Strong ground-motion spectral statistical properties of the East Anatolian Fault region, Türkiye: heterogeneous attenuation, stress drop, and site variations

This study analyzes the ground-motion characteristics of the East Anatolian Fault (EAF) and its adjacent regions, using a dataset comprising 7,972 horizontal and vertical accelerograms from 415 small-to-moderate earthquakes recorded at 170 strong motion stations between 2018 and 2024. This process is implemented by the generalized inversion technique (GIT) combined with the diffuse field concept (DFC). To reduce the GIT uncertainty, we consider the site effect of the outcrop bedrock and the spatial heterogeneity of the path attenuation. First, significant variations in geometric attenuation across the EAF region are observed at different frequencies with increasing distance. The greatest attenuation follows the R⁻¹, while the smallest attenuation shows almost no reduction. The quality factor Q_s exhibits significant spatial heterogeneity, primarily due to the influence of late Cenozoic volcanic regions, with an average value of 145 f^0.96. Next, we decompose the source spectra and site effects from the path-corrected records. The degree of high-frequency decay in the non-parametric source spectra increases with earthquake magnitude. The stress drop ranges from 0.35 to 50.8 MPa. The stress drop exhibits a slight positive dependence on magnitude and focal depth. The site effect in the EAF region demonstrates moderate amplification at frequencies exceeding 7 Hz. At frequencies below 2 Hz, a maximum horizontal amplification of 6 times is observed, specifically at the northwestern end of the EAF. Additionally, a maximum vertical amplification of 3 times occurs around 1 Hz at this same location. The fundamental site period (T₀) in the region ranges from 0.07 to 1.91 s. Horizontal near-surface attenuation (κ_0H) varies from 0.0106 to 0.0777 s, while vertical near-surface attenuation (κ_0V) ranges from 0.0102 to 0.0838 s. Spatially, both T₀ and κ_0V are inversely correlated with V_s30, while the correlation between κ_0H and V_s30 is weaker.