Fault structure-related variability of seismic activity: a result of local geology or universal physical mechanisms?

Seismic activity is the result of progressive strain accumulation in the brittle crust due the action of tectonic processes. Stress increase depends on several physical and geological parameters such as temperature, fluid percolation and tectonic setting. Fault and rock rheology are also recognized to play a role in producing different fault slip modes. Lithology is also believed to be involved in feedback mechanisms connecting fault formation and maturation and energy dissipation. Fault rocks are indeed different from those in the walls, being fault rocks weaker than those in intact volumes producing more and more localized ruptures along prone-to-break interfaces. However, limited seismic catalogs and poorly constrained geophysical data contribute to achieve different and sometimes conflicting results. Moreover, associating even minimal variations in seismicity to local physical quantities and geology has resulted in many empirical relationships linking rheological properties, fluid circulation, state of stress and characteristics of seismic events with sometimes unclear physical significance. Therefore, we wonder whether it would be possible to explain the variability of observed seismological patterns in terms of few universal physical mechanisms also to get testable and statistically significant results, which are difficult to be achieved using geophysical parameters, usually poorly constrained at seismogenic depths. Also using high-resolution machine-learning-based earthquake catalogs to prove our conclusions, we show that some statistical properties of seismicity such as the b-value of the Gutenberg-Richter law and earthquake clustering can be explained by few universal physical mechanisms such as fractality, positive feedback and fault memory of previous earthquakes. We demonstrate that a chain of connections exists between spatial and fractal organization of fault systems, frequency-size statistics of seismic events, earthquake clustering, seismogenic potential, tectonic settings, earthquake moment tensor and fault rheology.