Body waves as normal and leaking modes, 3. Pseudo modes and partial derivatives on the (+ −) sheet

In a multiple elastic wave guide, the phase velocity curves of the Π and Σ modes on the (+ −) sheet combine to form two families of almost continuous curves. The curves of the first family, called Π pseudo modes, are formed by the plateaus of the dispersion curves; they have a high group velocity, a low attenuation, and a low excitation; they depend solely on the P velocities. The curves of the second family, called Σ pseudo modes, are formed by the slopes of the dispersion curves and have the opposite characteristics. The Π pseudo modes make up the oscillatory part of the seismogram between P and S . The study is based on a simplified CIT 11 GB model, which consists of 6 layers, is 653 km thick, and contains a 54‐km low‐velocity P channel. The frequency range studied is from 0 to 0.146 Hz. The influence of the P channel is very restricted. At wavelengths smaller than ⅓ of its thickness, the layer below the channel (α = 8.19 km/sec) supports what is essentially a head wave.