Articles | Volume 23, issue 6
Nonlin. Processes Geophys., 23, 419–433, 2016
https://doi.org/10.5194/npg-23-419-2016
Nonlin. Processes Geophys., 23, 419–433, 2016
https://doi.org/10.5194/npg-23-419-2016

Research article 16 Nov 2016

Research article | 16 Nov 2016

Conditions for the occurrence of seismic sequences in a fault system

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

Abstract. We consider a fault system producing a sequence of seismic events of similar magnitudes. If the system is made up of n faults, there are n! possible sequences, differing from each other for the order of fault activation. Therefore the order of events in a sequence can be expressed as a permutation of the first n integers. We investigate the conditions for the occurrence of a seismic sequence and how the order of events is related to the initial stress state of the fault system. To this aim, we consider n coplanar faults placed in an elastic half-space and subject to a constant and uniform strain rate by tectonic motions. We describe the state of the system by n variables that are the Coulomb stresses of the faults. If we order the faults according to the magnitude of their Coulomb stresses, a permutation of the first n integers can be associated with each state of the system. This permutation changes whenever a fault produces a seismic event, so that the evolution of the system can be described as a sequence of permutations. A crucial role is played by the differences between Coulomb stresses of the faults. The order of events implicit in the initial state is modified due to changes in the differences between Coulomb stresses and to different stress drops of the events. We find that the order of events is determined by the initial stress state, the stress drops and the stress transfers associated with each event. Therefore the model allows the retrieval of the stress states of a fault system from the observation of the order of fault activation in a seismic sequence. As an example, the model is applied to the 2012 Emilia (Italy) seismic sequence and enlightens the complex interplay between the fault dislocations that produced the observed order of events.

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Short summary
We devise a model of a fault system and study the conditions under which it can generate a sequence of seismic events. We also point out the conditions determining the order of the events in a sequence. A significant conclusion of our analysis is that the features of seismic sequences originated by a given fault system (order of events, duration of the sequence, duration of interseismic intervals) typically change from one sequence to another, due to the complexity of fault interaction.