3, pp. 63-75, 

 Giittingen, 1923. 



" Wadati, K., Shallow and deep earthquakes, Geophys. Mag., vol. 1, No. 4, pp. 162-202, 

 Tokyo, 1928. 



