# Vulnerability and Seismic Exposure of Residential Building Stock in the Historic Center of Alcamo
## 1 Introduction
Hybrid methods are formulated by attempting to integrate the potential of the two previously mentioned categories of methods, e.g., by using mechanical models to define correlations between typological characteristics and expected damage, recalibrating them on damage data observed during major seismic events [16; 18]. Mechanical methods for assessing seismic vulnerability at a territorial scale employ extremely simplified models of the built environment, most often capable of representing only some of the essential characteristics determining the seismic vulnerability of buildings [19; 20], such as the height or number of floors, material mechanical characteristics of vertical structural elements, stiffness and strength of horizontals, and sometimes characteristics of connections between vertical-resistant elements or between the latter and horizontals. Thus _\"the reduced capability of a prototype (or a set of them) of representing a more complex building stock and the simplifications inevitably introduced by the structural modelling are the main limitations of the analytical approaches. As a consequence, also the identification of different sources of uncertainty (e.g., in capacity, demand and damage state thresholds) and their quantification can affect the reliability of analytical results. On the contrary, analytical approaches permit to simulate the seismic response of different building types, also analyzing them under high intensity values, for which no or poor empirical data is generally available\"_ (quoted verbatim from [21]). Empirical methods, based on a typological classification of the built environment, conducted using qualitative information, still play a prominent role in the vulnerability characterization at the territorial scale, since they prove to be especially effective for the vulnerability statistical assessment at the territorial scale, aimed at the allocation of reduced economic resources available for seismic risk mitigation, or for evaluation of damage scenarios for post-earthquake emergency management [19; 22; 23]. Proof of this is the fact that the Italian Civil Protection Department (ICPD) decided to conduct the latest National Seismic Risk Assessment (NSRA) through a multi-model methodology, where three of the six models are based on an empirical approach [24; 25; 26]. Two adopt an analytical approach to derive the fragility curves [18; 27] and a third a hybrid approach [23], i.e., a recalibration according to a post-earthquake damage database [28] for masonry buildings only of the macroseismic model [20] used in the present paper. Regarding exposure, a European model has recently been proposed by [29]. In Italy, at present, NSRA [30] is still evaluated on the basis of the ISTAT database, where construction material, floor number, and construction year only are available. Thus, many researches are trying to supplement the ISTAT census database, collecting data for large-scale assessments (e.g., [31; 32]). In this regard, aiming at gathering information specifically devoted to seismic vulnerability assessments that can detect and reveal local structural features, the Italian Protection Department promote, through RELUIS (Italian Network of Academic Laboratory of Earthquake and Structural Engineering), uses of the interview-based CARTIS form. To date, the CARTIS form has been used to survey approximately 6% of all Italian municipalities, and some Italian researchers are engaged in studies of seismic vulnerability assessment at the territorial scale on the basis of information through the CARTIS card, which has been made available on an online platform [33; 34; 35]. The CARTIS form provides a rich description of the structural features of residential building typologies, provided by interviews with technicians with experience in the field of structural construction, who are rooted in the territory and have a thorough knowledge of the construction techniques of the place.