  The analysis of the classical and mirror triads of the sequence of
earthquakes has been carried out in order to find the equations of evolution of
foreshocks and aftershocks. The differential equation with cubic (quadratic)
nonlinearity has been proposed to describe the activity of foreshocks
(aftershocks). The solution of the governing equation for foreshocks indicates
the explosive instability of rocks, leading to the main rupture and the main
shock of the earthquake. It is suggested that explosive instability leads to a
first-order phase transition, in which the main parameters of the earthquake
source as a thermodynamic system change abruptly. A direction for further
research has been outlined. Keywords: Omori law, evolution equation, earthquake
triads, foreshocks, main shock, aftershocks , rise and fall in seismic
activity.
