Blackwell Publishing Journal Backfiles 1879-2005
The group velocities of Rayleigh waves, as measured by Rohrbach on the Göttingen records of two Asian earthquakes for which the azimuth of arrival was from the East, have been discussed in terms of a two-layer model, in which the upper layer, of granite of thickness T1, rests on a basaltic layer of thickness T2, with ultrabasic material below extending to a great depth. The wave velocity equation, in the form of a ten-row determinant, was solved by means of the EDSAC electronic computing machine of the Mathematical Laboratory, Cambridge; the ratio c/β2 of wave velocity to distortional wave velocity in the T2 layer was calculated for a sequence of values of KT1, for the three values I, 1/2, 1/3 of the ratio T2/T1. The values for T2, = o on slightly different data were available from earlier work. The group velocity was obtained from the wave velocity by numerical differentiation.For the earthquake of 1930 July 2 in Eastern Bengal the mean values of T1 over the whole range of periods from 65.3 sec to the shortest periods that the dispersion curves permitted (24 to 27 sec) were respectively 27.1, 30.6, 32.7 km on the three hypotheses, with 37.8 km for T2= o. The internal consistency of the results is in favour of the third case, with T1= 32.7 km; T2=10.9 km. The independent evidence of other lines of investigation favours a continental structure with a granitic layer 30 to 35 km thick and a basaltic layer of small or zero thickness; studies of near earthquakes and of large explosions suggest that the intermediate layer may be of only local occurrence. These determinations fit closely an empirical formula T1+0.35T2=36.3 km.The corresponding thicknesses derived from the shock of 1924 July 11, in the Kuen Lun Mountains, are a little smaller and are closely represented by the formula T1+0.646T2=36.6 km. Since the waves from Eastern Bengal traverse the Himalaya region this difference is to be expected.The results fully justify the use of seismograms that record the vertical component of motion, and indicate the desirability of further research along these lines.
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