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

Abstract

Abstract. A fault containing two asperities with different strengths is considered. The fault is embedded in a viscoelastic 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 the 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 postseismic 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.

Complex interplay between stress perturbations and viscoelastic relaxation in a two-asperity fault model

“…We propose a means to estimate the stress transfer from the knowledge of the relative positions and faulting styles of the two faults. As a further novelty with respect to 10 the work presented by Dragoni and Lorenzano (2015), we show how the knowledge of the time interval elapsed after the 1999 earthquake can be used to constrain the admissible set of states that may have given rise to the 1992 event. We discuss the possible subsequent evolution of the Landers fault after the stress transfer from the Hector Mine earthquake, pointing out the main differences with respect to an unperturbed scenario.…”

“…The specific location of P 0 inside the sticking region allows the prediction of the first slipping mode involved in the next seismic event on the fault. In fact, Dragoni and Lorenzano (2015) illustrated the existence of a transcendental surface Σ within H, expressed by the equation…”

“…The coordinates of P 4 can be calculated from the solution to the equations of mode 00 given by Dragoni and Lorenzano (2015) and taking into account that the time intervalt elapsed between the 1992 Landers and 1999 Hector Mine earthquakes amounts to about 7.3 years: we get ∆F C −0.069. Since ∆F C < 0, we conclude that the stress perturbation is such that: states P 4 ∈ H 1 are moved to H 1 ; the state P 4 ∈ Σ enters H 1 ; states P 4 ∈ H 2 are shifted towards the Σ surface and some of them enter H 1 .…”