Gypsum as a potential tracer of earthquake: a case study of the Mw7.8 earthquake in the East Anatolian Fault Zone, southeastern Turkey

Abstract. Obvious macroscopic anomalies of geothermal fluids were observed before and after the Mw 7.8 earthquake in Turkey. In order to find out the relationship between geothermal fluid anomalies and earthquakes, we performed a systematic hydrogeochemistry and isotopic analysis of the geothermal fluids in the East Anatolian Fault Zone (EAFZ). The results show that these geothermal fluids were reconstructed (including: energy and materials) by earthquakes. Based on chlorine – enthalpy model, the temperature of the deep geothermal fluid has been increasing to 382 °C on the strength of the energy released by the seismic activity. However, the information of the deep geothermal fluid was eventually covered due to the infiltration of a large amount of shallow cold water after the earthquake. The abnormal concentrations of Ca²⁺ (54.04~501.58 mg/L), Mg²⁺ (6.58~116.20 mg/L), SO₄^2– (6.37~287.74 mg/L), Sr (34.78~3244.8 μg/L), and Ba (1.89~196.48 μg/L) in geothermal water shown that the geothermal water has undergone complex water-rock interaction processes such as gypsum, calcite, dolomite, anorthite and serpentinization. Specially, significant gypsum dissolution was observed at HS05, HS09 and HS14 before and after the earthquake, suggesting that the earthquake broke the balance of water-rock reaction and promoted the dissolution of gypsum. Combined with geological background and previous studies, we propose that shallow sedimentary minerals, such as gypsum, have the potential to be used as earthquake warning indicators. However, shallow minerals are controlled by many external factors (e.g., temperature, pressure, climatic conditions, seasonal changes etc.), which greatly weakens their practical value in earthquake early warning.