Estimating strong ground motions at great depths
Résumé
In this paper, ratios of peak ground acceleration (PGA), velocity (PGV) and displacement (PGD) and response spectral acceleration (SA) of the ground motion at the surface and at different depths are estimated using shear-wave velocity profiles of rock sites, information on Q in the upper crust and recorded ground motions from rock sites. The results are compared with those reported in the literature for those sites with strong motion instruments at great depths. As expected estimated ground motions at depth are, in general, lower than those on the surface. Also it is found that the estimated ratios do not show much variation with depth except for high-frequency strong-motion parameters for profiles with high attenuation (Q=10) where ground motions at great depths can be higher than those at the surface. However, it is unlikely that such a low value of Q will occur in practice at depths down to 2km. When details of the near-surface velocity and attenuation structure are not known it is found that the commonly used factor of two (accounting for the effect of the free surface) is a reasonable conservative first estimate at all depths.
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