A prediction model for vertical-to-horizontal ratios of PGA, PGV, and 5%-damped response spectra (0.01–10 s) for Iran

Abstract

New models are developed for the prediction of vertical-to-horizontal (V/H) spectral ratios of peak ground acceleration (PGA), peak ground velocity (PGV), and 5%-damped elastic response spectra for periods ranging from 0.01 to 10 s. The models use moment magnitude, source-to-site distance, style-of-faulting (SoF), and site classification terms as independent variables. The study is based on a well-compiled Iranian ground-motion databank, consisting of 1350 (*3) three-component accelerograms generated by 370 earthquakes. Only records with known measured time-averaged shear-wave velocity in the uppermost 30 m (VS30) are used for regression analyses. The presented models are valid for moment magnitude ranging from 4.5 to 7.4, distance up to 200 km, and for earthquakes with reverse and strike–slip fault mechanisms. Four alternative mathematical forms are evaluated in terms of magnitude and distance in order to find the optimal functional form for reliable prediction of V/H ratios. Out of four proposed forms, two were chosen for further investigation. The point source (hypocentral and epicentral) as well as extended source (rupture and Joyner-Boore) distance metrics are employed to derive four models. The overall validity of the proposed models is examined by detailed residual analyses as well as comparison with some of the local and regional predictive V/H models. Our proposed models showed significant reduction in the total standard deviation across all periods. The proposed V/H models facilitate assessment of vertical components of response spectrum where horizontal component of response spectrum is known, using V/H spectral ratios as scaling factors.