BLOCK TYPE MOVEMENTS AT THE PRIHRAZY PLATEAU (BOHEMIAN MASSIF)

J. Zvelebil & J. Stemberk (Institute of Rock Structure and Mechanics, Academy of Sciences, V Holesovickach 41, Praha, Czech Republic)

The Prihrazy plateau is built up by a nearly subhorizontal plate of Upper Cretaceous sandstone overlying marl of the same age. The plate thickness reaches 90 m. At the edge of the plateau extensive failures are present. These are deep-seated slope block movements and shallower landslides. Block movements within the sandstone layer are considered Pleistocene or early Holocene in age, and fossile under present conditions. The analysis of dilatometric records together with the occurrence of ground sinking and crack openings indicate a renewed fase of activity, stating from at least 1989. A significant, human induced, increase of ground water level, due to changes in the vegetation cover on the plateau, and the effect of the temporal increase of precipitations above their long-time average in the period 1977-1988, are considered the primary causes of the block movement reactivation.