Rheology heterogeneities on fault surfaces inferred from the time history of afterslip events

Abstract

A model of shallow afterslip on transcurrent vertical faults is presented. The creeping region is represented with a 2‐D, quasi‐static crack, open to the Earth surface; the fault gouge is treated as a viscoplastic layer with vertically heterogeneous Bingham rheology and a deep earthquake is assumed to generate a two‐level stress distribution on the fault plane: in the deeper level the stress drops to values less than the plastic threshold of the fault gouge while, in the shallower level, the earthquake concentrates stress to values greater than the plastic threshold; in this region aseismic slip takes place. Initially the creeping region is assumed to extend from the Earth surface to the depth of transition between stress levels, but subsequently it penetrates deeper, thanks to the stress concentration created by the crack beyond its tip. Slip at the Earth surface is computed for some stress‐drop distributions and viscosity profiles, and the results are compared with afterslip observations on three California faults. The whole evolution is sensibly affected by the initial stress profile variations, mainly because the crack gradually extends to increasing depths. The initial part of the evolution is also affected by the viscosity profile assumed on the fault plane. In particular the advanced phase of the evolution is not affected by a shallow region with low viscosity on the fault plane. The fast relaxation of this region in fact provides a quick displacement superimposed from the beginning on a slip evolution very similar to that with a uniform viscosity. After removal from the data of this initial displacement, treated as a free parameter since it is not well constrained from data themselves, a suitable choice of the stress parameter values gives a good reproduction of the data in all the cases analysed even if a uniform viscosity on the fault plane is assumed. In all the analysed cases these parameter values are consistent with a temporal deepening of the creeping region, even if the computed displacement at the Earth surface along a direction orthogonal to the fault strike is scarcely sensitive to maximum depth of creep. Copyright © 1994, Wiley Blackwell. All rights reserved