The analysis of the deformation and seismicity data recorded in the Phlegraean caldera between 2000 and 2023 has highlighted the close relationship between ground uplift and the number of earthquakes
A team of researchers from the National Institute of Geophysics and Volcanology (INGV) analyzed the evolution of ground deformation and seismic activity at Campi Flegrei from 2000 to 2023, quantifying their acceleration over time and deriving an exponential relationship between the maximum uplift of the caldera and the cumulative number of seismic events.
The results, presented in the article “Accelerating upper crustal deformation and seismicity of Campi Flegrei caldera (Italy), during the 2000-2023 unrest” just published in the journal 'Communications Earth & Environment' of Nature, derive from the study of the main geophysical signals recorded at Campi Flegrei by the monitoring networks of the Vesuvius Observatory of the INGV that the recent evolution of bradyseism has allowed to be analyzed in depth.
“The study conducted,” explains Augusto Neri, INGV researcher and coordinator of the research, “does not adopt specific physical models a priori but rather aims to represent the evolution of the crisis in the most objective and neutral way possible through a rigorous mathematical analysis of the data from the geophysical monitoring networks. Through this analysis it is possible to represent, albeit in synthetic and approximate terms, the behavior of the volcano and highlight its changes over time with the ultimate goal of improving the understanding of its functioning.”
In particular, the study highlighted the continuation of a long-term (decennial) acceleration of geophysical variables that began in 2005 and how this acceleration has not been constant over time. "On a decadal scale", explains Andrea Bevilacqua, researcher at INGV and first author of the study, "the ground uplift follows a parabolic trend with an average acceleration of about 0,7-0,8 cm/year² with reference to the GNSS station of Rione Terra in Pozzuoli, at the center of the caldera. The temporal trend of the earthquake rate is instead supra-exponential, that is, faster than an exponential trend. However, it is important to highlight how these trends are not constant over time but subject to oscillations of various frequencies. The main periods of these oscillations vary from about 2 to 5 months (for the shortest periods) up to about 1,5 and 3 years (for the longest periods). In recent years, a trend of reduction of these periods by about 10-15% has also been observed. One implication of these results is that periods of reduced seismic activity are not necessarily indicative of a change in the decadal behavior of the volcano".
A particularly interesting aspect of the study concerned the relationship between ground deformation and the number of recorded earthquakes.
“The study has highlighted,” Neri continues, “a clear exponential relationship between the maximum uplift of the caldera and the cumulative number of recorded earthquakes. This relationship is different from the linear one observed during the last bradyseismic crisis of 1982-1984. Furthermore, the exponential relationship has become stronger starting from around the year 2020, that is, as the uplift of the caldera approached the maximum level reached during the crisis of 1982-1984. The relationship explains why the uplift of the caldera recorded in recent years has been accompanied by more intense seismic activity compared to previous years. This behavior is similar to that of quasi-elastic materials subjected to increasing stress and can be interpreted as a progressive deterioration of the mechanical properties of the uppermost crust of the Campi Flegrei”.
The published study uses data recorded up to November 2023 but is still continuously updated based on new monitoring data recorded.
“The analyses updated up to the end of October 2024,” concludes Flora Giudicepietro, researcher at INGV and co-author of the study, “confirm that the trends and relationships identified in the period 2000-2023 are still valid. This means that as the uplift speed increases, the probability of earthquakes in Campi Flegrei in the following months also increases. Another implication of this study is that, if these trends were to continue with the same characteristics in the future, further uplift of the caldera could be associated with seismic activity rates higher than those recorded in 2023, as already happened in May 2024. This scenario represents a possible future evolution if the bradyseismic crisis currently underway were to persist. On the other hand, it is also possible that the uplift process of the Phlegraean soil will slow down over time, and this would also lead to a reduction in seismic activity.”
The published research has an essentially scientific value, without immediate implications regarding the aspects of civil protection at the moment.
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(Figure 1. (a) Maximum vertical ground displacement at Campi Flegrei (CFc) in the period 1983-11/2023. The data combine leveling measurements collected at reference point 25A and GNSS data from the RITE station. (b) Number of earthquakes per month recorded at Campi Flegrei (CFc) in the period 1983-11/2023. In both figures (a) and (b), the data relating to the period 2000-11/2023 are highlighted in blue. The data are adapted from the periodic bulletin of Campi Flegrei of INGV-OV (http://www.ov.ingv.it) (Bevilacqua et al 2024).
Figure 2. Relationship between the maximum vertical uplift recorded at the RITE GNSS station and the cumulative number of seismic events in the period 2000-11/2023. The graph refers to the number of earthquakes with Md > 0,5. The data are represented by red dots. The years are indicated in the labels and as black dots and show the temporal evolution of the uplift. The green and blue curves indicate the two exponential functions that fit the data in the graph, together with their exponents (from Bevilacqua et al. 2024).
Figure 3. The Temple of Serapis (Macellum), located in the city of Pozzuoli in the center of the Campi Flegrei caldera. Over the centuries, the Temple of Serapis has become a sort of indicator of the bradyseism of the Campi Flegrei. In fact, its monolithic columns have been used to measure the variation in sea level through the observation of particular marine molluscs (lithodomes) that have progressively damaged the lower half of the columns. Today, the temple is located just above sea level (see the Gulf of Pozzuoli in the background) (photo by Augusto Neri, INGV).