In previous studies, short‐period (0.2–5.0 s) ground motions in Mexico City were investigated by a systematic analysis of surface and downhole accelerograms, and it was found that short‐period Love waves were very dominant at the predominant periods of grounds. Although the average vertical amplification of ground motions with long duration was interpreted fairly well at soft‐soil sites, the large time variations in the vertical amplification remain unexplained. The present study addresses two issues on the vertical amplification at two soft‐soil sites, which are important for engineering applications. The first issue is the interpretation of the large time variations in the vertical amplification of ground motions, and the influence of the depth of the structure in which surface waves are trapped is examined. The second issue is the interpretation of inertial force excitation, which is usually used in the engineering community, in terms of seismic waves. The second issue is also estimated by the vertical amplification. The conclusions are as follows: (1) the large time variations in the vertical amplification of ground motions can be explained sufficiently by both the mixture of S waves and Love waves and the depth of the structure in which Love waves are trapped. The structure corresponds to the Mexico City basin. (2) Although inertial force excitation is considered to be valid for S waves, it proves to be invalid for Love waves.