Flandoli, Franco

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.

At Mt. Etna volcano, flank eruptions are often accompanied by seismic swarms with damaging earthquakes; the most recent case-history is the 2018 flank eruption, associated with a destructive earthquake (Mw 5.0). In this paper, we analyze the earthquake and eruptive catalogs from 1800 to 2018, to produce quantitative estimates of the earthquake rate under the influence of flank eruptions. We quantify that 30% of the flank eruption onsets precede a major (i.e., damaging, Ix ≥ V–VI EMS) earthquake by 30 days or less; 18% of the major earthquakes follow a flank eruption onset in 30 days or less. Thus, we show that the probability of major earthquakes increases 5–10 times after the onset of flank eruptions and this effect lasts for 30–45 days. This is also observed after the end of the eruptions. Results indicate different relationships depending on the location of the volcano-tectonic systems considered individually (eruptive fissures, seismogenic faults). For instance, we describe a 10–20 times increased probability of earthquakes for 65–70 days after eruptions on the northeastern flank, and of new flank eruptions for 45–70 days after earthquakes of the Pernicana fault.

This compressed folder contains supporting information related to the manuscript: "Quantifying the statistical relationships between flank eruptions and major earthquakes at Mt. Etna volcano (Italy)". Data Set S1. Catalog of flank eruptions Historical catalog of flank eruptions of Mt. Etna from 1600 to 2018. Data Set S2. Catalog of major earthquakes Macroseismic catalog of Etnean earthquakes from 1800 to 2018. Data Set S3. GIS dataset of Eruptive fissures GIS shapefiles of eruptive fissures at Mt. Etna from 1800 to 2018. UTM WGS84, Zone 33 N. Data Set S4. Tests with ±2 months maximum inter-event time Histograms of the inter-event time of earthquakes and flank eruptions lesser than ±2 months. Pie charts of the positive values (dark colors), negative values in [-2.5, 0] days (light colors), and lower than -2.5 days (white) are reported. Data Set S5. Tests with dt = 5 days moving window Conditional rates of major earthquakes less than ±4 months from flank eruptions, obtained assuming dt = 5 days instead of dt = 10 days. A solid line marks the average annual rate of the earthquakes, and bold lines threshold rates 2, 5, and 10 times larger than the average value. Data Set S6. Tests on the eruptions end, including earthquakes in ±2.5 days from the onset Conditional rates of major earthquakes less than ±4 months from flank eruptions end, obtained without excluding the earthquakes occurred in ±2.5 days from the onset. A solid line marks the average annual rate of the earthquakes, and bold lines threshold rates 2, 5, and 10 times larger than the average value. Data Set S7. Summary of inter-event time histograms Histograms of the inter-event time of earthquakes and flank eruptions lesser than ±4 months, also decomposed according to spatial groups E1-E4 and fault systems F1-F4. Light-colored bars highlight the eruptions > 2850 m.a.s.l.

The problem of evaluating the statistical dependence among different series of events is often encountered in geophysics. On active volcanoes, the typical question is to relate different phenomena having significant impact in terms of multi-hazard implications. At Mt. Etna volcano, flank eruptions are often related to the occurrence of damaging earthquakes; the last case-history is the 2018 flank eruption, accompanied by a seismic swarm producing a destructive earthquake (Mw 4.9). The statistical analysis of the seismic and eruptive records on a significant time-span may contribute to defining potentially hazardous conditions and enhancing civil protection preparedness.

Episodes of slow uplift and subsidence of the ground, called bradyseism, characterize the recent dynamics of the Campi Flegrei caldera (Italy). In the last decades two major bradyseismic crises occurred, in 1969/1972 and in 1982/1984, with a ground uplift of 1.70 m and 1.85 m, respectively. Thousands of earthquakes, with a maximum magnitude of 4.2, caused the partial evacuation of the town of Pozzuoli in October 1983. This was followed by about 20 years of overall subsidence, about 1 m in total, until 2005. After 2005 the Campi Flegrei caldera has been rising again, with a slower rate, and a total maximum vertical displacement in the central area of ca. 70 cm. The two signals of ground deformation and background seismicity have been found to share similar accelerating trends. The failure forecast method can provide a first assessment of failure time on present‐day unrest signals at Campi Flegrei caldera (Italy) based on the monitoring data collected in [2011, 2020] and under the assumption to extrapolate such a trend into the future. In this study, we apply a probabilistic approach that enhances the well‐established method by incorporating stochastic perturbations in the linearized equations. The stochastic formulation enables the processing of decade‐long time windows of data, including the effects of variable dynamics that characterize the unrest. We provide temporal forecasts with uncertainty quantification, potentially indicative of eruption dates. The basis of the failure forecast method is a fundamental law for failing materials: ẇ^-α ẅ = A, where ẇ is the rate of the precursor signal, and α, A are model parameters that we fit on the data. The solution when α >1 is a power law of exponent 1/(1 − α) diverging at time Tf , called failure time. In our case study, Tf is the time when the accelerating signals collected at Campi Flegrei would diverge if we extrapolate their trend. The interpretation of Tf as the onset of a volcanic eruption is speculative. It is important to note that future variations of monitoring data could either slow down the increase so far observed, or suddenly further increase it leading to shorter failure times than those here reported. Data from observations at all locations in the region were also aggregated to reinforce the computations of Tf reducing the impact of observation errors.

Sub-Task 2 del Task 2: "Realizzazione di un sistema di monitoraggio in tempo reale delle deformazioni del suolo dell'area vulcanica napoletana (Campi Flegrei, Vesuvio ed Ischia) tramite misure GNSS ad alta frequenza (HR-GNSS) e sviluppo di modelli statistici e numerici per la mappatura della probabilità eruttiva a breve termine della caldera dei Campi Flegrei"

Sub-task 2 del Task 7: "Stima quantitativa e modellazione della dipendenza statistica di sismicità ed eruzioni all’Etna, sulla base dei dati storici, con particolare attenzione al possibile legame tra terremoti maggiori ed eruzioni laterali"

In questo studio, usando il failure forecast method si fornisce una stima preliminare del failure time dei segnali di unrest della caldera dei Campi Flegrei, concentrandoci sul dato di deformazione orizzontale registrato in 11 stazioni GPS nel periodo [2011, 2020]. In particolare, si applica un approccio probabilistico che modifica il metodo classico incorporando un rumore stocastico nelle equazioni linearizzate e una proprietà di mean reversion per modularne gli effetti. La formulazione stocastica permette di analizzare i dati registrati in ca. 10 anni di monitoraggio, includendo, tramite il rumore, gli effetti della dinamica variabile che caratterizza l’unrest della caldera. Si forniscono previsioni temporali con quantificazione dell’incertezza, cioè informazioni su un intervallo di possibili failure times.