Present-Day Soil Gas Geochemistry Along the 1975 Ms 7.3 Haicheng Earthquake Rupture Zone: Fault-Controlled Migration Five Decades after the Earthquake

ABSTRACT

Although substantial gas emissions from postearthquake rupture zones have been documented, their underlying mechanisms, sources, and fault-related migration pathways remain uncertain. Here, we present a systematic tectonogeochemical survey across the surface rupture zone of the 1975 Ms 7.3 Haicheng earthquake, northeast China. A total of 233 sampling sites were established for soil gas concentration measurements, among which nine were further selected for carbon isotope analysis. Synchronous in situ measurements of Rn, CO2, and H2 concentrations were performed at all sites to elucidate the spatial coupling between soil gas concentration anomalies, seismic activity, and fault structures. The results indicate that soil gas concentrations along the rupture zone remain higher than those in background areas 50 yr after the Haicheng earthquake. The peak concentrations of Rn, CO2, and H2 reach 376.60  kBq/m3, 9.98 vol.%, and 1292.00 ppm, respectively, implying distinct geochemical anomalies along the fault zone. The carbon isotope results reveal that the contribution of deep-derived CO2 ranges from approximately 5% to 12%, indicative of the potential upward migration of deep crustal fluids. According to the analysis of gas emission-controlling factors, fault offset, persistent small-to-moderate seismicity, and deep fluid activity are likely the primary controls on these geochemical anomalies. Furthermore, the pronounced H2 anomalies and unique geological conditions of the Haicheng area may be useful for regional geochemical screening of fault-controlled gas migration. In conclusion, this study provides a representative case for understanding decadal-scale postseismic gas migration and its primary controls along the rupture zone of a major continental earthquake.