GNGTS 2015 - Atti del 34° Convegno Nazionale

Zoning fault displacement hazard around capable faults: a first assessment in Italian urban areas A.M. Blumetti, L. Guerrieri, V. Comerci, P. Di Manna, G. Leoni, E. Vittori ISPRA, Geological Survey of Italy, Roma, Italy Introduction. Fault Displacement Hazard (FDH) is a component of seismic hazard that focus on the potential of coseismic surface tectonic rupture/deformation. Although the Italian territory is characterized by a great number of capable faults (i.e., faults able to produce significant ruptures or deformations at or near the topographic surface), nowadays, this hazard is not yet taken into account in official seismic hazard maps and building codes in Italy. Nevertheless this hazard it has been considered in the guidelines and criteria for seismic microzonation (Working Group MS, 2008), and more recently in specific studies (e.g. Peronace et al. , 2013) and technical guidelines still under preparation. This paper aims at providing a general indication of Fault Displacement Hazard in Italy in the 73 most populated urban areas (population > 60,000 inhabitants) with the aim to point out the cities where this hazard does exist and is more relevant (in terms of maximum expected displacements) and what is directly threatened by surface faulting. This is only a first indication of the areas where this problem is more critical. Of course, a FDH assessment helpful for siting and land planning purposes, will require the delineation of setback areas (i.e. the distance from the fault trace within critical facilities and structures designed for human occupancy cannot be built, Bryant and Hart, 2007) through more detailed site investigations (Quaternary geology and paleoseismology), aimed at characterizing at larger scale the local pattern of capable faults and the age of last movements. Zoning Fault Displacement Hazard around capable faults. In order to respond to the need of a specific knowledge regarding the Fault Displacement Hazard, the Italian Agency for Environmental Protection (ANPA, later APAT, now ISPRA) in the second half of the 90s started the project ITHACA (ITaly HAzard from CApable faults). The project is aimed at building a tool for summarizing and making easily available information on capable faults, based on published sources, field checks and ad hoc studies (for more details, see Comerci et al., 2013). Currently, the ITHACA database is available at the GeoMapViewer of Geological Survey (http://sgi.isprambiente.it/GMV2/index.html ) and contains about 2000 records (mapped in Fig. 1) including faults that exhibit at least one evidence of capability among the following: a) historical coseismic surface faulting; b) creep or surficial tectonic deformation; c) Late Pleistocene-Holocene paleoseismic evidence of ground rupture; d) displacement of Quaternary deposits/landforms. Moreover, the faults are classified according to the age of the last ascertained movement. At the moment, the ITHACA database, although still incomplete and not homogeneous in terms of resolution and reliability of supporting data, is the most reliable tool for a first characterization of Fault Displacement Hazard in the entire Italian territory. Among previous studies focused on this topic, Guerrieri et al. (2009) aimed at estimating the extent of urban areas exposed to surface faulting hazard within the ZS9 seismotectonic zonation of Italy. The analysis was conducted for each seismotectonic zone that was considered homogeneous also in terms of Fault Displacement Hazard, through the intersection of ITHACA and CORINE Land Cover databases. The results of the spatial analysis have been weighted through the introduction of a Fault Class parameter which takes into account the expected maximum displacement associated to capable faults in each zone. For this assessment a standard 300-m wide buffer area around capable faults was considered. More recently, Guerrieri et al. (2013, 2014) proposed a more refined zonation of the area around themappedcapable faults,whose shape andwidthdependon the seismotectonicbehaviour (i.e., type, style and amount of faulting) and the severity of the maximum expected earthquake. These two factors control also the amount of maximum expected surface displacements. 190 GNGTS 2015 S essione 2.2