Computational Tools for Relaxing the Fault Segmentation in Probabilistic Seismic Hazard Modelling in Complex Fault Systems

Use of faults in seismic hazard models allows us to capture the recurrence of large-magnitude events and therefore improve the reliability of probabilistic seismic hazard assessment (PSHA). In the past decades, fault segmentation provided an important framework for quantifying fault-based PSHA. Recent complex coseismic ruptures (e.g., 2010 Mw 7.1 Canterbury, 2012 Mw 8.6 Sumatra, 2016 Mw 7.8 Kaikōura, 2016 Mw 6.5 central Italy) have shown the need to consider different possible combinations of rupture scenarios in PSHA. Here we present two new methodologies that model rates of ruptures along complex fault systems, one based on a floating rupture approach (FRESH) and another one based on assumed rupture scenarios (SUNFISH). They represent alternatives to a recently proposed approach (SHERIFS), and further step to overcome the segmented and un-segmented approaches commonly used in PSHA in Europe. Differences among SHERIFS, SUNFiSH and FRESH are related to the way slip rate, rupture geometries and magnitude–frequency distributions are modelled. To quantify the differences between these three methodologies, we compared PSHA results based on geometries and slip rates of a fault system located in northeastern Italy, assuming a given maximum magnitude and the same seismic moment rate target. Differences up to 20–30% in the peak ground acceleration at 2% and 10% in 50 years are observed. Finally, we show that the three methodologies are able to solve for the long-term rate of ruptures with resulting PSHA that reflect the fault system geometry and slip rates, without any assumption on segment boundaries. Using fault-based approaches in PSHA requires collecting as much local geological information as possible. Now that multi-fault rupture approaches are available, simplifying assumptions often made to model complex fault systems (uniform slip rate, segmentation hypothesis) are no longer necessary. On the other hand, local data collection should be strongly encouraged to better characterize the actual fault slip rate variability and the complex 3D geometries.