Upper crustal magma chamber properties using P-wave tomography at Santorini Volcano

Santorini Volcano is an arc volcano in the Hellenic subduction zone that has been active for at least 600 kyr and has a history of alternating explosive and effusive, shield-forming eruptions. The most-recent Plinian event is the well-studied Late Bronze Age (Minoan) eruption. From 2011-2012 the volcano experienced inflation and increased seismic activity thought to be due to an intrusion of magma into a shallow crustal magma chamber (4-5 km depth). To investigate processes that control movement and ponding of magma in arc volcanoes and provide estimates on the shape, volume, and melt content of upper crustal magma accumulation, we use a high-resolution, P-wave, seismic velocity model plus geological and geochemical constrains provided by previous studies. We collected dense, 3D, active-source marine-land seismic data to determine the velocity structure of the entire crust surrounding Santorini. The experiment utilized 90 ocean bottom seismometers, 65 seismic land stations, and over 14,000 controlled-sound marine sources from the R/V Marcus Langseth with 360° ray coverage through the caldera. We use seismic tomography to invert over 180,000 P-wave first-arrivals from the experiment that sample the crust beneath the Santorini caldera to 8km depth. The near-surface velocity structure of the preliminary velocity model correlates well to mapped geology of the island. A potential shallow magma body is imaged below the northern caldera basin, where a low-velocity anomaly causes seismic attenuation and delayed first-arrivals. The low velocity anomaly extends 3.5-5 km below sea level with a width of 3-5 km and is elongated 15 km NE-SW, perpendicular to tectonic stresses. The recovered velocity anomaly ranges from -0.3 to -1 km/s with the largest amplitude coinciding with the 2011-2012 Mogi inflation source. We will interpret low velocities in the region of the inferred magmatic intrusion in terms of porosity, composition, elevated temperatures and/or the presence of melt. We will use estimates of physical properties to determine if the anomaly is a recent intrusion of magma and/or remnants of the pre-Minoan magma body. Our results will clarify the size and state of a shallow magma chamber below the Santorini caldera, which has important hazard implications for future eruptions.