Physics
Scientific paper
May 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991pepi...67..162n&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 67, Issue 1-2, p. 162-179.
Physics
1
Scientific paper
In the absence of local scattering the ground motion due to major phases at an array should be perfectly coherent between different seismometers. We have studied the coherency of ground motion for the regional phase Lg as a function of frequency and spatial separation for the NORESS, ARCESS and FINESA arrays. The events examined are quarry blasts at a distance of 200-400 km from the array, and coherency was estimated for 10-25 s windows containing Lg. In the 1-10 Hz range coherency decreases with increasing spatial separation. The decrease is faster for higher frequencies, but if the separation is scaled to the wavelength then the decay curves are similar and indicate that coherency decreases to < 0.5 within about a wavelength. To study this problem we have stimulated synthetic seismograms by two methods. One is to start with an observed trace and produce new traces by successive random perturbations of the Fourier amplitude and/or phase. Perturbations of travel time (phase) have a much greater effect on coherency than perturbations of the amplitude, and scale with frequency in the same manner as the observations. One possible physical explanation is the influence of velocity perturbations. To test this, finite difference calculations have been made for a layer over a half-space model where the source is located in the layer and a receiver array is located at the surface. The layer and the half-space contain random velocity variations. Models using self-affine autocorrelations for the velocity perturbations reproduce the major features of the observed coherency relations, although computational limitations prevent testing the wavelength dependence.
Charrette Edmond E.
Dainty Anton M.
Nafi Toksoz M.
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