## 1 Introduction  
Turkey, located in one of Europe's major earthquake-prone regions, experiences seismic activity due to the compression of the Anatolian Plate as it moves toward the Eurasian Plate [1]. This movement is further influenced by the advancing Arabian and African Plates. Significant earthquakes, such as the 6.9 Erzincan (1992), 7.4 Kocaeli (1999), 7.2 Duzce (1999), 7.2 Van (2011), 6.8 Elazib (2020), 6.9 Seferhiisar (2020), 7.7 Pazarck (2023), and 7.6 Elbistan (2023) earthquakes, have caused extensive destruction of buildings and significant loss of life. The presence of industrial cities along these fault lines, combined with increasing urbanization, places a substantial portion of the country's population at high risk from earthquakes [2]. Consequently, it is critical to design structures based on site-specific conditions and to thoroughly analyze the interaction between structures and soil. For earthquake-resistant structural design, understanding the behavior of soil layers under seismic loads is crucial for mitigating damage in earthquake-prone areas. The forces acting on structures during an earthquake vary depending on the properties of the soil layers and their response to dynamic loads [3]. To determine site-specific soil dynamic behavior, obtaining undisturbed soil samples from the region and conducting shear wave velocity (Vs) measurements in the field are of great importance. These measurements provide valuable information about how soil profiles change with depth, making them essential for site-specific analyses [4; 5]. In Turkey and other parts of the world, structural damage often occurs due to soil amplification [6]. In such analyses, earthquake acceleration records are as critical as determining the dynamic parameters of soils. The selection of these records must comply with regulatory requirements. Earthquakes are selected based on factors such as the fault type of the region and local soil conditions, followed by detailed analyses. National regulations typically outline the procedures for selecting and scaling earthquake acceleration records and deriving design spectrum curves [7]. In studies conducted in earthquake-prone regions, earthquake records are chosen according to the seismic characteristics of the area. These records are scaled using various methods and applied in analyses as bedrock acceleration. When scaling is performed in the time-history domain--a common method in the literature--the frequency content of the acceleration remains unchanged. However, when scaling is conducted in the frequency domain, an equivalent acceleration record is generated based on the design acceleration spectrum, resulting in changes to the frequency content. As a result of site-specific earthquake scaling, the response spectrum differences observed at the surface affect superstructure designs. Additionally, the examination of soil amplification effects in the region also varies depending on the scaling method used. Therefore, the choice of scaling method is crucial for analyses conducted in earthquake-prone areas. Turkey's earthquake code studies began in 1975 and were subsequently revised in 1998 and 2007, following significant earthquakes and the accumulation of new knowledge. The latest regulation, published in 2018, further emphasized the impact of local soil conditions. The Turkish Earthquake Code (TEC 2018), which has been in effect since 2019, introduced recommendations for calculating earthquake ground motion spectra based on the location of the structure, its distance from the fault, and soil properties. This regulation also expanded the section on site-specific soil classifications compared to previous codes. As part of this study, laboratory tests and computer analyses were conducted using undisturbed soil samples collected from field studies in Sakarya, one of Turkey's critical earthquake regions. The amplification effect of soil was particularly evident during the region's 1999 earthquakes, with magnitudes of M7.2 and M7.4.