SEISMIC-WAVE PROPAGATION IN COMPLEX CRUST UPPER-MANTLE MEDIA USING 2-D FINITE-DIFFERENCE SYNTHETICS

A description is given of the numerical FD scheme used to solve the el astic wave equation, including a few remarks on the source functions u sed. Our FD method has been used for computing synthetic seismograms f or 2-D crust/upper mantle models of size 150 x 400 km(2), with options for free-surface topography. The strategy was to introduce successive ly more complex lithosphere models for generating the synthetics; the reference model was laterally homogeneous lithosphere. The interface s cattering was visualized through displays of free-surface synthetic wa veforms and snapshots for models with a corrugated Moho only and free surface topography only. Near the free surface the latter seems to dom inate, in the form of P-to-Rg and S-to-Rg conversions. Lithosphere ran domizations were introduced through von Karman functions of order v = 0.3, with rms velocity fluctuations of 3-4 per cent and correlation di stances (horizontal and vertical) at 2.5 or 10 km. In case of a medium with only sub-Moho heterogeneities, those with horizontal anisotropy (a(x) = 10 km; a(z) = 2.5 km) produced relatively strong Pn and Sn pha ses. The respective codas were dominated as in most of our experiments by P-to-S and S-to-S scattering wavelets excluding Rg scattering at a free surface with topography. For a medium with crustal heterogeneiti es, the distortions of the P and S wave trains with distance were clea rly demonstrated. For full-scale heterogeneous lithosphere models, cha racteristic features of the synthetics were quantitatively similar to observational records of local events. Dominant attributes were a pron ounced P coda consisting mainly of P- and Rg-scattered wavelets, and a relatively strong S coda consisting mainly of P-to-S and S-to-S scatt ered wavelets. The P and S waveforms are severely distorted pointing a t the futility of reliably picking many secondary arrivals in local ev ent recordings. Most of the scattering wavelets are confined to the cr ustal waveguide and to surface waves, since coda excitations for senso rs at a depth of 100 km were weak and, moreover, consisted mainly of S wavelets. This implies that a strong teleseismic P coda does not refl ect scattering within the crust in the source region but, rather, a co mplex source. Observational results from analysis of NORESS and ARCESS local event recordings are also presented. Clearly the lithosphere is not isotropically inhomogeneous. The essence of our 2-D FD synthetic seismogram experiments is that a simple lithosphere model, being moder ately heterogeneous, gives rise to complex seismograms which are gross ly similar to the observational recordings. In contrast, complex model s derived from profiling surveys (but lacking the fine-scale random va riations) give simple, 'ray tracing' like synthetics, not necessarily similar to the observed records.