Imaging of the Hellenic subduction zone with S-P converted phases

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The seismological images of the Hellenic subduction zone derived so far with P-wave receiver functions have large gaps in the Sea of Crete and the central Aegean. The crustal structure in areas not covered by P-wave receiver functions can be probed with S-P converted waves from both nearby local earthquakes and distant earthquakes. It can be shown that the conversion points of S-P converted waves lie up to 70-80 km off the stations for conversion depths of 25 km because the upgoing converted phases are refracted away from the vertical at seismic boundaries. This offers an opportunity to sample seismic boundaries in sea areas with data from nearby land stations at low costs as compared to the deployment of ocean-bottom seismographs for the study of steeply propagating P-S phases that are converted at the same point of a seismic boundary. We will use existing station data from intermediate-depth local and shallow distant earthquakes for a comprehensive study of S-P converted phases that appear as precursors to S and SKS and that offer an alternative method to infer depths to, and velocity contrasts across seismic boundaries in the area of the Sea of Crete and the central Aegean. This will complement our P-S receiver function images of the Hellenic subduction zone and provide, on a regional scale, a complete structural frame for the discussed drilling site in central Crete.