“…In saturated or partially-saturated rocks, the content and physical state of fluids has a greater effect on P-wave velocities than S-wave velocities (Vanorio et al, 2005). Fluid phase transitions induce changes in fluid compressibility and thus bulk modulus (Ito et al, 1979;Wang & Nur, 1986).…”
Section: Interpretation Of V P /V S Variationsmentioning
confidence: 99%
“…Shear moduli are little affected by fluid phase transitions and hence S-wave velocities change insignificantly due to a density effect, meaning that low V P /V S ratios tend to characterise gas-bearing rocks (i.e. those with high fluid compressibility) whereas liquid-bearing rocks (with low fluid compressibility) are characterised by high V P /V S ratios (Vanorio et al, 2005).…”
Section: Interpretation Of V P /V S Variationsmentioning
confidence: 99%
“…Overpressure is one explanation for this. If the fluid pressure is higher than the normal hydrostatic fluid gradient for a given depth, it prevents the effective stress from increasing with depth as it usually would (Vanorio et al, 2005).…”
Section: Lowmentioning
confidence: 99%
“…Knowledge of the seismic velocity structure can therefore help to identify melt-bearing regions, hydrothermal fluids and over-pressurised gases beneath a volcano (e.g. Londoño & Sudo, 2003;Vanorio et al, 2005;Lin, 2013;Wilks et al, 2020). These observations are crucial to understanding volcanic unrest, characterising future volcanic hazard and assessing geothermal potential.…”
Understanding the crustal structure and the storage and movement of fluids beneath a volcano is necessary for characterising volcanic hazard, geothermal prospects and potential mineral resources. This study uses local earthquake traveltime tomography to image the seismic velocity structure beneath Nabro, an off-rift volcano located within the central part of the Danakil microplate near the Ethiopia-Eritrea border. Nabro underwent its first historically-documented eruption in June 2011, thereby providing an opportunity to analyse its post-eruptive state by mapping subsurface fluid distributions. We use a catalogue of earthquakes detected using machine learning methods to simultaneously relocate the seismicity and invert for the threedimensional P-and S-wave velocity structures (Vp, Vs) and the ratio between them (Vp/Vs). Overall, our model shows higher than average P-and S-wave velocities, suggesting the presence of older consolidated volcanic deposits or intrusive magmatic rocks in the crust. We identify an aseismic region of low Vp, low Vs and high Vp/Vs ratio at depths of 6-10 km b.s.l., interpreted as the primary melt storage region that fed the 2011 eruption. Above this is a zone of high Vs, low Vp and low Vp/Vs ratio, representing an intrusive complex of fractured rocks partially-saturated with over-pressurised gases. Our observations identify the persistence of magma in the subsurface following the eruption, and track the degassing of this melt through the crust to the surface. The presence of volatiles and high temperatures within the shallow crust indicate that Nabro is a viable candidate for geothermal exploration.
“…In saturated or partially-saturated rocks, the content and physical state of fluids has a greater effect on P-wave velocities than S-wave velocities (Vanorio et al, 2005). Fluid phase transitions induce changes in fluid compressibility and thus bulk modulus (Ito et al, 1979;Wang & Nur, 1986).…”
Section: Interpretation Of V P /V S Variationsmentioning
confidence: 99%
“…Shear moduli are little affected by fluid phase transitions and hence S-wave velocities change insignificantly due to a density effect, meaning that low V P /V S ratios tend to characterise gas-bearing rocks (i.e. those with high fluid compressibility) whereas liquid-bearing rocks (with low fluid compressibility) are characterised by high V P /V S ratios (Vanorio et al, 2005).…”
Section: Interpretation Of V P /V S Variationsmentioning
confidence: 99%
“…Overpressure is one explanation for this. If the fluid pressure is higher than the normal hydrostatic fluid gradient for a given depth, it prevents the effective stress from increasing with depth as it usually would (Vanorio et al, 2005).…”
Section: Lowmentioning
confidence: 99%
“…Knowledge of the seismic velocity structure can therefore help to identify melt-bearing regions, hydrothermal fluids and over-pressurised gases beneath a volcano (e.g. Londoño & Sudo, 2003;Vanorio et al, 2005;Lin, 2013;Wilks et al, 2020). These observations are crucial to understanding volcanic unrest, characterising future volcanic hazard and assessing geothermal potential.…”
Understanding the crustal structure and the storage and movement of fluids beneath a volcano is necessary for characterising volcanic hazard, geothermal prospects and potential mineral resources. This study uses local earthquake traveltime tomography to image the seismic velocity structure beneath Nabro, an off-rift volcano located within the central part of the Danakil microplate near the Ethiopia-Eritrea border. Nabro underwent its first historically-documented eruption in June 2011, thereby providing an opportunity to analyse its post-eruptive state by mapping subsurface fluid distributions. We use a catalogue of earthquakes detected using machine learning methods to simultaneously relocate the seismicity and invert for the threedimensional P-and S-wave velocity structures (Vp, Vs) and the ratio between them (Vp/Vs). Overall, our model shows higher than average P-and S-wave velocities, suggesting the presence of older consolidated volcanic deposits or intrusive magmatic rocks in the crust. We identify an aseismic region of low Vp, low Vs and high Vp/Vs ratio at depths of 6-10 km b.s.l., interpreted as the primary melt storage region that fed the 2011 eruption. Above this is a zone of high Vs, low Vp and low Vp/Vs ratio, representing an intrusive complex of fractured rocks partially-saturated with over-pressurised gases. Our observations identify the persistence of magma in the subsurface following the eruption, and track the degassing of this melt through the crust to the surface. The presence of volatiles and high temperatures within the shallow crust indicate that Nabro is a viable candidate for geothermal exploration.
“…CFc hosts a densely inhabited urban area home to about 350,000 people, including the western neighborhoods of the city of Naples 37 , 82 , 90 . Hazard assessment is thus particularly critical 15 , 22 , 86 , 103 , and the interaction of the active magmatic and hydrothermal systems complicates the interpretation of monitoring data 26 . Figure 1 ( a ) Campi Flegrei caldera GPS monitoring network (small squares, 21 permanent stations + 4 buoys offshore) 27 .…”
Ongoing resurgence affects Campi Flegrei caldera (Italy) via bradyseism, i.e. a series of ground deformation episodes accompanied by increases in shallow seismicity. In this study, we perform a mathematical analysis of the GPS and seismic data in the instrumental catalogs from 2000 to 2020, and a comparison of them to the preceding data from 1983 to 1999. We clearly identify and characterize two overlying trends, i.e. a decennial-like acceleration and cyclic oscillations with various periods. In particular, we show that all the signals have been accelerating since 2005, and 90–97% of their increase has occurred since 2011, 40–80% since 2018. Nevertheless, the seismic and ground deformation signals evolved differently—the seismic count increased faster than the GPS data since 2011, and even more so since 2015, growing faster than an exponential function The ground deformation has a linearized rate slope, i.e. acceleration, of 0.6 cm/yr2 and 0.3 cm/yr2 from 2000 to 2020, respectively for the vertical (RITE GPS) and the horizontal (ACAE GPS) components. In addition, all annual rates show alternating speed-ups and slow-downs, consistent between the signals. We find seven major rate maxima since 2000, one every 2.8–3.5 years, with secondary maxima at fractions of the intervals. A cycle with longer period of 6.5–9 years is also identified. Finally, we apply the probabilistic failure forecast method, a nonlinear regression that calculates the theoretical time limit of the signals going to infinity (interpreted here as a critical state potentially reached by the volcano), conditional on the continuation of the observed nonlinear accelerations. Since 2000, we perform a retrospective analysis of the temporal evolution of these forecasts which highlight the periods of more intense acceleration. The failure forecast method applied on the seismic count from 2001 to 2020 produces upper time limits of [0, 3, 11] years (corresponding to the 5th, 50th and 95th percentiles, respectively), significantly shorter than those based on the GPS data, e.g. [0, 6, 21] years. Such estimates, only valid under the model assumption of continuation of the ongoing decennial-like acceleration, warn to keep the guard up on the future evolution of Campi Flegrei caldera.
We review pre-eruptive dynamics and evidence of open-system behavior in the volcanic plumbing system beneath Campi Flegrei Caldera, together with estimates of magma residence time, magma ascent, and mixing-to-eruption timescales. In detail, we compile pre- and syn-eruptive dynamics reported in the literature for (a) the Campanian Ignimbrite ~ 40 ka, (b) the Neapolitan Yellow Tuff (~ 15 ka), and (c) the recent activity within the Phlegrean area. We first summarize geochemical and textural evidence (e.g., magma mixing, crystal disequilibria, vertical zonings, and isotopic records) of open-system behavior for the pyroclasts erupted in the last 40 ky at Campi Flegrei Caldera. We show that the fingerprint of open-system dynamics is ubiquitous in the deposits associated with the volcanic activity at the Campi Flegrei Caldera in the last 40 ky. Then, we describe the results of geophysical and petrological investigations that allow us to hypothesize the structure of the magma feeding system. We point to a trans-crustal magmatic feeding system characterized by a main storage reservoir hosted at ~ 9 km that feeds and interacts with shallow reservoirs, mainly placed at 2–4 km. Finally, we define a scenario depicting pre-eruptive dynamics of a possible future eruption and provide new constraints on timescales of magma ascent with a physical model based on magma-driven ascending dyke theory. Results show that considerably fast ascent velocities (i.e., of the order of m/s) can be easily achieved for eruptions fed by both shallow (i.e., 3–4 km) and deep (i.e., ~ 9 km) reservoirs. Comparing the results from experimental and numerical methods, it emerges that mixing-to-eruption timescales occurring at shallow reservoirs could be on the order of minutes to hours. Finally, we highlight the volcanological implications of our timescale estimates for magma ascent and mixing to eruption. In particular, explosive eruptions could begin with little physical ‘warning’, of the order of days to months. In this case, the onset of volatile saturation might provide pre-eruptive indicators.
Graphical Abstract
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