SECT. 1] SUB-OCEANIC STRUCTURAL EXPLORATION BY SEISMIC SURFACE WAVES 



127 



12). The short-period wave train on the horizontal transverse component 

 frequently occurs as a continuation of the long-period Love -wave train toward 

 shorter periods and lower group velocities. The short-period waves, when 

 recorded, are frequently found in all three components of ground motion, 

 sometimes with approximately equal amplitudes in the later portions of the 

 train (see seismograms of Fig. 11) or sometimes predominantly on the trans- 



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Fig. 11. Honolulu seismograms of a shock in the Gulf of Alaska. Long-period Love waves 

 begin on the east-west at about 0401 h and at 0402 h change into waves of almost 

 constant short period which persist for many minutes. Rayleigh waves begin on the 

 north-south and vertical at about 0402 h. At about 0405 h there is an abrupt increase 

 in the dominant frequency as the short -period train begins on the latter components 

 as well. A dispersion analysis of these seismograms is given in Fig. 13. 



verse as in Fig. 12. Sometimes long-period waves are recorded on the longitudi- 

 nal and vertical components and short-period waves on the transverse, as in 

 the seismograms of Fig. 10. Furthermore, the beginnings of the wave trains on 

 the transverse component usually have longer periods and precede the short- 

 period train on the other two components, as in Fig. 11. When observable, the 

 periods are always about the same on all three comj^onents at the same time, 

 but particle motion is ajjparently not coherent, i.e. the relative phases between 



