Trinity or Verity ?

Abstract

Single-station three-component analysis is applied to seismic recordings in Norway from quarry blasts on the Kola peninsula. Even for P-waves, non-linear particle motions are observed, implying phase shifts between the horizontal and vertical components. Such instantaneous phase-lags provide direct and useful information on the arrival of different wave types, and this is illustrated. For P-waves, the most probable origin of these phase shifts is scattering and P-to-S conversion. Phase-lags are also observed for later phases. Using signal particle motion models which allow for phase shifts we have been successful in analysing several different phases (P, S and Lg) in these complex regional events. Such results contrast with various previous studies of this and similar data, in which three-component analysis was found to be relatively ineffective. We attribute this significant difference in results in part to sub-optimal processing parameters and/or three-component analysis methods in some of the earlier studies e.g. analysis based on visual inspection of particle motion figures (very difficult where phase-lags are involved) or on eigenvector analysis where the diagonal elements of the covariance matrix, and thus uncorrelated noise, have a large influence. Incorporating seismic wavefield physics leads to signal models which can be expressed primarily in terms of off-diagonal elements and such models are therefore inherently more robust.

The relative information content and thus cost-effectiveness of single-component arrays and three-component data are compared, and our results indicate that in many circumstances the three-component configuration is by far the most cost-effective.