Vertical elastic response spectra for low and high seismicity regions

Abstract

This paper is focused on the definition of the vertical elastic response spectra. This study follows the recent development of the horizontal response spectrum in the framework of the revision of the Algerian seismic code RPA99 (Laouami and Slimani 2025). The same ground motion database is used, which comprises 773 3-component records, from magnitude ranging from 3.0 to 7.4, and hypocentral distances less than 200 Km. A statistical method is used to estimate constant spectral acceleration branch limits, attenuation indexes, and the ratio of the vertical to horizontal response spectra, at two seismicity levels: weak to moderate seismicity (wms) and moderate to high seismicity (mhs). The results of the analysis showed significant differences between the control periods of the elastic response spectra as a function of both the site class and the magnitude. The results reveal that the control period T_Cv increases as the site moves from rock to soft classes and from low to high earthquake magnitude, whereas the attenuation index decreases with increasing earthquake magnitude. The findings reveal also that the vertical to horizontal spectral ratio increase with magnitude and can exceed unity at near field for vertical vibration periods in the range 0.05–0.1 s. The recommended values of the ratio of vertical to horizontal design acceleration, \(\:{C}_{v/h}\), for wms and mhs seismicity levels, are 0.60 and 0.80 respectively. Comparison to the vertical spectra of the ASCE7-16 and the new generation EC8-draft2022 standards, reveals that period Tc of the proposed spectra is in agreement with the ASCE7-16 for the wms seismicity level, and with EC8-draft2022 for the mhs seismicity level. Finally, two spectral shapes are proposed for the two seismicity levels wms and mhs. This solution enables a significant upgrade over the present version of the national seismic design code RPA99, which does not offer elastic response spectra for the vertical component of seismic motion.