Influence of topography on coseismic displacement induced by the Friuli 1976 and the Irpinia 1980 earthquakes (Italy) analyzed through a two-dimensional hybrid model

We investigate the perturbations introduced by the local topography on the coseismic displacement components induced by crustal seismic faults. The approach, proposed by Tinti and Armigliato [2002], is applicable to homogeneous two-dimensional domains with arbitrary topography of the free surface. It uses both analytical results, valid for homogeneous spaces and half-spaces bounded by a flat free surface (FFS), and a numerical code solving the equations of equilibrium of linear elasticity through a finite element (FE) scheme. A major characteristic of our "hybrid" method is that it allows for the correct representation of the displacement components in correspondence with the fault, which does not need to be explicitly introduced in the FE mesh. We show that, in the case of homogeneous domains, this makes our approach preferable to pure FE schemes. Our hybrid approach is here applied to two different disastrous events of the recent Italian seismic history, for which topography is expected to play an important role, namely the 6 May 1976 (MS = 6.5) Friuli and the 23 November 1980 (MS = 6.9) Irpinia-Basilicata earthquakes. For each event, we selected hypotheses for the parent fault proposed in the literature and compared the coseismic displacements computed through the widely used analytical models dealing with FFS and through our hybrid approach, accounting for the effect of realistic topographies. The most relevant effects are observed in the case of the Irpinia earthquake: depending on the profile chosen, the computed misfits can be as high as 19 and 13% for the horizontal and vertical displacements, respectively, with absolute differences up to 9 and 10 cm. Copyright 2003 by the American Geophysical Union.