Structural geology and shallow site characterization using multi-geophysical methods in the Edremit Basin (Balıkesir) and its surroundings, Northwestern Türkiye

Because the Edremit Basin is situated at a complex junction of strike-slip and extensional tectonics, its seismic hazard is governed by the interactions among the Yenice–Gönen, the Edremit, and Havran–Balya fault zones (HBFZ), which generated the major 1944 (Mw 6.8) and 1953 (Ms 7.4) earthquakes. This study refines seismic hazard assessment of the region and identifies a transtensional regime characterized by dominant normal faulting along the northern margin. It identifies right-lateral strike-slip motion along the southern margin, integrating updated structural mapping, paleostress data, and high-resolution near-surface geophysical data. Empirical scaling suggests the westward continuation of the HBFZ could host events up to about Mw 7.2 earthquake. Data from 504 HVSR measurement sites and 181 MASW and ReMi surface-wave profiles provide estimates of the site period, amplification factor, seismic vulnerability index, shallow and 3D Vs structure, and shear-strain estimate for multiple PGA scenarios. Coastal and deeper basin areas show low Vs30 (≤250 m/s), longer site periods, and high amplification factors, while higher topographic elevations rest on engineering bedrock (Vs >760 m/s). Zones with Kg >20 particularly Çoruh, Çıkrıkçı, Havran could undergo plastic deformation at 0.25 g, with deformation spreading across much of the basin at 0.50–0.75 g. Most sites are classified as ZC–ZD soil classes, with localized ZE soft-soil zones according to the Turkish Building Earthquake Code (TBEC), highlighting strong spatial variability in site effects and emphasizing the need for micro-zonation and site-specific response analyses to support resilient against potential Mw ≥7 earthquakes.