122 



OLIVER AND DORMAN 



[chap. 8 



The crustal Love-wave train for oceanic paths is usually short in duration 

 and normally dispersed, arriving before the beginning of the Rayleigh-wave 

 train. I)is])ersion is small as indicated in Fig. 8. Occasionally this train continues 

 into the "short-period train" (see Fig. 13) as discussed below under that title. 

 Wilson (1940) based two important conclusions on Love-wave dispersion data: 

 (a) that the Pacific, Atlantic and Indian Ocean basins were similar in structure 

 and (b) that the high- velocity mantle was much closer to the surface beneath 

 the oceans than beneath the continents. Coulomb (1952) i^ubUshed data 



10 



20 



30 



T 



40 



50 



60 



Fig. 8. Love-wave dispersion observations (group velocity versus period) for paths across 

 the Pacific to Honolulu (Oliver, Ewing and Press, 1955). Agreement witli group 

 velocity for the single-layer theoretical case sho\\Ti by the solid curv^e indicates the 

 relative thinness of the oceanic crust. These data reveal significant variations in 

 velocity tlepending on azimuth from Hawaii. In particular, the low velocity of waves 

 from the southeast indicates lower shear velocities or a thicker crust, or both, which 

 may be localized in the Easter Island Rise. 



derived from the great Queen Charlotte Islands earthquake A\"hicli were con- 

 sistent with Wilson's data, as were the data of Caloi and Marcelli (1952) for the 

 Atlantic. Oliver, Ewing and Press (1955) studied dispersion of Love waves 

 recorded at Hawaii from a number of Pacific shocks and at Atlantic stations 

 from several Atlantic shocks. Their results confirm Wilson's conclusions. Some 

 scatter in Love-wave velocities of given periods was observed, which appears 

 to relate to variations in the crustal structure. Low velocities were recorded at 

 Honolulu from shocks southeast of Hawaii, as shown in Fig. 8, indicating that 

 a somewhat different structure exists in the vicinity of the Easter Island rise. 

 Data from the Queen Charlotte Islands recorded at Honolulu give the highest 

 CTrou]) velocity for any Pacific ])ath with equally high velocities for Love waves 

 recorded at Bermuda from Mid-Atlantic Ridge shocks. Other studies by 

 Evernden (1954) and by DeNoyer (1959) based on Berkeley seismograms of 



