In cases of large deformations of the ground underlying a structure, mat and grid foundations are two options to reduce probable differential settlements, due to their rigidity in the perpendicular directions and their relatively larger area compared with single spread foundations. A case study of large settlements of an 8-storey structure resting on soft saturated fine-grained soil due to seismic loading is numerically investigated herein. The foundations were matching in plan dimensions (10*18 m), and the grid foundation was 30 cm thicker than the 60-cm-thick mat, reaching nearly similar volume (110 m3 in the case of grid foundation and 108 m3 in the case of mat foundation). To decide on the more economical foundation type between mat and grid, the amount of reinforcing steel bars and the cost of construction, including concrete molding should be considered. Herein, based only on the amount of foundation settlement, it is shown that the performance of a grid foundation is better than or identical to mat foundations, which makes it an alternative with probably a more economical option to be preferred to mats in the design of foundations in areas of high seismicity. It is also shown that assessment of the frequency content of the input motions by applying the white Gaussian noise added to a sine sweep of frequencies to the model and inspecting the results, is useful in understanding the results.