At a depth of 5 to 50 kms in the earth, some seismic waves are reflected back - Also, these waves appear to speed up below this. B. From about 100 km down to about 350 km below surface.
Seismic waves move slowly because material behaves more plastically than elastically.
This is the asthenosphere. C. S waves don't make it through the earth. P waves that move more deeply speed up and are refracted at 5100 km down. To understand this, we must try to understand the structure of the earth itself and the materials that make it up. A. Mass of the Earth.