Spatiotemporal Dynamics of Clusters in the Bridge Zone Linking L'Aquila 2009 and Central Italy 2016 Seismic Sequences

We analyze the spatiotemporal evolution of earthquake clusters in the Campotosto area, located between the L'Aquila 2009 and Central Italy 2016 seismic sequences. This region has experienced several moderate earthquakes (Mw 5–5.5) and persistent low-level seismicity. Using a hierarchical density-based algorithm on high-resolution catalogs, we identify clusters lasting from days to months and migrating at rates of meters to kilometers per day. These clusters alternate between phases of rapid expansion and slower diffusion, reflecting complex interactions among fluids, aseismic slip, and seismic rupture across fault segments. Energy release within clusters is low, with effective stress drop ranging from 0.01 to 1 MPa, suggesting diverse driving processes. Clusters with larger spatial extents tend to exhibit lower effective stress drop, implying a significant aseismic slip component, consistent with remote sensing observations. Our findings indicate that deep fluids promote multiphase slip and fault reactivation, influencing seismicity across fault segments with varying criticality. Variations in fault orientation and dip further contribute to heterogeneous slip distribution, affecting both energy release and cluster formation.