Seismic imaging of the upper crustal magma system and caldera at Santorini volcano, Greece.

Santorini is the most active arc volcano of the Hellenic subduction zone and has a history ofalternating effusive, shield-forming eruptions and explosive caldera collapse. The most recentcaldera-forming event is the well-studied 3.6 kyr Late Bronze Age (LBA/Minoan) eruption.In 2011-2012, the volcano experienced inflation and seismicity attributed to magmaintrusion between 3.3 and 5.9 km depth. To investigate processes that control calderaformation and the accumulation of magma at arc volcanoes, we use a high-resolution,P-wave, seismic velocity model together with existing geological and geochemicalconstrains. The PROTEUS experiment collected dense, 3D, active-source marine-landseismic data using 90 ocean bottom seismometers, 65 seismic land stations, and∼14,300 controlled-sound marine sources from the U.S. R/V Marcus Langseth.We use seismic tomography to invert >220,000 Pg travel times to image of theupper crustal structure to 7 km depth. Regional geology and tectonic features arereflected in the near-surface velocity structure. Magmatism has localized along NE-SWbasin-like structures that connect the Christiana, Santorini, and Kolumbo volcaniccenters, indicating a strong interaction between magmatism and tectonism. In theupper 3 km beneath the north-central caldera, we find a ∼3-km-wide, cylindricallow-velocity anomaly (-2 km/s) that lies directly above the pressure source of 2011-2012inflation and between two NE-SW-striking tectonic lineaments. Vents that formedduring the first three phases of the LBA eruption locate near the edges of the imagedstructure. We infer that collapse of a limited area of the caldera floor resulted ina high-porosity, low-density cylindrical volume, which formed by either chaoticcollapse along reverse faults, wholesale subsidence and infilling with tuffs andignimbrites, phreatomagmatic fracturing, or a combination of these processes. LBAphase 4 eruptive vents are located along the margins of the topographic calderaand the velocity structure indicates that the wider topographic caldera formed bycoherent down-drop following the more limited collapse in the northern caldera. Thisprogressive collapse sequence is consistent with models for multi-stage formationof nested calderas along conjugate reverse and normal faults. The upper-crustaldensity differences inferred from the seismic velocity model predict differences insubsurface gravitational loading that correlate with 2011-2012 edifice inflationindicating that sub-surface density anomalies may influence present-day magmaaccumulation. Directly beneath this structure, we image a second low-velocityanomaly that is 3-6 km below the northern caldera basin, 3-5 km wide, and elongated∼15 km NE-SW, parallel to tectonic structures. The largest-amplitude recoveredvelocity anomaly (-1 km/s) is consistent with 4-10% melt and coincides with the2011-2012 Mogi pressure source. Significant ray bending indicates that the recoveredsize and amplitude of the low velocity volume are minimum bounds. We infer anupper-crustal intrusion rate (∼2.6 km3/ky) consistent with the inferred long-termgrowth of the shallow magma system at Santorini. Future analysis of waveforms,reflectors, and shear waves, and full waveform inversion, will better constrain themelt content and distribution of crustal magma accumulation beneath Santorini. [ABSTRACT FROM AUTHOR]