An innovative model of bradyseism was presented at the world's pre-eminent geochemistry conference at the Goldschmidt 2016 conference in the Japanese city of Yokohama. A geochemical and geophysical reappraisal to the significance of the recent unrest at Campi Flegrei caldera (Southern Italy), based on geochemicals and geophysics collected by the Vesuvius Observatory in recent decades. To realize it, three Italian volcanologists: Giuseppe De Natale and Claudia Troise of the Naples section 'Vesuvian Observatory' of the National Institute of Geophysics and Volcanology (INGV-OV), Roberto Moretti of the Second University of Naples (SUN).
The new model, mainly based on a re-analysis of all the geochemical data collected by the INGV-OV from the 80s to today, overturns the hypotheses formulated in recent years to explain the current uplift phenomena and shows that the hypothesis more likely to explain the recent uplift phenomena, which have persisted for about 10-12 years, is not an ascent of magma in the surface layers but, on the contrary, a flow of fluids generated by the degassing of magma which resides at great depths. The model shows an effective rise of magma in the 70s and 80s, coinciding with the major episodes of bradyseism, with a maximum ground uplift of about 3,5 meters from 1969 to 1984, recorded in the port of Pozzuoli; but the slight and persistent uplift of the last decade would instead be caused by the migration of fluids coming from great depths (7-8 km), which began coinciding with the cooling and consequent solidification of the magma injected into the surface layers (3-4 km) in the 70-80s.
This model has important implications in the study of the Phlegrean area, characterized by a high volcanic risk, also given the extreme density of the resident population, and can substantially modify the interpretation of the unrest phenomena in all the active caldera areas of the world, representing therefore a significant step forward in our knowledge of these phenomena.
Image:
Aerial photo of the Phlegrean Caldera, with a diagram of the deep structure and of the CFDDP drilling site built in 2012. The area is affected by large vertical displacements of the ground (ups and downs). From 1969 to 1985 the ground had a maximum observed uplift of 3,5 meters in the port of Pozzuoli. After about 20 years of successive subsidence, in the last 10-12 years the ground has started to rise again, at an average rate of about 2-3 cm per year. Since 2013, due to this phenomenon, the alert level in the area has risen from the green (basic) level to the yellow level.
Conference abstracts
A geochemical and geophysical reappraisal to the significance of the recent unrest at Campi Flegrei caldera (Southern Italy) ROBERTO MORETTI1, GIUSEPPE DE NATALE1 AND CLAUDIA TROISE2 1 DICDEA, Seconda Università degli Studi di Napoli, Aversa(CE), Italy (This email address is being protected from spambots. You need JavaScript enabled to view it. ) 2 INGV-Vesuvian Observatory, Naples, Italy
Volcanic unrest at calderas involves complex interaction between magma and geothermal fluids. Campi Flegrei caldera (CFc), located in the Neapolitan area and characterized by the highest volcanic risk on Earth for the extreme urbanisation, undergoes unrest phenomena involving several meters of uplift and intense shallow seismicity since several decades. Despite the presently on-going unrest displays moderate ground deformation and seismicity, geochemical variations point to a highly pressurized hydrothermal system. We show that at CFc, for which an exceptional 35 years long geochemical and geophysical dataset exists, the usual assumption about vapor-liquid coexistence in the fumarolic fluidplume leads to inevitably estimate high hydrothermal pressures, which conditions unrest interpretation. By relaxing the unconstrained 'a priori' assumptions generally made for geochemical interpretations, we get a likely agreement between geophysical and geochemical observations, and enlighten the discrepancies between what observed 1) for two decades since the 1982-84 bradyseism, when shallow magma was supplying heat and fluids to the hydrothermal system, and 2) in the last decade. The post-2005 unrest is marked by much lower aquifer overpressure and magmatic involvement, with respect to the one occurred in the '80s, just as indicated by geophysical data and despite large changes in geochemical indicators. Our interpretation points to a model in which shallow sill intrusions, occurred during 1970-1984, have quickly cooled, so that fumarole emissions are affected now by deeper gases released from a 8 km-deep magma. Our results have important implications on the short term risk in the area and the monitoring of hydrothermal pressure build-up.