Universal physical mechanisms behind the differences between fault structure-related variability of seismicity

Seismicity showcases wide variability in its spatio-temporal occurrence and size. Thanks to the recent advances in the amount and quality of seismic catalogues due to machine learning techniques, nowadays it is possible to assess several physical and statistical quantities with unprecedented detail. At the same time, previously undetected spatial and temporal changes can be highlighted, which need explanation. In fact, they turn out to be not fully compatible with variations of stress and rheology at depth as suggested by models developed at time when observational resolution was weaker. Here, we suggest explaining the observations of different b-value, fractal dimension, clustering and temporal evolution of seismicity within the same tectonic setting using a few physical laws. We think that, even though rheology and stress gradients certainly play a role in shaping seismicity, most of the observed differences between “in-fault” and “distributed” seismic activity, i.e., earthquakes occurring along major faults vs clustered within localized crustal volumes with dense fracturing, can be explained by universal properties and self-organization of the seismic process via simple feedback mechanisms between the structure of the seismogenic source and energy dissipation within crustal volumes. We consider recent seismicity (2016-2017) in Central Italy as a case study.