H2, CO2, 222Rn, and Hg degassing from soil gas after a strong earthquake with implications for concealed fault detection in the northeastern Tibetan Plateau

Subsurface fault structures are generally difficult to delineate, particularly in areas covered by thick sediments. Soil-gas geochemical surveys can provide useful constraints on concealed structures after major earthquakes and complement conventional geophysical methods in sediment-covered basins. This study investigates the 2023 Jishishan M 6.2 earthquake as a representative case. Previous studies have suggested that the nearby Lajishan fault alone cannot fully account for the seismogenic structure of this earthquake, while no obvious surface rupture has been identified within the epicentral basin, complicating post-earthquake reconstruction planning and fault-avoidance measures. The spatial distribution of coseismic damage suggests the possible presence of previously unrecognized concealed structures. To investigate post-seismic soil-gas degassing and its spatial relationship with these structures, we measured Rn, Hg, H 2 , and CO 2 along two profiles across the severely damaged area. Two prominent gas-anomaly zones were identified and showed spatial correspondence with the inferred concealed structures. Integration of the soil-gas anomalies with aftershock relocation, shallow seismic reflection, and ambient-noise surface-wave tomography provides preliminary constraints on the location and geometry of a concealed active structure spatially associated with the broader seismogenic fault system. Hg and H 2 anomalies were more closely associated with the inferred major structural zone than those of Rn and CO 2 , suggesting that their distributions may be preferentially influenced by permeable fault-related pathways. These findings provide new constraints on concealed active structures associated with the Jishishan earthquake sequence and highlight the potential value of integrating soil-gas geochemistry with geophysical observations as a complementary approach for post-earthquake concealed-fault investigation in sediment-covered basins.