Shallow marine volcanism can produce violent phreatomagmatic eruptions that pose a significant hazard to surrounding coastal communities. Kolumbo volcano, located 7 km northeast of Santorini, is one of the most hazardous volcanic centers in the Aegean Sea. Kolumbo last erupted explosively in 1650, causing over 70 casualties and forming the 2500 m wide and 500 m deep present-day crater. While 2D seismic reflection data have provided a general overview of its the temporal evolution, the full complexity of Kolumbo’s internal structure can only be reconstructed using three-dimensional (3D) data. Here, we use high-resolution 3D seismic reflection data covering most of Kolumbo’s edifice, and present a detailed reconstruction of its internal architecture and the processes that have controlled its evolution. We show that Kolumbo’s edifice was formed by more than the previously assumed five major explosive eruption.  Furthermore, we present evidence for at least one additional explosive eruption that formed a crater larger than the present-day crater from the 1650 eruption. Together, the increased explosive potential of past eruptions and the higher eruption frequency indicate a greater than was previously appreciated. Considering its proximity to Santorini, one of the most visited islands in the eastern Mediterranean, our results emphasise the need for more comprehensive natural hazard monitoring strategies at the central Aegean Sea.

How to cite: Blanch Jover, M., Karstens, J., Kopp, H., Berndt, C., Crutchley, G. J., Preine, J., and Nomikou, P.: Reconstructing Kolumbo’s explosive history: insights from 3D seismic data (Santorini), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10907, https://doi.org/10.5194/egusphere-egu25-10907, 2025.

The Bathymetrists Seamounts (BSM) are an elongated intra-plate volcanic province in the equatorial Eastern Atlantic Ocean. They are situated north of the Sierra Leone Rise, a smooth, aseismic seafloor elevation, believed to have formed above a hot spot at the Mid Atlantic Ridge. The arrangement of the 40 elongated seamounts, some of which are over 100 km long, is enigmatic. The strike direction of the seamounts is either parallel to the intersecting fracture and transcurrent zones or tilted by about 30° to 60°, which has been interpreted as evidence of Riedel shears. Previous age models are based on the geochemistry of a few dredged (surface) rock samples.

With the first set of ca. 4000 km of high-resolution multichannel seismic reflection data, we provide new insights into the structural and temporal evolution of the BSMs. Subsurface images reveal stacked seamount structures, offering direct evidence for their relative order of formation. Based on the DSDP site 366 findings, we establish a chrono-stratigraphic model for the sediment basins between the seamounts. This allows us to approximate the age of the underlying volcanic flanks and to refine the spatial and temporal evolution of BSM edifices through the Paleocene to Eocene. Furthermore, our data reveal signs of recent hydrothermal and magmatic activity, including intrusions, mud diapirism, and fluid chimneys penetrating the volcanic flanks. These findings highlight the BSMs as a dynamic system with both ancient origins and ongoing activity, offering new perspectives on understudied intra-plate volcanism and its associated processes.

How to cite: Hartge, M., Hübscher, C., Seidel, E., and Preine, J.: Volcanic evolution of the Bathymetrists Seamounts (equatorial Atlantic Ocean) since the Paleocene, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2894, https://doi.org/10.5194/egusphere-egu25-2894, 2025.

Campi Flegrei is an active caldera in a densely populated area, currently experiencing significant ground uplift and seismicity. Leveraging precise relocations of extensive seismicity since 2014, we determined high resolution (250 m), 3D P- and S-wave seismic images of the inner caldera which we combine with a novel rock-physics experiment to characterize the primary features of the caldera’s 3D structure: a gas-rich reservoir below 2 km depth, a deformed caprock at 1 to 2 km depth, and a funnel-shaped, (thermo-metamorphic) basement below 3.5 km depth. Seismicity migrates downwards from the caprock to the reservoir, and, following reservoir depletion, stress loading triggers deeper, larger magnitude events along the inner-caldera boundary faults. The reservoir extent and the seismicity distribution closely correlate with the area of maximum uplift, where accelerating deformation due to pore-fluid pressure is corroborated by laboratory experiments using site-relevantin-situ rock samples. These findings suggest that coupling between gas-reservoir pressure and the fibrous microstructure of the confining caprock drives the ground uplift. This structural-dynamic reconstruction of interconnected seismic and ground deformation processes provides a framework for forecasting the evolution of unrest, which is crucial for enhancing medium- and short-term multi-hazard assessment and mitigation strategies. Our results indicate that seismic activity and the potential for a phreatic explosion should be considered as plausible scenarios, prompting a reevaluation of the hazard assessment for the area.

How to cite: De Landro, G., Vanorio, T., Titouan, M., Russo, G., Lomax, A., Virieux, J., and Zollo, A.: 3D Structure and Dynamics of Campi Flegrei Enhance Multi-Hazard Assessment, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11536, https://doi.org/10.5194/egusphere-egu25-11536, 2025.

Integrating seismic and petrological analyses is crucial for comprehensively understanding volcanic systems. In this study, we used the Receiver Function (RF) technique to investigate the crust and upper mantle beneath the Campi Flegrei Caldera (CFc), one of the world’s most active and complex volcanic systems. Over the past decades, this caldera has exhibited signs of unrest, including significant ground deformation, degassing, and seismicity.

RF analysis is highly susceptible to seismic discontinuities, enabling the detection of significant structural features and providing critical insights into P and S-wave velocity distributions. This study extends previous research by incorporating petrological constraints to understand better the relationship between seismic velocity anomalies and the magmatic system beneath CFc.

Using data from thirteen seismic stations located within the caldera, we applied a multi-taper deconvolution method to derive RFs. We then employed Bodin et al.'s (2012) transdimensional inversion approach to retrieve 1D velocity profiles and determine the probability of seismic discontinuities. To connect geophysics and petrology, we used petrological modelling tools to estimate the liquid fraction and the rock temperature in the identified velocity anomalies, offering a robust interpretation of the magmatic system's physical state.

Our analysis revealed two significant Low-Velocity Zones beneath CFc. The first is a crustal reservoir located beneath Nisida island, extending from 8 to 16 km depth with dimensions of approximately 4 x 5 x 12 km. The Vs values are between 3.5 – 4 Km/s, and a melt fraction ranges between 0 to 5%. Below this anomaly, a deeper magma source zone was identified at depths ranging from 16 to 33 km. This layer is characterised by Vs values between 2.3 – 3.3 km/s and melt fraction ranging from 15% to 30%. These seismic models and petrological data suggest that the shallow reservoir contains only partially molten material. At the same time, the deeper zone represents a more significant magma source, potentially feeding the volcanic system. Our results also indicate the presence of an older, dense intrusive complex within the crust, which may influence the migration and storage of magma.

This study demonstrates the value of integrating seismic and petrological analyses to enhance our understanding of volcanic plumbing systems. The identified velocity anomalies provide critical evidence of the magmatic system's geometry and physical properties, highlighting the interplay between crustal and upper mantle structure and magmatic processes. These findings contribute to the ongoing monitoring efforts and hazard assessment of Campi Flegrei. 

How to cite: Ortega-Ramos, V., D'Auria, L., Granja-Bruña, J. L., Cabrera-Pérez, I., Pappalardo, L., Buono, G., and Pérez, N.: Integrating seismic and petrological data unveils the magmatic system beneath Campi Flegrei Caldera., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14066, https://doi.org/10.5194/egusphere-egu25-14066, 2025.

Terceira is a volcanic Island of the Azores Archipelago, located in a tectonic triple junction expression of the intersection of the Mid-Atlantic Ridge with the complex western sector of the Eurasian-Africa Boundary. The island sits on the so-called Terceira rift, the Azorean segment of the triple junction, oblique to the main spreading direction with ultra-slow spreading rate.

The main active volcanic structure is the Santa Barbara volcano to the west of the island, geologically the younger part. The island is divided by a roughly WNW-ESE fissural volcanic system, that intersects Santa Bárbara volcano to the north, crosses the active Guilherme Moniz volcano, in the mid of the island, and the extinct Serra do Cume volcano to the south. Pico Alto, another active volcano, is located north of the fissural system, whereas to the NE lies the significant tectonic structure of the Lajes Graben.

We will present a 3D tomographic model of Terceira Island, obtained from coupled inversion of local earthquake and ambient noise tomography, using a dense temporary seismic deployment (2019-2020) coupled with data from the permanent IPMA Seismic Network (2000-2019).

The Santa Bárbara volcano presents a distinct tomographic signature, with a high Vp and Vp/Vs anomalies located beneath the caldera and extending ~4-6 km, and significant seismicity up to ~6 km depths. On the other hand, although with the same signal the Vp and Vp/Vs anomalies beneath the Pico Alto volcano are much weaker, with few seismicity associated. The shallow layers beneath Guilherme Moniz present weak perturbations on both Vp and Vp/Vs, but a stronger deep high Vp anomaly is clear around 7 km depths with no significant Vp/Vs variation associated. The fissural system signature is usually associated with shallow anomalies, with high Vp north of Santa Bárbara volcano, but low Vp and very low Vp/Vs ~1.6 beneath the area between Pico Alto and Guilherme Moniz volcanoes; the low Vp/Vs anomaly in the center of the island is very shallow <3 km and is probably due to the presence and migration of geofluids in the vicinity of the geothermal powerplant.

The fissural system in this area presents a strong seismic activity, with the deepest events recorded by the networks ~9 km; a relatively high number of very shallow and low magnitude events were recorded around the area of the geothermal powerplant.

The SW area of the Serra do Cume volcano presents high Vp values and usually a normal Vp/Vs ~1.77, suggesting an already frozen volcanic system. It has some significant seismicity associated probably due to the intersection of the fissural system that splits the island. To the NE, a bit surprisingly, no significant seismicity was recorded along the Lajes Graben; the lack of seismicity also gave a low tomographic coverage.

This is a contribution to RESTLESS (DOI:10.54499/PTDC/CTA-GEF/6674/2020) and GEMMA (DOI: 10.54499/PTDC/CTA-GEO/2083/2021) and funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC): UID/50019/2025 and LA/P/0068/2020 https://doi.org/10.54499/LA/P/0068/2020), and UIDB/04683 and UIDP/04683 – Instituto de Ciências da Terra.

How to cite: Dias, N. A., Fontiela, J., Silveira, G., Moreira, M., and Matias, L.: Terceira island, Azores: crustal imaging and correlation of volcano-tectonic structures with seismicity, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12383, https://doi.org/10.5194/egusphere-egu25-12383, 2025.

The Soufrière Hills Volcano on the island of Montserrat in the Lesser Antilles was activated in 1995 after centuries of dormancy. Since then and until 2010, five major phases of activity led to explosive eruptions, dome building and collapse events, pyroclastic flows and ash clouds, which buried the capital, Plymouth, caused loss of life and extensive destruction of infrastructure, rendering the southern part of the island uninhabitable. Montserrat is currently rebuilding, with efforts focusing among others in the importance of better understanding the volcanic system and its associated geothermal field for both risk management and potential resource development.

In this work we jointly invert 7,112 P-wave and 1,376 S-wave arrival times recorded on 18 seismic stations from 1039 local events, classified by the Montserrat Volcano Observatory as tectonic or volcano-tectonic earthquakes, to determine the three-dimensional compressional and shear wave velocities and to simultaneously improve the event locations. Finite frequency sensitivity kernels are used instead of geometrical rays-paths to account for finite frequency effects, providing a more realistic representation of data’s sensitivity. To evaluate the quality and the uncertainty of the tomographic images, we utilize high performance direct sparse algorithms and graph-theory based techniques that allow the efficient calculation of the model’s resolution and posterior covariance matrices, allowing to assess the robustness of the features revealed in the tomographic models.

Our results show significant velocity anomalies associated with the volcanic centres and the associated geothermal field. Fast seismic P- and S- wave velocities are imaged beneath the active volcano of Soufrière Hills and the older dormant Central Hills volcanic centre. The fast velocity anomaly becomes more prominent and wider beneath Soufrière Hills at depths approximately between 1 and 3 km below sea level. These regions correspond to present and older volcanic cores and are possibly comprised from andesitic crystalized rocks of dome cores and intrusive magmatic bodies such as dikes and sills. The surrounding regions appear slower, possibly associated with deposits of volcanic ash, lava fragments, pyroclastic flows and lahars. A significant P- and S-wave low velocity anomaly is observed to the western side of the Island to the area of a high-temperature geothermal field that was inferred from previous geophysical studies and confirmed by three geothermal wells. In our models this feature is dipping towards E/SE, and extends down to ~3.5 km. These results are in good agreement with previous studies in the region. The Vp/Vs ratio map derived from the two velocity models reveals low Vp/Vs ratio at the volcanic centre, and positive anomalies at the flanks of Soufrière Hills. The low-velocity region that is associated with the geothermal field is characterized by low Vp/Vs ratio values too.

How to cite: Bogiatzis, P., Kendall, J.-M., Baird, A. F., Blundy, J. D., Stewart, R. C., and Ryan, G. A.: Insights on the structure of the Soufrière Hills volcano and the associated geothermal field using joint inversion of passive P- and S-wave travel time data, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19072, https://doi.org/10.5194/egusphere-egu25-19072, 2025.

In this study, we develop a multiparameter ambient noise adjoint tomography method, and apply it for the first time to image the crustal magmatic system and regional deformation of the Toba region. Using Rayleigh and Love waves at periods of 5-20 s extracted from ambient noise, we construct a new multiparameter 3D crustal model that includes shear-wave velocity, radial and azimuthal anisotropy. The isotropic component of our model reveals 1) over 30% Vs reductions beneath the Toba caldera with a melt fraction ranging from 14.5% to 18.5%, and 2) two low Vs bodies located in the middle crust (10-20 km) beneath the Helatoba volcano and the upper crust beneath the Lubukraya volcano, suggesting a large transcrustal magmatic mush model beneath this volcanic arc region. Our anisotropic model shows > 10% positive radial anisotropy (Vsh > Vsv) in the middle crust of the volcanic regions, indicating the presence of horizontally layered melt sills. In the upper crust, we find predominantly weak negative radial anisotropy and significant azimuthal anisotropy, suggesting subvertical rock fabrics dominate upper crustal anisotropy. The orientation of fast velocity directions (FVDs) mostly aligns with the Sumatran Fault due to fault fabrics resulting from shearing deformation along the plate boundary. In the Tarutung region with rich geothermal resources, FVDs shift to being fault-perpendicular probably due to the alignment of stress-induced, fluid-rich microcracks. Our study provides new insights into crustal magmatic architecture and deformation regimes of the Toba region shaped by regional tectonics and magmatic processes.

How to cite: Wang, K., Tong, P., Zhang, Z., and Su, L.: Mapping the crustal magmatic system and regional deformation of the Toba region by multiparameter ambient noise adjoint tomography, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5325, https://doi.org/10.5194/egusphere-egu25-5325, 2025.

Changbaishan volcano (CBV), located on the border between China and North Korea, has experienced multiple large-scale eruptions in the past. The well-known millennial eruption occurred in 946 and is one of the largest eruptions in the world. In 2002~2005, the CBV experienced an episode of unrest with intensive seismicity and some ground deformations, which has raised great concerns for the public. Many seismic tomography studies have been conducted in the CBV region, but due to the sparse distribution of seismic stations or smaller spatial coverage of some dense temporary stations, the crustal magma plumbing system for the CBV is still not well resolved.

To characterize the fine crustal structure of the CBV area and characterize its magmatic system, we deployed a dense seismic array over a one-month period, which consists of 277 short-period stations. The array covers an area of 200 km in the east-west direction, and 190 km in the north-south direction, with an average station spacing of 10 km. We also assembled continuous data from 14 permanent volcanic monitoring stations operated by the Jilin Earthquake Agency.

To extract high-quality empirical Green’s functions (EGFs) from one month of continuous ambient noise data, we first followed the conventional ambient noise data processing flow to compute hourly cross-correlation functions (CCFs) for each station pair. Subsequently, the template-matching-based selection method was applied to select CCFs with distinct surface wave signals for some time segments. These selected CCFs were then denoised using the SVD-based wiener filter (SVDWF) to further enhance the signal-to-noise ratios (SNRs) of CCFs. Finally, phase-weighted stacking (PWS) was employed to obtain the final CCFs for each station pair. This processing workflow significantly improved the SNRs of EGFs, enhanced the quality of the dispersion spectra and extended the surface wave dispersion frequency band from ~10 s to ~20 s.

After extracting high-quality dispersion curves, we employed the direct surface wave tomography method to invert the crustal velocity structure beneath the CBV region. The inverted Vs model reveals a prominent low-velocity anomaly in the mid-to-upper crust beneath the Tianchi crater, and the presence of a wide-spread low-velocity layer approximately 10 km thick in the middle crust. In addition, our Vs model also indicates low-velocity anomalies in the upper crust beneath the Wangtian’e crater. A unified low-velocity body connects the Tianchi crater with the Namphothe crater in the mid-to-lower crust, which suggests that these volcanic systems may originate from a common deeper source.

How to cite: Pan, D., Gao, J., and Zhang, H.: Ambient Noise Tomography Reveals Magmatic Plumbing System Beneath the Changbaishan Volcano Field, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16194, https://doi.org/10.5194/egusphere-egu25-16194, 2025.