A model distribution of anomalous masses in the southwestern part of Elysium Planitia under the surface of the site of the InSight mission seismometer has been obtained. Numerical simulations are per- formed with the S-approximation method by expansions of the data on the Martian topography and gravita- tional field (the MRO120D model) in spherical harmonics to the 90th degree and order. Inhomogeneity of the density structure in the form of zones, containing blocks of higher and lower density in the subsurface lay- ers of the study area, has been revealed.