The impact of long-term seismic coupling on fault-based seismic hazard models: insights from the central Apennines (Italy)

The slip rate, derived from the total offset accumulated over geological time, defines the long-term activity of a fault and is a crucial parameter for fault-based seismic hazard models. Traditionally, this rate has been assumed to represent the seismicity rate of the fault. Still, geological and geodetic evidence shows that only a fraction of the tectonic offset is released seismically, and precisely this fraction matters to seismic hazard modelers. Quantifying this fraction requires estimating long-term seismic coupling, the ratio between the seismically released slip rate and the total tectonic slip rate. We present a probabilistic framework that combines fault throw-rate data with the historical earthquake catalogue of the central Apennines to derive tectonic and seismic moment rates and the associated long-term seismic coupling. These values are then used to rescale fault moment rates and compute earthquake recurrence parameters and hazard estimates. Our results show that neglecting seismic coupling biases hazard estimates across spectral periods, underscoring its importance in fault-based PSHA.