A Predictive Equation for Vertical‐to‐Horizontal Response Spectral Ratios in Northern Iran

Abstract

Abstract The importance of the vertical component of ground motion in damage infliction on a variety of structural systems has been shown in recent years. However, the available models for ground‐motion prediction equations (GMPEs) for this component of earthquakes are limited, and most of the existing GMPEs have been specifically developed for the horizontal component of ground motion. Alternatively, in some cases, it is proposed to scale the horizontal spectrum of the earthquake to the vertical one using vertical‐to‐horizontal response spectral ratios (VHSRs). In this work, the applicability of some global and regional VHSR models for northern Iran are evaluated using several existing statistical approaches. The results of these evaluations show inadequate correlation between these models and the observed VHSRs in this region. Consequently, a new VHSR model has been proposed to convert the horizontal earthquake spectrum to the vertical one in the studied region. In order to determine the regression coefficients for the proposed VHSR model, some 712 three‐component records (with distances less than 300 km from the source) from 79 earthquakes with the magnitude ranging from M w 4.1 to 7.3 were used. Regression coefficients for peak ground acceleration and pseudospectral acceleration (for 5% damped systems) are determined using a random‐effect approach. A series of comparative studies on the proposed model with respect to the existing VHSRs was performed using the median residuals for the spectral periods in the 0.1–3 s range. The results of such investigation show significant reduction of the variability for the proposed model (at least 50%) compared with the other VHSRs. Online Material: Database of strong‐motion records.