Continuous monitoring of hydrogen and oxygen stable isotopes in a hot spring: Significance for distant earthquakes

Earthquakes can change the physical and chemical properties of groundwater, and the changes can be identified by isotope techniques. Here, the hydrogen and oxygen stable isotope ratios of groundwater from the Banglazhang #1 hot spring in Yunnan Province, Southwest China, were collected from April 2015 to March 2019. The results show that the stable isotope ratios of hydrogen and oxygen are distributed around the local meteoric water line (LMWL), indicating that the hot spring recharge source is meteoric water with slight water-rock interaction. Temporal variations in the isotope ratios show that the recharge sources are relatively stable and the groundwater residence time is relatively long. However, in two periods, values exceed the background ratios recharged from meteoric water and are associated with distant earthquakes with magnitudes 6.8, 4.6 and 4.4. The stable isotope ratios changed within three months before the earthquakes due to permeability decreases in aquifers associated with stress and strain increases before the earthquakes. The changing permeability led to reduced recharge by deep hot water and the isotopes shift. These results reveal that the continuous observation of hydrogen and oxygen stable isotopes can indicate the recharge characteristics of groundwater and reflect the regional stress and strain before distant earthquakes. Hydrogen and oxygen stable isotopes may provide information for the analysis of earthquake precursors.