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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.
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Melt distribution and Vp/Vs for the Mid Crust at the Santorini-Kolumbo Volcanic System
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.
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- Award ID(s):
- 2023338
- PAR ID:
- 10567273
- Publisher / Repository:
- 2023 Fall Meeting, American Geophysical Union
- Date Published:
- Format(s):
- Medium: X
- Location:
- https://agu.confex.com/agu/fm23/meetingapp.cgi/Paper/1316502
- Sponsoring Org:
- National Science Foundation
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Santorini volcano has a history of caldera‐forming eruptions, most recently in the Late Bronze Age, at 3.4 kya, and remains volcanically active. The Kolumbo submarine volcano, located 7 km to the northeast of Santorini, erupted in 1650 AD in a deadly phreatomagmatic eruption. Ongoing seismic activity and active hydrothermal venting at Kolumbo indicate this volcano is a significant hazard to the Santorini region. The magma source for Santorini and the Kolumbo edifice are considered separate in the shallow crust, though their deeper magma distribution is not yet constrained. In this study, we improve constraints on the mid‐crustal magma system of Santorini caldera and the nearby Kolumbo volcano using local earthquake tomography. We use 1515 P‐wave and 1435 S‐wave arrival times from 63 local earthquakes with magnitudes from 0.5 to 3.0 that occurred between 5 and 15 km depth together with an existing data set of active source Pg arrivals. The upper crustal magma system beneath Santorini is imaged to at least 6 km depth, and to 12 km depth beneath Kolumbo. We recover a high P‐wave velocity layer (∼6–8 km) under the Kolumbo magma reservoir that we infer is a rheologically strong seismogenic layer. We also recover a mid‐crustal magma body below 8 km depth located to the NE of Santorini and Kolumbo.more » « less
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null (Ed.)Abstract 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.more » « less
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