Development of a GIS-Based Predicted-VS30 Map of Türkiye Using Geological and Topographical Parameters: Case Study for the Region Affected by the 6 February 2023 Kahramanmaraş Earthquakes

Abstract

Abstract The time-averaged shear-wave velocity in the upper 30 m of a site (VS30) is virtually essential in characterizing local soil conditions for multiple purposes, including estimation of site effects, anticipated ground-motion levels, seismic hazards, and the shape of design spectra. Considering the significance of this proxy and that a number of the Disaster and Emergency Management Presidency of Türkiye (AFAD) strong ground-motion stations across Türkiye lack assigned VS30 values, a comprehensive study was performed herein to develop empirical equations for estimating VS30 values in Türkiye based on relationships between 432 VS30 measurements at the AFAD stations, geologic units, and topographic data. Initially, units in the geological digital maps were reclassified into four geological periods. Statistical relationships between geological period classes and VS30 samples were interpreted to determine the VS30 boundaries for each period class. Second, VS30 estimations with topographic parameters by utilizing a 2D trend surface analysis method were performed. The resultant two-parameter polynomial coefficients were associated with VS30 according to the least squares principle, leading to the development of topographic functions for VS30 estimation under each geological period class (R2=0.601). Thereby, digital VS30 estimation maps were produced in grid (90 m) format that may be queried in a Geographic Information Systems environment. Moreover, the quantile regression method was also utilized to determine the coefficients of the envelope curve corresponding to a given exceedance probability (p) for the worst case scenario. Finally, to evaluate the accuracy of the proposed equations, the verifications performed with the VS30 data at the selected AFAD stations in the region affected by the 6 February 2003 earthquakes have also presented successful outcomes. Considering the availability of VS30 maps derived from digital elevation data in the literature, this study offers novel equations that take into account geological units and provide crucial background data for the regional seismic hazard-based risk assessments in Türkiye, especially for site effect studies using VS30 as a regional site classification parameter.