128 OLIVER AND DORMAN [CHAr. 8 



the components change throughout the train. The dispersion data plotted in 

 Fig. 13, which are derived from the seismograms in Fig. 11, show that, following 

 the long-period Love-wave train, motion of about 9-sec period was recorded on 

 all components for group velocities between 4 and about 1 km/sec. There is a 

 tendency for the period to decrease slightly with time. 



The beginning of the short-period train which has transverse particle motion, 



—'-~^'"*^"^ 13 July 58 A-44IOkm ' ^"^ 



EU^08 10 01 58"N 137 W 



AMMARMMs UMMMta I \ I T ^^^^^ ii^^^^^^ua^^^^j^^^uj^^ ^^^^^^^^^^ ^^A^^i^^^^ ^^^^^UU^ui ^^^^^At^^^^^^^^i^^^^i^ i^^^^^^^^u. u^^^^^^^^^ j^^^ii^^^u^ tt^^^^^ ^ ^ ^ ..... 



^^ff^f<^Mi^^*^>fm 



(WMMMHMHM 



'V^N*VyV^»S^i^W 



Fig. 12. Honolulu seismograms of a shock in the Gulf of Alaska. These seismograms are 

 characterized by relatively strong Love waves, beginning at about 0827 h on the 

 east-west, compared with weakly recorded Rayleigh waves on the vertical and north- 

 south components. The period of the Love waves decreases rapidly at first to an 

 almost constant period of about 9 sec, which pei'sists for several minutes. Vertical 

 motion at about 0833 h is a part of the short-period train. The seismograms of Figs. 

 10, 11 and 12 illustrate the long- duration, constant -frequency characteristics of the 

 short-period train. 



as shown in Fig. 11 or Fig. 12, has been shown previously to correspond to a 

 portion of the fundamental Love-mode dispersion curve (see DeNoyer, 1959; 

 Oliver, Dorman and Ewing, 1959). The remainder of the short-period train on 

 all three components has not been explained previously, and it has been sug- 

 gested that the waves correspond to higher-mode propagation of the Love 

 and/or Rayleigh types. The authors have recently compared data of the type 

 described above with dispersion computations based on layered models of 

 oceanic structure derived from seismic refraction measurements. 



An attempt to explain the short-period data by including a mud layer with 

 low shear velocity in the theoretical model was made as shown in Fig. 13. The 

 data of Nafe and Drake (1957, and in Chapter 29) on the physical properties of 

 marine sediments provided a useful basis for these investigations. The disper- 

 sion curves in Fig, 13 show that the first Love mode and first shear mode are 

 closely coincident and reach very low group velocities. Both of these features 



