## 2 Materials and Methods
### Earthquake Database  
The second main data source is composed of the earthquakes database provided by the USGS containing the magnitude, UTC time, depth, and location of the epicenter, among other parameters. It is important to remark that the magnitude is a complex variable to describe, as there are different methods of computation and scales, which depend on the region of the Earth where earthquakes are studied, the composition of the lithosphere, or the measuring technique used. Originally, the Richter scale (M\({}_{\rm I}\)) was developed for earthquakes in the California region by measuring the frequency of the seismic waves received at a seismographic station. Later on, when trying to use the same scale for other regions, it was found that the Richter method was not valid for all frequencies and distance ranges, in particular for larger magnitude earthquakes. Other methods to measure the magnitude were developed, designed to match the Richter scale in their range of validity [28; 29]. The m\({}_{\rm b}\), using the body wave, and the M\({}_{\rm S}\), using the surface wave are two extensions of the Richter magnitude, but they were not valid for larger earthquakes. The moment magnitude scale M\({}_{\rm W}\) (usually noted as m\({}_{\rm\_}\)ww) is the one with a wider range of application, extending above M5, and in particular, for large earthquakes (M8 and greater events) is the most accurate one. In the USGS database, each earthquake in the period studied is given by a single magnitude type [30], but, as stated before, they are all designed to be consistent. This is why, in our study, the magnitude provided is used for all the earthquakes, without taking into account the magnitude type. Table 1 shows the relative abundance of each magnitude type in the period studied:  
Figure 3 shows the distribution of all earthquakes in the period as a function of their depth and magnitude, and showing the magnitude type with different symbols and colors. Figure 4 shows the position of all the earthquakes occurring in the six months studied, from March to August 2019. Please note that the color scale and size of the circles represent their magnitudes. It is important to remark that most of them happen in the middle of the oceans, near the faults between tectonic plates. With the method presented here, only these earthquakes in oceanic regions or near the shore can be monitored, because the land \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Magnitude Type** & **Description [30; 31]** & **Counts** & **Percentage** \\ \hline mb & Short-period body wave (\(m_{\rm b}\)) & 5268 & 86.25\% \\ mww & Moment W-phase (\(M_{\rm W}\)) & 516 & 8.45\% \\ mwr & Moment magnitude (regional) & 127 & 2.08\% \\ mlr & Revised local (Richter) magnitude & 83 & 1.36\% \\ mw & Moment magnitude & 66 & 1.08\% \\ ml & Richter (local) magnitude (\(M_{\rm I}\)) & 43 & 0.70\% \\ md & Duration magnitude & 4 & 0.07\% \\ mb\({}_{\rm\_}\)lg & Short-period surface wave & 1 & 0.02\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Magnitude types found in the USGS earthquakes database, indicating the counts and relative abundance for each of them. Figure 2: Temporal evolution of the region under study after applying the filters (LT and quality), and the detrending described in this section. The graph shows the latitudes covered at least by one pixel (gray), and the interquartile (green), and median (blue line) of these latitudes. The average period of this evolution is 50 days. reflectivity is highly variable, and the possible ionospheric scintillation would become masked. For example, the largest earthquake in the period studied, with a magnitude of 8.0, on 26 May in Peru, was located on land, 500 km away from the closest seashore, which is larger than the maximum distance considered in this study. The total number of earthquakes that took place in the period studied is 6108, and 29.4% of them are located on land, but many of them could happen on small islands or very near the shore so they are included.