Abstract
The Chuan–Dian region, situated in the middle part of the north-south seismic zone of mainland China in a highly deformed area of the eastern margin of the Tibetan Plateau, is one of the principal areas for monitoring earthquake activities in China. In this study, the geochemical characteristics of soil degassing (of CH4, H2, CO2, Rn, and Hg) and, the relationship between degassing and fault activity, were investigated in the Xianshuihe–Xiaojiang fault system (XXFS) at the eastern boundary of the Chuan–Dian rhombic block. The mean soil-gas concentrations of CH4, H2, CO2, Rn, and Hg in the XXFS were 8.1 ppm, 9.9 ppm, 0.5%, 15.1 kBq/m3 and 12.9 ng/m3, respectively. The δ13CCO2 and δ13CCH4 values of the hot-spring gases varied from −11.9‰ to −3.7‰ and −62.5‰ to 17‰, respectively. The He-C isotopic ratios indicate that the carbon in the northern and middle parts of the XXFS may have originated from deep fluids, whereas the carbon in the southern part of the XXFS is of organic origin. The high concentrations of soil gas were distributed near the faults, indicating that the faults could act as channels for gas migration. The distributions of the high soil-gas concentrations in the XXFS coincide with the highest stress and maximum strain rates, indicating that the fault activity enhanced permeability and increased the emission rates of the gases. The results of this study will be helpful for degassing in active fault zones and earthquake monitoring.
Highlights
Degassing from the solid Earth to the atmosphere can be enhanced by varying the crustal stress (King, 1986; Toutain and Baubron, 1999; Du et al, 2008; Bohnhoff and Zokack, 2010)
Rn concentrations higher than the upper anomaly thresholds (UAT) in the XXFS might be related to the upward migration of Rn along the
From the spatial distributions of soil gases in the XXFS (Figure 5), the anomalously high concentrations of CH4, H2, CO2, Rn, and Hg appeared near the fault, which might indicate that the fault slip-rate influenced degassing from the active fault zone
Summary
Degassing (of gases such as Rn, He, CO2, CH4, CO, H2, N2, and Hg) from the solid Earth to the atmosphere can be enhanced by varying the crustal stress (King, 1986; Toutain and Baubron, 1999; Du et al, 2008; Bohnhoff and Zokack, 2010). The Chuan–Dian region, in the southern part of the north-south seismic zone of mainland China, is tectonically located in the highly deformed area of the eastern Tibetan Plateau. The XXFS is located at the eastern boundary of the Chuan–Dian rhombic block (Figure 1B) and has a curved left-lateral strike-slip structure extending at least 1,400 km into the eastern margin of the Tibetan Plateau (Allen et al, 1991). The NW-striking XSH fault is a highly active strike-slip fault in the eastern margin of the Tibetan Plateau that begins in Luding, passes through Kangding and Luhuo, and extends into Qinghai Province (Figure 1B). The northern segment of the ANH fault is characterized by thrust and ductile shear, whereas the middle segment is characterized by deep faulting Seismic activities in these segments are severe, with many M ≥ 6 earthquakes occurring (BGMRSP, 1993). The Jinkagu (1), Kama (2), and Guanding (3) hot springs in the XSH fault; the Zimakua (4), Xide (5), and Hongmo (6) hot springs in the ANH fault; the Liutie (7) hot spring in the ZMH fault; and the Xundian (8), Yiliang (9), and Tangi (10) hot springs in the XJ fault (Figure 1B) are distributed near the soil-gas measurement sites (Figure 1B); bubbling gas from these hot springs was sampled
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