## 3 Results  
Figure 1 reports the analysis performed by the MASS algorithm of the magnetic data from _Swarm_ Bravo track 5 acquired on 6 July 2019. The figure shows a map (in panel e) with the Earth's surface projection of the satellite track; the colour is related to Flags: brown when the VFM instrument and satellite are in nominal condition, or light blue when Flag_attitude is equal to 18 (and the others are nominal). We note that this track preceded the earthquake occurrence by about 15 min. It presents two highlighted behaviours in the Y magnetic field component (panel b), underlined by a red circle and an orange one. The red circled anomaly is closer to the latitude of the earthquake, and it is entirely inside the [PERSON]'s area (yellow circle on the map). The geomagnetic field conditions during this time were sufficiently quiet (Dst = 2 nT, a\({}_{p}\) = 4 nT, AE without particular activity during and in the hours before--as well as the Dst and a\({}_{p}\)--the passage of the satellite above the investigated region, so excluding any possible penetrating electric field from the auroral regions). All the samples are acquired with Flags that indicate data of good quality for science, as indicated by ESA [21]. As marked in Figure 1, the flagged sections of the track (in light blue in panel e) are due to a bright object (e.g., the Sun) in one of the three-star cameras. The other two cameras were nominal, and no other issues were detected on this track, so we confirm that all the samples can be considered good for the purpose of the present paper (as two cameras are more than sufficient to properly rotate the data from the instrument to NEC frame). The _Swarm_ Alpha, Bravo and Charlie Y magnetic field component data have been systematically analysed. Here we have enough data to extend back the analysis until 1000 days before the earthquake, i.e., from 10 October 2016 to 6 July 2019. As significant concentrations of anomalies were found in the closest 3.34 degrees from the epicentre of the earthquakes by [PERSON] et al. (2015), we decided to select a circular area of the same extension around the California earthquake. We applied the MASS algorithm, and for the analysis, we selected a threshold of \(\mathrm{k_{t}}=2.5\) within the sliding window of the 7-degree length in latitude. Figures 2 and 3 show the cumulative number of _Swarm_ Alpha, Bravo and Charlie anomalies in a circular area of 3.34 degrees around the Mw = 7.1 California 2019 epicentre (blue line) compared with other validation areas in the US East Coast and Europe (EU), respectively, at the same magnetic latitude (red line) and with the same extension centred on geographic coordinates 32.92\({}^{\circ}\) N, 82.5\({}^{\circ}\) W and 39.45\({}^{\circ}\) N, 3.30\({}^{\circ}\) W, respectively. The comparison regions have been chosen on a similar context (i.e., above continental areas and so excluding fully oceanic ones) in order to compare potential similarities surely not due to California earthquake. We checked that in both comparison areas, and United States Geological Survey (USGS) reports no M4.5+ earthquakes during the analysed time (i.e., from 10 October 2016 to 6 July 2019). The cumulative number of anomalies around the earthquake epicentre presents several changes of slope that underline probably a particular geomagnetic activity, despite the fact that the data are selected only in quiet geomagnetic field time. Indeed, most of these behaviours happened also in the comparison areas, in some cases with some delay. In particular, the part of the cumulates between \(-600\) and \(-500\) days presents a very similar behaviour in all the considered areas (and an even steeper increase in the US East Coast around 550 days before the earthquake), pointing to a global effect affecting all the analyses.