The 1997 Umbria-Marche earthquakes (Italy): relation between the surface

Abstract

We classified the most outstanding ruptures of the 1997 Umbria-Marche seismic sequence as secondary tectonic effects that occur within the zone of deformation induced by the deep displacement on the seismogenic structure. The trend of the surface deformation is homogeneous within the entire area of interest and consistent with NE-oriented extension evidenced by CMT focal solutions of the three main shocks. We extrapolate the discontinuous sites of break measurements and suggest that the local deformation concentrates along four narrow bands. Location and direction of these bands are locally controlled by pre-existing structures. The comparison between our data with the seismological data - such as main rupture planes and spatial aftershock distribution - highlights that three bands mark part of the boundaries of the NW-SE elongated aftershocks area and the fourth occurs where this area is widest. Moreover, the analysis of the structural setting of the area suggests that N-S shear zones have strongly controlled the extension of the main rupture segments and the aftershock distribution. The surface ruptures are located within the area of coseismic deformation resulting from DInSAR data; we propose that they represent the localized response to the vertical ground deformation of the area. Finally, we discuss the contribution of the pattern of the 1997 surface breaks to the characterization of the seismogenic source.