Seismic fragility assessment of historic masonry façades: integrating micro-CT material characterization and large-scale vulnerability modeling

Founded in 1541 and designated a UNESCO World Heritage Site in 1991, the historic center of Morelia, Mexico, includes more than 1000 architectural monuments that are vulnerable to severe seismic damage because of their age, structural types, and material degradation. This study assesses the seismic vulnerability and fragility of the façades of 577 residential and commercial masonry buildings at this heritage site. Unlike standard large-scale investigations, this study uses a refined, parameter-based methodology to incorporate microstructural material properties, digital building-height assessment, and detailed conservation-state data from visual inspections. Field data are supplemented by 3D micro-computed tomography (micro-CT) of architectural stone samples to evaluate stone porosity, which correlates with degradation of mechanical strength. Vulnerability indices are subsequently categorized using the Jenks natural breaks optimization method to minimize intra-class variance and enhance spatial mapping. Structural fragility curves are derived for four distinct damage states by coupling an adapted macroscopic damage model with localized seismic demand scenarios. The results indicate that, while the evaluated building inventory exhibits homogeneous vulnerability and acceptable performance under a standard 475-year-return-period design earthquake, an extraordinary seismic event would trigger extensive nonlinear structural damage across the building façades. These findings offer a high-resolution, georeferenced technical framework to help decision-makers implement targeted structural retrofitting, urban prevention policies, and risk-mitigation strategies in heritage cities.