  It is useful to consider the earthquakes in terms of catastrophe theory. In
the paper, we illustrate this statement by analysis foreshocks preceding the
strong earthquakes. We focused on the so-called catastrophe flags, and on the
triggers that cause the critical transitions. The analysis shows a sharp
increase of foreshock activity 3 h before the mainshock. This result is
reminiscent of the well-known activation of the ULF magnetic precursors of
earthquakes. Furthermore, we found that the characteristic frequency of
foreshock sequence decreases a few hours before the mainshock, which is
consistent with a prediction of the catastrophe theory. Finally, the analysis
testifies that the strong foreshocks seems may be triggers of mainshocks. The
idea is that the surface waves propagating outwards from the foreshock return
back to the vicinity of the epicenter after having made a complete revolution
around the Earth and induce there the mainshock.
