414 Notices of Memoirs — Australia, 1914 — 



the waves that travel through the Earth, has thus given a certain amount of 

 information regarding the state of the Earth's interior which E. D. Oldham 

 aptly regards as analogous to that given by the spectroscope ^ with regard to the 

 inaccessible atmosphere of the Sun. 



The existence of two groups of earthquake waves— those passing through, 

 and those passing near the surface around the Earth — has long been recognized ; 

 but B. D. Oldham^ has shown that the waves passing through the Earth are 

 of two kinds, travelling at two different speeds. 



The record on the distant seismograph thus shows three well-marked phases : 

 the first phase, due to waves of compression passing through the Earth's 

 interior; the second phase, due to waves of distortion,^ also passing through 

 the Earth's interior ; and the third phase, recorded by the waves which pass 

 around the arc along the superficial crust. 



The third phase is always recorded at a time after the occurrence of the 

 shock proportional to the arcual distance of the recording seismograph from the 

 earthquake centre, the records of several large earthquakes showing an average 

 speed for the waves of about 3 kilometres per second. The rates of propa- 

 gation of the waves giving the first and second phases are both much greater 

 than of those forming the third phase ; and up to an arcual distance of about 

 120° from the earthquake's centre the rate of their propagation increases with 

 the distance. It is thus assumed that the waves giving rise to the first and 

 second phases in each distant seismographic record, by following approximately 

 along the chord of the arc between the place of origin and the instrument, 

 pass through deeper layers of the Earth when the seismograph is farther away, 

 the material at greater depths being presumably more elastic as well as denser. 



But Oldham ■* has shown that when the seismograph is as much as 150° from 

 the earthquake centre there is a remarkable decrease in the mean apparent 

 rate of propagation of the waves giving the second phase in the record, 

 from over 6 to about 4 J kilometres per second. There is also a drop, although 

 not nearly so marked, in the apparent speed of the waves of the first 

 phase when transmitted to a seismograph 150° or more distant from the earth- 

 quake origin. Oldham concludes that this decrease of apparent rate for waves 

 travelling through the Earth to places much more than 120° distant is due to 

 their passing into a central core, four-tenths of the radius in thickness, com- 

 posed of matter which transmits the waves at a markedly slow speed. Thus 

 the earthquake waves which emerge at a distance not greater than 120° from 

 their origin do not enter this central core, while those which pass into the Earth 

 to a greater depth than six-tenths of the radius are supposed to be refracted on 

 entering, and again on leaving the postulated core, in which the rate of trans- 

 mission of an elastic wave of distortion is very much slower than in the main 

 mass of the Earth around. In consequence of the refraction of these waves on 

 passing through the central core, places situated at about 140° from an earth- 

 quake origin should be in partial shadow, due to the great dispersion of the 

 distortional waves, and the few records made so far by seismographs thus 

 situated with regard to great earthquakes show that there is either no, or at 

 most a doubtful, record for the second phase, which is known to be due to the 

 so-called distortional waves. 



^ In his Presidential Address to the Geological Society of London in 1909, 

 Professor W. J. Sollas (Proc. Geol. Soc, 1909, p. Ixxxvii) credits H. Benndorf 

 (Mitth. Geol. Ges. Wien, i, p. 336, 1908) with this pretty analogy, but 

 Oldham has the precedence by just two years (cf. Quart. Journ. Geol. Soc, 

 vol. Ixii, p. 456, 1906). 



2 Phil. Trans., ser. A, vol. cxciv, pp. 135-74, 1900. 



^ There is more complete agreement regarding the fact that two distinct 

 sets of waves give rise to the so-called preliminary tremors indicated by a 

 seismographic record than about the nature of the waves. Of. E. D. Oldham, 

 Phil. Trans., loc. cit., and 0. Fisher, Proc. Camb. Phil. Soc, vol. xii, 

 pp. 354-61. 



■* Quart. Journ. Geol. Soc, vol. Ixii, pp. 456-75, 1906. 



