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Bulletin of the Seismological Society of America; December 1976; v. 66; no. 6; p. 1855-1872
© 1976 Seismological Society of America
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Mode conversion of Love waves at a continental margin

S. GREGERSEN and L. E. ALSOP

DEPARTMENT OF SEISMOLOGY GEODETIC INSTITUTE, GAMLEHAVE ALLE 22, DK 2920 CHARLOTTENLUND Denmark
LAMONT-DOHERTY GEOLOGICAL OBSERVATORY OF COLUMBIA UNIVERSITY, PALISADES, NEW YORK 10964 IBM THOMAS J. WATSON RESEARCH CENTER, YORKTOWN HEIGHTS, NEW YORK 10598

Abstract

Lateral inhomogeneities in the Earth's crust and upper mantle have a large effect on the amplitudes of seismic surface waves. Numerical calculations have been made for Love waves propagating from a continental structure into an oceanic structure, and vice versa. The results of these calculations are compared with observations at the seismic stations BKS, lying on a continental structure, and OBS, lying on an oceanic structure. Calculated, as well as observed, ratios of Love-wave amplitudes at the continental surface and at the ocean bottom show two prominent trends: There is almost no dependence on angle of incidence; and the amplitude ratios increase with decreasing period in the period interval 60 to 20 sec. For periods shorter than 25 sec, mode conversion between fundamental and first-higher modes is significant. This is due to differences in the amplitudes of the particle motions as a function of depth for the fundamental and first-higher modes in the oceanic and continental structure. When the path of the Love wave from the source to the receiver is across one or more continental margins, the dispersed Love-wave train is distorted by this mode conversion. Since the group velocities in the continental structure for the fundamental and first-higher modes are different, the two modes will be separated in time, when they pass through a continental structure. As a consequence, the energy of the periods between 20 and 25 sec is spread out in time, when the continental part of the path is long and a number of continental margins are encountered. Since mode conversion is insignificant for periods longer than 30 sec, these periods become dominant in the Love-wave train.




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