Abstract
Despite their importance for eruption forecasting the causes of seismic rupture processes during caldera unrest are still poorly reconstructed from seismic images. Seismic source locations and waveform attenuation analyses of earthquakes in the Campi Flegrei area (Southern Italy) during the 1983–1984 unrest have revealed a 4–4.5 km deep NW-SE striking aseismic zone of high attenuation offshore Pozzuoli. The lateral features and the principal axis of the attenuation anomaly correspond to the main source of ground uplift during the unrest. Seismic swarms correlate in space and time with fluid injections from a deep hot source, inferred to represent geochemical and temperature variations at Solfatara. These swarms struck a high-attenuation 3–4 km deep reservoir of supercritical fluids under Pozzuoli and migrated towards a shallower aseismic deformation source under Solfatara. The reservoir became aseismic for two months just after the main seismic swarm (April 1, 1984) due to a SE-to-NW directed input from the high-attenuation domain, possibly a dyke emplacement. The unrest ended after fluids migrated from Pozzuoli to the location of the last caldera eruption (Mt. Nuovo, 1538 AD). The results show that the high attenuation domain controls the largest monitored seismic, deformation, and geochemical unrest at the caldera.
Highlights
The intrusion of magma of 3 m thickness as a sill in this depth range and connected to the simultaneous strain of the crust in the ESE-WNW direction[2]
The seismic recordings produced by the SERAPIS active seismic experiment have been used to image crustal structures down to 8 km depth[9, 24, 25] and have depicted few seismic structures spatially related to ground deformation or magmatic sources
The resolution of the attenuation model is poor at 4.25 km depth, our tests indicate the existence of a 1 km-wide area of high attenuation SE of Pozzuoli (Figs 3a,b and S3–S5)
Summary
The intrusion of magma of 3 m thickness as a sill in this depth range and connected to the simultaneous strain of the crust in the ESE-WNW direction[2]. Until (or just before) April 1, 1984, a slow decrease in concentration of CO2/H2O was paired with a steep decrease in CH4/H2O concentration After this date, the CO2/H2O ratio drastically increased while the CH4/H2O ratio remained constant until the end of 1984. Large amplitude seismic reflections show the top of a fluid-bearing rock formation extending across the caldera at depths of about 3–3.5 km[25]. These are interpreted as a “basement top”, as defined by the study of rock physical properties on Campi Flegrei samples[14]. A second smaller-amplitude reflection constrained a ~7.5 km deep, 1-km-thick, low-velocity layer, associated with a mid-crust, partially molten zone beneath the caldera[25], compatible with the deepest magmatic source modelling the geochemical unrest[16, 18]
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