Linear and Nonlinear Site Responses Revealed by Single-Station Time–Frequency Analysis: Application in Southeastern Türkiye

ABSTRACT Understanding seismic hazards requires increased attention to the site effects, particularly in southeastern Türkiye, where the 6 February 2023 earthquakes revealed complex local amplification. This study presents a detailed regional assessment of site effects from seismic recordings, considering both linear and nonlinear site responses. We apply two time–frequency methods: the previously introduced time–frequency resonance analysis (TFRA) and an extended method, the stacked Stockwell power spectral density (sS-PSD). sS-PSD is primarily used to investigate linear site response because it provides higher low-frequency resolution than TFRA, whereas TFRA is applied to characterize nonlinear behavior under strong ground motions. Both methods rely on single-station recordings from weak to strong motions, require no reference site, and provide resonance information rather than absolute amplification. For linear site response, sS-PSD captures resonance characteristics over a broad frequency range (0.1–30 Hz) and yields results comparable to those obtained with generalized inversion techniques. For nonlinear analysis, resonance curves obtained from TFRA show that the most significant changes appear in higher frequency bands relative to the fundamental frequency. These frequency shifts vary among sites, reflecting a nonlinear site response that is strongly controlled by local geological conditions and that also differs across frequency bands, even within the same site. In southeastern Türkiye, nonlinear frequency shifts can reach up to 63% during strong shaking. On average, shifts of approximately 5% occur at a peak ground acceleration threshold of ∼25 cm/s2, and 10% at ∼70 cm/s2. Compared to the well-documented nonlinear effects observed at KiK-net sites in Japan, sites in southeastern Türkiye appear more susceptible to frequency shifts under strong ground motion.