The relationship between seismic deformation and deep seated gravitational movements during the 1997 Umbria-Marche (Central Italy) earthquakes

Abstract

This paper re-evaluates the origin of some peculiar patterns of ground deformation observed by space geodetic techniques during the two earthquakes of September 26th of the Colfiorito seismic sequence. The surface displacement field due to the fault dislocation, as modeled with the classic Okada elastic formulations, shows some areas with high residuals which cannot be attributed to unsimulated model complexities. The latter was investigated using geomorphological analysis, by recognising the geologic evidence of deep seated gravitational slope deformations (DSGSD) of the block-slide type. The shape and direction of the co-seismic ground displacement observed in these areas are correlated with the expected pattern of movement produced by the reactivation of the identified DSGSD. At least a few centimetres of negative Line of Sight ground displacement was determined for the Costa Picchio, Mt. Pennino, and Mt. Prefoglio areas. A considerable horizontal component of movement in the Costa Picchio DSGSD is evident from a qualitative analysis of ascending and descending interferograms. The timing of the geodetic data indicates that the ground movement occurred during the seismic shaking, and that it did not progress appreciably during the following months. In this work it has been verified the seismic triggering of DSGSD previously hypothesized by many authors. A further implication is that in the assessment of DSGSD hazard it is necessary to consider the seismic input as an important cause of acceleration of the deformation rates.