# Vulnerability and Seismic Exposure of Residential Building Stock in the Historic Center of Alcamo
## 1 Introduction  
The building fabric of historic town centers is predominantly characterized by masonry constructions, built using construction techniques and criteria handed down over the years by the traditions of good building or, at most, by the \"rule of art\" [1, 2, 3]. In areas where intense seismic events have not frequently occurred, building methods have not developed effective devices to ensure stable buildings against horizontal seismic actions and, for this reason, the constructions are particularly vulnerable. Thus, the planning of retrofitting interventions in these areas cannot disregard an assessment of the seismic risk and vulnerability of the built environment [4, 5]. More specifically, a seismic risk assessment should link the built environment's vulnerability to the seismic hazard of the site and its exposure, the latter being described through a conventional measure of the value of assets and human activities potentially affected by a seismic event. Methods for the assessment of seismic vulnerability are largely varied and some of them concern specific sub-assets (RC or masonry buildings); they utilize very different fragility models with accompanying uncertainties, which would provide biased final risk estimations. Seismic vulnerability analyses can be performed at the building scale, using structural analysis methods, similar to those employed in modern new building designs, which require the use of detailed and accurate structural models [6]. Although their implementation of computational codes can provide reliable seismic-response predictions, it requires a knowledge of the building's geometric and mechanical characteristics, which can only beknown through the in-depth development of the knowledge pathway [7], which involves high economic and time burdens. A comprehensive review of the existing methodologies for the seismic vulnerability assessment of RC and masonry structures can be found in [8; 9; 10; 11; 12; 13]. Seismic vulnerability assessment of large portions of man-made territories, such as neighborhoods, historic centers, entire urban centers, or even regions or entire national territories, can be conducted more rapidly, based on information gathered from general census campaigns, such as the ISTAT [14] database, and complementary information obtainable from cursory survey campaigns. Methods for vulnerability assessments at the territorial scale that have been proposed in the past can be divided into three main categories, namely, empirical, mechanical/analytical, or hybrid methods [8]. Empirical methods are based on the statistical analysis of the damage observed after a seismic event. In general, these methods refer to typological classes and vulnerability indices, which are based on several structural parameters and on geometrical and mechanical characteristics [15]. A review of these methods can be found in [8; 16; 17]. Mechanical/analytical methods are based on the use of a simplified representation of building mechanical features [15], the latter being the key issue hindering the implementation of this method at the territorial scale. Correlations between building mechanical features, expected damage, and seismic intensity can be assessed through analytical or numerical approaches. To this aim, both approaches evaluate the response of either archetypes of the structures chosen in order to be representative of a building typology or class of building typologies, or a simplified structural scheme with geometrical and mechanical features randomly generated. Numerical methods based on numerical analyses provide more accurate seismic-response estimates of the expected damage level for each structural type during seismic events of a predetermined intensity, but they require elevated computational efforts, such that these methods are more suitable for the vulnerability assessment of a single building; on the other hand, the analytical approach has less accuracy in terms of seismic-response assessment.