Assessing the longevity and stationarity of surface velocities for seismic hazard in the central Apennines (Italy) by combining InSAR and fully dynamic earthquake cycle modeling

The central Apennines (Italy) are characterized by active normal faulting that is largely clustered in space and time, as documented by both historical and paleoseismic records. The 2016-2017 central Italy earthquake sequence, comprising a series of Mw 5 to Mw 6.5 events within half a year, exemplifies this behavior. Over longer timescales, 36Cl dating of Holocene fault scarps reveals earthquake clustering in the Fucino basin. Although the central Apennines have dense geodetic and seismological observations, these instrumental datasets only cover a small portion of the seismic cycle. This raises fundamental questions about how representative the present-day deformation signals are of long-term tectonic loading and seismic hazard. Here, we address the following questions: How representative is the current geodetic signal over multiple earthquake cycles in an area characterized by a dense fault network? How do surface velocities evolve through the earthquake cycle, and how does the spatial and temporal distribution of earthquakes relate to this evolution?

We combine new InSAR observations with newly developed seismo-thermo-mechanical models with an invariant rate-and-state friction (STM-RSF) and a visco-elasto-plastic rheology in a geodynamic framework. This fully dynamic earthquake cycle model resolves the inter-, post- and co-seismic periods, as well as cumulative deformation over several seismic cycles. We build on previous STM modeling in the central Apennines (Fonteijn et al., in prep). Faulting is localized on pre-defined weak zones from geology and the Fault2SHA active faults database, but can also occur outside the weak zones.

We analyzed InSAR time-series to study interseismic surface deformation in the central Apennines. We detect significant short wavelength velocity variations across faults of 0.5 to 2 mm/yr, which could possibly be explained by bookshelf faulting. Additionally, we simulated an earthquake sequence of six large normal-faulting earthquakes over ~8000 years in the central Apennines. These earthquakes occur on different normal faults in sequence before faults are reactivated, with rupture on one fault transferring stresses to adjacent faults. We also find rupture of a spontaneously arising antithetic fault and accumulated vertical displacement shows block-faulting behavior. We assess the variability of interseismic surface displacements and compare with InSAR interseismic displacements. Preliminary results show significant variations in vertical velocities in both duration and intensity over 8000 years, with alternating periods of subsidence and uplift in the orogen. This new modelling approach for the first time allows for a comparison of surface displacements over multiple earthquake cycles with short-term geodetic observations. The outcome of this study will have important implications for how to use geodetic data for seismic hazard assessment.