Abstract The Cascadia subduction zone (CSZ) hosts major seismic and tsunami hazards, yet key questions persist about the relationship between margin structure, fluid distribution, episodic tremor and slip, shallow megathrust behavior, shaking and tsunamigenesis, and the resulting hazard estimates. Addressing these problems requires an empirically grounded, three-dimensional seismic velocity model to illuminate subsurface structure and properties and to provide a basis for geophysical studies such as earthquake simulations and ground-motion estimation. In May 2024, the National Science Foundation-funded Cascadia Region Earthquake Science Center (CRESCENT) community velocity model (CVM) working group, with U.S. Geological Survey and regional partners, convened a workshop to identify priorities for such a model. Participants emphasized the features necessary for addressing key science questions, including implementing findability, accessibility, interoperability, and reusability (FAIR) access, capturing along-strike and along-dip structural heterogeneity, resolving shallow offshore–onshore structure, constraining elastic properties and quantifying their uncertainties for numerical wave propagation simulations, their validation benchmarks, and supporting associated accurate earthquake ground-motion simulations and hazard assessments. This article describes the priorities defined in the workshop, and a description of how, guided by these needs, CRESCENT plans to develop multiple generations of a CVM to advance CSZ science and improve seismic and tsunami hazard modeling across the Pacific Northwest. The CVM will span the CSZ from the surface to ∼100 km depth, offshore and east of the Cascades into Idaho (∼132°–110° W) and the southern and northern tectonic regime transitions (∼36°–52° N) to capture the entire tectonic system as well as its surroundings.

Accurate characterization of subducting slab geometry is fundamental to understanding the distribution of earthquakes, the dynamics of arc volcanism, and the assessment of seismic hazards. Well-constrained slab structures also serve as critical inputs for geophysical imaging and geodynamic modeling efforts that aim to resolve key processes in subduction zones. In this study, we present a comprehensive, margin-wide model of the Moho associated with the subducting oceanic plate beneath the Cascadia subduction zone, developed through the integration of publicly available offshore and onshore datasets. We integrate high-resolution seismic reflection data from the offshore CASIE21 expedition with three previously published, lower-resolution onshore slab models (McCrory et al., 2012; Hayes et al., 2018; Bloch et al., 2023) to construct a unified Moho surface. This synthesis produces six alternative Moho geometries, enabling flexibility for studies that require varying structural assumptions. The accompanying open-source workflow offers a transparent and adaptable approach for combining heterogeneous datasets. In areas lacking direct constraints, Moho depths were estimated through interpolation from adjacent regions. The resulting models provide a valuable foundation for analyzing along-strike variations in slab structure and their implications for Cascadia geodynamics.

Deep‐crustal magma plumbing at arc volcanoes controls the volume, frequency, and composition of magma being transported to and stored in the upper crust. However, the mid‐to‐lower crust remains a challenging region to image. We explore the mid‐to‐lower crustal velocity structure beneath the Christiana‐Santorini‐Kolumbo Volcanic Field (CSKVF) to better understand how an established stratovolcano and flanking volcano (Santorini and Kolumbo) are fed through the mid‐to‐lower crust. We use active‐source seismic data to obtain a P‐wave velocity model of the crust below the CSKVF. We invert direct and reflected P phases to cover the entire depth extent of the crust and solve for the Moho interface depth. Our model requires a curved Moho interface representative of crustal thickening via underplating. Results show a high V p anomaly in the lower crust under Santorini and a mid‐crustal low V p anomaly offset from both Santorini and Kolumbo. We find that accumulation of magma is located under the local extensional basin in the upper mid‐crust (<10 km) but is offset at deeper depths. We find evidence for melt storage at 11–13 km depth feeding volcanism at the Kolumbo volcanic chain. This melt is also a plausible source for the 2025 seismic swarm and dike intrusion. Resolution is limited in the mid‐crust below the Santorini caldera, leaving Santorini's mid‐crustal magma plumbing unconstrained. We think it likely that Santorini and Kolumbo have entirely separate crustal plumbing systems and mantle sources, but allow the possibility of a connection in the mid or lower crust.

This study addresses a significant gap in understanding the features of the south-central Cascadia subduction zone, a region characterized by complex geologic, tectonic, and seismic transitions both offshore and onshore. Unlike other segments along this margin, this area lacks a 3-D velocity model to delineate its structural and geological features on a fine scale. To address this void, we developed a high-resolution 3-D P-wave velocity model using active source seismic data from ship-borne seismic shots recorded on temporary and permanent onshore seismic stations and ocean-bottom seismometers. Our model shows velocity variations across the region with distinct velocity-depth profiles for the Siletz, Franciscan, and Klamath terranes in the overlying plate. We identified seaward dipping high-velocity static backstops associated with the Siletz and Klamath terranes, situated near the shoreline and further inland, respectively. Regions of reduced crustal velocity are associated with crustal faults. Moreover, there is significant along-strike depth variation in the subducting slab, which is about 4 km deeper near the thick, dense Siletz terrane and becomes shallower near the predominantly less-dense Franciscan terrane. This highlights a sudden tectonic and geologic transition at the southern boundary of the Siletz terrane. Our velocity model also indicates slightly increased hydration, though still minimal, in both the oceanic crust and the upper mantle of the subducting plate compared to other parts of the margin.

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 similar to 3.3x10(11) 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.

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.

Catastrophic, caldera-forming explosive eruptions generate hazardous ash fall, pyroclastic density currents and, in some cases, tsunamis, yet their dynamics are still poorly understood. Here we use scaled analogue experiments and spectral analysis of well-preserved concentric terracing of seafloor deposits built by submarine caldera-forming explosive eruptions to provide insights into the dynamics governing these eruptions and the resultant hazards. We show that powerful submarine eruption columns in collapsing regimes deliver material to the sea surface and seabed in periodic annular sedimentation waves. Depending on the period between successive waves, which becomes shorter with decreasing jet strength, their impact and spread at the sea surface and/or seabed can excite tsunamis, drive radial pyroclastic density currents and build concentric terraces with a wavelength that decreases with distance, or deposits that thin monotonically. Whereas the Sumisu (Izu–Bonin arc) caldera deposit architecture is explained by either a subaerial or deep-water model involving no interaction between sedimentation waves and the sea surface, those of the Macauley (Kermadec arc) and Santorini (Hellenic arc) calderas are consistent with a shallow-water model with extensive sedimentation wave–sea surface–seabed interactions. Our findings enable an explicit classification of submarine caldera-forming explosive eruption dynamics and quantitative estimates of eruption rates from their terraced deposits. Submarine terraced deposits of some caldera-forming explosive eruptions result from periodic collapses of the eruption column and can be used to estimate their source eruption rate, according to an analysis of such terraces and analogue experiments.

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 modeling 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 similar to 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.

Volcanic activity is dictated by crustal pathways and reservoirs through which magma ascends and collects. The Cascades are a natural labora-tory to study the influence of the crust on erupted magmas and subter-ranean reservoirs. The interaction of ascending magmas with the variable subduction geometry and tectonics of the overriding North American plate has given rise to a diversity of magmatic storage conditions. The confluence of geochemical and geophysical investigations emphasizes that most magmatic systems in the Cascades have been built at multiple levels in the crust, as deter-mined by tectonics, pre-existing structure, and magmatic flux from the mantle.

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.

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 km3) and medium-sized (<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.

Many of the most hazardous volcanoes lie in rift systems, where tectonics often seems to exert control on magma emplacement. However, our current knowledge of the interplay between volcanism and tectonics is immature due to the lack of observations on geological time scales. Located in the southern Aegean Sea, the Christiana-Santorini-Kolumbo (CSK) volcanic field lies in a prominent continental rift zone caused by back-arc extension along the Hellenic Arc. Covered by numerous geophysical surveys, this area offers the unique possibility to reconstruct a volcanic rift in time and space. Previous studies have revealed that the CSK volcanic field developed during four distinct volcanic phases, which initiated in the Pliocene and only recently matured to form the vast Santorini edifice. Here, we combine P-wave velocity tomography models and high-resolution reflection seismic data to reveal the internal architecture and the spatio-temporal evolution of the rift basins as well as their relation to the evolution of the CSK volcanoes. Our joint analysis reveals a distinct NE-SW-directed horst-structure separating the volcanic rift into a volcanically active northwestern zone and a volcanically inactive southeastern zone. Using a refined seismo-stratigraphic framework of the internal architecture of the rift basins, we identify four distinct phases of the rift system that correspond to the volcanic phases of the CSK field. These phases reflect the gradual development of a Pliocene-Pleistocene NE-SW oriented fault system overprinting an older Miocene-Pliocene ESE-WNW oriented fault system. The latest volcanic phase, during which volcanism focussed on Santorini and became highly explosive, corresponds to a distinct shift in the tectonic behavior of the rift system after which enhanced subsidence at the Santorini-Anafi and Amorgos faults occurred that was rapidly filled up by thick volcano-sedimentary deposits. We conclude that the volcanism of the CSK field is fundamentally controlled by NE-SW-directed rifting, which lies parallel to the Pliny and Strabo trends of the southeastern Hellenic Arc. This volcanic system is bounded to the southeast by the Akrotiri-Anhydros horst, which seems to be a deep-rooted structural boundary for the volcanic plumbing system. The shift from ESE-WNW directed faulting to NE-SW directed faulting is an indication that the dominant direction of slab-rollback driving the extension of the CSK rift shifted from the southwestern to the southeastern Hellenic Arc with Santorini lying at the hinge of these trends.

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, no such a chamber has ever been imaged unambiguously, suggesting that large chambers responsible for caldera-forming eruptions are too short-lived to capture. Here we use a high-resolution imaging method based on finite-length seismic waveforms to detect a small, high-melt-fraction magma chamber embedded in a melt reservoir extending from ~2 to at least 4 km b.s.l. beneath 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. Though too small to be detected by standard seismic tomography, the chamber is large enough to threaten the nearby islands with tsunamigenic eruptions. Our results suggest that similar reservoirs (relatively small but high melt-fraction) may have gone undetected, and are yet to be discovered, at other active volcanoes.

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 still hardly understood. We present a ten-month long microseismicity dataset for the period 2006-2007. The majority of earthquakes clusters 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, that is interpreted as a crustal melt reservoir. Our dataset includes several earthquake swarms, of which we analyse the four strongest in detail. The swarms occupy near-vertical volumes of the crust and together they form a zone of fracturing elongated in SW-NE direction, parallel to major regional faults. All four swarms show a general upward migration of hypocentres 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 either by obstructions in the melt conduits in a rheologically strong layer between 6-9 km depth or by dynamic triggering from purely tectonic earthquakes. 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 feasible and monitoring of the Kolumbo volcanic system is highly advisable.

Santorini is the most active volcano of the Southern Aegean Volcanic Arc, with volcanic material that has accumulated during the last 600+ Kyrs on top of the pre-volcanic Santorini island. The geometry of the pre -volcanic basement not only provides constraints on the volcanic history, but is also fundamental for seismic hazard assessment, as it partly controls site-effects on strong ground motions. We investigate the geometry of the metamorphic (Cycladic) basement of Santorini using information from both passive (noise) and active source data. We performed a large number (-280) of single-station ambient noise measurements and collected addi-tional data from 42 previous studies to compute the Horizontal-to-Vertical Spectral Ratio (HVSR) curves. The majority of the recovered HVSR curves show prominent spectral ratio peaks (locally larger than 7-8) indicating a large impedance contrast between pyroclastic volcanic strata and bedrock metamorphic formations. We also performed active (MASW) and passive (Noise Array) surface wave measurements at 4 selected sites to determine local dispersion curves. To find the 1D Vs structure from the surface down to the metamorphic bedrock we performed a joint Monte Carlo inversion of the dispersion curves and the fundamental frequency (fa SR) values from the HVSR curves. These models, as well as deep borehole information and HVSR data along the caldera cliff, where the thickness and stratigraphy of the volcanic formations above the bedrock can be directly observed, allowed us to determine a linear relationship between the thickness of the pyroclastic formations and their fundamental resonance period for central-southern Santorini. The resulting geometry of pre-volcanic island shows deep basins around the pre-Alpine bedrock outcrop (Profitis Ilias) and generally agrees with recent results from larger-scale tomographic and gravity data but with some significant local differences. The basins are 200+ meters thick in the Kamari and Perissa area (southeastern Santorini) and >400 m thick in the central (Fira-Imerovigli) area, while there is a thick (-70-130 m) pyroclastic layer on top of a metamorphic basement "plateau" in the Megalochori area.

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.

Geologic maps are foundational products for natural hazard assessments but developing them for submarine areas is challenging due to a lack of physical access to the study area. In response, submarine geomorphologic maps are used to provide geologic context and spatial information on landforms and related geo-hazards for risk management. These maps are generated from remotely sensed data, e.g. digital elevation models (DEMs), which introduce unique hurdles to submarine mapping. To address this issue, we produced a workflow for applying planetary geologic mapping methods to submarine data. Using this, we created an onshore-offshore geomorphologic map of the Christiana-Santorini-Kolumbo Volcanic Group, Greece. This product can be used to enhance hazard assessments on Santorini, which is a tourist hot-spot at high risk for volcanically- and seismically-induced hazards. We present this workflow as a tool for generating uniform geomorphologic map products that will aid natural hazard assessments of submarine environments.

The U.S. Pacific Northwest (PNW) is one of the largest Internet infrastructure hubs for several cloud and content providers, research networks, colocation facilities, and submarine cable deployments. Yet, this region is within the Cascadia Subduction Zone and currently lacks a quantitative understanding of the resilience of the Internet infrastructure due to seismic forces. The main goal of this work is to assess the resilience of critical Internet infrastructure in the PNW to shaking from earthquakes. To this end, we have developed a framework called ShakeNet to understand the levels of risk that earthquake-induced shaking poses to wired and wireless infrastructures in the PNW. We take a probabilistic approach to categorize the infrastructures into risk groups based on historical and predictive peak ground acceleration (PGA) data and estimate the extent of shaking-induced damages to Internet infrastructures. Our assessments show the following in the next 50 years: ∼65% of the fiber links and cell towers are susceptible to a very strong to a violent earthquake; the infrastructures in Seattle-Tacoma-Bellevue and Portland-Vancouver-Hillsboro metropolitan areas have a 10% chance to incur a very strong to a severe earthquake. To mitigate the damages, we have designed a route planner capability in ShakeNet. Using this capability, we show that a dramatic reduction of PGA is possible with a moderate increase in latencies.

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.

We investigate the geometry of the metamorphic basement of the Santorini volcanic island using ambient noise data to determine the pre-Alpine/pre-volcanic bedrock structure. The geometry of pre-volcanic Santorini is important in order to constrain the recent volcanic history of the island and also to study the site-effect of the volcanic formations on seismic motions. Santorini is the most active volcano of the Southern Aegean Volcanic Arc and is the southernmost island of the Cyclades islands metamorphic complex. As a result, the volcanic material that has accumulated during the last 600+ Kyrs has been superimposed on the pre-volcanic Santorini (Cycladic) island. To map the thickness of volcanic material, we have performed a large number (>200) of single-station noise measurements in the Santorini area. Measurements were mainly performed using conventional acquisition systems (Guralp-40T 30sec seismometer and Reftek-130A digitizer). We also employed additional single-station noise data from several previous studies (Dimitriadis et al. 2006, PROTEUS Project 2015), as well as permanent stations from the Hellenic Seismological Network in the same region. HVSR curves were calculated using single-station noise data and were used to estimate the fundamental frequency, f0, as well as the corresponding maximum HVSR amplitude, A0HVSR. The majority of HVSR curves showed prominent peaks (A0HVSR locally larger than 7-8), indicating a clear impedance contrast between volcanics and metamorphic formations. To map the bedrock depth, we estimated the thickness of the upper volcanic formations using the quarter-wavelength approximation for each site. For this assessment, the average shear-wave velocity (Vs) of the volcanic formations was estimated from the inversion of several passive ambient noise array data, as well as additional constraints from selected MASW measurements. Where possible, the reliability of the spatial variation of volcanic formation thickness was checked with independent geological information. Using the digital elevation model and the volcanic formation thickness for each site of the single-station noise data, we estimated the spatial distribution of the pre-Alpine, metamorphic bedrock depth. The resulting geometry of the pre-volcanic Santorini island shows very deep basins (now filled with volcanic formations) around the pre-Alpine bedrock outcrop in the southern part of Santorini (Profitis Ilias), increasing to 100+ meters in the Kamari-Perissa basin area (southeastern Santorini) and to more than 400+ in the central (Fira-Imerovigli) and the north Santorini areas (Oia), in agreement with recent larger-scale tomographic results (Heath et al., 2019). The results are also in very good agreement with the pre-Alpine bedrock geometry independently inferred from gravity data inversion (Tzanis et al., 2019.) This research is co-financed by Greece and the European Union (European Social Fund- ESF) through the Operational Programme «Human Resources Development, Education and Lifelong Learning» in the context of the project “Strengthening Human Resources Research Potential via Doctorate Research” (MIS-5000432), implemented by the State Scholarships Foundation (ΙΚΥ), the Hellenic Foundation for Research and Innovation (HFRI) under the “First Call for HFRI Research Projects to support Faculty members and Researchers and the procurement of high-cost research equipment grant” (Project Number: 2924) and the Institute for the Study and Monitoring Of the SAntorini Volcano (ISMOSAV).