The Role of Viscoelastic Relaxation in Triggering of Aftershocks, Inferences from two Californian Earthquakes

A number of studies indicate that a first order causal relationship exists between the location of aftershocks and the areas that have received positive coseismic static stress changes following a major earthquake. Some studies have also argued the influence of dynamic stress triggering on aftershocks for a short period of time (up to several weeks). Aftershocks, however, typically last several years, which may indicate the time dependent role of postseismic processes. In particular, the contribution of viscoelastic (VE) relaxation of the ductile lower crust and upper mantle to the aftershock triggering has not been studied in detail. Here we investigate the contribution of VE relaxation process for two Californian earthquakes, the 1992 Landers and 1994 Northridge events, where best possible crustal models and aftershock data exist. They also represent different tectonic regimes in which the stress transfer to the upper crust due to the relaxation processes may differ, and hence the VE contribution may be different. In our analysis the aftershocks are binned in 30-day periods and both coseismic and postseismic stress changes are resolved on the nodal planes of the focal mechanisms of aftershocks in order to see which model does better with time. For Landers, our preliminary results show that the model with total stress changes (coseismic + VE) explains the aftershock triggering about 5 percent better than coseismic alone at the start of the aftershock period, and this improvement increases over time. For Northridge, the VE contribution becomes only significant after 5 months following the mainshock.