CRUSTAL STRUCTURE AND SURFACE-WAVE DISPERSION* By MAURICE EwING and FRANK PRESS

A RECENT seismic refraction profile1 in 2,800 fm. (5.1 km.) at latitude 34° 00' N, longitude 66° 30' W, in the Atlantic Ocean proved that basement rocks having a velocity of 7.58 km/sec. for compressional waves were covered by 1.37 km. of sediment. We tentatively identified this velocity with either the P n velocities used in earthquake seismology for the ultra basic layer immediately beneath the Mohorovicic discontinuity, or with the deepest subdivision of the intermediate layer, indicating the absence of the granitic and upper part of the intermediate layers. This velocity is higher than most of the values deduced for the uppermost suboceanic rocks from studies of the velocity and the dispersion of earthquake surface waves over oceanic paths. If this refraction measurement is representative of ocean-wide conditions, it is evidently in conflict with the crustal layering inferred from previous studies on surface waves. The present study indicates that the observed surface-wave dispersion is to be expected if proper account is taken of the influence of the water and sediment over an ocean bottom consisting of a very thick ultrabasic2 layer, as indicated by the seismic refraction measurements. INFERENCES FROM THE AIRY PHASE