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. It can last several years and is normally accompanied by thousands of small tremors and sometimes larger earthquakes or a volcanic eruption.

The area around the Solfatara volcano near Naples is especially noted for it. The town of Pozzuoli contains three Roman marble columns 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 1.6 metres between 1982 and 1984, with a shallow (4 km deep) earthquake on October 4, 1983 which left around 30,000 people homeless.