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Elimination of near-source ellipticity corrections to body-wave travel times by use of equidistant latitudes

DEPARTMENT OF GEOLOGY AND GEOPHYSICS UNIVERSITY OF CALGARY, CALGARY, ALBERTA, Canada , T2N 1N4

Abstract

The use of equidistant latitudes has been proposed by the author to eliminate discrepancies between angular and kilometric epicentral distances. This is done in combination with a path-length correction which depends on the inclination of the great ellipse containing the epicenter-receiver path. If there were a one-to-one correspondence between source-receiver surface arc length (in kilometers) and, say, P-wave travel time (for constant focal depth) for a standard spheroidal Earth, the ellipticity (time) correction could then be replaced by the distance correction described. However, one would only expect this to be approximately valid for small epicentral distances .

In this paper, the travel-time corrections made by using equidistant latitudes (and the great-ellipse correction) are compared with the "true" ellipticity corrections due to Dziewonski and Gilbert. It is seen that the present equidistant-latitude method gives P-wave correction values that, for example, are always within 0.05 sec of the "true" values for 14° and normal focal depth (h 40 km). For large ( 45°) and/or great focal depth (h 475 km), these values may differ by more than 0.2 sec. This equidistant-latitude method of correcting body-wave travel times is thus not recommended for routine use, but it could be used to advantage in special studies involving smaller and h.