## 4 Discussion  
Tidal triggering of earthquakes is still a debated theme in Solid Earth Geophysics. In particular, there are three sources of discussion among geophysicists:  
* From a statistical viewpoint, earthquake catalogs are often insufficient to detect significant modulations of seismic activity over time. Tides are tiny perturbations of the gravitational field (0.1-100 kPa) with respect to typical earthquake stress drops (1-50 MPa), but nonetheless they are able to generate significant stress variation rates (\(\sim\)100 mbar/day compared with 1-10 mbar/day due to tectonic stress, [39]). However, the actual impact on the stability of rock volumes largely depends on the tectonic setting, the spatial orientation of the fault, the depth, and the hypocentral latitude; finally, also the magnitude of the impending event modifies the response of the system to the tidal perturbation. Therefore, a wide range of results were found in several geographical regions. * It is sometimes difficult to distinguish between the effects of Solid Earth tides from those of ocean tides. Even though the stress amplitude of liquid tides (up to 100 kPa) often exceeds that of the Solid Earth tide (usually \(\sim\)0.1-1 kPa), the mechanism of action is quite different. In the first case, tides are concentrated on limited surfaces, at most of the order \(10^{5}\) km\({}^{2}\), and act mainly through the \(\sigma_{rr}\) vertical component, which is transmitted to the horizontal components (almost symmetrically) thanks to the elastic properties of the lithosphere. However, the incremental stress decreases exponentially as depth increases, therefore it can strongly modulate shallow oceanic small magnitude seismicity, for example, at ocean ridges or submarine volcanoes, but it is unlikely that intermediate and deep earthquakes might be triggered by liquid tides. Solid tides, on the other hand, deform the outer layers of the planet mainly in the horizontal components, acting on very large surfaces. For this reason, solid tides have a dominant role in triggering earthquakes except for the just cited peculiar cases. For these reasons, liquid tides are neglected in this work. * Seismic response to tidal loading strongly depends on the duration of earthquake nucleation. Beyond the aforementioned issues, well-established scientific evidence exists about tidal synchronization in seismic catalogs, as already discussed in the introduction. Both global and regional seismic series show semiannual, annual, biennial, with approximately 9-, 19-, 37-, and 56-years-long periods activity modulations. While the first three frequencies are generally associated with seasonal patterns, the others have no explanation other than lunisolar tidal loading. The FFT of European magnitudes (SHEEC 2020 catalogue) in Figure 6 clearly attests this phenomenon. Since M\({}_{\rm c}\approx 6\) for the SHEEC catalogue, the nonuniform FFT is applied to include at least M\({}_{\rm midf}>5.0\). Instrumental recordings are rarely available before the 1950s also for violent earthquakes, and therefore, macroseismic intensity data are widely used combined with epicentral macroseismic intensities and other parametric data sources. Therefore, parametric catalogs must be used with caution and results must be interpreted according to their reliability. Even if an accurate analysis cannot be performed for the aforementioned reasons, the power spectrum of the European seismicity shows typical tidal frequencies such as 8-10- or 18-19-years-long periodicities and some multiples. Moreover, local seismic rates are noticed to be directly correlated with the phase of tidal shear stress or the Coulomb failure stress change in submarine volcanic seismicity [31] and seismic tremors [6]. Finally, it is reasonable to expect that the triggering power of tides is affected by the following variables:  
* The critical stress needed for crack propagation depends on the tectonic setting.