TESTS OF MODELS OF AFTERSHOCK RATE DECAY

Six rate decay functions have been fit to 14 aftershock sequences, mos tly from southern California. The best fit was usually by the modified Omori function, but five of the sequences were best fit by new varian ts of the stretched exponential model, and three more were best fit by the modified Omori with a steady background rate added. Background ra te terms in the modified Omori model never improved the fit to sequenc es best fit by exponential models. The variety of decay laws observed can be explained if the underlying aftershock generation process follo ws stretched exponential relations with spatial inhomogeneities in q v alue. Tests of a large number of synthetic sequences show that if diff erent parts of a fault zone produce aftershocks decaying with stretche d exponentials having a wide variety of q values, the combined sequenc e will be best fit by a variant of the modified Omori relation. The se quences best fit by modified exponential may be associated with more h omogeneous material properties or simpler geometry.