We consider a model of a fault containing two regions (asperities) whose slip is associated with earthquakes on the fault. The stress field generated by seismic events is relaxed in the following post-seismic interval, due to the properties of rocks in the upper mantle. The sequence of asperity slips in an earthquake is controlled by several elements of the model, e.g. the intensity of seismic waves radiation, the coupling between the asperities and the difference in their frictional strengths.

We devise a model to investigate the interplay between two common phenomena affecting the evolution of a seismogenic fault: the occurrence of earthquakes on neighbouring faults and the partial degree of anelasticity of rocks in the upper mantle, a feature that is often manifested by a post-seismic process known as viscoelastic relaxation. We show how this process dramatically complicates the way an earthquake may alter the magnitude and timing of a seismic event on the perturbed fault.

The paper presents new analytical solutions for both the coseismic slip and the interseismic evolution of a fault with two asperities of different strengths. It enlightens the relationship between the state of the fault before a seismic event and the number and sequence of slipping modes in the event. It shows that the knowledge of the source function of a seismic event constrains the subsequent evolution of the system. The model is applied to the fault that generated the 1964 Alaska earthquake.