Gravimetric study of a retrogressive landslide in southern Italy

Post-failure activity of the December 1993 Senerchia slump-earthflow was characterised by intermittent recession of the headscarp and earthflow movements. The retrogression showed considerable spatial variability, depending on the properties of the geological materials. The retrogressive failures were preceded by intense fissuring of the ground in the crown zone. Two microgravimetric surveys were carried out in order to detect possible spatial-temporal density variations in an area upslope of the headscarp. Although it was not possible to recognise any significant temporal density changes, this surveying revealed the presence of a negative anomaly which coincided with the area of maximum headscarp retreat. The gravity modelling was constrained by borehole information and new headscarp exposures produced by a series of retrogressive failures suggested that the origin of the anomaly might be associated with a hollow in an underlying clay-rich bedrock which had been subsequently filled by coarse colluvium. A possible concentration of groundwater in the hollow and its discharge towards the headscarp area controlled the local slope instability. The results of this study showed that microgravimetric surveys conducted upslope of retrogressive landslides can provide useful information on subsurface lithological heterogeneities that may control the amount and preferential direction of upslope landslide enlargement.