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LAMONT GEOLOGICAL OBSERVATORYCOLUMBIA UNIVERSITY
Abstract
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.
Footnotes
* The research reported in this document has been made possible through support and sponsorship extended by the Geophysical Research Division of the Air Force Cambridge Research Center, under Contract AF19(122)441. It is published for technical information only and does not represent recommendations or conclusions of the sponsoring agency.
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