Abstract: Fluid geochemistry plays an important role in short-term and imminent earthquake prediction, earthquake precursor research and tectonic activity analysis. Based on field fixed-point observations and flow measurements of fault soil gas Hg and H₂ concentrations in the Songliao Basin, the relationship between short-term and imminent gas geochemical anomalies and small earthquakes in volcanic areas, satellite thermal red long-wave radiation, and the subduction dynamics of the Japan Sea plate before the Nenjiang M_L4.6 earthquake in Heilongjiang on February 7, 2020 is analyzed and discussed. The conclusions are as follows: ① There is a quasi-synchronous anomaly in soil gas Hg and H₂ concentrations in Zhaodong fault before the Nenjiang M_L4.6 earthquake. ② Before the Nenjiang M_L4.6 earthquake in 2020, the low value anomaly of soil gas H₂ concentration on the Longhe Town and Heping Town survey lines on the Nemoer River fault was related to the decrease in the frequency of small earthquakes in the Wudalianchi Volcanic Area. ③ The vorticity field of thermal red long-wave radiation before and after the Nenjiang M_L4.6 earthquake has obvious temporal and spatial evolution characteristics. In space, the vorticity field anomaly area gradually evolves along the Nenjiang fault to the epicenter area; In time, the low value anomaly period is synchronized with the low value of soil gas H₂ concentration and the annual low frequency of small earthquakes in volcanic areas. ④ Before the Nenjiang M_L4.6 earthquake, the fault soil gas anomaly is intrinsically related to tectonic activity.

Key words: Fluid geochemistry, Fault soil gas, Tectonic activity, the 2020 Nenjiang M_L4.6 earthquake