Abstract
The gas compositions (He, H2, CO2, CH4, Ar and N2) and isotope ratios (3He/4He and δ13C) were yearly investigated from April 2010 to April 2019 at the Luojishan spring located in the proximity of the Zemuhe Fault, eastern Tibetan Plateau. The continuous automatic monitoring of hydrogen concentrations in Luojishan hot spring bubbling gas for the purpose of earthquake prediction requires the discrimination of seismic precursor anomalies. Helium isotope ratios (3He/4He) in the bubbling gas of hot springs varied from 0.05 to 0.18 Ra (Ra = 3He/4He = 1.39 × 10−6 in the air), with a maximum mantle-derived He up to 2.2% of the total He measured in the Luojishan hot spring (assuming R/Ra = 8.0 for mantle). This suggests that Zemuhe Fault might act as a conduit for crustal-derived fluid. N2 concentrations in the majority of the hot spring was ≥80 vol%, and δ13CCO2 values varied from −13.2 to −9.3‰ (vs.PDB). Hydrogen concentration time series display a complex temporal pattern reflecting a wide range of different physical processes. There were short-term (5–60 h) seismic precursor anomalies of hydrogen concentration before natural earthquake. The anthropogenically-induced earthquakes provoke only post-earthquake responses. The concentration of hydrogen in bubbling gas of the Luojishan hot spring is sensitive to increase of stress in the Xianshuihe-Xiaojiang fault system. Monitoring the hydrogen concentrations with automatic gas stations may be promising tool for unraveling earthquake mechanisms and for predicting earthquakes.
Highlights
Earthquake precursors are elusive, and this elusiveness has hampered earthquake prediction (Donald, 1988; Cicerone et al, 2009; Gherardi et al, 2017)
Gas collected at the Luojishan hot spring from the bubbling pools was characterized from April 25, 2010 to April 19, 2019 in detail (Table 1)
The temperatures of Luojishan hot spring water ranged from 43°C to 44.8°C
Summary
Earthquake precursors are elusive, and this elusiveness has hampered earthquake prediction (Donald, 1988; Cicerone et al, 2009; Gherardi et al, 2017). The large change of gas geochemical anomalies as regards shape, duration, delay/ anticipation time and parameters-association with respect to foreshocks, main-shock and aftershocks are considered to be highly site specific (King, 1986; Sugisaki et al, 1996). Noble gas and their isotopes are very good natural tracers for exploring crust-mantle interaction in different geologic background because they are chemically inert and their isotopic signature is conservative with respect to the source of gas/fluid in crustal waterrock interactions (Sano and Wakita., 1985; Hilton, 1996). The isotopic signatures of He showed some correlations with seismic activities (Sano and Marty, 1995; Kissin, 2007)
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