Step-like displacements of a deep seated gravitational slope deformation observed during the 2016–2017 seismic events in Central Italy

Deep Seated Gravitational Slope Deformations are characterized by low deformation rates although they can experience partial-collapse phases or more rapid movements, especially in presence of active tectonic structures. In the Central Italy, considering the high seismicity rate, seismic activity must be considered to be an important trigger of deep slope movements. We aim to contribute to the research in this field by reporting the results of a monitoring program on a Deep Seated Gravitational Slope Deformation in this region that involves marly calcareous rocks. We documented the pre-earthquakes evolution of the phenomenon and measured its displacements during the seismic sequence in 2016 and 2017 in Central Italy, which largest events were Mw 5.0-to-6.5. A multidisciplinary approach that combines a field geomorphological survey, installation of permanent GPS stations, and InSAR elaborations was adopted for this study. The average ground motion rate of the slope deformation before the earthquakes was very low (<3 mm/y) and not spatially homogenous, as detected by GPS and InSAR. In detail, the uppermost area of the slope instability likely moves faster than the lowest sector. On the other hand, GPS and InSAR recorded significant step-like movements, one to ten times higher than the normal activity rate, triggered by the Mw 5.0-to-6.5 earthquakes. In detail, the movement mainly depended on the magnitude of the earthquake and the distance from the epicenter, and only secondarily on the number of larger magnitude earthquakes on a given day. In conclusion, we furnished monitoring data on the activity rate of a Deep Seated Gravitational Slope Deformation in seismic context, we indicated two sectors of the investigated deformation that resulted more unstable and we proved that the combination of InSAR and GPS data is a useful monitoring system for earthquake-activated, slow-moving slope instabilities