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Logic tree for ground motion characterization of the vertical component: a case study based on updated Turkish ground motion database

Agha Alipour, Nima
Two approaches are used to develop the vertical design spectra in Probabilistic Seismic Hazard Assessment (PSHA): performing the PSHA for the vertical component using vertical ground motion models (GMMs) and utilizing the V/H ratio GMMs to scale the horizontal spectrum during or after the hazard calculations for the horizontal component. This study intends to develop a new framework for building the vertical ground motion logic tree that combines both vertical and V/H ratio GMMs in the PSHA analysis. The main objectives of this study are: (i) to compile a reliable and up-to-date strong ground motion database for Turkey (TSMD), (ii) to choose the V/H ratio and vertical GMMs that are consistent with the magnitude, distance, depth, and site amplification scaling of the updated TSMD, and (iii) to combine the selected vertical GMMs with the V/H ratio models to provide the ground motion characterization logic tree for vertical ground motion component in Turkey. For this purpose, the TSMD is updated with the recordings from the earthquakes occurred between 2008 and 2015, including the Mw=6.1 Elazığ and Mw=7.2 Van earthquakes. Updated TSMD contains 2698 recordings with the earthquake metadata, source-to-site distance metrics for the recordings, measured shear wave velocity profiles for the recording stations, and spectral accelerations for horizontal and vertical components. Four candidate V/H ratio GMMs (Gulerce and Abrahamson, 2011; Gulerce and Akyuz, 2013; Akkar et al., 2014; Bozorgnia and Campbell, 2016) together with four candidate vertical GMMs (Bozorgnia and Campbell, 2016; Stewart et al., 2016; and Gulerce et al., 2017 and Çagnan et al., 2017 model) are selected and the model predictions are compared with the actual data in the updated TSMD using the analysis of the residuals and data-driven techniques. Analysis results of V/H ratio GMMs showed that the median predictions of the V/H ratio GMMs proposed by Gulerce and Akyuz (2013), Akkar et al. (2014), and Bozorgnia and Campbell (2016) are compatible with the V/H ratios in the updated TSMD. Analysis results of vertical GMMs depicted that the magnitude scaling of Çagnan et al. (2017) model and the depth scaling of the Bozorgnia and Campbell (2016) model are inconsistent with the updated TSMD. Additionally, small magnitude (SM) scaling of Stewart et al. (2016) and Gulerce et al. (2017) GMMs should be modified for a better fit with the Turkish dataset. Trellis plots of SM-modified Stewart et al. (2016) and SM-modified Gulerce et al. (2017) vertical GMMs and selected V/H ratio models multiplied by NGA-West 2 and TR-adjusted NGA-W1 horizontal models are used to determine the logic tree weights for the proposed vertical ground motion characterization logic tree for Turkey.