Earthquakes and cluster dynamics during Interseismic phases between the Northern and Central Apennines (Italy)

In the last thirty years, the Northern and Central Apennines (Italy) have been affected by three main destructive seismic sequences: the 1997 Colfiorito (three events $M_L > 5.5$), the 2009 L'Aquila (one event $M_L > 5.5$), and the 2016--2017 Amatrice--Visso--Norcia (three events $M_L > 5.5$). Several studies have analysed the spatio-temporal evolution and processes driving each sequence, focusing mainly on the foreshock--mainshock--aftershock periods. Here, we focus on the 2018--2024 interseismic phase, aiming to unravel the long-term seismogenic behaviour of this region. We first relocated the earthquake catalogue and identified clusters through a declustering algorithm. During this phase, background seismicity and most clusters were arranged in a 2--3 km thick low-angle layer. We found that (i) most clusters were driven by aseismic processes, (ii) the depth of both clusters and the seismicity layer increased toward the southeast, (iii) the volume of clusters decreased toward the southeast, and (iv) the low-angle layer almost disappeared in the L'Aquila area. Comparing two interseismic phases (2011--2016 and 2018--2024), we found striking similarities, with events occurring at the same sites and characterized by both foreshock--mainshock--aftershock and swarm-like behaviour. In addition, the L'Aquila area was seismically more silent compared to the northern sites during both interseismic phases. We propose that these different long-term seismogenetic behaviours reflect variations in the structure and rheology of the upper crust from the Northern to the Central Apennines. This highlights the important role of structural inheritance in controlling how active deformation affects the interseismic period.