Postseismic relaxation driven by brittle creep: A possible mechanism to reconcile geodetic measurements and the decay rate of aftershocks, application to the Chi-Chi earthquake, Taiwan
Article first published online: 3 FEB 2004
Copyright 2004 by the American Geophysical Union.
Journal of Geophysical Research: Solid Earth (1978–2012)
Volume 109, Issue B2, February 2004
How to Cite
2004), Postseismic relaxation driven by brittle creep: A possible mechanism to reconcile geodetic measurements and the decay rate of aftershocks, application to the Chi-Chi earthquake, Taiwan, J. Geophys. Res., 109, B02304, doi:10.1029/2003JB002488., and (
- Issue published online: 3 FEB 2004
- Article first published online: 3 FEB 2004
- Manuscript Accepted: 31 OCT 2003
- Manuscript Revised: 25 SEP 2003
- Manuscript Received: 10 MAR 2003
- postseismic relaxation;
- Omori law;
- Chi-Chi earthquake
 We evaluate the effect of coseismic stress changes on the fault slip at midcrustal depth, assuming a velocity-strengthening brittle creep rheology. We show that this model can help reconcile the time evolution of afterslip, as measured from geodesy, with aftershocks decay. We propose an analytical expression for slip of the brittle creeping fault zone (BCFZ) that applies to any dynamic or static stress perturbation, including shear stress and normal stress changes. The model predicts an initial logarithmic increase of slip with time. Postseismic slip rate decays over a characteristic time tr = that does not depend on the amplitude of the stress perturbation, and it asymptotically joins the long-term creep imposed by interseismic stress buildup . Given that the seismicity rate might be considered proportional to the sliding velocity of the BCFZ, the model predicts a decay rate of aftershocks that follows Omori's law, with a mathematical formalism identical to that of Dieterich  although based on a different mechanical rationale. Our model also differs from Dieterich's model in that it requires that aftershock sequences and deep afterslip, as constrained from geodetic measurements, should follow the same temporal evolution. We test this for the 1999 Chi-Chi earthquake, Mw = 7.6 and find that both sets of data are consistent with a model of afterslip due to the response of the BCFZ. The inferred relaxation time tr = 8.5 years requires a value for a = ∂μ/∂log(V) (μ being the coefficient of friction) in the range between 1.3 10−3 and 10−2.