Pseudo-prospective earthquakes forecasting experiment in Italy based on temporal variation of the b-value of the Gutenberg-Richter law.

The analysis of space-time variations of the b-value of the frequency-magnitude distribution of earthquakes can be considered an important indicator in understanding the processes that precede strong earthquake events. Variations in b-value can provide valuable information on the stress state and probability of earthquake occurrence in a specific geographical region. By analyzing spatial variations in b-value, changes in local tectonic conditions can be identified, highlighting areas where seismic risk may increase. Similarly, the analysis of temporal variations in b-value can reveal patterns preceding seismic events, providing a possible precursor signal. Such variations could be the result of complex geological processes, such as the progressive accumulation of stress along active faults or the presence of underground fluids that influence fault dynamics. In fact, as it has been observed in many cases, the b-value tends to descend in the preparatory phases of a strong earthquake, and it increases suddenly after the mainshock occurrence. To evaluate such a hypothesis, in this work, an alarm-based forecasting method that uses b-value space-time variations as a precursor signal is implemented. The forecasting method has been retrospectively calibrated and optimized for the period 1990-2011 to forecast Italian shallow earthquake (Z<50 km) of magnitude larger than 5.0. The method has been than applied pseudo-prospectively over the period 2011-2022 and the forecasting skills have been assessed using specific test and statistics for alarm-based models. Such forecasting skills have been also compared with those of another alarm-based earthquake forecasting model that use the occurrence of potential foreshock as precursor signal.