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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) 

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. 

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. 

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. 

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. 

Seismic imaging methods have provided detailed three-dimensional constraints on the physical properties of magmatic systems leading to invaluable insight into the storage, differentiation and dynamics of magma. These constraints have been crucial to the development of our modern understanding of magmatic systems. However, there are still outstanding knowledge gaps resulting from the challenges inherent in seismic imaging of volcanoes. These challenges stem from the complex physics of wave propagation across highly heterogeneous low-velocity anomalies associated with magma reservoirs. Ray-based seismic imaging methods such as travel-time and surface-wave tomography lead to under-recovery of such velocity anomalies and to under-estimation of melt fractions. This review aims to help the volcanologist to fully utilize the insights gained from seismic imaging and account for the resolution limits. We summarize the advantages and limitations of the most common imaging methods and propose best practices for their implementation and the quantitative interpretation of low-velocity anomalies. We constructed and analysed a database of 277 seismic imaging studies at 78 arc, hotspot and continental rift volcanoes. Each study is accompanied by information about the seismic source, part of the wavefield used, imaging method, any detected low-velocity zones, and estimated melt fraction. Thirty nine studies attempted to estimate melt fractions at 22 different volcanoes. Only five studies have found evidence of melt storage at melt fractions above the critical porosity that separates crystal mush from mobile magma. The median reported melt fraction is 13% suggesting that magma storage is dominated by low-melt fraction crystal mush. However, due to the limits of seismic resolution, the seismological evidence does not rule out the presence of small (<10 km 3 ) and medium-sized (<100 km 3 ) high-melt fraction magma chambers at many of the studied volcanoes. The combination of multiple tomographic imaging methods and the wider adoption of methods that use more of the seismic wavefield than the first arriving travel-times, promise to overcome some of the limitations of seismic tomography and provide more reliable constraints on melt fractions. Wider adoption of these new methods and advances in data collection are needed to enable a revolution in imaging magma reservoirs. 

Santorini is located in the central part of the Hellenic Volcanic Arc (South Aegean Sea) and is well known for the Late-Bronze-Age “Minoan” eruption that may have been responsible for the decline of the great Minoan civilization on the island of Crete. To use gravity to probe the internal structure of the volcano and to determine whether there are temporal variations in gravity due to near surface changes, we construct two gravity maps. Dionysos Satellite Observatory (DSO) of the National Technical University of Athens (NTUA) carried out terrestrial gravity measurements in December 2012 and in September 2014 at selected locations on Thera, Nea Kameni, Palea Kameni, Therasia, Aspronisi and Christiana islands. Absolute gravity values were calculated using raw gravity data at every station for all datasets. The results were compared with gravity measurements performed in July 1976 by DSO/NTUA and absolute gravity values derived from the Hellenic Military Geographical Service (HMGS) and other sources. Marine gravity data that were collected during the PROTEUS project in November and December 2015 fill between the land gravity datasets. An appropriate Digital Elevation Model (DEM) with topographic and bathymetric data was also produced. Finally, based on the two combined datasets (one for 2012–2014 and one for the 1970s), Free air and complete Bouguer gravity anomaly maps were produced following the appropriate data corrections and reductions. The pattern of complete Bouguer gravity anomaly maps was consistent with seismological results within the caldera. Finally from the comparison of the measurements made at the same place, we found that, within the caldera, the inner process of the volcano is ongoing both before, and after, the unrest period of 2011–2012.