The evidence of H-O-C isotopes in response to earthquake and the precursor anomaly index: A case study of geothermal fluids in the Xianshuihe fault

Abstract

Earthquake forecasting remains challenging due to the limited understanding of reliable precursory signals. While hydrochemical anomalies in geothermal fluids prior to seismic events show promise, the response mechanism of H-O-C isotopes (δD, δ¹⁸O, δ¹³C) during seismogenesis and their quantitative links to tectonic processes are poorly constrained. We conducted continuous hydrogeochemical monitoring and stable isotope mass spectrometry (δD, δ¹⁸O, δ¹³C_DIC) on geothermal fluids in the seismically active Xianshuihe fault zone (Western Sichuan, China). Time-series data from two hot springs (LTGS and EDQP) during the 2022∼2023 aftershock sequence of the Ms6.8 Luding earthquake were statistically analyzed using a Z-score method to quantify precursor anomalies. (1) Pre-seismic δD and δ¹⁸O exhibited rise-fall trends, driven by fracture-enhanced fluid mixing and water-rock interaction. δD anomalies were uniquely sensitive to earthquakes Ms ≥ 5.5. (2) Elevated δ¹³C originated from thermal decomposition of deep carbonate during tectonic stress accumulation, releasing ¹³C-enriched CO₂. Subsequent dissolution and isotopic exchange impart this high-δ¹³C signature to dissolved inorganic carbon (DIC); (3) A Z-score ≥ 2 effectively discriminated precursor anomalies for Ms > 4.0 earthquakes, with precursor response times of 10 days to 2 months. This study establishes H-O-C isotopes as dynamic indicators of seismogenic processes and proposes a Z-score method for short-imminent earthquake forecasting. Integrating these indicators into multi-parameter monitoring networks, contributing to the development of multi-parameter seismic monitoring in active fault zones.