A Joint Vp/Vs Model for the Mid to Upper Crust at Santorini Arc Volcano

The Santorini arc volcano in the Hellenic subduction zone in the Aegean Sea has a history of caldera-forming Plinian eruptions, most recently in the Late Bronze Age 3.4 kya, and it remains volcanically active. The island of Santorini receives up to 2 million tourists annually, in addition to local residents, so it is crucial to understand how this volcano is recharging and what steps should be taken locally. The PROTEUS experiment in 2015 recorded >14,000 controlled marine sound sources on 165 land and seafloor seismic stations. Analysis of this data provided a P-wave velocity model with low uncertainty in the upper 4 kilometers. Low velocities beneath the caldera and Kolumbo seamount were interpreted as the magma system beneath the Santorini volcanic system. However, the melt content in the study area was only weakly constrained because the geometry of melt distribution is unknown and yet strongly influence P-wave velocity. In this study we improve constraints on subsurface melt content with a tomographic P and S wave velocity structure jointly inverted using P-to-S coupling. The resulting Vp/Vs ratio, and hence Poisson's ratio, of the upper (5-10 km) crust of the Santorini region better constrains the fluid distribution and rock lithology. We invert arrival times of shear and compressional waves from ~100 local earthquakes with magnitudes from 0.5 to 3.0 that occurred between 5 and 15 km depth. The events were recorded on 142 3-component ocean bottom and island seismic stations that span the seafloor ~60 km west and east of the island and the nearby islands. The resulting Vp/Vs ratio of the mid to upper crust around Santorini volcano lends insight into the regional rock lithology and composition and crack distribution and the melt content of the magma system. Preliminary results beneath Kolumbo seamount suggest reduced Vs and increased Vp/Vs between 5 and 8 km depth consistent with elevated melt content.