New evidence for a change in physical conditions at depths near 100 kilometers

Comparative investigation of the seismograms of shallow earthquakes shows that the amplitudes of the first recorded longitudinal wave (Pn or P) vary considerably with distance from the epicenter. At distances up to 6° the seismograms are of "local shock" type, and the amplitude of Pn increases somewhat with distance. From 6° the amplitude decreases regularly, reaching a very small minimum near 14° (1500 km.). There is then a sudden increase, and the amplitudes continue large to beyond 20°. Together with the corresponding travel times, this suggests that at about 80 km. below the surface the velocity of longitudinal waves ceases to increase with depth, and probably decreases slightly through a limited range of depth, below which it again increases with depth. It is not yet settled whether the decrease is gradual throughout its range, or whether it takes place suddenly at a discontinuity.