On the use of IGM95 GPS network to study coseismic deformations due to the earthquakes occurred on september 26, 1997 in the umbria and marche area

After the occurrence of two main shocks Mw = 5.7 and Mw = 6.0, of September 26, 1997 that struck the Umbria-Marche region, the Istituto Nazionale di Geofisica in cooperation with the Istituto Geografico Militare reoccupied 13 stations of the IGM95 GPS network, located in the epicentral area with the aim to detect coseismic ground deformation. The comparison between the 1995 coordinate set and those ones obtained during the surveys performed in the early days of october 1997, shows maximum deformation values of 14 cm and and 24 cm in the horizontal and vertical components respectively, at the stations closer to the epicenters. The avalaibility of the IGM95 network in the epicentral area, dense and reliable in data quality, allowed to translate geodetic data in geophysical results of great importance. The determinations of new postseimic coordinates of 13 vertices make out, for the first time in Italy, significant coseismic displacements extremely useful to identify a fault model responsible of the seismic events and to understand the seismic source mechanics.