High Frequency Array Studies of Long Range Lg Propagation and the Causes of LgBlockage and Attenuation in the Eurasian Continental Craton. Volume 2

摘要

An investigation was conducted of the nature of Lg propagation from the Soviettest site at Novaya Zemlya as recorded by array sensors to the south, including the new regional arrays ARCESS and NORESS, and the teleseismic Graefenburg array, located near the recently installed GERESS regional array. This study is important because of the recently announced intention of the Soviet Union to shift their underground nuclear testing activities from the test site near Semipalatinsk to Novaya Zemlya. A second study follows up earlier ones of long range Lg propagation from nuclear explosions in the continental platform and shield regions of the USSR; the present study gives geological interpretations of the efficiency of Lg propagation. The overall conclusion of these studies is that heterogeneity in the upper earth crust, sometimes referred to as the 'granitic layer,' seems to be the primary factor in blockage of Lg. Whenever a large part of the granitic layer is replaced in a confined region as in a basin, by lower velocity sediments, such that there is a large velocity contrast between the basin sediments and the surrounding granitic rocks, Lg waves appear to be trapped in the basin and are attenuated by repeated reverberations in the basin. As a result, Lg waves are delayed, diverted, scattered, and blocked, such that their recorded amplitudes are much reduced compared with what is expected from anelastic attenuation. Effects of Lg blockages need to be taken into account whenever Lg amplitudes are used for yield estimation and event identification.