New vertical-to-horizontal ratio spectrum due to intraslab earthquakes for soft-soil sites of Mexico City

Abstract

This technical note introduces a vertical-to-horizontal ratio, V/H, spectrum for soft soil sites of Mexico City due to intermediate-depth intraslab earthquakes. The shape of the proposed V/H spectrum takes into account the empirical trend observed in V/H response spectra derived from 283 earthquake records gathered during six historical events from 1964 to 2017. Unlike the conventional V/H value of 2/3 used to derive the vertical acceleration design spectrum included in several seismic codes worldwide, the new V/H ratio spectrum includes three spectral regions that depend on the period of vibration in the vertical direction. To account for the influence of the predominant period of the site on V/H ratios at long periods of vibration, a continuous regression equation is also introduced in this paper. Both proposals aim at filling the lack of seismic specifications to take into account the vertical acceleration component in the current Mexico City Building Code.