The Omori Epoch: On the 100th Anniversary of the Death of a Famous Japanese Seismologist

One hundred years ago, Fusakichi Omori died. Our paper is dedicated to his memory. Omori made an outstanding contribution to the physics of earthquakes. In 1894 he formulated the law of aftershock evolution. Omori’s Law states that after the main shock of an earthquake, the frequency of aftershocks decreases hyperbolically with time. In this paper, we briefly describe one of the directions of modern aftershock research. We present Omori’s law in the form of a differential equation describing the evolution of aftershocks. The evolution equation allows us to solve the inverse problem of the earthquake source “cooling down” after the main shock. The solution of the inverse problem allowed us to reveal the existence of the so-called Omori epoch. It is established that at the end of the Omori epoch something similar to a bifurcation of the source state occurs. A logistic equation supposedly describing the bifurcation phenomenon is proposed.