Structural heterogeneities and spatial variations of seismicity drive temporal b‐value changes

For the 2016/2017 central Italy earthquake sequence, various studies reported spatiotemporal variations in the Gutenberg–Richter b-value, including its unexpected increase before the Mw6.5 Norcia mainshock. Here, we investigate this specific observation with a combination of machine learning techniques. Before the mainshock, we identify two seismicity volumes with asynchronous activity, disparate fault orientations, and distinct b-values. Throughout the sequence, both structures maintain their distinct b-values; only when combined, the b-value apparently increases before the mainshock (and drops afterward). Our observation suggests that a temporal b-value variation in a large volume, or the entire extent of this sequence, originates from individual structures with distinct b-values being active at different times. More generally, temporal b-value variations may also reflect changes in the spatial distribution of seismicity besides other underlying processes. Our findings highlight that b-value interpretation must acknowledge the structural heterogeneity, such as associated fault segments and their orientations.