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Azimuth and slowness deviations from the GERESS regional array

Institute of Geophysics Ruhr-University Bochum, 44780 Bochum, Federal Republic of Germany

Abstract

For high signal-to-noise ratio events, body-wave travel times at GERESS stations are well fit by a plane-wave model corresponding to travel time uncertainties of about 1/100 sec. Slownesses obtained in this study are accurate to about 0.5 sec/deg, while azimuth uncertainties are about 2° for regional events and about 5° for teleseismic nuclear events. For illustration, we demonstrate performance for Nevada and Tuamotu nuclear tests and for regional events from Poland. Unbiased measurement requires array topography (about 200 m for GERESS) to be taken into account. If ignored, these elevation variations give rise to a systematic shift of about 0.6 sec/deg to eastern directions, which is almost independent of source location. In principle, arrays extending in vertical direction ("3D array") can measure the vertical slowness and hence local material velocity c. For GERESS, we find c 5.2 km/sec.

Compared with given accuracies, the regional GERESS array finds statistically significant deviations of slownesses and azimuths. These may be used to investigate lateral heterogeneity at regional scale.