SECT. 1] SUB-OCEANIC STRUCTURAL EXPLORATION BY SEISMIC SURFACE WAVES 131 



waves are more likely to predominate at the closer station. Also, short-period 

 waves seem to be greatly attenuated whenever they cross a continental 

 margin to reach the recording station. 



The T-phase is a high-frequency (of the order of a few cycles per second), late- 

 arriving wave generated by some submarine or coastal earthquake and 

 recorded at large distances from the source, usually by stations near a coast. 

 The T-phase is due to propagation of energy as sound waves in the ocean with 

 a velocity of 1.5 km/sec, the sound velocity of water. It appears that energy 

 transfer between the solid bottom and the water takes place primarily along 

 submarine slopes. In some cases the energy is transmitted to the water at a 

 submarine slope immediately in the ejjicentral region (see Ewing, Press and 

 Worzel, 1952). In other cases it may travel as far as 51° as a body P wave 

 before striking a slope and entering the water (Shurbet, 1955). The duration of 

 the phase may be several minutes and the average velocity observed varies 

 according to the proportions of the path segments through rock and water. 



Tsunamis are earthquake-generated sea waves. The dominant wavelengths 

 are usually much greater than the depth of the ocean [h) so that the waves 

 propagate as shallow- water gravity waves with velocity \/{gh). The generation 

 of these waves necessarily involves a sudden change in configuration of the sea 

 bottom associated with the earthquake. Either vertical movements along 

 submarine fault outcrops or slumps or turbidity currents triggered by the earth- 

 quake are considered possible mechanisms. 



References 



Bath, M., 1959. Seismic surface-wave dispersion: A world-wide survey. Oeofis. pur. appl., 



43, 131-147. 

 Bath, M. and A. Vogel, 1957. Continental dispersion of seismic surface waves. Qeofis. pur. 



appl., 38, 10-18. 

 Bath, M. and A. Vogel, 1958. Surface waves from earthquakes in northern Atlantic -Arctic 



Ocean. Oeofis. pur. appl., 39, 35-54. 

 Benioff, H. and F. Press, 1958. Progress report on long period seismographs. Geophys. J . 



Roy. Astr. Soc, 1, 208-215. 

 Berckhemer, H., 1956. Rayleigh-wave dispersion and crustal structure in the East Atlantic 



Ocean Basin. Bull. Seism. Soc. Amer., 46, 83-86. 

 Biot, M. A., 1952. The interaction of Rayleigh waves and Stoneley waves in the ocean 



bottom. Bull. Seism. Soc. Amer., 42, 81-92. 

 Bullen, K. E., 1939. On Rayleigh waves across the Pacific ocean. Mon. Not. Roy. Astr. 



Soc, Geophys. Suppl., 4, 579-582. 

 Caloi, P. and L. Marcelli, 1952. Onde superficiali attraverso il bacino deH'Atlantico. Ann. 



Oeofis., 5, 397-407. 

 Coulomb, J., 1952. Love waves of the Queen Charlotte Islands earthquake of August 22, 



1949. Bull. Seism. Soc. Amer., 42, 29-36. 

 DeNoyer, J., 1959. Crustal structure of the North Pacific from Love-wave dispersion. 



Bull. Seism. Soc. Amer., 49, 331-336. 

 Dorman, J., 1962. Period equation for waves of Rayleigh type on a layered, liquid -solid 



half space. Bull. Seism. Soc. Amer., 52, 389-397. 

 Dorman, J., M. Ewing and J. Oliver, 1960. Study of the shear-velocity distribution in the 



upper mantle by mantle Rayleigh waves. Bull. Seism. Soc. Amer., 50, 87-116. 



