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 analysis has essentially been based on a GIS-aided cross-correlation of three 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 geodynamic 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 (Northern Apennines and Calabrian Are), with development of backarc basins (Northern 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 Apennines), with development of quite peculiar rift processes within the inactive thrust belt; - transpression (Northern sicily) due to the combined effect of plate convergence (Africa-Europe) and high-rate flexure-hinge retreat of an intervening plate (Adria microplate) with high angles between the respective slip vectors; - intraplate strain partition and fault activity (mainly combined strike-slip 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 Cornell-type hazard evaluations every polygon should be considered as a homogeneous source-zone, seat of randomly-distributed earthquakes. A homogeneous mechanical behaviour of an entire zone and a random earthquake-distribution within a single source zone obviously represent oversimplified assumptions since every zone includes one or more master-fault segments responsible for the greatest events in the area and several second-order associated faults 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 authors consciously paid to produce, in a reasonably short time, a homogeneous product relative to the entire national territory, suitable for earthquake hazard evaluation and for decisions regarding risk mitigation.