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Seismic ray direction anomalies caused by deep structure in Fennoscandia

INSTITUTE OF SEISMOLOGY UNIVERSITY OF HELSINKI, ET. HESPERIANKATU 4, 00100 HELSINKI 10, Finland

Abstract

The directions of arrival of P waves from distant seismic events were observed at three seismic array stations in Fennoscandia, NORSAR in Norway, Hagfors Observatory in Central Sweden, and the telerecording station at Helsinki in Finland. Anomalies in dt/d up to 1 sec/deg and in azimuth up to 7° were observed. Consideration of the upper limits of anomaly size which could be produced by various factors leads to the conclusion that lateral velocity gradients in the Earth's mantle, probably close to the receivers, are required to explain the size of the direction anomalies. The anomalies of ray directions are compared to the P-wave travel-time residuals reported for the conventional seismic stations on the Scandinavian peninsula. It is concluded that both sets of data favor an increase of the average vertical P-wave velocity in the upper mantle under the Scandinavian peninsula in the direction from oceanic toward continental areas.

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				K. A. BERTEUSSEN The origin of slowness and azimuth anomalies at large arrays Bulletin of the Seismological Society of America, June 1, 1976; 66(3): 719 - 741. [Abstract] [PDF]