Stress loading from viscous flow in the lower crust and triggering of aftershocks following the 1994 Northridge California, earthquake

Following the M-w 6.7 Northridge earthquake, significant postseismic displa cements were resolved with GPS. Using a three-dimensional viscoelastic mode l, we suggest that this deformation is mainly driven by viscous flow in the lower crust. Such flow can transfer stress to the upper crust and load the rupture zone of the main shock at a decaying rate. Most aftershocks within the rupture zone, especially those that occurred after the first several w eeks of the main shock, may have been triggered by continuous stress loadin g from viscous flow. The long-term decay time of aftershocks (about 2 years ) approximately matches the decay of viscoelastic loading, and thus is cont rolled by the viscosity of the lower crust. Our model provides a physical i nterpretation of the observed correlation between aftershock decay rate and surface heat flow.