Vertical Ground-Motion Prediction Equation and Vertical-to-Horizontal Spectral Ratios of Peak Ground Acceleration, Peak Ground Velocity, and 5% Damped Response Spectra in Southwest China

ABSTRACT Using strong ground-motion records from 116 mainshocks with moment magnitude (Mw) ranging from 4.0 to 7.9 in the active tectonic regions of southwest China, a vertical ground-motion prediction equation (V-GMPE) was developed. This V-GMPE estimates the vertical peak ground acceleration (PGA), peak ground velocity (PGV), and 5% damped absolute spectral acceleration (SA) for periods ranging from 0.01 to 8 s for shallow crustal earthquakes. To enable an unbiased estimation of the vertical-to-horizontal (V/H) spectral ratio, this V-GMPE adopts the same functional form as the horizontal GMPE ( V-GMPE) developed by Zhang et al. (2022). The V-GMPE is effective in predicting the vertical ground motion of earthquakes with Mw between 4.0 and 7.9, considering faulting styles, rupture distance (Rrup) ≤300 km related to Mw, and sites with VS30 values ranging from 140 to 1130 m/s in southwest China. Compared with the horizontal component, the vertical component demonstrates stronger anelastic attenuation. The influence of the nonlinear site response on the V/H spectral ratio is particularly prominent at short periods. When estimating the V/H spectral ratio, equal aleatory variability exists in both the vertical and horizontal components. The V/H spectral ratio in southwest China is significantly influenced by factors such as Mw, Rrup, site type, and period. There are also significant regional differences compared with the findings in regions such as the western United States and Taiwan. These factors should be considered when developing a vertical design spectrum suitable for southwest China.