Accession Number : ADP204509
Title : Effects of Explosion Depth and Crustal Heterogeneity of Lg Waves,
Corporate Author : PRINCETON UNIV NJ DEPT OF GEOLOGICAL AND GEOPHYSICAL SCIENCES
Personal Author(s) : Keers, H. ; Vogfjoerd, K. ; Nolet, G. ; Dahlen, F. A.
PDF Url : ADP204509
Report Date : 14 AUG 1995
Pagination or Media Count : 8
Abstract : We use ray theory to model the propagation of Lg waves through 2D and 3D layered crustal models. The layers are homogeneous, and the discontinuities are undulating. The Lg wave train is modelled by multiple S reflections within the crustal layers. The ray tracing system is reduced from a set of linear differential equations to a set of maps. If the medium has three or more discontinuities the number of multiples increases exponentially. Using a binary tree-searching method we systematically keep track of all multiples. The ray behavior is chaotic for large take-off angles, causing multipathing of the crustal multiples. However, in the presence of mid-crustal discontinuities the rms Lg amplitude is stable, because of the distribution of the energy over a large amount of rays. The simplest case, a one layer over a halfspace model, already contains all the characteristics of wave propagation through the media under consideration. To examine the source-depth and velocity-structure effects on amplitudes and character of Lg we use wavenumber integration methods in 1-D structures. In particular, we study the ability of shallow explosions to generate S and pS rays. Although S is a nongeometrical phase it does follow a well defined raypath. In velocity structures, where the peak in the S radiation pattern is confined to the crust, signals from explosions within 1 km of the surface, that do not generate spall or significant tectonic release, are likely to be dominated by waves with a very narrow ray-parameter range, coming from the radiation peak of S. Observation of Lg waves dominated by such a narrow ray-parameter range may therefore identify the source as explosive, and shallow. Dislocation radiation patterns, or the spalls CLVD term are likely to generate significant S waves over a wider range of ray parameters and therefore not to be confused with S.
Descriptors : *SURFACE WAVES, *SEISMIC DETECTION, *SEISMIC WAVES, *ARMS CONTROL, *NUCLEAR EXPLOSION DETECTION, *SEISMIC DISCRIMINATION, PEAK VALUES, SYMPOSIA, VERIFICATION, MONITORING, UNDERGROUND EXPLOSIONS, WAVE PROPAGATION, EARTH CRUST, TREATIES, RAY TRACING, SEISMIC VELOCITY, RADIATION PATTERNS, TECTONICS, SECONDARY WAVES, LINEAR DIFFERENTIAL EQUATIONS.
Subject Categories : Government and Political Science
Seismic Detection and Detectors
Distribution Statement : APPROVED FOR PUBLIC RELEASE