Active Tectonics and Paleoseismology of an Extensional Basin: Implications from the Büyük Menderes Graben (Western Anatolia, Türkiye)

Western Anatolia represents one of the most active continental extensional domains within the Alpine–Himalayan orogenic system. Ongoing NNE–SSW extension has produced a system of E–W–trending grabens and half-grabens controlled by active normal faults. These basins provide natural laboratories to investigate the interaction between fault-controlled deformation, sedimentary basin evolution, and seismic hazard. A key characteristic of such extensional basins is the presence of thick, unconsolidated basin fills overlying competent basement rocks. This strong mechanical contrast promotes seismic wave trapping and amplification, leading to prolonged ground-motion duration and increased shaking intensity. Similar basin-related effects have been documented in other extensional and transtensional settings worldwide (e.g., the Basin and Range Province, Central Apennines, and the Aegean region), highlighting their importance for seismic risk in densely populated areas.

The Büyük Menderes Graben is one of the largest and most mature extensional basins in Western Anatolia and hosts several major population centers. Paleoseismological investigations carried out on the main basin-bounding normal faults reveal repeated surface-rupturing earthquakes during the Holocene. These data show that fault segmentation, fault length, and basin geometry play a primary role in controlling earthquake magnitude, rupture characteristics, and recurrence patterns. At a regional scale (~100 km), several active faults have the potential to generate moderate to large earthquakes (Mw ~6.0–7.1). The combined effects of distributed fault deformation and basin amplification imply that seismic hazard in extensional provinces cannot be assessed solely based on proximity to individual faults. Instead, an integrated approach that considers fault interaction, basin geometry, and site effects is required.

In this study, trench-based paleoseismological investigations were carried out along the İncirliova, Umurlu, and Atça segments forming the northern margin of the Büyük Menderes Graben (BMG). In trenches excavated along all three segments, strong evidence was obtained for Holocene earthquakes that produced surface faulting. Preliminary findings suggest that the 22 February 1653 Menderes Valley earthquake (Ms6.7) may have originated from the İncirliova Segment, whereas the 20 September 1899 Menderes Valley earthquake (Ms6.9) was likely generated by the Umurlu and Atça Segments.

This study synthesizes active tectonic observations, paleoseismological trench data, and basin-scale geological constraints from the Büyük Menderes Graben to highlight how extensional basins amplify seismic risk beyond simple fault-based models. The results have broader implications for seismic hazard assessment in other active continental rift and graben systems worldwide, particularly where rapidly growing urban areas are built on young sedimentary basins.