Ozmen, O.T.; Yamanaka, H.; Arslan, H.I.; Beyhan, M.; Altiok, S.; Ozel, A.O.; Turkoglu, M.; Gokce, O.

This study presents a nationwide assessment of seismic site amplification across Türkiye, based on 7191 strong-motion records from 757 earthquakes recorded at 462 stations of the Türkiye National Strong Motion Network. Using a spectral separation technique, contributions of source, path, and local site conditions were separated within the 0.5–20 Hz frequency range. The results indicate spatially and frequency-dependent amplification patterns across the country. Low-frequency amplification within the 0.5–1.3 Hz band is primarily observed at sites situated in sedimentary surface settings in eastern and southwestern Türkiye, whereas amplification within the 8.0–20 Hz band appears more evident in parts of southern Türkiye, including Hatay. Site amplification factors were evaluated across four geological units and five site classes defined by the National Earthquake Hazards Reduction Program (NEHRP). Quaternary units and sites classified as Class E exhibit stronger low-frequency amplification, whereas Paleozoic units and Class C–D sites tend to show higher-frequency responses. A relationship between Vs30 (time-averaged shear-wave velocity in the upper 30 m) and amplification is observed, particularly within the 0.5–1.3 Hz frequency band. These findings contribute to understanding of local site effects and provide insights relevant to seismic hazard assessment, urban planning, and resilient structural design under local ground conditions in Türkiye.

Pamuk, Eren; Fırat, Seyhan; Büyüksaraç, Aydın; Çetin, Kemal Önder; Bektaş, Özcan; Işık, Nihat Sinan; Sarıtepe, Halil Erdim

On February 6, 2023, two devastating earthquakes (Mw 7.8 and Mw 7.6) struck southeastern Türkiye, two of the most destructive seismic events in the country's history. This study investigates the structural damage and seismic vulnerability in the Gölbaşı Basin, located in Adıyaman Province—one of the regions most severely affected by these events. Geophysical techniques, the HVSR (Nakamura) and spatial autocorrelation (SPAC) methods, were employed to develop shear wave velocity (Vs) profiles and evaluate the dynamic soil properties of the basin. Shear wave velocities within the Gölbaşı Basin, down to a depth of 300 m, range from 211 to 923 m/s, with the lowest values observed near the lake, indicating weak and loose soil conditions. Natural site periods vary between 0.1 s and 2.86 s, with the longest periods (T > 2.5 s) also concentrated in the vicinity of the lake. In areas where the engineering bedrock (Vs > 760 m/s) lies deeper than 250 m, natural periods frequently exceed 1.5 s. These findings suggest that zones with thick alluvial deposits and low Vs values are particularly susceptible to seismic hazards. Structural damage was most severe in areas where Vs is below 350 m/s, site periods exceed 1 s, and the engineering bedrock lies deeper than 50 m. Notably, low-rise industrial buildings and low-rise structures with basement floors remained intact despite poor soil conditions. In contrast, in areas with more competent ground conditions, structural collapses were more likely caused by deficiencies in engineering design or construction quality.