Coseismic displacement of the 27th September 1997 Umbria - Marche (Italy) earthquakes detected by GPS: campaigns and data

On September 26,1997 two earthquakes of Mw 5.7 (00.33 GMT) and Mw 6.0 (9.40 GMT), occurred in the Umbria-Marche region (Central Apennines, Italy). The epicentres were located in an area of the Apenninic chain that experienced historical earthquakes up to X degrees of the MCS scale. During the time span 1992-1996, the Italian Istituto Geografico Militare (IGM) set up a new national geodetic network measured by Global Positioning System space geodetic technique, consisting of more than 1200 vertices uniformly distributed on the Italian peninsula and islands. From October 7 to 11, 1997, a short while after the main shocks of the Umbria-Marche seismic sequence, we reoccupied thirteen stations belonging to the IGM and TYRGEONET networks to measure coseismic displacement. The determinations of the post-seismic coordinates at 13 GPS monuments detected significant coseismic displacements. The comparison between the preseismic and postseismic data sets show maximum displacements of 14 cm and 25 cm in the horizontal and vertical components respectively. In this paper, the GPS network, the field work, the data processing procedures and the computed coseismic displacements measured at the geodetic monuments are discussed with the aim to provide a data set useful to the scientific