A model of interseismic fault slip in the presence of asperities

Abstract

SUMMARY A 2-D model which represents a slipping fault with non-uniform Coulomb friction is studied. The fault plane is subject to a uniform ambient shear stress, slowly increasing with time. Aseismic fault creep is assumed to start in a weak zone, when the ambient stress reaches a strength threshold. The solution for the resulting dislocation is worked out analytically using a technique based on Chebyshev polynomials. It is found that the dislocation partially propagates into the adjacent asperities, concentrating stress onto them and preparing the conditions which will produce the asperity failure and the accompanying earthquake. Propagation is not self-similar and occurs at increasing velocity. A non-linear slip hardening effect is reproduced. The nearness to earthquake instability is measured by a dimensionless parameter which depends on Coulomb friction and ambient shear stress and decreases to zero with time. An upper boundary to the critical value of this parameter, at which instability may occur, is estimated and is found to depend on the ratio between the sizes of the asperity and the weak zone.