CALCULATION OF SYNTHETIC SEISMOGRAMS IN A LATERALLY VARYING MEDIUM BYTHE BOUNDARY-ELEMENT DISCRETE WAVE-NUMBER METHOD

We investigate the propagation of Lg waves in laterally varying crusta l structures by numerical simulation. The method of calculation is for mulated in terms of boundary integral equations where the Green's func tions are evaluated by wavenumber summation. The approach is well suit ed to study the propagation of seismic waves in a layered medium where the interfaces are flat in some regions and irregular in others. We i nvestigate the effect of a crustal fault with vertical offset and stud y the case of a lateral change in crustal thickness. The results show that the Lg wave amplitude is only slightly affected by the presence o f these heterogeneities. They confirm the robustness of Lg wave propag ation in presence of lateral heterogeneities observed in other numeric al simulations. They show that large-scale geometric features of the c rust cannot account alone for the strong attenuation of Lg waves obser ved in many regions. The results also suggest a possible relation betw een the level of Lg wave coda and the degree of roughness of the Moho. They further indicate the importance of backscattering and suggest a possible use of the backscattered wave field to map strong crustal het erogeneities.