Processi morfodinamici e movimenti del suolo nella Valle di Maratea (Basilicata)

Morphodynamical processes and ground displacement in the Valley of Maratea (Basilicata)

Abstract

The Valley of Maratea presents complex gravitational phenomena which are directly related to the seismic and tectonic activity in the area. The opening of the Valley itself may be seen as the result of a strike slip fault bend which gives rise to transtensive stresses in the central part of the valley, and to a structure of pull-apart type. In the restricted northern edge of the valley, whereas the fault bending has thrust the calcareous southern block against the northeastern one, there are transpressive and horse type structures. Within the valley imposing landsliding prevalently induced by seismotectonic activity can be observed. Sagging, sudden spreading failure, block-slide type (involving wide blocks and large calcareous units, breccia and detrital deep covers, bordered by a striate breccia fault plain) were moved along the south-western limit of the valley on a substratum of plastic clay, today inactive from long time. In fact at their crown local evidences, trenches containing rock falls, testify to sudden movement which took place as a consequence of a strong shock, probably related to the Upper Pleistocene tectonic activity. Translational sliding, involving detrital covers and shallow clayey colluvia on shale, whereas their depth is less than 40-50 m, were moved, with the leading role of the basal clay saturation and water table in the central part of the valley, by intense seismic and rainy events in historical time. Creep, spreading and other plastic deformations of shallow clayey covers (squeezing), are located in the same area previously described (sometimes showing closed shear plain as a tendency to translational slide phenomena) and are today active. No absolute dating on described phenomena are available today; general indications could be obtained considering that: the sedimentary sequences in the valley show changes in their paleoenvironmental facies and rests of two different thick ash outcroppings (related to the regional volcanic activity and covering landslide depressions and graphitic lenses); the last coast line, located 4-6 m above the sea level, shows a small displacement. The research carried out during the last fifteen years has allowed us to reconstruct the principal phases of morphodynamic development and to locate areas which show different aspects of stability in the valley’s recent history, as a base to understand a previsional scenery and to orient the human activity. The data collected, including a systematic analysis of damage sustained by the buildings during the last century, show a noticeable increase in deformation starting a few years before (1973-78) the earthquake of March 1982 in an area which is still subject to intense uniform movement (2-3 cm/year). A comparison between surficial and deep movements related to the level of the water table and rain, show a sliding activity scarcely influenced by these last factors. Even if limitedly to the strike slip fault significant displacements were detected (0,5 cm/year), deep creep related to tectonic or to deep slope deformations isn’t till proved. In any case, on the basis of obtained data and structural features of the valley, the hypothesis of a dependence of slide movements from tectonic stress is consequently formulated.