In the wake of death and destruction left by the 2017 earthquake in Mexico City, it is natural to ask whether the event was unexpected and anomalous. Although such an intraslab earthquake (M w 7.1; depth 57 km; epicentral distance = 114 km from the city) was considered likely, the recordings in the city during the last 54 yrs reveal that the ground motion during the 2017 earthquake was anomalously large in the critical frequency range to the city (0.4-1 Hz). The intraslab earthquakes occur closer to Mexico City, at greater depth, and involve higher stress drop than their interplate counterparts. Consequently, the ground motion is relatively enriched at high frequencies as compared with that during interplate earthquakes, which is dominated by lower frequency waves (f < 0:5 Hz). This explains the observed difference in the damage pattern during the 2017 and the disastrous interplate earthquake of 1985 (M w 8.0). Electronic Supplement: Figures showing spectral ratios, peak ground acceleration (PGA) and peak ground velocity (PGV) as function of distance R, comparison of observed response spectra SA, and predicted median and 1 s SA from a site-specific ground-motion prediction equation (GMPE) model at CU, plot of accelerograms, Fourier acceleration spectra, and SA at SCT of interplate 1985 M w 8.0 and intraslab 2017 M w 7.1 earthquakes, accelerographic stations in Mexico City, which recorded the 2017 M w 7.1 earthquake, SA of the 2017 earthquake at sites in and near Condesa and Roma colonies, and basis for the estimation of exceedance rate of PGA at CU in Mexico City from intraslab earthquakes, and tables providing a list of 20 (interplate and intraslab) earthquakes with largest recorded PGA at CU in the 1964-2017 period, significant pre-1975 intraslab earthquakes, and a comparison of observed PGA and PGV at CU during the 2017 M w 7.1 earthquake with predictions from GMPE.