# Successful Tests on Detecting Pre-Earthquake Magnetic Field Signals from Space
## 3 Results and Discussion
Figure 2 shows that for the Nepal Mw = 7.8 earthquake of 25 April 2015 (Earthquake number 120 as listed in Table S1), there is an obvious precursory anomaly in the Y component of the magnetic field from latitude 25\({}^{\circ}\) to 33\({}^{\circ}\), lasting about 120 s. Please note that in the same spatio-temporal interval, another M5+ earthquake occurred, as indicated by a red point. In Figure S2, the Swarm satellite shows a clear anomaly in the Z magnetic field component, lasting 45 s, before the Mw = 6.6 earthquake in Greece (Earthquake number 359 in Table S1). As shown in Figures S3 and S4, respectively, a 26 s lasting anomaly in Y was detected before the Mw = 5.9 earthquake in Iran (Earthquake number 572 in Table S1), and another lasting 15 s occurred before the Mw = 5.5 earthquake in Italy (Earthquake number 914 in Table S1). Notably, among the examples provided, the earthquake with the highest magnitude (Mw = 7.8) generated an anomaly lasting 120 s, while the earthquake with the smallest magnitude (Mw = 5.5) produced an anomaly lasting only 15 s. Intermediate magnitude earthquakes resulted in anomalies with durations falling between these extremes. The results clearly suggest a correlation between the duration of the anomaly and the magnitude of the earthquake. Other examples are given in Figures S5-S14 of the Supplementary Material. Generally, there is a trend where, as the magnitude of the earthquake increases, the duration of the corresponding satellite-detected anomaly also increases, indicating greater persistence. Similarly, the amplitude of the anomalies follows a similar pattern, increasing with the magnitude of the earthquake. In summary, the results, along with the figures in the Supplementary Material, lead to the conclusion that earthquakes of different magnitudes consistently produce satellite magnetic field anomalies within specific duration ranges. For instance, earthquakes with a magnitude of Mw = 5.5 typically exhibit anomalies lasting approximately 10 to 20 s, while those with Mw = 7.0 manifest anomalies lasting around 40 to 80 s. This range expands proportionally with increasing magnitude. The amplitude of anomalies also conforms \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Magnitude Range**} & \multicolumn{2}{c}{**Number of EQs That Exhibit Anomalies**} & \multirow{2}{*}{**Total Number of EQs**} & \multirow{2}{*}{**Percentage (\%)**} \\ \hline Full range & 615 & 786 & 78.2 \\ \hline
5.0–5.5 & 448 & 594 & 75.4 \\ \hline
5.5–6.0 & 110 & 130 & 84.6 \\ \hline
6.0–6.5 & 38 & 43 & 88.3 \\ \hline
6.5–7.8 & 19 & 19 & 100 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Number of earthquakes considering geomagnetic conditions. The ratio of the number of earthquakes that had magnetic anomalies to the total number of earthquakes is calculated for different magnitude ranges and for the entire magnitude range, taking into account quiet geomagnetic conditions. to this relationship. The maximum and minimum values within these ranges depend on the satellite orbit's distance from the earthquake epicenter and the timing of anomaly observation. Smaller distances and closer observation times to the event result in more pronounced and longer-lasting anomalies. The observed correlation between anomaly length (i.e., duration), orbital distance, anomaly energy, and earthquake magnitudes has motivated further exploration into the detailed relationships among these parameters.