A recent article (Tejeda-Jácome and Chávez-García, 2007) showed that significant differences in seismic ground-motion attenuation perpendicular to the Pacific coast exist between Guerrero and Colima, in western Mexico. For hypocentral distances larger than 100 km, larger ground motions are predicted using a model derived from local data in the northern section of the subduction zone (Colima) relative to ground-motion models derived using data from the southern section (Guerrero). In this article we consider two possible explanations for the differences. The first possible explanation is differences in the geometry of the subducting slab between Colima and Guerrero. The second is the varying shallow structure of the overriding plate due to the presence of the trans-Mexican volcanic belt (TMVB), oblique to the Middle American trench. We use 2D numerical modeling for P-SV waves to investigate which of the two factors can explain the differences in the observed attenuation. Our results show that it is the presence of the TMVB, closer to the coast in Colima, that better explains the differences.