Physics-based simulations of the foreshock and aftershock of the February 6, 2023 Turkiye earthquakes and ground motion validation

This study presents the first three-dimensional (3D) physics-based ground motion simulations conducted for southern Turkiye, focusing on the foreshocks and aftershocks associated with the February 6, 2023 Turkiye earthquake sequence. The simulations were performed using a fourth-order staggered-grid finite-difference method developed for elastic media, incorporating a point-source double-couple representation to model rupture processes. Moderate-magnitude earthquakes were selected to enable the investigation of site-specific effects with reduced influence from the complex source characteristics typically associated with large events and from nonlinear behavior of near-surface soils. Accordingly, a regional-scale velocity model was constructed to capture both the source locations of these events and the dense strong-motion observation network covering Kahramanmaraş, Gaziantep, Türkoğlu, Nurdağı, and Pazarcık. The simulation results for both the Nurdağı (Gaziantep) Mw 4.5 aftershock on February 19, 2023 and the Pazarcık (Kahramanmaraş) Mw 4.5 foreshock on October 20, 2022 are analyzed by incorporating variations in the deep crustal velocity structure and low-velocity shallow sedimentary layers, with shear-wave velocities as low as 800 m/s. Simulated ground motions were compared with observed strong-motion records, and validation was carried out using a comprehensive set of intensity and frequency-based ground motion metrics. These included waveform similarity, Arias duration, energy duration, Arias intensity, energy integral, peak ground acceleration (PGA), peak ground velocity (PGV), peak ground displacement (PGD), response spectra, and Fourier amplitude spectra, following the quantitative fit scoring methodology based on synthetic-to-observed motion comparison metrics proposed by Anderson (Earthquake Engineering Research Institute, 2004). Strong-motion recordings were largely consistent with the simulations, with intensity-based fit scores predominantly in the 6–8 range and coherent response spectrum residuals in the 1–10 s period band, demonstrating the reliability of the results up to 1.0 Hz for long-period ground-motion assessment and for resolving contributions from both shallow and deep structural features. The study highlights the critical importance of capturing irregular wave propagation patterns shaped by regional stratigraphy and sedimentary basin configurations. Overall, the research provides essential methodological and modeling infrastructure to improve seismic hazard assessment and support resilient infrastructure planning.