> looking for precursor patterns in an ad hoc way, with no physical theory in mind
A bit of skepticism is healthy but some of these threads are approaching downright dismissive.
There's a long history of research in this area to be aware of, though I know the paper in question doesn't elaborate on it.
Here's one physical theory for you, straight from NASA [1].
> In the ionosphere, the solar winds generate electrical currents. On the Earth surface, these currents cause magnetic field fluctuations. These fluctuations, penetrating the Earth interior, induce the electrical currents J, and, in the presence of the Earth magnetic field B, generate electromagnetic force, known as Lorentz force F = J × B. To study the relation of earthquakes and the Lorentz force, acting at the near onset times of strong earthquakes, we examine the Kp index, a logarithmic measure of the magnetic field deviation. The time varying Kp index gives us J, which in turn determines F.
> The Lorentz force tilts the subtle force balance in the earth crust towards triggering the release of stress strain energy, initiating an earthquake in a similar way as a mountain climber’s step can trigger the avalanches. The internal dynamics, however, are highly statistical.
> We find that the distinctive patterns of the Kp surges often strongly correlate to the onset of earthquake. This correlation depends on the seismic regions and the magnitudes of earthquakes. The stronger the earthquake is, more closely the Kp surge is associated. The statistical significance of nearly 100% is obtained for the Kp variations, synchronizing with more earthquakes in the Pacific Rim region.
[1] Geomagnetic Kp Index and Earthquakes (2018)
https://www.scirp.org/journal/paperinformation.aspx?paperid=...