Earthquakes evoked by lower crustal flow: Evidence from hot spring geochemistry in Lijiang-Xiaojinhe fault

Geothermal fluid is considered to be one of the important methods of earthquake forecasting, and some achievements have been made in previous studies. However, there is still a lot of work to be done before geothermal fluid can actually be used for earthquake forecasting. Lijiang-Xiaojinhe fault (LXF) which is a quakeprone region that lies on the border of Tibetan Plateau and Yangtze block is a good natural experimental field for earthquake research. In this contribution, we performed a systematic hydrogeochemistry and isotopic analysis of the geothermal waters and gases in the LXF zone. High Cl- (0.69-529.44 mg/L) concentrations in hot springs indicate that they are contaminated by deep fluid obviously. Further continuous observation (2020-2022) indicates that there are at least four times correspondence between ion concentration in two hot springs (Jinhe (94) and Xiaoshuiping (331)). Two earthquakes (ML5.2 earthquake on January 02, 2022 and ML4.6 earthquake on April 16, 2022) in Ninglang are only 50 km away from 331 hot spring, and obvious abnormal concentrations of Cl,- Na+ and SO4 2- were observed before the earthquakes. Combined with geological background and previous studies, we propose that lower crustal flow could be the reason why earthquake-prone in LXF. The lower crust flow through the LXF from northwest to southeast, causes internal deformation of the LXF zone. Then, a large amount of groundwater infiltrates deep underground along the fault, strengthens the pore fluid pressure, promotes the water-rock reaction, weakens fault zone, and trigger earthquakes. Ion concentrations in hot springs are sensitive to seismic activity in LXF. Unusually high or low ion concentrations in hot springs could represent the process of earthquake development, monitoring proper geothermal waters and gases in LXF will help us to study the pre-seismic hydrochemical precursors, and have a potential to provide important bases for earthquake forecasting.