  The earthquake source after the main shock can theoretically be represented
as a black box without an entrance. At the output, there is a signal in the
form of aftershocks, the frequency of which decreases on average with time
according to the Omori law. The task of the researcher is to evaluate the
structure, state and mechanism of functioning of a dynamic system simulating
the earthquake source based on the output signal. In this paper, we outline an
approach to a partial solution of this general problem. Omori's law is
presented as a differential equation of aftershock evolution. An inverse
problem has been posed and solved, the essence of which is to determine the
source deactivation coefficient from the observed frequency of aftershocks. The
existence of the so-called Omori epoch, during which the deactivation
coefficient remains constant, has been discovered. At the end of the Omori
epoch, the deactivation coefficient experiences complex variations. A
hypothesis has been put forward that the end of the Omori epoch indicates a
bifurcation, i.e. about the transition of the source from one state to a
qualitatively different state. Our paper is written for the 100th anniversary
of the death of Fusakichi Omori. Keywords: aftershocks, Omori law, evolution
equation, deactivation coefficient, inverse problem, Omori epoch, bifurcation
point.
