## 5 Discussion
### Interpretation of Earthquake-Induced Anomalies  
#### 5.3.1 Major Ions and Isotopic Perspectives on Earthquake Anomalies  
Tectonic stress accumulation in fault zones progressively alters permeability structures and fluid migration pathways, disrupting hydrogeochemical equilibrium prior to seismic rupture [8; 30; 67]. Continuous monitoring at S13 captured systematic anomalies in Na+, Cl-, and SO4\({}^{2-}\) concentrations and \(\delta\)18O, \(\delta\)D, and \(\delta\)13C DIC isotopic values months before the Atushi \(M_{5}\)5.4 earthquake and Wushi \(M_{5}\)7.1 earthquake (Figure 8). Between May 29 and August 15, 2023, stress accumulation along the MDF triggered aseismic creep and intermittent stick-slip events, causing pre-seismic fluctuations in major ion concentrations without surpassing the shear strength of the fault's locked segment. During this period, \(\delta\)13C DIC values remained stable (\(-2.84\%\) to \(-4.37\%\)), indicating a persistent deep-sourced fluid regime. Figure 8: Long-term monitoring of Na+, Cl-, SO4\({}^{2-}\), \(\delta\)18O, \(\delta\)D, and \(\delta\)13C DIC concentration changes at S13 hot springs and the magnitude and date of \(M>3.0\) earthquakes in related tectonic region. Forty-one days pre-Atushi \(M_{5}\)5.4 earthquake, a transient drop in Na\({}^{+}\) concentrations (below 2\(\sigma\)) indicated the activation of shallow secondary fractures, allowing infiltration of low-temperature meteoric water, likely from snowmelt, into the hydrothermal system [23; 25; 75]. Thirty-nine days pre-Atushi \(M_{5}\)5.4 earthquake, a sustained decline in \(\delta^{18}\)O marked the onset of fracture opening, enhancing the influx of deep-seated fluids and intensifying water-rock interaction. Thirteen days pre-Atushi \(M_{5}\)5.4 earthquake, \(\delta\)D, \(\delta^{18}\)O, and Na\({}^{+}\) concentrations rose concurrently, indicating the simultaneous activation of both shallow and deep fractures systems under transmission tectonic stress. This sequential pattern delineates the rupture preparation phase of the Atushi fault: initial shallow fracture dilation triggered by meteoric water infiltration, followed by deeper fracture preparation and ascent of high-temperature fluids. These findings also suggests that MDF was approaching a critical stress threshold, with the Atushi \(M_{5}\)5.4 earthquake functioning as a foreshock adjustment for the Wushi \(M_{5}\)7.1 earthquake. It is worth noting that in both events, Na\({}^{+}\) concentrations initially dropped below the \(-2\sigma\) threshold, followed by a gradual rebound prior to the mainshock; however, this rebound was more sustained and pronounced in the Wushi sequence, reflecting deeper fracture activation and stronger fluid ascent. Forty-two days pre-Wushi \(M_{5}\)7.1 earthquake, \(\delta^{13}\)C DIC peaked beyond \(+2\sigma\) (\(-0.78\)%), indicating the expansion of deep fracture that facilitated upwelling of \({}^{13}\)C-rich CO\({}_{2}\)--potentially derived from high-temperature decarbonation of carbonate rocks or mantle-sourced fluids--leading to \(\delta^{13}\)C DIC enrichment [29]. Thirty-eight days pre-Wushi \(M_{5}\)7.1 earthquake, Na\({}^{+}\) and SO\({}_{4}\)\({}^{2-}\) concentrations and \(\delta^{18}\)O exceeded the \(-2\sigma\) threshold and showed progressive increases, reflecting synchronized rupture of deep and shallow fracture and complex mixing with high-salinity deep fluids and cold meteoric waters. Between 23 days pre-Wushi \(M_{5}\)7.1 earthquake and the mainshock, these parameters exhibited secondary decline-increase fluctuations, interpreted as multi-phase stress release and dynamic fracture network adjustments. Persistent CO\({}_{2}\) from depth continued to promote intense water-rock interactions, accelerating mobilization of major ions. Twenty-four days post-Wushi \(M_{5}\)7.1 earthquake, fracture closure curtailed deep fluid input, causing negative anomalies in ion concentration and \(\delta^{13}\)C DIC (\(-4.89\)%), signaling reestablishment of a shallow hydrogeochemical equilibrium.