How Long Do Aftershocks Last?
How Long Do Aftershocks Last? Wednesday November 4, 2009 The classroom treatment of aftershocks is fairly simple: An earthquake hits, and then a series of aftershocks occurs that is well summarized by three mathematical expressions called the Gutenberg-Richter relation, Bath’s law and Omori’s law. The first two relate the aftershocks to the mainshock in terms of number and size (G-R says their numbers rise by an order of magnitude with each unit fall in seismic magnitude, and Bath says the largest aftershock averages about 1.2 magnitude units smaller). Omori’s law describes how aftershocks tail off to a steady background rate as the reciprocal of time (that is, 10 years later aftershocks occur at 1/10 the rate). The great majority of earthquakes and their aftershocks are easy to deal with because they occur near plate boundaries. There the seismic background rate is high and aftershock sequences therefore are short, around 10 years (because the background noise swamps their long tails)
