INFLUENCE OF THE LOWER CRUSTAL STRUCTURE ON THE EARLY CODA OF REGIONAL SEISMOGRAMS

We use synthetic seismograms for a flat-layered Earth model to study the influence of the widely observed strong reflectivity of the lower crust on the propagation of seismic regional phases. The lower crustal layering induces a clear increase in the duration of the Pg and Lg phases on both the vertical and radial components beyond the group velocity of their terminations in a model with a homogeneous lower crust. The transverse component is weakly affected. The amplitude and the spectral content of the early Lg coda are strongly dependent on the distribution of layer thicknesses. The predominance of the low-frequency components (below 2 Hz) observed in the Lg coda of real data is correctly accounted for with 1-km-thick layers in the lower crust. From particle motions at various depths and the analysis of the dispersion curves of normal modes, we hypothesize that the late arrivals result from the interaction of plate-type disturbances of the high-impedance layers. These waves leak energy in the mantle at a low rate. This mode of generation of the coda waves explains the puzzling observation that the spreading coefficient of Lg seems to be applicable to the early coda of regional seismograms. The study of the influence of the lamellae on the average Q factor of the continental crust shows that the lower crust layering does not account for the observed low values of the apparent Qs. Another origin is needed for this strong attenuation, for example scattering by randomly distributed heterogeneities.