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A seismic hazard can be quantified by using probabilities. Modern seismic forecasting models (e.g., Operational Earthquake Forecasting systems) allow us to quantify the short-term variations in such probabilities. Indeed these probabilities change with time and space, in particular after strong seismic events. However, the short-term seismic hazard could also change during seismic swarms, i.e., a sequence with several small-/medium-sized events. The goal of this work is to quantify these changes, using the Italian Operational Earthquake Forecasting system, and also estimate the variations in the Gutenberg–Richter b-value. We focus our attention on three seismic swarms that occurred in Central Italy in October–November 2023. Our results indicate that short-term variations in seismic hazard are limited, less than an order of magnitude, and also that b-value variations are not significant. Placing our findings in a more general context, we can state that according to currently available models and catalogs, the occurrence of seismic swarms does not significantly affect the short-term seismic hazard. [ABSTRACT FROM AUTHOR]

SUMMARY: We estimate the b-value parameter of the Gutenberg–Richter law for earthquake magnitudes in the early stage of the Costa Marchigiana (Italy) seismic sequence, starting on 2022 November 9, with an ML 5.7 event in the Adriatic sea. In particular, we estimate both the completeness magnitude Mc and the b-value within the first 4 and 7 d after the initial strong event in the sequence. Our work represents a practical example of b-value estimation in 'true' real time, that is, during the seismic sequence, and its possible interpretation in terms of short-term forecasting. We highlight some critical issues to consider both in estimating/intepreting the b-value, and in evaluating the real time estimation of Mc. These issues are mainly due to the fact that preliminary catalogues available in real time are quite different from the revised ones, which are usually delivered after a few months. The criticalities are linked to the raw data recorded at an early-stage, an unreliable evaluation of the Mc with statistical approaches, the Short Term Aftershock Incompleteness entailed after the initial strong event, and the magnitude binning. Our results show that real time estimation of the b-value can give insights into the evolution of an ongoing seismic sequence, when attention is paid to data quality and quantity. [ABSTRACT FROM AUTHOR]

This study investigates the relationship between the b value of the Gutenberg–Richter law and the so-called k–M slope, a topological parameters, derived from the visibility graph analysis, that recently has been gaining great attention in describing the time dynamics of seismicity. The relationship has been already found to be nearly linear for several seismic areas. In this paper, we investigate such relationship in two of the most seismically active areas of the world: Italy and Taiwan. Our findings indicate that the relationship between the Gutenberg–Richter b value and k–M slope is nearly linear with a slope of regression line of about 0.07. Furthermore, analysing the seismicity by using the directed visibility graph, the role of the aftershocks is evidenced by the value of two new parameters, defined as the difference and the ratio between the forward directed and the backward directed k–M slopes, which are higher for the whole catalogues than for the aftershock-depleted ones. [ABSTRACT FROM AUTHOR]

Abstract: The Hyblean Foreland (South Eastern Sicily) represents the northern extremity of the African Plate in collision with the Euro-Asiatic one. It is mainly made up of quite thick carbonate formations and volcanic units, which are in subduction under the Tertiary allochthonic terrains and Quaternary clays, on which the volcano Etna is formed. In the recent past, researchers have analysed both the historical and instrumental seismicity of the Hyblean Foreland as a phenomenon independent of the dynamics to which Etna is prone; in fact, it was presumed that the particular seismic style of the Hyblean area, characterised by large releases of seismic energy in a brief time span after centuries of relative quiescence, was only related to the dynamics of plate-tectonics in the Mediterranean area. The detailed analyses and the results obtained in this study, based on the instrumental seismicity of the Hyblean area between 1983 and 2002, partially modify this point of view. Although the current convergence between the African and Euro-Asiatic plates does play a significant role in the tectonic activity of south-eastern Sicily, the authors believe that the behaviour of the mantle under Mt Etna does seem to have a controlling influence on the seismicity of the area under study. In particular, on comparing the development between the eruptive activity of Etna and the seismic energy of the Hyblean Foreland in the period between 1983 and 2002, the b-value trend and the characteristics of the stress field in the period 1994–2002, the authors deduce that the rising of the mantle in the Etnean area may have disturbed the stress distribution in the Hyblean area. The modifications produced in the stress field trigger earthquakes of a higher magnitude in the areas near the volcanic one. Finally, it is hypothesised that the disastrous seismic phenomena of the last 1000 years have been set off by the interaction between the local dynamics of the mantle and the regional dynamics associated to the plate-tectonics. [Copyright &y& Elsevier]

Abstract: In this study we discuss the relationship between the in-soil radon concentration and the effective stress variation at Mt. Etna volcano in the period January 2003–April 2005. The acquired radon concentration trend was compared with the one obtained by the b-value variation analysis in the frequency–magnitude relationship for earthquakes that occurred at Mt. Etna volcano (Sicily) during the investigation period. The b-value calculated using both the maximum likelihood and the least square methods shows a similar trend and, in particular, an increase starting a few months before the 7th September 2004 eruption and a decrease during the following 9 months. A similar trend was recorded during 2003 when a recharging magma phase of the volcano occurred. Results obtained by studying the b-value trend and the comparison with in-soil radon concentration are also discussed. [Copyright &y& Elsevier]

Seismicity preceding the September–October 1989 flank eruption at Mt. Etna volcano has been analysed by means of the fractal two-point correlation dimension Dc relative to epicentres distribution and the scaling exponent D inferred from the seismic b-value. Time evolution of Dc and D preceding the eruption onset revealed: a general decrease of Dc and increase of D (9–4 months prior the eruption onset), intermittent rates of Dc increase and D decrease (4 to 1 month), steady high values of Dc and low values of D (1 month) and sharp decreases of Dc and D marking the eruption onset. Results allowed to constrain the increment of seismic damage by recognising negative (mechanical hardening) and positive (mechanical softening) feedback prior the eruption onset. [Copyright &y& Elsevier]

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. • A multi-parametric description of seismicity changes is proposed and interpreted as the expression of rocks damage evolution. • North-eastern Italy, for its complex tectonics and rather high seismic hazard, is considered as test area. • Shannon entropy, b-value, fractal dimension, nearest neighbour distance, and Principal Component Analysis are investigated. • Sharp variations of rock mechanical characters control stress concentrations and localization of seismicity. [ABSTRACT FROM AUTHOR]