Anatomy of seismicity clustering from parametric space-time analysis

A multi-parametric study of the space-time evolution of the seismicity from 2015 to the beginning of 2020 is performed within a well-focused area, located between the Alps and the Prealps, in the central part of the Friuli Venezia Giulia region (Northeastern Italy). The study area is characterized by a complex tectonic pattern resulting from the interference of differently oriented fault systems and involving mechanically heterogeneous rocks. From 2018 to 2019, the area experienced a significant increase and clustering of radiated seismic energy. The estimated damage pattern emphasizes the tectonic complexity. Notably, the most energetic events are located in correspondence with the sharp transitions from zones of low damage to zones of intermediate damage. The evolution of seismicity is analyzed through the temporal variation of the Shannon entropy, b-value, fractal dimension, nearest neighbour distance, and changing orientation of the planes inferred from Principal Component Analysis (PCA). The PCA analysis is applied along a geological cross-section to infer the geometry and the time-evolution of the fracturing. It reveals best-fit planes mainly subvertical. Two distinct temporal phases are recognized, characterized by different orientation of planes and propagation of fracturing. The two phases correspond to the changes in the seismic activity, highlighted by the variations of the entropy, b-value, fractal dimension and nearest neighbour distance. The observed spatio-temporal evolution of seismicity is interpreted within the frame of damage evolution in a heterogeneous medium subjected to an applied remote stress.