The Po Plain, a foreland basin formed by the downflexing of the crust by the loading of the Apennine thrust sheets, overlies and mainly conceals the active front of the Northern Apennines fold and thrust belt, across which there is about one millimeter (0.04 in) per year of active shortening at present. Information from hydrocarbon exploration demonstrates that the area is underlain by a series of active thrust faults and related folds, some of which have been detected from anomalous drainage patterns. These blind thrust faults are roughly WNW-ESE trending, parallel to the mountain front, and dip shallowly towards the south-southwest. Several damaging historical earthquakes, such as the 1570 Ferrara earthquake, have occurred in the area.