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Conversion of Sn to Lg at a continental margin

DEPARTMENT OF GEOLOGICAL SCIENCES CORNELL UNIVERSITY, ITHACA, NEW YORK 14850

Abstract

Examination of the seismic phase Sn from earthquakes in the West Indies as recorded by numerous stations in eastern North America reveals that a substantial fraction of the short-period energy carried by Sn across the suboceanic lithosphere of the Atlantic is fed into the continental crust near the continental margin and travels into North America as the crustal phase Lg. As distance within the continent increases, the Lg part of the short-period wave train becomes predominant, and can be identified at stations in northern Canada as far as 58° from the sources. Several estimates of the average Q for the attenuation of Lg in eastern North America agree upon values in the range of 600 to 1,400. Hydrophone recordings at Bermuda indicate an average Q as high as 4,000 for the attenuation of Sn in the suboceanic lithosphere. Conversion of Sn to Lg also appears to occur near the margin between the continental U.S. and the Gulf of Mexico. In this case, Sn travels northward across the Gulf from earthquakes located near the border between Mexico and Guatemala.

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