For the 1992 Landers mainshock, the lateral heterogeneity of the lithosphere combined with an extended earthquake source may account for the complexity of the strong-motion observations that cannot be easily interpreted by a simplified point-source model in a laterally homogeneous medium. Both the effect of an extended source and that of lateral heterogeneity are examined using a forward modeling technique that combines distributed nonuniform slip on a fault plane in a 3D medium. Surface-wave synthetics are constructed using surface-wave Gaussian beam method, which allows for a mild degree of lateral heterogeneity. We analyze the amplitude and waveform data of the Landers strong-motion recordings in southern California in order to identify the path effects and the effect of an extended source. The overall source parameters of the Landers earthquake are obtained by comparing synthetics with recordings at near-source stations which lie within relatively homogeneous path structures. This procedure yields a unilateral rupture propagating northwesterly over two distinct rupture segments with somewhat different focal mechanisms. The surface-wave phase velocity map is modified to fit the data recorded at larger hypocentral distances. The final improved model generates a very good agreement between observations and synthetics. In the presence of lateral heterogeneity, the focusing, defocusing, and multipath interference are found to contribute significantly to the strong-motion shaking in a 3D basin.