Aftershocks are the small tremors that follow shortly after a major earthquake occurs. It is an important nuance, since if the intensity of one is greater than that of the initial earthquake, this is considered a premonitory earthquake.
They appear because the earth's crust is adapting in the vicinity of the fault where the first earthquake originated. Their number varies enormously and can exceed a hundred; For example, in the 24 hours after the Alaska earthquake on November 30, 2018, there were 550.
The frequency of aftershocks declines as time goes by, although they tend to last for several days.
In some cases, they can continue for years, as happened with the New Madrid (USA) earthquake of 1811: they lasted for almost two centuries.
Since 1894, such a thing can be calculated thanks to Omori's law, named after its discoverer, the Japanese seismologist Fusakichi Omori: whatever the chances of an aftershock occurring on the first day, the second will be cut in half. ; to the third, to the third part … Now, the quantity, the location or the exact moment in which the aftershocks will take place is unpredictable.
