Articles | Volume 22, issue 3
Nonlin. Processes Geophys., 22, 349–359, 2015
https://doi.org/10.5194/npg-22-349-2015
Nonlin. Processes Geophys., 22, 349–359, 2015
https://doi.org/10.5194/npg-22-349-2015

Research article 30 Jun 2015

Research article | 30 Jun 2015

Stress states and moment rates of a two-asperity fault in the presence of viscoelastic relaxation

M. Dragoni and E. Lorenzano M. Dragoni and E. Lorenzano
  • Dipartimento di Fisica e Astronomia, Alma Mater Studiorum Università di Bologna, Viale Carlo Berti Pichat 8, 40127 Bologna, Italy

Abstract. A fault containing two asperities with different strengths is considered. The fault is embedded in a shear zone subject to a constant strain rate by the motions of adjacent tectonic plates. The fault is modelled as a discrete dynamical system where the average values of stress, friction and slip on each asperity are considered. The state of the fault is described by three variables: the slip deficits of the asperities and the viscoelastic deformation. The system has four dynamic modes, for which analytical solutions are calculated. The relationship between the state of the fault before a seismic event and the sequence of slipping modes in the event is enlightened. Since the moment rate depends on the number and sequence of slipping modes, the knowledge of the source function of an earthquake constrains the orbit of the system in the phase space. If the source functions of a larger number of consecutive earthquakes were known, the orbit could be constrained more and more and its evolution could be predicted with a smaller uncertainty. The model is applied to the 1964 Alaska earthquake, which was the effect of the failure of two asperities and for which a remarkable post-seismic relaxation has been observed in the subsequent decades. The evolution of the system after the 1964 event depends on the state from which the event was originated, that is constrained by the observed moment rate. The possible durations of the interseismic interval and the possible moment rates of the next earthquake are calculated as functions of the initial state.

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Short summary
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.