S and the Structure of the Upper Mantle

The European as well as the north-eastern American observations of S at small epicentral distances indicate the presence of a low velocity layer. In Europe its upper boundary seems to be at a depth of about 140 km. Since late S phases are observed at epicentral distances down to about 10° there is likely to be an abrupt increase of velocity (as well as of velocity gradient) at the lower boundary of the layer at about 220 km depth. Late S phases beyond 20° can be accounted for if a further strong increase of velocity gradient at a greater depth is assumed.