Feedback processes and fault memory as rulers of seismicity

The simplest model to describe seismic activity assumes that earthquakes occur because of strain accumulation within the brittle crust to be released along weak frictional interfaces, i.e., faults, whose dynamic behaviour can be described by stability conditions, so that large seismic events would occur almost periodically if faults were perfectly smooth and energy dissipation uniform during each earthquake. However, faults are not isolated, but strongly interacting systems and earthquakes are known to take place clustered in time and space at both short and long scales with evident long-lasting memory effects. This evidence encourages us to set up new models of seismic occurrence to consider of such nonlinear phenomena. Two possibilities exist: the first one consists in finding empirical relationships between lithology and frictional properties of faults and hosted seismicity in the framework of classical physics producing ensembles of observations depending on several parameters, while the other tries to encompass all of them by modelling seismic dynamics in the light of few physical laws. We show that the second approach can explain the first order properties of faulting and seismicity as consequences of universal physical mechanisms such as fault memory, positive feedback processes and fractality in dissipative environments. We utilise them to discuss a chain of relationships corroborated by systematic, regional and global analyses between some physical quantities describing fault system structure, tectonic setting and the seismic source and geophysical observables of seismic events and seismic sequences. The advantage of this approach is that it does not require the in-depth knowledge of local geophysical properties of faults (e.g., frictional properties, fluid percolation, fault stress), which are usually poorly constrained at seismogenic depths, to describe seismological observations, being them just passive elements of a complex, self-organized earthquake machine.