Two slow surface waves across North America*

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.