At extensional volcanic arcs, faulting often acts to localize magmatism. Santorini is located on the extended continental crust of the Aegean microplate and is one of the most active volcanoes of the Hellenic arc, but the relationship between tectonism and magmatism remains poorly constrained. As part of the Plumbing Reservoirs Of The Earth Under Santorini experiment, seismic data were acquired across the Santorini caldera and the surrounding region using a dense amphibious array of >14,300 marine sound sources and 156 short-period seismometers, covering an area 120 km by 45 km. Here a P wave velocity model of the shallow, upper-crustal structure (<3-km depth), obtained using travel time tomography, is used to delineate fault zones, sedimentary basins, and tectono-magmatic lineaments. Our interpretation of tectonic boundaries and regional faults are consistent with prior geophysical studies, including the location of basin margins and E-W oriented basement faults within the Christiana Basin west of Santorini. Reduced seismic velocities within the basement east of Santorini, near the Anydros and Anafi Basins, are coincident with a region of extensive NE-SW faulting and active seismicity. The structural differences between the eastern and western sides of Santorini are in agreement with previously proposed models of regional tectonic evolution. Additionally, we find that regional magmatism has been localized in NE-SW trending basin-like structures that connect the Christiana, Santorini, and Kolumbo volcanic centers. At Santorini itself, we find that magmatism has been localized along NE-SW trending lineaments that are subparallel to dikes, active faults, and regional volcanic chains. These results show strong interaction between magmatism and active deformation.

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Despite multidisciplinary evidence for crustal magma accumulation below Santorini volcano, Greece, the structure and melt content of the shallow magmatic system remain poorly constrained. We use three-dimensional (3-D) velocity models from tomographic inversions of active-source seismic P-wave travel times to identify a pronounced low-velocity anomaly (–21%) from 2.8 km to 5 km depth localized below the northern caldera basin. This anomaly is consistent with depth estimates of pre-eruptive storage and a recent inflation episode, supporting the interpretation of a shallow magma body that causes seismic attenuation and ray bending. A suite of synthetic tests shows that the geometry is well recovered while a range of melt contents (4%–13% to fully molten) are allowable. A thin mush region (2%–7% to 3%–10% melt) extends from the main magma body toward the northeast, observed as low velocities confined by tectono-magmatic lineaments. This anomaly terminates northwest of Kolumbo; little to no melt underlies the seamount from 3 to 5 km depth. These structural constraints suggest that crustal extension and edifice loads control the geometry of magma accumulation and emphasize that the shallow crust remains conducive to melt storage shortly after a caldera-forming eruption.

Highlights: (3-5 bullets, 85 characters max) 1. There is a shallow low-velocity, high-porosity volume in the north-central caldera 2. Vents of the first 3 LBA eruption phases correlate with this inner structure 3. Inner collapse involved reverse faults, volcanic deposits, and/or rock fractures 4. The low-density volume may have caused 2011-2012 inflation to localize beneath it 5. The outer topographic caldera formed by relatively coherent down drop *Manuscript Click here to view linked References

In this paper, we combine active and passive source P wave seismic data to tomographically image the magmatic system beneath Newberry Volcano, located east of the Cascade arc. By using both travel times from local active sources and delay times from teleseismic earthquakes recorded on closely spaced seismometers (300–800 m), we significantly improve recovery of upper crustal velocity structure (<10 km depth). The tomographic model reveals a low‐velocity feature between 3 and 5 km depth that lies beneath the caldera, consistent with a magma body. In contrast to earlier tomographic studies, where elevated temperatures were sufficient to explain the recovered low velocities, the larger amplitude low‐velocity anomalies in our joint tomography model require low degrees of partial melt (∼10%), and a minimum melt volume of ∼2.5 km3. Furthermore, synthetic tests suggest that even greater magnitude low‐velocity anomalies, and by inference larger volumes of magma (up to 8 km3), are needed to explain the observed waveform variability. The lateral extent and shape of the inferred magma body indicates that the extensional tectonic regime at Newberry influences the emplacement of magmatic intrusions. Our study shows that jointly inverting active source and passive source seismic data improves tomographic imaging of the shallow crustal seismic structure of volcanic systems and that active source experiments would benefit from longer deployment times to also record teleseismic sources.

We show that seismic autocorrelations provide new depth constraints on upper crustal magmatic systems. Autocorrelations of both ambient noise recorded on seismometers and geophones and teleseismic earthquake coda recorded on seismometers elucidate the structure of Newberry Volcano. These autocorrelations result in the two‐way, body wave Green's function beneath a station. Within the caldera, a reproducible, coherent P wave reflection is inferred to come from the top of a magma body at ~2.5 km depth and maps with the lateral extent of an anomalously low‐velocity body imaged tomographically. On the west flank of the volcano, a reflection that deepens with distance from the caldera is inferred to result from a temperature‐dependent change in metamorphic facies and may map the thermal structure of the edifice. Our results show that the autocorrelation of diffuse seismic energy reconstructs reflections from seismically sharp boundaries associated with the upper crustal magmatic structure.

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 (&lt;10 km3) and medium-sized (&lt;100 km3) 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.

Arc volcanoes are underlain by complex systems of molten-rock reservoirs ranging from melt-poor mush zones to melt-rich magma chambers. Petrological and satellite data indicate that eruptible magma chambers form in the topmost few kilometres of the crust. However, very few chambers have ever been definitively located, suggesting that most are too short-lived or too small to be imaged, which has direct implications for hazard assessment and modelling of magma differentiation. Here we use a high-resolution technology based on inverting full seismic waveforms to image a small, high-melt-fraction magma chamber that was not detected with standard seismic tomography. The melt reservoir extends from ~2 to at least 4 km below sea level (b.s.l.) at Kolumbo – a submarine volcano near Santorini, Greece. The chamber coincides with the termination point of the recent earthquake swarms and may be a missing link between a deeper melt reservoir and the high-temperature hydrothermal system venting at the crater floor. The chamber poses a serious hazard as it could produce a highly explosive, tsunamigenic eruption in the near future. Our results suggest that similar reservoirs (relatively small but high melt-fraction) may have gone undetected at other active volcanoes, challenging the existing eruption forecasts and reactive-flow models of magma differentiation.