Imaging Crustal Magma Systems and Mantle Dynamics with Large Amphibian OBS Arrays

Amphibian geophysical experiments are transforming our ability to image Earth’s interior beneath the oceans, yet only a limited number of deployments have been designed at the scale and density required to address specific scientific questions from the crust to the mantle. In this invited talk, I synthesize results from two large ocean-bottom seismometer (OBS) experiments, at Santorini and the Galápagos, that demonstrate a range of science questions, spatial scales, depth ranges, and physical targets.

To image magma transport throughout the entire crust at an arc volcano, the PROTEUS experiment deployed a uniquely dense amphibian short-period array of 89 OBS and 65 land stations at the Santorini-Kolumbo volcanic system. The design combined active-source and passive seismic observations to resolve crustal magma plumbing and volcanic structure at kilometer-scale resolution. The data revealed the depth, volume, and melt extent of shallow magma accumulation beneath the Santorini caldera and demonstrated that caldera-collapse structures formed during the Late Bronze Age Plinian eruption continue to control present-day magma recharge, including the 2011-2012 and 2024-2025 inflation episodes. Imaging further showed how the evolution of regional extension during the Neogene to Quaternary formed a complex of faulted basins.  This tectonic system has interacted with magmatism, influencing magma pathways, storage geometry, and anisotropic crustal structure.

A key advance was the discovery of a small, high–melt-fraction magma chamber beneath the adjacent Kolumbo volcano at ~2–4 km depth using full-waveform inversion; an approach enabled by the first use of dense source–receiver coverage at a volcanic system. The smaller size of this eruptible magma reservoir meant it could not be detected using traditional travel-time tomography. More recent tomography leverages the large aperture of the array to extend the velocity structure to greater depths, revealing a mid-crustal magma storage region (8-15 km depth) laterally offset from both Santorini and Kolumbo, as well as deep accumulation of mafic–ultramafic material that thickens the crust beneath Santorini. This crustal framework is central to interpreting the February 2025 seismic swarm, whose migration pattern is consistent with a substantial magma intrusion interacting with extensional faulting.

In contrast, the Marine IGUANA experiment was designed to address mantle-scale questions, including plume-ridge interaction and lithosphere-asthenosphere coupling beneath the Galápagos. These questions require broad spatial coverage and long-duration recordings of natural earthquakes, motivating a 15-month deployment of 53 broadband OBS spanning a ~650 × 800 km region around the archipelago and the nearby Galápagos Spreading Center. Preliminary results reveal low-velocity mantle structures in both P- and S-waves associated with hot plume material, high-velocity material consistent with foundering lithosphere, and plume transport to both the eastern and western Galápagos spreading centers. In addition, spatial variations in seismic anisotropy indicate mantle flow due to absolute plate motion that is modified by the mantle plume.

Together, these experiments illustrate how science-driven experimental design enables insights into processes ranging from crustal magma systems to mantle dynamics, highlighting the power of dense, large-aperture amphibian OBS arrays to address Earth science questions across different scales.