Seismicity associated with the 2004-2006 renewed ground uplift at Campi Flegrei caldera, Italy

Following the significant ground uplift (∼1.8 m) of the 1982–1984 bradyseismic crisis, the recent history of Campi Flegrei volcaniccomplex (Italy) has been dominated by a subsidence phase. Recent geodetic data demonstrate that the subsidence has terminated, andthat positive ground deformation renewed in November 2004, at a low but accelerating rate leading to about 4 cm of uplift by the end of October 2006. As in previous episodes, ground uplift has been accompanied by swarms of micro-earthquakes (M≤1.4) in threedistinct episodes: October 2005, October 2006 and December 2006. Hypocenters of these earthquakes are mainly located beneath theSolfatara Volcano at depths ranging between 0.5 and 4 km. Inversion of S-wave spectra indicates source radius and stress drop on the order of 30–60mand 104–9×105 Pa, respectively. Fault plane solutions indicate predominantly normal mechanisms. Accompanyingthe October 2006 swarm, we detected intense long-period (LP) activity for about 1 week. These signals consist of weak, monochromaticoscillations whose spectra exhibit a main peak at frequency ∼0.8 Hz. This peak is common to all the stations of the network, and not present in the noise spectra, suggesting that it is a source effect. About 75% of the detected LPs cluster into three groups ofmutually similar events. Adjustment of waveforms using cross-correlation allows for precise alignment and stacking, which enhancessignal onsets and permits accurate absolute arrival picks, and thus better absolute as well as relative locations. Locations associated with the three different clusters are very similar, and appear to delineate the SE rim of the Solfatara Volcano at a depth of about 500 m. The most likely source process for the LP events involves the resonance of a fluid-filled, buried cavity. Quality factors of the resonator cluster in a narrow interval around 4, which is consistent with the vibration of a buried cavity filled with a water-vapour mixture atpoor gas-volume fractions.We propose a conceptual model to interpret the temporal and spatial patterns of the observed seismicity.© 2007 Elsevier B.V. All rights reserved.