## 3 Results
To further check the detected anomalies, we propose to calculate the percentage of anomalies in the California area with respect to the ones in the comparison (US and EU) areas, normalised with respect to the potential tracks by the following expression:  
\[\%=\frac{\left(\text{anomalies}_{\text{California}}-\text{anomalies}_{\text{ comparison}}\right)}{\text{anomalies}_{\text{comparison}}}.\frac{\text{Total Windows}_{\text{comparison}}}{\text{Total Windows}_{\text{California}}}.\]  
The \"total windows\" are the number of windows whose centres fall inside the investigated area during the quiet geomagnetic time. Table 1 summarises these values for each satellite and for the constellation as a whole. We noticed that the constellation presented more anomalies in the epicentral area with a normalised percentage from about 27% to 82%. The single satellite generally shows more anomalies in the epicentral area with respect to the comparison ones, with the exception of Alpha, which presents 22% less anomalies in the epicentral area with respect to the US East Coast comparison one. In the other cases, the satellites present more anomalies in the epicentral area with a normalised percentage from 42% up to 220%. Charlie is the satellite with the highest percentage of anomalies in the epicentral area with respect to both comparison ones. All the differences between the California and EU areas, reported in Table 1, are statistically significant. Considering all the satellites, in the epicentral area, the number of anomalies is 15 more than in the US East Coast comparison area and 32 more than in EU one, which corresponds to 27% and 82% more, respectively. We also compared the distribution of the anomalies with respect to their local time in all the areas. A difference in local time distribution can be considered to support the possible link with the seismic activity. In particular, we checked whether the anomalies are concentrated at a particular time of the day. To be sure that any possible conclusion will not be affected by the influence of the _Swarm_ constellation orbital parameters on the local time, the latter has been checked by analysing how it is distributed in the analysed period, as shown in Figure 4a. All the analysed windows with quiet geomagnetic field conditions (as above defined) are reported as dots: the colours for Alpha and Charlie satellites have been chosen to be the same for simplicity, considering that their local time differences are few minutes. The local time distribution of the analysed window confirms that the epicentral and comparison areas are equally covered for the same period. Figure 4b-d report the local \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & **Alpha** & **Bravo** & **Charlie** & **All** \\ \cline{2-7}  & **Alpha** & **Bravo** & **Charlie** & **All** \\ \cline{2-7}  & **Alpha** & **Bravo** & **Charlie** & **All** & **Brand** \\ \hline California area & 20 & 1268 & 24 & 1138 & 26 & 1033 & 70 & 3439 \\ \hline Comparison area (US East Coast) & 26 & 1210 & 14 & 1186 & 15 & 1000 & 55 & 3396 \\ \hline Normalised percentage of anomalies w.r.t. US comparison area & \(-\)22.0\% & \(74.4\%\) & \(71.0\%\) & \(26.9\%\) \\ \hline Comparison area (Europe) & 6 & 1193 & 14 & 1203 & 18 & 971 & 38 & 3367 \\ \hline Normalised percentage of anomalies w.r.t. Europe comparison area & 219.5\% & \(75.5\%\) & \(41.8\%\) & \(82.4\%\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Number of anomalies, total windows and normalised percentage of anomalies detected in the epicentral area with respect to (w.r.t.) the comparison areas centred in US East Coast and in Europe, using the _Swarm_ satellites. The anomalies have been obtained by considering 1000 days before the California earthquake that occurred on 6 July 2019.  
time histogram distributions of the detected anomalies for the California area, US East Coast and EU regions.