Modelling nonlinear site coefficients and predominant periods for southern coasts of İstanbul by geotechnical downhole arrays

Abstract

This study investigates nonlinear response spectral amplification factors (RSAF) and predominant periods (T-p) for the southern coast of the Istanbul's European Side by employing a Monte Carlo simulation-based approach. The methodology incorporates one dimensional (1D) site response analyses of simulated random shear wave velocity (V-s) profiles, along with the optimization of these profiles through least-squares error between the response spectra of recorded weak ground motions and the modelled ones. The recorded ground motions were selected from a database of near-field earthquakes with epicentral distances R-epi < 100 km and magnitudes in the range 3.9 = 4.8, a scaling procedure for bedrock PGAs was applied to model the nonlinear RSAF and T-p. In this study, the local site coefficients specified in TEBC (2018) were tested for the southern coastal soils of the Istanbul's European side for the first time, an area broadly characterized by poor soil conditions (i.e., time averaged shear wave velocity for the top 30 m, 180 < V-s30 < 360 m/s). The findings indicate that the estimated RSAFs (i.e., local site coefficients) particularly at 1.0 s period (T = 1.0 s), exceed those defined in TEBC (2018). Furthermore, the T-p values at seismic station locations under weak amplitude ground motions differ substantially from those under strong ground motions. The in-situ methods (e.g., microtremor, etc.) for determining the site T-p may be misleading, as they primarily capture linear behavior. The proposed method provides a basis for revising the short-period (T = 0.2 s) and 1.0 s period RSAFs in TEBC (2018) and highlights T-p variations for the loose granular and clayey soils of Istanbul