ON SEISMOLOGICAL INVESTIGATIONS. 255 
finding the thicknesses of the upper layers. It appears that in the reports of many 
stations these later pulses are given as 8. or earthquakes in oceanic regions their 
_ behaviour should be markedly different on account of the absence of the granitic layer. 
Denp-rocus HARTHQUAKES, 
By R. SToNELEY. 
In the preceding report Prof. Turner called attention to the distribution of deep- 
focus earthquakes. These earthquakes are relatively rare occurrences, only 141 being 
noted during the years 1918 to 1926; the epicentres nearly all lie on an oval curve 
which approximately girdles the Pacific. 
There has been some reluctance on the part of both geophysicists and geologists 
to accept as genuine these great depths, amounting in one instance to 0-09 of the 
earth’s radius below the normal focal depth, for at great depths it is generally believed 
that yield occurs through plastic flow and not by fracture of a material of considerable 
strength. Further, Dr. Jeffreys has pointed out that, in view of a general dynamical 
theorem, the amplitudes of the surface waves of earthquakes of great focal depth 
should be small or insensible ; that this crucial test had not been applied, and that 
the occurrence of readings for L and M in the alleged deep-focus earthquakes appeared 
_ prima facie to disprove the occurrence of great focal depths. 
- The occurrence of deep-focus earthquakes has in the last few months been definitely 
established by two quite independent methods. Mr. F. J. Scrase (Proc. Roy. Soc. A, 
_ Vol. exxxii., 1931) has recently obtained a very beautiful confirmation of the reality 
_ and order of magnitude of Turner’s additional focal depths by noticing that, in general, 
additional phases should be produced by reflection at points near the epicentre. The 
additional waves are designated by Mr. Scrase as pP, pPP...sP,sS... p[P], p[S], &c., 
in order to distinguish them from the corresponding PP, PPP ...SP, SS... P[P], 
P[S], &c. Mr. Scrase has made the lengthy computations required to obtain the 
travel times of the new waves, and has found the pulses to be prominent in the 
_ Eskdalemuir and Kew records examined by him. The agreement with observation 
is extremely good. The phases pP and s§ are the easiest to recognise ; in the case 
_ of a focal depth of 0-06 of the earth’s radius below normal and at epicentral distances 
greater than 60°, pP follows P by an interval of about 80 sec., while sS arrives about 
150 sec. after S. for other focal depths the time intervals are roughly in proportion 
to the depth. Mr. Scrase states further that the surface waves of the deep-focus 
earthquakes examined by him are very small in amplitude. The International 
Summary shows that these waves have in the past often been mistaken for P and S ; 
some seemingly anomalous waves given in the ‘ Additional Readings’ fit well into 
the predicted places. 
__ The smallness of the amplitudes of the surface waves has been noted by Mr. R. 
Stoneley (Gerlands Beitrage zur Geophysik, 29 (1931), 417) from an examination of 
records taken at Kew, Eskdalemuir, Oxford and Stonyhurst. The effect is strongly 
marked for earthquakes given by Turner as 0-04 and over. Nearly the whole of the 
energy is transported by the body waves, so that P, PP, PPP ...8S, SS, SSS... 
have enormous amplitudes, and the large amplitudes of S, SS, SSS, &c., might easily 
be mistaken for those of surface waves. A graph of the L and M readings in the 
International Seismological Summary shows at once that this is what has happened. 
_At the calculated time of arrival of M there is usually a movement of rather small 
_ amplitude, quite unlike the characteristic long-wave phase, and this is attributable 
_ to the dying down of the body waves. Further, there is a great scarcity of L and M 
readings, especially of M, in these very deep-focus shocks. ‘The scarcity of normal M 
teadings confirms the assumption of great focal depth. Theory indicates that sudden 
beginnings for L, corresponding to the arrival of infinitely long Love waves 
(0-41 min./degree) and Rayleigh waves (0-47 min./degree), should appear, though 
possibly of smaller amplitude than in a normal shock ; actually the former seems to 
occur more frequently than the latter, and their infrequency doubtless arises from the 
L oaaiaag of the typical train of long waves that follows these two onsets in normal 
cks. 
_ The genuineness of Prof. Turner’s deep foci may accordingly be considered as 
established ; the actual values of these additional depths, which depend in the end 
on the Zéppritz-Turner tables, may require some modification. 
__ The depth of focus of a normal earthquake was believed by Prof. Turner to be 
about 0-04 of the radius of the earth. The agreement of the Byerly-Jefireys time 
