The 400-km Discontinuity

On the NTS-NE profile Julian and Anderson observed prominent late P arrivals indicative of a deep ‘discontinuity’ in the velocity, possibly at about 400 km depth. Additional readings of these phases were extracted from shot reports. The phases were present also in the records of two stations in azimuth 71° and 76°, but when records obtained on the eastern profile were examined the phases were no longer very clearly indicated. In the records on the western Early Rise profile corresponding phases are clearly seen, but they are not very prominent. Two European examples are given in which late S phases indicate deep discontinuities in the S velocities, but where there are no corresponding P phases. It is concluded that the nature of the deep discontinuity in the P velocity varies and that it may not be present everywhere.