Investigating the complex relationship between b-value changes and seismic activity in Central Italy

Spatiotemporal variations of the b-value (the slope of the Gutenberg-Richter relation) are commonly associated to many disparate factors, such as critical stress conditions, the tectonic regime, and the incompleteness of earthquake catalogs.

During the 2016/17 central Italy earthquake sequence, several studies reported notable b-value variations, including an unexpected increase prior to the Norcia event (Mw 6.5). Such observations highlighted the complex relationship between b-value changes and seismic activity.

To get a better understanding of this relation, we reanalyze this sequence with a focus on the spatiotemporal evolution of seismicity near the Norcia mainshock. We temporally divided the seismic sequence into subperiods separated by the largest events and employ a combination of three machine-learning algorithms: DBSCAN for performing event clustering, OPTICS for analyzing spatially nested dense zones within clusters and PCA for inferring the planar geometry of those zones as fault surfaces. We identified two specific zones and reconstruct two separate fault planes. Those two zones exhibited asynchronous activity before the mainshock. We used the two-sample Kolmogorov-Smirnov Test to investigate similarity between the magnitude-frequency distribution of earthquakes associated with these planes. The results show that the b-values associated with these fault planes remained stable over time. Yet, their temporal changes exhibited a correlation with spatial variations of seismicity. In particular, the analysis indicated a relationship between the b-value and the geometry of the active fault.  

These findings suggest that temporal variations of the b-value during an earthquake sequence may not necessarily reflect changes in underlying stress conditions, but rather the activation of different earthquake sources throughout the sequence, each with different lithological and geometrical properties. This finding highlights the importance of understanding the fine-scale structure of earthquake sources in a sequence for correctly interpreting b-value variations.