Bradyseism

Bradyseism is the gradual uplift (positive bradyseism) or descent (negative bradyseism) of part of the Earth's surface caused by the filling or emptying of an underground magma chamber and/or hydrothermal activity, particularly in volcanic calderas. It can persist for millennia in between eruptions and is normally accompanied by thousands of small tremors and sometimes larger earthquakes.

The area around the Phlegraean Fields (Campi Flegrei) caldera near Naples, which includes the noted Solfatara volcano, is especially noted for bradyseismic uplift and subsidence. This inflation and deflation of the this caldera is especially well documented due to its seaside location and a long history of habitation and construction in the area. In particular the town of Pozzuoli contains three marble columns (in the Roman Temple to Serapis) which have boreholes made by marine molluscs. These occur up to 7 metres up the columns, showing how bradyseism in the area lowered the land to at least this depth under the sea and subsequently raised it again. More recently, between 1968 and 1972 the town suffered an episode of positive bradyseism and rose by 1.7 metres. There was another rise of 1.8 metres between 1982 and 1984, that was correlated with a shallow (4 km deep) earthquake on October 4, 1983 which left around 30,000 people homeless.