**2. Data and Methods**

The ground magnetometers used for the present analysis come from both INTER-MAGNET and SUPERMAG magnetometer array networks, which are consortium of observatories guaranteeing a common standard data release to the scientific community, leading to possible comparison among measurements at different observation points. In our analysis we have used 1 s time resolution data.

To evaluate the *f* ∗, we studied the cross-space spectrum [17] between the North-South magnetic field components observed at two geomagnetic observatories close enough to the EQ epicenter location (see Table 1). It is well known that, at the eigenfrequency of a field line centered between two neighboring stations having almost the same magnetic longitude, the phase difference maximizes [17,18]. Waters et al. [19] showed that the patterns of the maximum phase differences in the cross-phase spectrograms were observed consistently from day to day in the dayside region over baselines of about 100 km in the magnetic meridian. Green et al. [18] also reported that, among the several methods that determine the resonant frequency, the phase shift is least affected by geologic inhomogeneity and consistently defines the resonant frequencies.


