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



129 



seem to be required by the data. However, the period of the strongly dispersed 

 waves in both these modes is too short in this computation. Additional recent 

 study of the dispersive effects of the mud parameters by the authors, as well 

 as the latest data quoted by Nafe and Drake, indicate that a mud layer of 

 lower shear velocity than that used in Fig. 13 may provide a model of bottom 

 structure consistent with dispersion data as well as data derived from other 

 sources. However, conclusive results are not yet available. 



DATA OF 24 SEPT 1958 

 GULF OF ALASKA 



TO HONOLULU 



+ Transverse component 

 o Vertical component 



o 



> 



Q. 

 O 



Dormon 8055 

 a(km/sec) /3 (km/sec) p[ 9 /ml) h(km ) 

 1.52 - 1.03 5.0 



2.10 



1.0 



2.84 



5.0 



3.27 



20 25 



Period (sec) 



30 



Fig. 



13. Typical oceanic dispersion data, derived from the seismograms of Fig. 11, are 

 plotted with theoretical surface-wave group -velocity dispersion data. The theoretical 

 curves were computed on the basis of the layered structure indicated. Agreement 

 between theoretical and experimental data is good in the Rayleigh mode owing to 

 proper choice of the thickness of water plus mud (layers one and two), which deter- 

 mines the period of the steep portion of the curve. However, poor agreement in the 

 first Love and first shear modes is due mainly to an incorrect choice of parameters 

 to represent the mud. 



Factors related to the presence or absence of various surface-wave types on 

 seismograms were discussed by Oliver, Ewing and Press (1955) who compiled 

 the geographical data of Fig. 14. Primarily, on seismograms written at Hawaii 

 short-period waves predominate for shocks in the area including the Easter 

 Island Rise, the west coast of North America and the Aleutians. For shocks 

 elsewhere on the Pacific margins, long-period waves predominate at Hawaii. 

 When two stations are on the same azimuth from a shock, the appearance of 

 long-period waves is favored at the more distant station while short-period 



