Significance of the 1982–2014 Campi Flegrei seismicity: Preexisting structures, hydrothermal processes, and hazard assessment

Abstract

We find that the double-difference relocated seismicity, which occurred over the last 30 years at Campi Flegrei, was triggered by the uprising of fluids preferentially concentrated along onshore and offshore NW striking preexisting caldera faults. Focal volumes of the 2005-2014 seismicity do not overlap that of the 1982-1984 period, when a major uplift of 1.8 m occurred in the central sector of the caldera. This indicates a transition from an elastic to a plastic behavior due to fluid saturation and heating of the rocks in the hydrothermal reservoir. The 2012-2014 deeper earthquakes are located in a low V P ∕V S zone at the western boundary of the hydrothermal reservoir, where a volume increase from a magmatic body at 3.5 km depth has been recognized. The progressive rheological change from elastic to plastic in the upper 4 km of the crust implies that a slow upward migration of magma may not necessarily be preceded by earthquakes or swarms.

Reservoir Structure and Hydraulic Properties of the Campi Flegrei Geothermal System Inferred by Audiomagnetotelluric, Geochemical, and Seismicity Study

“…Our finding of a fractured and high‐temperature steam/gas‐bearing reservoir just under the Solfatara reinforces previous inferences of Di Luccio et al (). Based on our multidisciplinary conceptual model we propose that two main factors could contribute to the lack of seismicity underneath the Solfatara during the ongoing unrest episode: (i) increasing temperatures suggested by an evident thermal anomaly beneath the Solfatara (Chiodini et al, ; Gresse et al, ) could favor ductile deformation of the hydrothermal reservoir; (ii) enhancement of the reservoir fracture permeability induced, during the 1982–1984 unrest episode, by the sustained shallow seismicity clustered under the Solfatara area.…”

“…The resistive plume matches the zone of high clusterization of earthquakes during the 1982–1984 unrest episode (supporting information Figure S9), whereas the 2005–2016 seismicity clusters at the eastern boundary of the 1982–1984 focal volume within the relative conductive zone detected in the depth range 0.5–2.5 km (Figure ). Di Luccio et al () suggest that events distribution and fault geometries are consistent with a funnel‐like structure within which brittle deformation have occurred. Also, to explain the lack of recent seismicity underlying the Solfatara, the authors speculate that this sector of the hydrothermal system could not represent a zone of brittle deformation at present.…”

“…Conversely, recent seismicity to the east of the Solfatara could be induced by pore pressure increase within the relative conductive zone that might represent fractured liquid‐bearing rocks, as inferred by the CF23 well and the V p pattern of the LET models (Vanorio et al, ). In particular, Di Luccio et al () linked recent seismicity to NW‐SE striking faults crossing the Pisciarelli degassing area. We remark that our AMT survey favors this interpretation because the lateral resistivity variations resolved in the shallow and deep parts of the section provide evidence of the continuity at depth of NW‐SE active faults hypothesized previously by more authors (Isaia et al, among others, Figure ).…”

Increasing CO<sub>2</sub> flux at Pisciarelli, Campi Flegrei, Italy

“…Our results related to the CO 2 degassing are compatible with findings which state that the elastic rock matrix of CF is transitioning to inelastic behavior under long-term stress accumulation, accompanied by a permeability increase in the shallow crust, disguising any direct indicator of unrest, such as rapid ground uplift or enhanced seismicity (Bodnar et al, 2007;Di Luccio et al, 2015;Kilburn et al, 2017). In line with this prospect is a clear seismic velocity decrease since 2012 (Zaccarelli and Bianco, 2017), which could be due to, for instance, a softening bulk or increase in CO 2 saturation in the CF aquifer (Queißer and Singh, 2012) that may also explain the strong seismic attenuation observed (De Siena et al, 2017b).…”

Magmas near the critical degassing pressure drive volcanic unrest towards a critical state

“…6). Furthermore, comparison of the locations of earthquakes in 1983–1984 and 2005–2014 highlights the recent disappearance of shallow seismicity in the Solfatara area, suggesting a transition from an elastic-like to a plastic-like behaviour, which is probably due to heating of the rocks52.…”