Fault-zone structure and fluid migration control aftershock distribution during the L'Aquila 2009 seismic sequence

The 2009 L’Aquila seismic sequence offers a key opportunity to investigate how fault architecture controls the aftershocks spatio-temporal evolution. We integrate detailed geological cross-sections of the Paganica (PAG) and San Demetrio (SD) fault systems with high-resolution seismic catalogue to examine the relationship between long-term structure and short-term seismicity. The PAG and SD sectors display contrasting structural architectures, with deformation localized in a narrow (3 km) zone in the PAG and distributed across a wider (6–8 km) zone in the SD. The cumulative long-term vertical throw profile shows a bimodal distribution with maxima corresponding to the PAG and SD systems and a minimum in their linking zone, where additional complexity arises from E–W-striking inherited structures. Seismicity patterns mirror this architecture. Areas of highest throw accommodated by a few closely spaced major faults of the PAG system host on-fault aftershocks with Omori-like decay and low b-values (b = 1.04), whereas the complex SD system and the linking zone exhibits shallow, spatially distributed seismicity. This distributed activity comprises non-clustered events with moderately high b-values (b = 1.15) and clustered swarm-like sequences with higher b-values (b = 1.36). We propose a two-stage process in which mainshock-induced stress change firstly activates the distributed SD fault network and its linkage zone, generating non-clustered aftershocks; subsequently delayed clustered swarm-like activity is triggered by the migration of pressurized fluid as supported by high Vp/Vs anomalies. These results demonstrate that multiple interacting processes—fault-zone structural complexity, coseismic stress changes, and fluid-mediated stress perturbations—govern the aftershock evolution.