Deciphering tectonic driving mechanisms of seismicity in the central Apennines

The central Apennines (Italy) are located within the geodynamically complex Central Mediterranean. Subduction and continental collision of the Adriatic plate underneath the Tyrrhenian appear to have ceased and the region is undergoing large-scale extension of 3-4 mm/yr accompanied by large normal faulting earthquakes. The main drivers of seismicity, extension and surface deformation remain unresolved, inhibiting a fundamental understanding of Apennine geology and progress towards seismic hazard assessment. Multiple driving mechanisms have been proposed, including differences in gravitational potential energy (GPE), independent motion of the Adria microplate, and large-scale uplift related to slab detachment. In terms of structure, debates continue about whether the slab has detached and whether the continental Moho's overlap. 

We systematically test these driving mechanisms and hypotheses by exploring different structures, forcings and rheologies through cross-scale numerical modelling. We adopt the seismo-thermo-mechanical (STM) modelling approach in a realistic 2D setup ranging from the surface to 800 km depth. The model uses a visco-elasto-plastic rheology and a strongly slip-rate dependent friction to spontaneously simulate fault growth and earthquake-like events. We start from the present-day setup in the central Apennines, integrating a geological cross-section, receiver function data and tomography. The initial temperature is based on long-term STM models and geothermal data. 

Results indicate that an attached slab induces thrust earthquakes onshore, uplift in the orogen and subsidence above the Adriatic downgoing plate, all of which are inconsistent with observations. Shallow slab detachment, leaving no Moho overlap, also fails to reproduce the observed surface deformation, as it lacks a driving force within the model. Among hundreds of tested models, a model with a detached slab, slab rebound in the undetached slab remnant and Tyrrhenian/Adriatic Moho overlap explains most observations in the central Apennines. This model successfully reproduces normal faulting earthquakes within the orogen and slight compression offshore in the Adriatic Sea, driven by eduction of the partially subducted upper crust. However, the resulting horizontal surface velocities are lower than observed, indicating that external forces also drive part of the extension in the Apennines. We model this by imposing an eastward motion of the Adriatic plate of 3-4 mm/yr, representing the pull by the Adria microplate. Removing the topography shows that GPE slightly contributes to near surface extension, but its influence is minor compared to other parameters. Finally, the power law rheology of the mantle plays a key role in allowing upward mantle flow near the base of the lithosphere, thereby counteracting compression induced by the downward pull of the sinking detached slab. 

To conclude, far-field Adriatic plate pull, eduction of the subducted upper crust and slab rebound drive extension and seismicity in the central Apennines. Knowing these drivers provides a basis for modeling the seismic cycle and advancing seismic hazard assessment.