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Mexico City experienced a large amount of damage during the September 19, 1985, earthquake. Although the whole city experienced different destruction degrees the region of maximum damage was constrained to a densely populated area west from the international airport, where water saturated sediments are still abundant. Other regions within the city, also with saturated sediments, did not experience a similar destruction. Among the mechanisms proposed to explain such large amount of destruction, one was proposed involving the interaction of incoming and reflected seismic waves in, or close to, the saturated surface. The existence and location of seismic wave reflecting surfaces within the basin of Mexico is thus of basic importance. It is herein proposed that high-density geologic structures within the basin may constitute those reflecting surfaces. 2-D gravity modeling is performed across the basin between Sierra del Tepeyac(N) and Xochimilco (S); some lines intersect and model volcanic structures in this region. A 3-D inversion of the gravity field shows that Sierra del Tepeyac-Peñón de Los Baños is a dense structure potentially capable of reflecting seismic waves towards the west, while Sierra de Santa Catarina would preferentially reflect them towards the south. The high-density regions are defined and mapped in 3-D space. These findings support the possibility of strong interactions between seismic waves travelling in opposite directions in selected regions of the Mexico basin. The reflection mechanism will obviously be proportional to the magnitude of the originating seismic disturbance.