## 5 Discussion
### Interpretation of Earthquake-Induced Anomalies
#### 5.3.2 Trace Element Signatures of Earthquake Anomalies  
This section examines trace element dynamics in hot spring waters associated with the Wushi \(M_{\text{S}}7.1\) earthquake to elucidate the geochemical mechanisms underpinning seismic stress-strain processes. Trace elements are categorized based on their post-seismic trends: (1) Li, B, Al, Ba, Ge, Rb, and Cs exhibited notable declines (Figure 9a); (2) Be, V, Mo, Cr, Sr, and Sb showed significant enrichment (Figure 9b). These divergent element behaviors, coupled with major ion anomalies, reflect the interplay between deep-seated and near-surface fracture systems [30; 37], which jointly drive hydrogeochemical re-equilibration during seismic cycles. For the first group, five days before the Atushi \(M_{\text{S}}5.4\) earthquake, concentrations of Ba, Al, and B dropped sharply (\(<\!-2\sigma\)), indicating rapid expansion of shallow fractures and subsequent mixing with low-temperature meteoric water [25; 76; 77]. This influx of cold snowmelt diluted trace element concentrations, while oxidation of pyrite generated \(\text{SO}_{4}{}^{2-}\), inducing \(\text{BaSO}_{4}\) oversaturation and precipitation, further contributing Ba depletion. These anomalies coincided with \(\delta\)D and \(\delta^{18}\)O depletions (cold water influx) and a Na\({}^{+}\) concentration drop (dilution signature), supporting shallow fracture-mediated mixing. Between 38 and 12 days pre-Wushi \(M_{\text{S}}7.1\) earthquake, these elements fluctuated near baseline, suggesting partial deep-fracture connectivity. The positive \(\delta^{13}\)C DIC anomaly reflects continuous deep-sourced CO\({}_{2}\) influx. During this phase, the input of high-temperature geothermal fluids partially offset dilution effects, maintaining elevated Li, B, and Rb levels [17]. After the  
Figure 9: Long-term monitoring of trace elements concentration changes at S13 hot springs and the magnitude and date of \(M>3.0\) earthquakes in related tectonic regions. **(a)** Li, B, Al, Ba, Ge, Rb, and Cs concentrations, (b) Be, V, Mo, Cr, Sr, and Sb concentrations. Wushi \(M_{5}7.1\) earthquake (post-day 24), negative \(\delta^{13}\)C DIC anomaly and pH drop (to 6.3) indicated reduced deep fluid input due to deep fracture closure. This led to significant depletion of Li, B, Rb, and Cs. Under mildly acidic conditions, Al and Ge were adsorbed onto Fe-hydroxides, while sustained SO4\({}^{2-}\) enrichment (Figure 9) promoted BaSO4 precipitation. By day 34 after the Wushi \(M_{5}7.1\) earthquake, concentrations of these elements returned to their average levels. For the second category of elements, five days before the Atushi \(M_{5}5.4\) earthquake, Sr and Sb concentrations surged above \(+2\sigma\), likely driven by stress-induced dissolution of near-surface carbonates (calcite, aragonite) [17] and plagioclase, releasing Sr\({}^{2+}\). This enrichment persisted until 12 days pre-Wushi \(M_{5}7.1\) earthquake, indicating sustained activation of secondary fracture network [65; 78]. Concurrent oxygen influx into shallow fractures oxidized stibnite (Sb\({}_{2}\)S\({}_{3}\)), mobilizing Sb as Sb(OH)\({}_{6}\)\({}^{-}\). Twelve days pre-Wushi \(M_{5}7.1\) earthquake, Be concentrations exceeded \(+2\sigma\) and continued rising, likely due to the propagation of secondary fracture within the principal fault zone. These fractures facilitated both mechanical disruption and acidic leaching of Be-bearing minerals (e.g., beryl), with mildly acidic conditions (pH 6.3; Table S2) enhancing Be\({}^{2+}\) release. Post-Wushi \(M_{5}7.1\) earthquake (day 24), further mechanical and geochemical weathering of Be-rich minerals occurred within shallow fractures, resulting in additional Be\({}^{2+}\) mobilization. At this stage, Sr also slightly increased, reflecting contributions from both deep granitic sources (as inferred from pre-Wushi \(M_{5}7.1\) earthquake \(\delta^{18}\)O anomalies) [8; 13] and near-surface carbonate dissolution.