A physical appraisal of a new aspect of bradyseism: The miniuplifts

Abstract

The Campi Flegrei caldera is characterized by well known episodes of fast uplift, called ‘‘bradyseism,’’ the last of which produced 1.8 m of maximum vertical displacement in the period 1982–1984. Less known is a particular aspect of Campi Flegrei unrests, detected only in the last 20 years, namely, the occurrence of ‘‘miniuplift’’ episodes, characterized by relatively small positive vertical displacements (in the range 0.03–0.11 m). Miniuplift episodes occurred in 1989, 1994, and 2000 and, probably, one or two times in the period 1972–1982. These small ground uplifts were also accompanied by moderate seismicity. Each of them was followed by a short phase of relatively quick subsidence, after which the previous rate of subsidence of the period was resumed. An order-of-magnitude analysis of the forces, energy, and power that can cause the onset and determine the evolution of these events is derived on the basis of a thermal-fluiddynamical approach. It allows the quantitative correlation of the power input from the energy source with the rate of energy dissipation in the geologic system during the miniuplifts. In order to assess the likelihood of our fluid-dynamical model for these episodes, theoretical ground displacements have been computed. These displacements result from the changes in pressure as a function of depth predicted by the fluid-dynamical model for reasonable perturbations of the geothermal system, described as changes of the Pe´clet number. Theoretical results agree extremely well with the observed amounts of the miniuplifts.