Blackwell Publishing Journal Backfiles 1879-2005
A recent surge of interest in short-period fundamental mode Rayleigh waves (Rg) propagation is motivated by their usefulness for structural mapping and estimation of attenuation (Q) properties of the uppermost crust, their importance in scattering studies and as a nuisance (false alarms) in detection seismology. We have investigated Rg phase velocity dispersion characteristics, using recordings from 16 out of 22 subarrays of the NORSAR array of aperture 100 km. The observed phase velocity in combination with a one-layer and two-layers model over halfspace models were inverted using a maximum likelihood scheme. In view of the smoothness and simplicity of the dispersion curves, the number of unknowns were limited to three parameters, namely shear velocities and thickness, using a Poisson's ratio of 0.25 for P-S velocity coupling and density fixed. In the general array siting area the derived low velocity layer has an average thickness of 1.02 km, and the associated shear velocity is 2.82 km s-1. The underlying model half space appears to be uniform, and its derived shear velocity is 3.55 km s-1, which is typical of the upper crust. In contrast to other similar studies, no obvious correlation was found with local geology, which mainly consists of Precambrian crystalline rock sequences. Group velocities were calculated from the structural models and compared to the observational ones. The agreement between calculated and observed group velocities was reasonable.
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