Earthquake scaling: the effect of a viscoelastic asthenosphere

Viscous creep in the crust and mantle can have important consequences for earthquake patterns in time and space. Here, numerical models of repeating earthquake rupture are used to investigate how viscoelasticity can affect size distributions of repeated earthquakes on vertical, planar fault segments. The earthquake model is based on exact solutions of static 3-D elasticity theory. Viscous flow beneath the seismogenic crust is modelled as a Maxwell relaxation process. Two different fault strength models are used: a smooth rate- and state-dependent friction model and a strongly heterogeneous asperity model. In a previous paper it was shown that the characteristic scale of fault segmentation is proportional to the vertical width of a seismogenic fault. For both the heterogeneous and smooth friction models, viscous relaxation of the substrate modifies the spatio-temporal distribution of cumulative slip and slip events. For the heterogeneous models, the resulting quake size distributions are independent of the viscous relaxation time. In contrast, for the smooth models, it is shown that the characteristic event size is inversely proportional to the relaxation time of the substrate. These results imply a connection between the spatial complexity of fault zones and the maximum earthquake size a fault can sustain. Whereas earthquake sizes on complex fault zones depend only weakly on the viscous rheology beneath the seismogenic crust, the sizes of large events on relatively simple faults can be substantially increased due to viscous relaxation and partial decoupling of the seismogenic crust from the deeper crust and mantle.