The coseismic ground deformations of the 1997 Umbria-Marche earthquakes: a lesson for the development of new GPS networks

After the occurrence of the two main shocks Mw=5.7 (00.33 GMT) and Mw=6.0 (09:40 GMT) on September 26, 1997, which caused severe damages and ground cracks in a wide area of the Umbria Marche region, the Istituto Nazionale di Geofisica in cooperation with the Istituto Geografico Militare Italiano set out to detect the coseismic ground deformation and reoccupied the available geodetic monuments placed across the epicentral area, belonging to the first order Italian GPS network IGM95 and to the Tyrgeonet network. The comparison between the pre and post-earthquakes coordinate set, the latter obtained from the surveys performed in the early days of October 1997 in the Umbria Marche earthquake area, showed maximum displacements values at the closest stations to the epicentres, up to 14.0±1.8 and 24.0±3.0 cm in the horizontal and vertical components, respectively. The availability of the IGM95 stations allowed geodetic data to be translated into relevant geophysical results. For the first time in Italy, the evaluation of post-earthquake coordinates at 13 vertices provided the estimation of a significant deformation field associated with a seismic sequence. Unfortunately, the same actions could not be applied to the October 14, 1997, Mw=5.6 Sellano earthquake, whose epicentre was located a few tens of km south of the previous ones, due to a lack of available geodetic vertices of Tyrgeonet and IGM95 networks in the surroundings of the epicentral zone. This fact, which prevented the estimation of coseismic deformation and seismic source modelling for this earthquake, clarified the need to set up tailor made GPS networks devoted to geophysical applications, able to capture a possible coseismic signal, but also interseismic and post-seismic signals, at the surface of the Earths crust at the scale of the expected magnitudes and fault length. Here we show and discuss the development of the Discrete GPS and Continuous GPS (CGPS) networks in the Italian region started since the early 1990s, which greatly increased after the 1997 Umbria Marche earthquakes, and the insights gained from this action which can be also integrated as Global Observing Strategy to monitor our Environment from Earth and Space