Seismic Structure of the Mid to Upper Crust at the Santorini-Kolumbo Magma System from Joint Earthquake and Active Source Vp-Vs Tomography

The Santorini arc volcano has a history of caldera-forming Plinian eruptions, most recently in the Late Bronze Age 3.4 kya, and remains volcanically active. Located 7 km to the northeast of Santorini, the Kolumbo seamount 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 separate in the shallow crust, though their deeper magma nature is not yet constrained. In this study we improve constraints on the deeper magma system of Santorini and the nearby Kolumbo seamount with 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 dataset of active source Pg arrivals. The upper crustal magma system beneath Santorini is imaged to at least 6 km depth, deeper structure was not recovered. We recover the structure beneath Kolumbo to 12 km depth, and image a high P-wave velocity layer (~6-8 km) under the magma reservoir that we infer is a rheologically strong seismogenic layer of blueschists of the Cycladic metaxmorpic complex. We also recover a mid-crustal magma body below 8 km depth located to the NE beneath the Kolumbo volcanic lineament.