Examination of the vertical earthquake ground motion component during the September 19, 2017 (Mw = 7.1) earthquake in Mexico City

This technical note aims at investigating the earthquake ground motion and spectral features of the vertical component motions recorded at soft soil sites in Mexico City during the strong September 19, 2017 Puebla-Morelos (Mw = 7.1) earthquake by means of examining ground motion (EQGM) features, vertical-to-horizontal (V/H) spectral ratios, and comparison with design spectra ordinates. EQGM features of vertical components recorded at 10 accelerographic stations revealed that they have shorter mean periods than their corresponding horizontal components, and long significant duration. In addition, V/H spectral ratios are larger than one in 6 stations for periods of vibrations between 0.22 s and 0.56s. Mean V/H ratios confirmed that the trend depends on the period of vibration and the vertical component could be more important for short-period structures than medium-to-long period structures. Since current Mexico City seismic regulations do not include recommendations to account for the vertical component, it is showed that the simple rule of reducing the horizontal acceleration design spectrum prescribed in the recently released 2017 Mexico City seismic regulations by 2/3 is enough to cover the mean + one-standard deviation vertical acceleration response spectrum.