Soil Gas Investigations in Hatay-Reyhanlı (Türkiye): Implications for Buried Fault Detection and Seismic Hazard Assessment

This study presents a soil gas geochemical survey in the Kırcaoğlu and Reyhanlı regions of Hatay Province, southeastern Türkiye, following the 2023 Kahramanmaraş earthquake doublet. The aim was to identify concealed faults and assess seismic hazard through analysis of soil CO₂ flux, CO₂ and ²²²Rn concentrations, and carbon and helium isotopic compositions. A total of 98 sites were surveyed, and graphical statistical methods were used to establish geochemical anomaly thresholds. In Kırcaoğlu, two prominent gas anomaly zones were delineated with dominant NW-SE and NE-SW trends, likely representing buried faults linked to the Yesemek Segment, including one beneath the Reyhanlı Dam. In Reyhanlı, an east-west gas anomaly suggests a possible westward extension of the Reyhanlı Fault. These findings refine the region's structural framework and highlight seismic risks from buried faults. Isotopic analyses show CO₂ derives from biogenic and deep crustal reservoirs, with ⁴He/²⁰Ne and ³He/⁴He ratios confirming up to 7.1% crustal helium and <1% mantle helium. Heavier δ¹³C values and elevated crustal helium in Kırcaoğlu support deep gas migration along fault zones. Natural CO₂ emissions are estimated at 66 t/d in Kırcaoğlu and 60 t/d in Reyhanlı. Regionally, emissions from the Amik Basin (∼15,586 t/d) comprise ∼1.5% of Türkiye's daily anthropogenic CO₂. The overlap between gas anomalies and surface ruptures and liquefaction zones from the 2023 earthquakes confirms the effectiveness of soil gas surveys for buried fault detection. These results highlight the utility of soil gas geochemistry as a non-invasive tool for fault detection and seismic hazard assessment.