Recurrent fault surface ruptures and Probabilistic Fault Displacement Hazard Assessment: case studies from Taiwan and northern Calabria, Italy

The aim of this work is to investigate the fault surface rupture phenomenon and its hazard assessment through Probabilistic Fault Displacement Hazard Analysis (PFDHA). Surface rupture is a natural phenomenon occurring during an earthquake, representing an important hazard for critical infrastructures, that must be addressed in our society. PFDHA is the main methodology used in the literature to estimate this hazard but, despite many efforts to collect new data and develop new models, fault displacement hazard remains poorly integrated in the national and international safety guidelines. This approach is based on observed past surface rupture and on the assumption that future ruptures will be similar to past ones. This assumption is examined through the Longitudinal Valley Fault system in Taiwan, where surface ruptures have been documented multiple times on the same faults in the last decades (1951–present). Comparison of two surface-rupturing earthquake sequences shows that fault surface expressions were broadly similar, with observed differences largely attributable to the aleatory uncertainty of earthquake nucleation (epicentral location, magnitude). A PFDHA study is then conducted for the northern Calabria region in Italy, one of Europe's most seismically active regions, using a range of existing empirical models. This study provides a comprehensive hazard assessment through a new approach, at the regional-scale instead of the common site-specific approach. The hazard is mostly concentrated on and around the main tectonic structures in the area: the Crati and the Laghi-Cecita faults where significant surface displacement (> 10 cm) can be expected with return periods of 10-100 ka. This new approach is beneficial to explore various fault configuration, to highlight the spatial distribution of the hazard around the main fault traces and could be used as a first-order tool for decision-making authorities or for land-use planning.