We propose a ``multifractal stress activation'' model combining thermally
activated rupture and long memory stress relaxation, which predicts that
seismic decay rates after mainshocks follow the Omori law ∼1/tp with
exponents p linearly increasing with the magnitude ML of the mainshock and
the inverse temperature. We carefully test this prediction on earthquake
sequences in the Southern California Earthquake catalog: we find power law
relaxations of seismic sequences triggered by mainshocks with exponents p
increasing with the mainshock magnitude by approximately 0.1−0.15 for each
magnitude unit increase, from p(ML=3)≈0.6 to p(ML=7)≈1.1,
in good agreement with the prediction of the multifractal model.Comment: four pages and 2 figure