ESTIMATING EARTHQUAKE SITE AMPLIFICATION USING LOCAL SEISMIC PHASES FROM A DENSE NODAL SEISMIC ARRAY IN SOUTHEASTERN TURKIYE

Türkiye is one of the most tectonically active regions in the world, situated generally at the confluence of the African, Arabian, and Eurasian plates. Running roughly along Türkiye’s southeastern border with Syria is the East Anatolian Fault, a left-lateral strike-slip boundary between the Arabian and Anatolian plates. On February 6, 2023, two earthquakes occurred nine hours apart in southeastern Türkiye, one of M_w 7.8 along the East Anatolian Fault and one of M_w 7.5 along the Çardak Fault, about 30 km north. In total, these events caused nearly 60,000 confirmed deaths and left another 1.5 million homeless throughout Türkiye and Syria. The damage was not wholly proportional to the proximity to the epicenter; for example, the city of Antakya is located 100 km from the epicenter but incurred substantially more damage than the epicenter’s nearest city, Gaziantep. This is thought to be largely the result of variations in the geological characteristics underlying the respective cities, namely the hardness of sediment and the depth of bedrock. A basin, additionally, will act as a lens and focus seismic energy, causing greater destruction. It is therefore critical to understand the seismic hazard potential of a given location, as well as how to build accordingly, in order to minimize loss of life and property. This study was done in collaboration with TÜBİTAK (the Turkish equivalent of the National Science Foundation) and the Georgia Institute of Technology. The data was drawn from a network of 200 nodal stations, including 70 stations deployed on a linear profile with an average spacing of approximately 200 m. Because of the density and linearity of this profile, the effects of path-based attenuation and radiation on the amplification of ground motions are negligible, thereby isolating the site amplification. We are also able to measure the peak frequency of the site amplification, which is an important parameter for structural engineers. Preliminary results indicate patterns of greater amplification within basins and possibly the damage zone (the region of broken rock associated with fault slip) on both sides of the fault. The frequency of peak amplification appears generally to be 3.0 Hz. Future plans involve using reverse two-station paths to isolate site amplification for all 200 nodal stations.