Anatomy of seismicity clustering from parametric space-time analysis

A multi-parametric approach, based on five different parameters quantifying seismicity, is proposed for investigating the space-time evolution of earthquakes occurrence in areas characterized by complex tectonics, namely by the interference of differently oriented faults and by the heterogeneous mechanical strength of the rocks. Specifically, the variations of entropy, the b-value from the Gutenberg-Richter law, the changes in fractal dimension, and the Nearest Neighbour distance (η) are used for assessing changes in the temporal patterns of seismicity. In parallel, the Principal Component Analysis (PCA) in 4D (space and time) is used to define the hypocentres distribution geometry and the propagation directions.In particular, we applied the methods mentioned above in a multi-parametric study of the seismicity space-time evolution from 2015 to the beginning of 2020 in a well-focused area. The study area, centred on the town of Tolmezzo, in Northeastern Italy, between the Alps and the Prealps, is characterized by a complex tectonic pattern resulting from the interference of differently oriented fault systems and involving mechanically heterogeneous rocks. After a long period of low seismic activity, lasting about 15 years, in 2018-2019, the area experienced a significant increase of radiated seismic energy, spatially clustered, with four sequences induced by earthquakes with MD (coda-duration magnitude) from 3.7 to 4.0 (http://www.crs.inogs.it/bollettino/RSFVG). Notably, the most energetic events are located in correspondence with the sharp transitions from zones of low damage to zones of intermediate damage. Two distinct periods of the seismic activity are identified, as revealed by the b-value and the fractal dimension, which show relevant fluctuations since the beginning of 2017. The temporal variation of the b-value can be related to crustal stress changes in the medium, which is characterized by different mechanical properties. The fractal dimension time evolution indicates a prevailing clustering of the earthquakes with a tendency to propagate linearly. The temporal variations of the Shannon entropy and η quantify the evolving organization and correlation of seismicity within an area; hence, they reflect a process of damage evolution in heterogeneous rocks that changes with time due to continuous strain energy redistribution. According to this view, the Shannon entropy and η can be considered parameters related to each other that reflect the memory of past deformations. The recovery of Shannon entropy and η to values preceding the crisis of 2018-2019 suggests that the system has reached a temporary new equilibrium.The solutions provided by the PCA analysis along a cross-section close to Tolmezzo confirm such observations. They reveal mostly vertical and sub-vertical planes changing orientation along the cross-section considered. The fracture propagates within the fracturing plane in the southernmost and northernmost parts of the cross-section. In contrast, the results suggest the activation of parallel planes in the central part of the section, closer to Tolmezzo. The orientation of the planes inferred from PCA analysis agrees with secondary NNE-SSW and E-W trends present in the region considered.