Dispersion of Rayleigh waves produced by nuclear explosions. Crustal structure of western Europe

Abstract

Most of the nuclear explosion fired near Novaya-Zemlya<br />island from September 1961 to J a n u a r y 1963 (21 in total) have been recorded<br />on the seismographs of Toledo Observatory. The study of these records,<br />mainly concerning the dispersion of Rayleigh waves, has been the purpose<br />of this paper.<br />A crust-mantle s t r u c t u r e for t h e Zemlya-Toledo p a t h has been determined<br />by means of group velocity curves and especially by the phase velocity<br />ones obtained from Rayleigh waves of explosions. This structure supposes<br />a crust of about 40 kms thick with an upper sedimentary layer with a<br />thickness of about 5,5 kms and a shear velocity of 2,3 km/sec.<br />The average shear velocity in the granitic and basaltic layers jointly,<br />is about 3,65 km/sec, permitting a small ambiguity at the position of the<br />Conrad discontinuity between them.<br />A velocity of 4,5 km/sec has been assigned for the underlying crust<br />material, but a better agreement with the data recorded is obtained by<br />taking 0.28 for the Poisson ratio value.<br />Dispersion of Rayleigh waves of these explosions has been compared<br />to the Rayleigh dispersion of some earthquakes of Eurasia, three of them<br />with epicentral distances similar to those of the explosions and other four<br />with the same azimuth in respect to that of Toledo-Zemlya, but more<br />distants.<br />The results do not show any notable difference either in dispersion<br />between explosion and earthquakes or in structure of the path considered.<br />The phase velocity between Toledo and Malaga Observatories supports<br />t h e same above structure for this short path.<br />The velocity of Lg waves, which clearly appears on the record of the<br />explosions, confirms this admitted structure, which serves to deduce t h e more<br />probable transmission mechanism for these channel waves.<br />Also atmospheric pressure waves have been recorded on the three<br />components with very notable amplitudes.