Postseismic quasi-static fault slip due to pore pressure change on a bimaterial interface

Abstract

We theoretically study the mechanism of afterslip, generally observed after the occurrence of large shallow earthquakes, taking account of poroelastic effects including fluid flow. A two-dimensional in-plane shear fault is assumed on a bimaterial interface that separates mechanically different poroelastic media. We first derive analytical expressions for the stress tensor components and fluid pressure as integrals of fault slip; the rigidity and diffusivity of the two media separated by the fault are assumed to be equal in value to derive the solution analytically. We then numerically solve these integral equations assuming stress boundary condition on the fault and obtain the spatiotemporal evolution of fault slip. The Coulomb failure criterion is assumed for the quasi-static growth of fault. We find that the positive feedback between the fluid pressure raised at the extending fault tip and evolving fault slip promotes the quasi-static fault tip extension; the amount of afterslip is found to be larger for smaller value of the Biot-Willis coefficient. It is also found that the poroelastic bimaterial effect favors unilateral extension of fault. Our calculations show that the patch of postseismic fault slip does not overlap that of coseismic slip significantly. We also observe a rapid decrease in postseismic moment release rate with time. These are in harmony with geodetic observation related to afterslips. We will have to resort to purely numerical analysis if we remove the assumptions about the rigidity and diffusivity; however, our present study can provide a reference point for such analysis. Copyright 2007 by the American Geophysical Union.