Characterization of seismic activity in Northern Apennines (Italy) during the interseismic period.

The Northern and Central Apennines of Italy are regularly struck by moderate to strong seismic activity, such as the 2009 L’Aquila (Mw 6.1 mainshock) and the 2016-2017 Amatrice-Visso-Norcia (AVN, Mw 6.0 to 6.5 mainshocks) sequences. Many studies have been conducted to understand the processes driving the seismicity of these two seismic sequences. However, much less is known about what happens during the interseismic period in this region. We focused on the seismic activity since the 2016-2017 AVN seismic sequence and retrieved the seismic catalogue from the INGV website for the period 01/01/2018 to 31/08/2024. A total of 55 278 events were detected. We relocated the seismicity using a double-difference relocation program and obtain 54 151 relocated events. Using a declustering method, we define 134 clusters (9 285 events) having more than 10 events. In order to gain insights into the physical processes driving the seismicity in each cluster, we use a set of parameters bearing information about their spatio-temporal evolution and magnitude distribution (e.g. Seismic-to-total-moment ratio, Skewness and Kurtosis, Seismic Active Volume, Estimated injected fluid volume). Based on this parametric analysis we can concluded that: 1) the majority of the clusters behaved like swarms, 2) the clustered seismicity did not occur on-fault but rather off-fault, activating a volume rather than discrete fault structures, 3) the clusters are likely driven by fluids. All these analyses helped us to characterize the seismic activity during the interseismic period.