Time-dependent changes in failure stress following thrust earthquakes
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Two-dimensional (2-D) viscoelastic finite element models were used to calculate the time-dependent changes in Coulomb failure stresses following thrust earthquakes due to respective effects of relaxation of viscous lower crust or upper mantle and postseismic creep on the main fault or its downdip extension. Results suggest that thrust earthquakes cause a coseismic increase in Coulomb stress along antithetic lobes normal to the slip plane. Following a quake, creep processes that reduce stresses in a ductile lower crust or upper mantle are calculated to cause a transfer of stress to the upper crust. Under certain conditions, transfer of stress may lead to a further buildup of high Coulomb stress along the base of the upper crust, potentially shortening the time to failure of other faults in the region. The conditions under which an antithetic lobe of high Coulomb stress are favored to expand at the base of the upper crust postseismically within a few decades include the following: the lower crust or upper mantle has an effective viscosity not greater than 10(19) Pa s; the thrust fault has a moderate dip angle (40 degrees-50 degrees); the brittle/ductile transition is deep enough to provide a corridor at the base of the upper crust for expansion; and the crust has a low apparent coefficient of friction (<0.2). Postseismic increases in Coulomb stress within the upper crust may also be caused by aseismic creep on the fault. Stress changes due to this mechanism are maximized with a high apparent coefficient of friction. Analysis of experimentally determined non-Newtonian flow laws suggests that wet granitic, quartz, and feldspar aggregates mat have a viscosity of the order of 10(19) Pa s. The calculated rate of stress transfer from a viscous lower crust or upper mantle to the upper crust becomes faster with increasing values of the power law exponent and the presence of a regional compressive strain rate. Results of this 2-D analysis suggest a potentially important role of viscous flow in controlling time-dependent postseismic stress changes that warrant further investigation using 3-D viscoelastic analysis.