  The decay rate of aftershocks has been modeled as a power law since the
pioneering work of Omori in the late nineteenth century. Considered the second
most fundamental empirical law after the Gutenberg-Richter relationship, the
power law paradigm has rarely been challenged by the seismological community.
By taking a view of aftershock research not biased by prior conceptions of
Omori power law decay and by applying statistical methods recommended in
applied mathematics, I show that all aftershock sequences tested in three
regional earthquake catalogs (Southern and Northern California, Taiwan) and
with three declustering techniques (nearest-neighbor, second-order moment,
window methods) follow a stretched exponential instead of a power law. These
results infer that aftershocks are due to a simpler relaxation process than
originally thought, in accordance with most other relaxation processes observed
in Nature.
