Vertical ground motion prediction equations and vertical-to-horizontal (V/H) ratios of PGA and PSA for Algeria and surrounding region

Abstract

The paper presents a ground motion prediction equation (GMPE) for the vertical pseudo spectral acceleration for periods varying from 0.02 to 4 s. The model is derived from the same strong motion database as previously considered for the horizontal GMPE for Algeria (Laouami et al. 2018). 583 vertical records homogeneously processed having magnitude and distance intervals of 3–7.4 and 1–150 km respectively are used. The functional forms are similar to those used for our horizontal GMPEs. To consider the soil factor, three soil classes are defined based on the horizontal over vertical spectral ratios: rock, firm and soft. Estimates of period-dependent site coefficients reveal negligible vertical site amplifications compared to the horizontal ones. This work also presents a model for vertical over horizontal (V/H) response spectral ratio derived from the vertical and the horizontal GMPEs. One found, particularly for small distances and large magnitudes, that the V/H ratio has a large peak, which may exceed unity, around the period range 0.04–0.1 s. This result may be very important for seismic behavior of stiff structures which vertical fundamental period’s lies within a period range 0.04–0.15 s. The derived horizontal and vertical GMPE’s are used to compute predicted H/V spectral ratios for given scenarios. Compared to the recorded mean H/V spectral ratios, this tool allows checking the confidence level of the site classification scheme. Comparison of the median V/H response spectral ratios with period predicted from the model of this study with those from the Algerian seismic code RPA99 (2003) and the EC-8 (2004) (type 1 and type 2) reveals that the definition of the V/H spectral ratio from the RPA99 is closer to the scenario (M = 7 and Rjb = 100 km). Unfortunately, this scenario is not the worst for the vertical component. The present study has shown that the worst case scenario is related to high magnitude and short distance. For this scenario, for which the RPA99s do not provide an adequate V/H ratio, the EC-8 type 1 appears to give higher values. The proposed vertical GMPE for Algeria is compared with recent published models. It results that the present model with those from Cagnan et al. (2016) and Stewart et al. (2016) are the most recommended for the seismic hazard analysis in Algeria.