Construction of a seismotectonic model: The case of Italy

Procedures for constructing a seismotectonic model of Italy, designed to be used as a basis for hazard assessment, are described. The seismotectonic a nalysis has essentially been based on a GIS-aided cross-correlation of thre e data sets concerning: - the 3-D structural model of Italy and surrounding areas; - the space distribution of historical and present seismicity; - the kinematic model of the Central Mediterranean region, referred to the last 6 Ma and including the available information on the present-day plate motion and stress field. The seismicity pattern in the study area is controlled by a guile complex g eodynamic framework which includes: - continent-continent convergence (Alps and Dinarides) with development of a neutral are bordering the plate margins; - plate divergence across margins characterized by passive slab sinking (No rthern Apennines and Calabrian Are), with development of backarc basins (No rthern Tyrrhenian Sea and Southern Tyrrhenian Sea) flanked by forelandward migrating thrust bett-foredeep systems; - plate divergence across a margin previously characterized by lithosphere sinking and afterwards discharged from the subducted slab (Southern Apennin es), with development of quite peculiar rift processes within the inactive thrust belt; - transpression (Northern sicily) due to the combined effect of plate conve rgence (Africa-Europe) and high-rate flexure-hinge retreat of an intervenin g plate (Adria microplate) with high angles between the respective slip vec tors; - intraplate strain partition and fault activity (mainly combined strike-sl ip and thrust motions), possibly in correspondence of inverted structures. The results of the seismotectonic analysis are synthesized in a zonation of Italy in which every delimited zone corresponds to the surface projection of a kinematically-homogeneous segment of a seismogenic fault system. In Co rnell-type hazard evaluations every polygon should be considered as a homog eneous source-zone, seat of randomly-distributed earthquakes. A homogeneous mechanical behaviour of an entire zone and a random earthquake-distributio n within a single source zone obviously represent oversimplified assumption s since every zone includes one or more master-fault segments responsible f or the greatest events in the area and several second-order associated faul ts responsible for the background minor seismicity. Therefore, major faults and background seismicity should be treated separately. Nevertheless, the oversimplified assumption of homogeneous seismic zones was the price the au thors consciously paid to produce, in a reasonably short time, a homogeneou s product relative to the entire national territory, suitable for earthquak e hazard evaluation and for decisions regarding risk mitigation.