The 1997-1998 Umbria-Marche post-earthquakes investigation: Perspective from a decade of analyses and debates on the surface fractures

The Umbria-Marche sequence represents the first case in Italy for which the surface effects of a moderate seismic event (5<M<6) were systematically investigated and documented. Surface fractures attributed to the events were mapped by several groups of researchers. The coseismic features were expressed mainly by NW-SE trending, discontinuous, linear open fractures, as well as fracture swarms, locally with centimetric vertical displacements. They affected rocks and loose deposits, buildings, and roads. Because of the earthquake sizes (M?6), the deformation was faint, making the detection difficult and giving rise to different seismotectonic interpretations: specifically, surface rupture of the seismogenic fault, triggered slip on secondary faults, and shaking-induced sliding of debris. Consequently, different models of connection between surface offsets and displacements at depth were proposed that integrated geology with other geophysical data. However, whether direct or indirect expression of the deep dislocation, the geometry of these fractures reflected the seismogenic structure. Even in the case of these moderate-earthquakes features, geomorphical and paleoseismological studies provide data for reconstructing the recent tectonic evolution of the region and for determining the frequency and size of earthquakes. Finally, we learned a more efficient way to prepare for post-earthquake emergency response, particularly for long-lasting sequences in highly vulnerable built environments, such as the ancient villages within the Apennines.