After the main shock of an earthquake, a stream of aftershocks that does not
subside for a long time is usually observed. Fusakichi Omori found that the
frequency of aftershocks decreases hyperbolically with time. It has recently
been observed that Omori's law can be viewed as a solution to a differential
equation describing the evolution of aftershocks. An alternative way of
describing is based on Utsu law, which states that the frequency of aftershocks
decreases with time according to a power law. The presented paper is polemical.
We discuss the issue of the applicability of each of the three alternative ways
of describing aftershocks. The Omori law has a limited scope. The law is valid
only in the so-called Omori epoch, after which the earthquake source undergoes
a bifurcation. In the Omori epoch, the Utsu law is also valid, but it does not
differ in this epoch from the Omori law. The general conclusion is that the
existence of the Omori epoch and the phenomenon of bifurcation exclude the
possibility of describing by a continuous smooth function. At the same time,
the differential evolution equation is applicable both before and after the
bifurcation point. Key words: earthquake, main shock, evolution equation,
deactivation factor, Omori epoch, bifurcation.Comment: 7 pages, 1 figur