Surface shear waves (Lg) with initial period about 1/2 to 6 sec. with sharp commencements and
amplitudes larger than any conventional phase have been recorded for continental paths at distances
up to 6,000 km. These waves have a group velocity of 3.51±.07 km/sec. and for distances
greater than 20° they have reverse dispersion. For distances less than about 10° the periods shorten
and Lg merges into the recognized near-earthquake phase Sg.
An additional large amplitude phase in which the orbital motion of the particle is retrograde
elliptical and the velocity is 3.05±.07 km/sec. has also been observed for continental paths.
It is believed that these phases are propagated through a wave guide formed by a superficial
sialic layer. The problem of explaining the propagation of these surface waves is that of finding a
crustal structure which is consistent with the other data of geology and geophysics and which will
provide a suitable wave guide for the new phases. A possible nature of the wave guide is described
