Abstract
IN a recent investigation1 I found the travel times of the seismic P waves (longitudinal waves) up to an epicentral distance of about 22° to conform with a structure below the Mohorovicic discontinuity that consists of an upper layer extending to a depth of about 220 km. in which the velocity is about 8.1 km./sec., increasing slightly with depth, and a lower layer in which there is a considerable increase of velocity with depth; at the boundary of the two layers there is an abrupt increase of velocity to about 8.4 km./sec. A discontinuous increase of velocity has to be assumed because the amplitudes of the waves refracted in the lower layer, observable as first arrivals from an epicentral distance of about 15°, increase gradually with epicentral distance, and at the lower end of their time-distance curve there is no focal point at which the amplitudes are very large.
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