Synthetic seisomograms constructed by addition of surface-wave modes in a layered half-space are compared to Cagniard-de Hoop calculations of Heaton and Helmberger (1977, 1978) and to ground displacement recordings near El Centro, California to examine the applicability of modal superposition as a means of simulating ground motion of possible engineering interest. Modal solutions of flat earth problems are desirable because of the modest cost involved and the versatility of the method in simulating extended sources and anelastic damping. P-SV and SH motions can be computed with almost equal ease. The comparisons show that in sedimentary structures surface waves can dominate ground displacement motion at epicentral distances of only a few source depths. Superposition of the higher modes often approximates quite well impulsive arrivals with analogies to refracted and reflected rays.
Ground displacement recordings of El Centro from the 1968 Borrego Mountain earthquake are modeled using a multi-layered geological structure and a source model based on independent studies. The gross character of the records appears to be insensitive to the details of the source. Both point sources and propagating sources with horizontal dimensions larger than half the epicentral distance give reasonable fits to the observed transverse motion. This insensitivity appears to be due to a complex interaction between rupture propagation and the surface-wave dispersion. By using the integrated El Centro accelerogram, which may have more reliable amplitude information than the Carder displacement record used in other studies, the moment is estimated to be 12 × 1025 dyne-cm. This is similar to values found from studies of teleseismic data.