Magmatic fluids uprise through ring faults at Campi Flegrei caldera

doi:10.21203/rs.3.rs-4998085/v1

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Magmatic fluids uprise through ring faults at Campi Flegrei caldera

https://doi.org/10.21203/rs.3.rs-4998085/v1

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The progressive increase of ground deformation, seismicity, and gas emission is marking a remarkable unrest at Campi Flegrei caldera. The direct involvement of magma has been invoked to explain the deformation and space/time changes of velocity anomalies at shallow crustal depths. A challenging aspect is to forecast possible scenarios for the upward migration of magmatic fluids from the source at depth. Here, we show that the most recent seismicity (period 2023–2024), derived by a machine-learning-based earthquake detection procedure, aligns on a continuous set of caldera rim faults and on top of an inflating magma source. Direct channeling of magma through such ring faults can be a way to feed future eruptions, as observed in other calderas and inferred for the Mt. Nuovo historical eruption.

Earth and environmental sciences/Natural hazards

Earth and environmental sciences/Solid Earth sciences/Volcanology

Earth and environmental sciences/Solid Earth sciences/Seismology

The Campi Flegrei (CF) caldera was produced by at least two main large explosive eruptions that occurred at 39 ka (Campanian Ignimbrite, CI) and 14.5 ka (Neapolitan Yellow Tuff, NYT). It is characterized by a nested structure, hosting about 70 minor volcanic eruptions since the NYT^1,2. Three distinct caldera rims (Fig. 1a) and associated fault zones accommodated the CI caldera collapse and were reactivated during the subsequent NYT collapse and the younger volcano-tectonic activity^3,4. In the last 15ka, volcanic eruptions generally occurred across the ring faults, highlighting their primary role in controlling the upward magma migration and localisation of several eruptive vents, magmatic dikes, and hydrothermal fluids³.

The Campi Flegrei seismicity⁵ of the ongoing and previous (1982–1984) unrest episodes shows an overall stationary distribution on an elliptical region centered above the central resurgent zone (Fig. 1a). Since the beginning of the unrest in 2005, the tight correlation between the increasing uplift rate (up to 2 cm/month) and seismic rate (up to 1500 events/month) is emerging^6–8, supporting the idea that it is generated by the same local source producing ground deformation and that the stress changes associated with overpressure in the shallow magma/magmatic fluids reservoir are the primary cause for local seismicity. Since 2023, both moment magnitude and seismic rate have escalated (Fig. 1b), together with the uplift rate.

The source of deformation responsible for ground deformation and seismicity is thought to be located at 3–5 km depth, based on deformation data^9,10 and tomographic studies^11–13. Neither the precise depth nor the composition (magma/magmatic fluids) of the source is definitively clear. A new injection of magma into the reservoir located at about 5 km depth, started in 2019, has been hypothesized based on recent tomographic data¹³. This epoch is contemporary with the beginning of the deepening of earthquakes (Fig. 1c), which progressively activated fracture zones at the margin of the resurgent dome⁸.

The uprise of deep fluids within the caldera is revealed by the huge amount and the composition of gas emissions in the Solfatara-Pisciarelli zone¹⁴. In the shallow hydrothermal system, the key role of fluids in triggering seismicity is well-established, supported by the observed correlation between seismic energy release, gas emission and increasing P-T¹⁴, and transient fluids flows revealed by ambient noise studies¹⁵. At greater depth, the fluid role is more questionable. Deep fluids injection from depth can be of course effective in increasing the stress and reactivating pre-existing weakness zones, but the causative process relating the deeper microseismicity to the source of deformation responsible for the bradyseism is indeed still poorly understood.

Our understanding of the interrelations between seismicity, pre-existing volcano-tectonic structures, deep fluid release, and source of ground uplift is also hampered by uncertainties in earthquake locations. These are often related to the completeness of information derivable from seismic data, to the quality of seismic phase arrival times pickings, and the accuracy of velocity models. In this study, we show that a machine-learning (ML)-based earthquake detection procedure, combined with detailed and updated velocity models, allows a more comprehensive view of the causal process of seismicity at the Campi Flegrei caldera.

We applied machine learning techniques to build a new seismic catalog for the period Jan-2023/June-2024 and computed improved hypocentral determinations by relocating the seismicity with both absolute and relative procedures (see Methods). The overall catalog consists of about 8,400 seismic events (Fig. 1a), 2,200 of which are characterized by a good hypocentral distribution after the 1D absolute locations, with low hypocentral errors and RMS (Figs. 2c, S1). We computed the local magnitude (Ml) of the overall catalog using the parameters in the GaMMA code¹⁶ and compared determinations with official magnitude reported by OV, based on the earthquake duration (Md). Ml ranges between − 1.08 and 4.5, mostly picked in the M1-2 band (Fig. 2b). We also compared the GaMMA-derived Ml with local magnitude derived from the official Md (see Methods, Fig. S3). The Magnitude range is similar, with a general consistency in the scaling between the two estimates. Anyway, a strong concentration of events with Md-derived ML in the range M0-1, corresponding to GaMMA-derived Ml in the range M1-2, suggests a possible underestimation for some of the Md < 1 events in the catalog. Since this potential underestimation has been already observed¹⁷, and the magnitude conversion relationship¹⁷ has been defined for Md > 1.5, a revision of estimates and calibration for small magnitude earthquakes, potentially influenced by location uncertainties at such local scale, is needed.

In the refined relative DD-locations (Figs. 2d and S2), seismicity is clustered between 2 to 4 km depth and the Caldera rings are better visible, while there is a tendency of excluding less clustered and deeper events. Nevertheless, two structures of the caldera ring are clearly visible: one shallower located from 2 to about 4 km depth and the other one deeper located between 4–6 km depth (sections 4 and 7–8 in Fig. S2).

Relocated events and ring fault structures

The overall seismicity is distributed over an elliptical region centered above the maximum uplift of the resurgent dome, with a remarkable concentration of shallow events (1-2.5 km) events in the northeastern region (Fig. 2d). This region is delimited to the east by the Astroni and Agnano craters, to the north by the Cigliano-Agnano E-W-striking S-dipping fault and to the south by the E-W-striking, N-dipping master fault⁴. Both these faults are far from the northern inner margin of the caldera.

The deeper seismicity (depth > 3 km Fig. 2d) defines an almost continuous ring, with some sectors characterized by relatively higher concentrations of clustered earthquakes, and with the deeper events confined offshore. Here, the clear ring-like distribution and the inward-dipping plane defined by seismicity (Fig. 2e) suggest that earthquakes occur along the prolongation at the depth of the inner caldera ring faults imaged by seismic data³. Although in agreement with previous studies^7,8, differences regarding the maximum depth reached by the earthquakes and their longer extent are more likely revealed in this study. The high accuracy of earthquake relocations allowed us to reconstruct the ring-shape with unprecedented resolution, showing an enhanced narrowing and continuity of the structure, mostly in the offshore. Another main novelty of our results is the appearance of two separated branches in the easternmost sector (Fig. 2d), the outermost of which is located in correspondence with the inner margin of the caldera.

While in the offshore region, the ring-like distribution of deeper earthquakes (> 3 km) is well-reconstructed, its in-land continuation seems more scattered, with a gap to the east of the Monte Nuovo crater and at the western boundary of the Agnano Plain, in the same region where the most shallow seismicity is observed. The more continuous portion of the ring highlighted by the deep seismicity, which in this sector does not exceed 4 km depth, correlates with the position of the E-W-striking, S-dipping Cigliano-Agnano fault (Fig. 2d). This fault could represent the on-land “main” ring fault activated during the ongoing unrest, while in the offshore the main ring fault coincides with the inner ring faults that accommodated the CI and NYT collapses.

In vertical cross-sections centered over the maximum deformation of the resurgent volume, earthquakes clearly show a bell-shaped distribution (Fig. 2e). Also in this case, despite the clear presence of clustered events, the distribution of seismicity shows an enhanced uniform distribution above the resurgent area. In the WSW-ENE cross-section earthquakes seem to delineate two outward-dipping planes. The eastern deep cluster of seismicity defines a slightly outward-dipping plane, the site of one of the larger magnitude (Md 4.2) events. This structure has been already identified in recent investigations (La Pietra fault⁸) but its relation with the inner caldera margin is poorly constrained due to the lack of deep-penetrating seismic profiles.

Ring faults and caldera velocity structure

The analyses of the DD-relocated seismicity overlapped on the 3D velocity models¹³ allow new insights into the relationship between the occurrence of seismicity, pre-existing tectonic structures and the velocity structure of the caldera.

At depth greater than 3 km, there is a tight correlation between the ring defined by seismicity offshore and the steep velocity gradient between the internal region (low Vp, low Vs, low Vp/Vs anomaly B Figs. 3–4), corresponding to the infilling caldera gas-bearing overpressured formations^13,18, and the external region (high Vp, high Vs, anomaly A Figs. 3–4), corresponding to hydro-thermalized volcanic formations^19–21.

At 5 km depth (Fig. 3), the low Vp, low Vs, and high Vp/Vs volume (Anomaly C), interpreted as the magma filled reservoir underneath the caldera¹³, is clearly confined inside the ring defined by seismicity. In particular, it is confined to the east by the innermost branch of the ring. Such a clear correspondence between seismic anomalies, ring structure and seismicity has never been highlighted before at the Campi Flegrei caldera.

From a general point of view, and without requiring an active role of hydrothermal fluids, the relative position between the ring faults and the magma reservoir is in agreement with the stress field changes produced by a source of overpressure, in the presence of pre-existing ring faults at the top of the source, and with a background regional tensile stress^22–24. According to this model, positive Coulomb stress changes favoring seismic release occur in the area enclosed by the ring faults and atop the source of deformation. The main effect of the ring faults is to focus the shear stress along the ring fractures, relieving the shear stress from the doming caldera center. The difference in the maximum depth of seismicity offshore (ca. 5–5.5 km) compared to the inland (ca. 3.5–4 km) can be explained by the different depths of the pre-existing ring faults, formed during major caldera collapses (IC and NYT) and minor volcano-tectonic events, respectively.

The reverse focal mechanism of the M_d = 4.2 event occurred along the La Pietra outward-dipping fault, which differs from the majority of normal-faulting earthquakes observed along the inward-dipping ring faults in the offshore⁸, is also consistent with the stress generated by an inflating magmatic source²⁴.

The distribution of seismicity compared to the velocity structure also corroborates the hypothesis that a magmatic reservoir at 5 km depth (anomaly C Figs. 3–4), together with the overlying overpressured gas-bearing cap formations (anomaly B Figs. 3–4), is the main source of deformation responsible for the bradyseism, while the pure poroelastic response of the shallow hydrothermal system^25,26 is unlikely.

Seismicity may be triggered by a coupled effect of pore pressure and elastic stress increase in the rock fabric^27–29. We think that the deep fluid uprise co-acts with the deformation source in creating conditions suitable for earthquake generations, by increasing the pore pressure and reducing the frictional resistance along the faults rings.

Inland ring-fault complexity

As previously noted, the ring faults are well-reconstructed by the seismicity distribution in the offshore region, while the in-land continuation seems more confused, with scattered seismicity at shallower depths (Z < 3 km). Moreover, the volcano-tectonic structures activated by the present seismicity seem to be located much more internally than the northern margin of the inner caldera collapsed area. Here, the fault network and then the ring fault's complexity are presumably increased by the younger volcano-tectonic events, such as the Agnano Pomici Principali Plinian eruption (12 ka³⁰) and the Plinian Averno - Monte Spina minor caldera collapse (AMS 4.55 ka³¹). The area involved in the last AMS minor caldera collapse³², which formed the Averno Plain, coincides with a strong high Vp, high Vs anomaly (anomaly D Figs. 3–4) located at 3 km depth, and also corresponds to the gap observed in the ring-shape deep seismicity distribution. Moreover, the northern and southern boundaries of the anomaly coincide with the position of the Agnano-Cignano E-W fault and a master fault showing reactivation during the AMS collapse⁴. Most of the seismicity is located in relatively lower velocity zones on both sides and atop the high Vp body (section 3, Fig. 4), which is directly above the strong low-velocity anomaly A associated with the gas-bearing overpressured formation. This suggests that the high-velocity body may act as a low-permeability caprock formation, which somehow controls the uprise of deep fluids, channeling them toward more fractured and permeable weakness zones (lower velocities). Many minor faults are mapped at the western boundary of the Astroni-Agnano craters and in the Solfato-Pisciarelli emission zone, where most of the shallower seismicity is observed, strongly supporting that the highly fracture density promotes and channels the migration and uprise of fluids from depth into the hydrothermal system³³. It is noteworthy that this is also the only region where a deviation from the typical bell-shaped deformation pattern has been observed, possibly indicating that complex structural/lithological heterogeneities create mechanical discontinuities able to modify the stress induced by the primary deformation source³⁴.

The automatic events detection and seismic phase picking by ML-based procedures represent an extraordinary resource for creating high-quality datasets in a very short time, especially from the perspective of real-time monitoring of caldera unrest. The high quality of phase pickings allowed us to obtain accurate earthquake relocations and to reconstruct the shape of the ring faults of the CF caldera with unprecedented resolution.

The comparison between earthquake relocations, velocity structure and preexisting fault zones, shows that offshore seismicity mainly occurs along the inner ring faults formed during the major CI and NYT caldera collapses. Differently, the relatively shallower and more complex distribution of seismicity in the inland appears to be controlled by minor fracture/fault zones and by volcanic intra-calderic formations, likely related to minor and younger tectonic-volcanic events.

The overall structure of the ring faults depicted by the seismicity well resembles that of other worldwide calderas (e.g., Rabaul, Papua Nuova Guinea^35,36; Tendürek volcano, Turkey³⁷), characterized by asymmetric inward and outward-dipping faults formed during different stages of the caldera evolution³⁸. The spatial correlation between the seismicity distribution along the ring faults and atop the magma reservoir revealed by seismic tomography indicates that this is likely the main source of deformation at Campi Flegrei, responsible for the localization of strain onto reactivated fault sets, well matching the stress field predicted by numerical modeling^23,24,34.

The clear reconnaissance of rings fault structure at CF and its full activation during unrest episodes like the active one pose significant new elements for the evolution of the system. Even if we can only speculate on where the formation of future vents is more likely and where more likely phreato-magmatic eruptions are, the joint interpretation of seismicity, velocity structure, and deformation gives valuable constraints. The volcanic history at Campi Flegrei, and in other calderas (e.g. Rabaul caldera^36–37), revealed that vent opening frequently occurs in correspondence of ring faults, suggesting that they are preferential pathways for fluids and magma uprise. A different ascent behavior between magma and magmatic fluids is to be expected due to the difference in density compared to the formations of the caldera center. The lower density of the magmatic fluids allows them to rise even in the central areas of the caldera, rising along fracture zones and/or creating local overpressures under low permeability caprock layers. On the contrary, given the high density of the magma, it is more likely to intrude at the caldera edges. The depth reached by the ring faults with respect to the magmatic reservoir could also play a crucial role. It seems reasonable to assume that the shorter this distance, the more likely the magma will rise. Conversely, shallower ring structures or shallower fractured zones may act as channels for fluid uprise stored in the gas cap, without a direct connection with the deeper magma reservoir.

This reasoning could explain why Monte Nuovo was the site of the last volcanic eruption (A.D. 1538^39,40), while Solfatara the site of the last phreato-magmatiic eruption (A.D. 1198^41,42). If this explanation was correct, and not considering any intermediate magma reservoirs, one could conclude that it is more likely to have the opening of new vents where the seismicity is deeper, while phreato-magmatic eruptions are to be expected in the north-eastern sector of the caldera, where the most shallow seismicity is observed.

Events detection

The new catalog of about 8,400 events was built with machine-learning detection algorithms, by combining the deep neural network (DNN) PhaseNet code⁴³ that picks P- and S- waves arrival times from continuous seismograms, with the GaMMA associator⁴⁴ that associates and estimates earthquake location and local magnitude. We extracted continuous waveforms from the EIDA portal for the period 2023/01/01–2024/06/18, for 26 seismic stations (network IV, managed by Istituto Nazionale di Geofisica e Vulcanologia) located in an area with a range of latitude between 40.5° − 41.16° and a range of longitude between 13.76° − 14.42°. The DDN PhaseNet associates the P- and S- waves probability distributions to the predicted P- and S- arrival times⁴³. The GaMMA associator uses these arrival times to cluster phase picks based on the physical constraints of arrival time moveout and amplitude decay with distance, using an unsupervised Bayesian Gaussian Mixture Model⁴⁴.

PhaseNet identified 869,877 and 689,613 P- and S- waves respectively in the period from 1st January 2023 to 18th June 2024. GaMMA associated 88,373 and 80,581 P- and S- waves from this initial dataset, respectively, and identified 16,334 seismic events. To select the best initial location, we filtered the seismic catalog considering only the seismic events with at least 4 P- and 2 S- arrival times, obtaining a final dataset of 8,374 events.

Events relocation

To improve hypocentral determinations, the seismicity was relocated with absolute (Hypoellipse⁴⁵) and relative (HypoDD^46–47) procedures. We computed 1D Vp and Vp/Vs models (Table S1) from the recent 4D local earthquake tomography¹³ performed at Campi Flegrei Caldera (5th epoch 2022). Then, we selected seismic events for HypoDD-relocations which have RMS < 0.2 sec, horizontal and vertical errors < 1 km; GAP < 240°, and a minimum residual of P- and S- waves arrival-times < 0.3 sec (Figs. 2a and S1 in the Supporting Information). In the relative locations, residuals between observed and theoretical travel-time differences (or double-differences) are minimized for pairs of earthquakes at each station while linking together all observed event-station pairs, obtaining high-resolution relative relocations. We used 46,299 and 242,301 P- and S- differential travel times. After 22 iterations, we obtained 2,053 clustered seismic events (Figs. 2d and S2 in the Supporting Information), considering a maximum distance between events of 20 km, a maximum number of neighbors of 40, a minimum of 8 links required to define a neighbor and a number of observations between 8 and 60. After the relocations, 94% of the initial dataset was clustered with a reduction of the rms of about 60%.

Computation of Local Magnitude (Ml)

The local magnitude of the new catalog is provided by GaMMA and estimated following ref. 16. In order to compare the local magnitude derived from the GaMMA parameters with already published local magnitude¹⁷, we selected 4200 events in the period from 1st January 2023 to 18th June 2024 that are present both in our catalog and in the official one. Then, the Md-derived Ml was computed from the official duration magnitude (Md) by applying the linear relationship Ml = 0.23 + 0.86Md¹⁷.

Competing interests

The authors declare no competing interests.

Author contributions

G.G. and C.C. conceived the idea of the work, performed data curation and wrote the manuscript. R.F. processed the seismic data and wrote the manuscript. P.DG. and A.G. performed statistical analysis and processed the seismic data.

Data availability

Waveforms data, recorded by the Istituto Nazionale di Geofisica e Vulcanologia (INGV) permanent and temporary networks are available at EIDA, the European Integrated Data Archive⁴⁹. Figures are generated by using the Generic Mapping Tools (GMT)⁵⁰.

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Magmatic fluids uprise through ring faults at Campi Flegrei caldera

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Abstract

Figures

Introduction

Results and discussion

Relocated events and ring fault structures

Ring faults and caldera velocity structure

Inland ring-fault complexity

Conclusions

Methods

Events detection

Events relocation

Computation of Local Magnitude (Ml)

Declarations

Competing interests

Author contributions

Data availability

References

Additional Declarations

Supplementary Files

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