The Kolumbo submarine volcano in the southern Aegean (Greece) is associated with repeated seismic unrest since at least two decades and the causes of this unrest are poorly understood. We present a ten‐month long microseismicity data set for the period 2006–2007. The majority of earthquakes cluster in a cone‐shaped portion of the crust below Kolumbo. The tip of this cone coincides with a low Vp‐anomaly at 2–4 km depth, which is interpreted as a crustal melt reservoir. Our data set includes several earthquake swarms, of which we analyze the four with the highest events numbers in detail. Together the swarms form a zone of fracturing elongated in the SW‐NE direction, parallel to major regional faults. All four swarms show a general upward migration of hypocenters and the cracking front propagates unusually fast, compared to swarms in other volcanic areas. We conclude that the swarm seismicity is most likely triggered by a combination of pore‐pressure perturbations and the re‐distribution of elastic stresses. Fluid pressure perturbations are induced likely by obstructions in the melt conduits in a rheologically strong layer between 6 and 9 km depth. We conclude that the zone of fractures below Kolumbo is exploited by melts ascending from the mantle and filling the crustal melt reservoir. Together with the recurring seismic unrest, our study suggests that a future eruption is probable and monitoring of the Kolumbo volcanic system is highly advisable.
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Abstract In extending volcanic arcs such as the Aegean, tectonic processes exert a significant control on magmatism. Spanning scales from 1 to 10s of km, volcanic vents, edifices, and eruptive centers follow the orientation of, and are located near, fault zones. Whether this tectonic control on magmatism results from individual faults/fractures weakening the crust or because regional stresses control magma input into the crust is debated. Here we investigate the scales of tectonic and magmatic interactions, specifically focusing on the role of local‐scale (<10 km) faults/fractures in controlling magmatism. We infer local‐scale fault/fracture orientations from anisotropic active‐source P‐wave travel‐time tomography to investigate tectonic and magmatic interactions in the upper crust of Santorini Volcano, Greece, and the actively deforming region to the east. We use the anisotropy magnitude and seismic velocity reduction to model the relative distribution of both consistently oriented and randomly oriented faults/fractures. Our results show that oriented faulting/fracturing resulting from regional‐scale (>10 km) tectonic stresses is distributed broadly across the region at 2–3 km depth, approximately paralleling volcanic/magmatic features. On a local‐scale, magmatism is neither localized in areas of higher oriented fault/fracture density, nor is it accommodating enough extensional strain to inhibit oriented faulting/fracturing of host rock. The alignment of magmatic features shows strong tectonic control despite the lack of correlation with local oriented fault/fracture density. These results suggest that magmatic processes are strongly influenced by regional‐scale, not local‐scale, tectonic processes. We infer regional processes have a greater impact on magmatism than local features due to their greater effect at depth.
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Explosive eruption jets rising through relatively shallow water layers form eruption columns that can deliver volcanic ash, gases, and entrained water to the atmosphere and ocean in sequence or simultaneously, depending on eruption source parameters (Gilchrist et al. 2023). Despite the mesospheric eruption column height of the January 15, 2022 eruption of Hunga Tonga-Hunga Ha’apai (HTHH), the majority of erupted material was delivered to the surrounding seafloor via submarine pyroclastic density currents (PDCs). Deposits of HTHH show evidence of axisymmetric terraced deposits, which we show are linked to the mass eruption rate and dynamics of column collapse. We use scaled analog experiments on multiphase sand-water fountains injected into water layers of varying depth to model the collapse dynamics of shallow water eruption columns and to link fountain source conditions to deposit topography. The source strength of multiphase fountains predicts whether they collapse periodically or continuously via sedimentation waves with varying frequency and momentum. In turn, the frequency and momentum of sedimentation waves impacting the tank base determines whether ground-hugging gravity currents flowing out of the sedimentation wave impact zone are initially erosive or depositional. On the basis of experiments, we propose that syn-eruptive shallow submarine caldera deposits that show evidence of terracing and proximal scouring are linked to relatively strong eruption jets in the regime where the jet is in partial collapse or total collapse. In these regimes, the eruption jet collapses periodically as sedimentation waves that erode the deposit in the impact zone and transition into submarine PDCs that deposit the sedimentation wave mixture into regularly spaced terraces thereafter (Fig. 1, black boxes). In contrast, we expect weak eruption jets to occur in the total collapse regime where sedimentation waves descend in rapid succession and effectively supply submarine PDCs continuously which, in turn, build deposits lacking terraces (Fig. 1, blue box). For common values of caldera eruption source parameters, we link submarine PDC deposit morphology to eruption jet strength and plausible mass eruption rates.more » « lessFree, publicly-accessible full text available December 1, 2025
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Understanding the complex dynamics of volcanic systems demands a multidimensional approach that combines geophysics, geology, and geodetics. In this study, we examine observed spatiotemporal gravity changes within the Santorini volcanic complex from 1975 to 2014. The historical data indicates that gravity has been increasing continuously since at least 1966 until our latest measurements in 2014, albeit with a decreasing rate of increase over time. Utilizing gravity inversion of various gravity datasets and evidence from other studies, we explore different scenarios to shed light on the underlying processes. Our preferred interpretation involves both a magmatic episode and continuous evolution of the shallow structure. We find that the 2011-12 unrest period resulted from the intrusion of ~3.3x1011 kg of basaltic magma at 3 km depth near the previously identified Mogi source. We attribute the continuous gravity increase beneath Nea Kameni to a density increases at about 1350 m depth. We infer these are a result of hydrothermal fluctuations, degassing, and/or vesicle collapse within the stored magma. Units: 1mGal = 10-5 m/s2 (SI)more » « less
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The Santorini arc volcano in the Hellenic subduction zone has a history of caldera-forming Plinian eruptions, most recently in the Late Bronze Age 3.4 kya, and it remains volcanically active. To inform volcanic hazard assessments, it is crucial to understand where melt is distributed. The PROTEUS experiment in 2015 recorded >14,000 controlled marine sound sources on 165 land and seafloor seismic stations. Tomographic inversion of this data revealed low P-wave velocities in the upper 4 kilometers beneath the caldera and nearby Kolumbo seamount interpreted as the magma system (McVey et al., 2020; Chrapkewiecz et al., 2022). However, structure of the magma system was only determined in the upper (<4-6km) crust and melt content is only weakly constrained. In this study we improve constraints on the deeper magma system and subsurface melt content with a tomographic P and S wave velocity structure. To do so, we add to the inverse problem arrival times from ~1500 local earthquakes with magnitudes from 0.5 to 3.0 that occurred between 5 and 20 km depth. The events were recorded on 142 3-component ocean bottom and island seismic stations that span the seafloor ~60 km west and east of the island and the nearby islands. Results beneath Santorini and Kolumbo suggest that the upper crustal magma reservoirs extend deeper than previously found, and we image a high Vp layer (~5-8 km) under the magma reservoir at Kolumbo. We identify this layer as strong, cooled, intruded magma and correlate it to the location of earthquakes, within which, swarms of rapidly upward propagating seismicity support prior inferences of melt conduits traversing a rheologically strong layer (Schmid et al, 2022). We give values for melt content of the upper crustal reservoirs using a scaled Vp/Vs model. Since the number of arrivals, apriori assigned uncertainty, and differences in ray geometry can result in P and S waves with different resolving power, we use measured resolution to scale the Vs perturbations and create a more realistic Vp/Vs model. The addition of earthquake arrivals allows us to map the magma reservoirs beneath the Santorini-Kolumbo magma system to 8 km depth and identify regions of elevated melt content.more » « less
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Santorini volcano in the South Aegean Volcanic Arc has a detailed history of ongoing volcanic and seismic activity, making it a prime location for studying magma storage and transport at arc volcanoes. The shallow magmatic system (<5 km depth) is well constrained by geophysical studies, but the deeper crustal structure is not. Located 15 km NE of Santorini, the Kolumbo seamount is also an active edifice, with consistently more seismicity and hydrothermal venting than Santorini. Geochemical studies indicate that Santorini and Kolumbo are fed by separate mantle and crustal magma sources, but prior seismic studies suggest otherwise (Dimitriadis et al, 2010; McVey et al, 2020). This study addresses the nature of lower-crustal magma structure beneath arc volcanoes and whether evolved volcanoes and nearby vents are connected through their plumbing. Tomographic inversion of P-wave Moho reflection (PmP) and turning P-wave (Pg) traveltimes is used to create 3-D models of Moho depth and P-wave velocity (Vp) down to depths of ~25 km. The PROTEUS experiment provides an exceptionally dense and large aperture traveltime dataset from an amphibious array of ~150 seismometers and ~14,000 active marine sources. The data are ~33,000 manually picked PmP arrivals and ~256,000 Pg arrivals from existing studies. Results show a low Vp anomaly extending from the Moho to the surface. This anomaly starts at the base of the crust under the NW Santorini caldera and extends up to the east. It is most pronounced at 10-15 km depth, where it is offset from both Santorini and Kolumbo. Limited resolution prevents imaging of a connection between this mid-crustal anomaly and the known shallow magma storage region under the Santorini caldera. A high-velocity core beneath Santorini is not found, a feature interpreted at other volcanoes as a cooled intrusive complex. Because no additional low Vp anomalies are found in the lower crust, we infer that a common mantle source and mid-crustal plumbing system is actively feeding both Santorini and Kolumbo. The spatial offset and elongated nature of magma storage implies a complex relationship between evolving magmatic structures and tectonics.more » « less
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Many studies focus on the best way to communicate volcanic information during a crisis event. Because of the urgency during crisis, many of crisis communication studies find that the issues that arise during volcanic crises can often be mitigated during the ‘quiet times’ between eruptions. This project addresses how to engage the population near a volcano that is in this period of quiescence. The goal is to synthesize peer-reviewed research that investigates volcano hazard communication when the threat of eruption is low. By doing this, we will provide scientists and others working with the public recommendations for communication materials. This synthesis will offer suggestions from the academic literature for effectively engaging the public in communication about volcanos, what content messages could include, and what mediums are available to reach different audiences. These recommendations are intended to provide a baseline for scientists to think about the multiple ways to engage with the variety of audiences that live around their volcano of study; they are not intended to be a rigid formula that applies to every population. We have systematically gathered peer reviewed articles from Web of Science, Georef, and Google Scholar, using specific search terms generated through consultation with a University of Oregon librarian. Through the use of specific exclusion criteria, we have narrowed down the 330 resulting papers to a final list of 34 studies that provide suggestions on volcano communications during periods of quiescence. This project will use the advice found in these studies to create a reference for scientists as they create communication materials to disseminate to the public regarding a volcano. The results found include different mediums, such as virtual reality, hazard maps, films, social media, and various online tools that a scientist can utilize to convey their findings. There are also recommendations for different audiences, such as tourists, children, rural communities, and indigenous populations. By synthesizing the findings of these studies into a single document for a scientist to reference, we can help scientists to best engage the public in learning about a volcano during quiescence.more » « less
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Despite their global societal importance, the volumes of large-scale volcanic eruptions remain poorly constrained. Here, we integrate seismic reflection and P-wave tomography datasets with computed tomography-derived sedimentological analyses to estimate the volume of the iconic Minoan eruption. Our results reveal a total dense-rock equivalent eruption volume of 34.5 ± 6.8 km³, which encompasses 21.4 ± 3.6 km³ of tephra fall deposits, 6.9 ± 2 km³ of ignimbrites, and 6.1 ± 1.2 km³ of intra-caldera deposits. 2.8 ± 1.5 km³ of the total material consists of lithics. These volume estimates are in agreement with an independent caldera collapse reconstruction (33.1 ± 1.2 km³). Our results show that the Plinian phase contributed most to the distal tephra fall, and that the pyroclastic flow volume is significantly smaller than previously assumed. This benchmark reconstruction demonstrates that complementary geophysical and sedimentological datasets are required for reliable eruption volume estimates, which are necessary for regional and global volcanic hazard assessments.more » « less