Earthquake-related hydrochemical changes in thermal springs in the Xianshuihe Fault zone, Western China

Abstract

Hydrochemical changes in three hot springs (GD, LTG and EDQ) were studied by following three large near-field earthquakes (2008 Wenchuan Ms 8.0, 2013 Lushan Ms 7.0 and 2014 Kangding Ms 6.3 earthquakes) in the Xianshuihe Fault zone, Western China. Each earthquake caused significant hydrochemical and isotopic changes in the three thermal springs. Hydrochemical ions (such as Ca2+ and HCO3−) decreased sharply following the Wenchuan earthquake and recovered after the Lushan and Kangding earthquakes. Although δD and δ18O data from before the Wenchuan earthquake are lacking, we find that δD and δ18O after the Lushan and Kangding earthquakes shifted toward the local meteoric water line (LMWL). Both the hydrochemical and isotope data indicate changes in the mixture of different waters, CO2 degassing and geothermal reservoir temperatures. Combining this information with the noble gas analysis, we interpret that new flow paths were created after the Wenchuan earthquake and were reclogged following the Lushan and Kangding earthquakes. More groundwater with low HCO3− and Ca2+ from the granite aquifer entered the hydrothermal system, and more CO2 escaped from the water following the Wenchuan earthquake, while less water from the granite aquifer entered the system following the Lushan and Kangding earthquakes. This study indicates that permeability can either increase or decrease in the near field following different earthquakes. Such changes should be considered significant factors in the management of groundwater supply and earthquake precursor monitoring in seismically active areas.