SECT. 1] SUB-OCEANIC STRUCTURAL EXPLORATION BY SEISMIC SURFACE WAVES 117 



is largely due to the shallow depth (about 11 km) of the high-velocity mantle 

 beneath oceans as compared to the relatively great depth (about 40 km) of the 

 mantle beneath the continents. 



At a period of about 20 sec, the effect of the water and low-velocity sedi- 

 mentary layer becomes important, the ocean curve drops steeply, and, for 

 periods shorter than about 17 sec, falls well below that for the continents. 



From earthquake sources, no waves corresponding to the fundamental Ray- 

 leigh mode for periods less than about 13-15 sec are observed. However, in the 

 case of certain special sources, i.e. nuclear explosions, some waves corresponding 

 to the shorter-period portions of the fundamental curve shown in Fig. 4 were 

 observed. 



At very short periods in the continental case, the effect of low-velocity 

 sedimentary layers becomes important, and the curve may fall to smaller 

 velocities, as indicated in the figure. Short-period waves associated with the first 

 higher mode, and in some cases still higher modes, have been identified for the 

 continental crust, and these waves account for much of the seismic disturbance 

 recorded by long-period instruments under certain conditions. 



The identification of higher-mode waves in the ocean is somewhat more 

 tenuous. Such waves appear to have been generated by a submarine nuclear 

 explosion as indicated in Fig. 4. Earthquakes sometimes generate a train of 

 short-period waves which have on occasion been identified as higher-mode 

 waves, but quantitative verification of this point has been difficult to obtain. 

 These two cases are discussed in considerably more detail in a later section. 



Fig. 5 shows similar data for Love waves. In general the curves are similar to 

 those of the Rayleigh type, with certain exceptions in addition to the generally 

 higher velocities. The minimum in the mantle-wave range is not so pronounced 

 in the case of Love waves, and may, in fact, be absent. The pronounced effect 

 which the water layer has on Rayleigh waves is, of course, lacking in the Love- 

 wave case, although the low- velocity sediments in ocean basins have an im- 

 portant effect on very short-period Love waves (about 8 sec and less). This 

 effect is also discussed in detail in a later section. Let us now consider various 

 sections of these curves for the oceanic case in considerably more detail. 



3. Evidence from Mantle Surface Waves 



The dispersive characteristics of oceanic surface waves of period greater 

 than about 30 sec are governed primarily by the properties of the mantle. In 

 general, studies of these waves, particularly of waves with periods greater than 

 about one minute, have been based on data from very large earthquakes, 

 although with modern instrumentation mantle waves may be detected in some 

 cases from much smaller shocks. 



The first identification of mantle Rayleigh waves and a study of their group 

 velocities was carried out by Ewing and Press (1954), using data obtained by 

 Benioff from three large earthquakes. A later study by Ewing and Press (1954a) 

 of the great Kamchatka shock of 4 November, 1952, included data on waves 



