Abstract
Several recent models that have been put forth to explain bradyseism at Campi Flegrei (CF), Italy, are discussed. Data obtained during long-term monitoring of the CF volcanic district has led to the development of a model based on lithological-structural and stratigraphic features that produce anisotropic and heterogeneous permeability features showing large variations both horizontally and vertically; these data are inconsistent with a model in which bradyseism is driven exclusively by shallow magmatic intrusions. CF bradyseism events are driven by cyclical magmatic-hydrothermal activity. Bradyseism events are characterized by cyclical, constant invariant signals repeating over time, such as area deformation along with a spatially well-defined seismogenic volume. These similarities have been defined as “bradyseism signatures” that allow us to relate the bradyseism with impending eruption precursors. Bradyseism is governed by an impermeable shallow layer (B-layer), which is the cap of an anticlinal geological structure culminating at Pozzuoli, where maximum uplift is recorded. This B-layer acts as a throttling valve between the upper aquifer and the deeper hydrothermal system that experiences short (1-102 yr) timescale fluctuations between lithostatic/hydrostatic pressure. The hydrothermal system also communicates episodically with a cooling and quasi-steady-state long timescale (103-104 yr) magmatic system enclosed by an impermeable carapace (A layer). Connectivity between hydrostatic and lithostatic reservoirs is episodically turned on and off causing alternatively subsidence (when the systems are connected) or uplift (when the systems are disconnected), depending on whether permeability by fractures is established or not. Earthquake swarms are the manifestation of hydrofracturing which allows fluid expansion; this same process promotes silica precipitation that seals cracks and serves to isolate the two reservoirs. Faults and fractures promote outgassing and reduce the vertical uplift rate depending on fluid pressure gradients and spatial and temporal variations in the permeability field. The miniuplift episodes also show “bradyseism signatures” and are well explained in the context of the short timescale process.
Highlights
About a billion people, roughly 15% of the world population, live in areas of volcanic risk because many population centers developed around dormant volcanoes that have not erupted in recent years
Data obtained during long-term monitoring of the Campi Flegrei (CF) volcanic district has led to the development of a model based on lithological-structural and stratigraphic features that produce anisotropic and heterogeneous permeability features showing large variations both horizontally and vertically; these data are inconsistent with a model in which bradyseism is driven exclusively by shallow magmatic intrusions
Bradyseism is governed by an impermeable shallow layer (B-layer), which is the cap of an anticlinal geological structure culminating at Pozzuoli, where maximum uplift is recorded
Summary
Roughly 15% of the world population, live in areas of volcanic risk because many population centers developed around dormant volcanoes that have not erupted in recent years. For the 198284 event, the interpretation of Troise et al [33] agrees with the Woo and Kilburn [41] model but argues that the sillshaped magma intrusion caused an uplift of ~110 cm and the subsequent uplift of ~70 cm (to achieve the total uplift of ~180 cm) was due to the influx of deep fluids which over the 20 years led to a slow subsidence. Chiodini et al [66] measured the fumarolic gas composition and CO2 fluxes in the Solfatara and Pisciarelli areas during the last 10 years and argued that magmatic fluids related to magma intrusion at depth cause volcanic unrest because they heat and pressurize the CF hydrothermal system which in turn transfers energy to the host rocks triggering low magnitude seismicity and gas emissions. Borehole sections indicate that carbonate country rocks continue to greater depths than the deepest wells
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