ULYSSE research program responds to the catastrophe of the unexpected occurrence of the Tohoku subduction giga-earthquake and tsunami in Japan by trying to complete on the SW Hellenic Arc the deep structural investigations missing in Japan, but successfully initiated in Greece (STREAMERS, SEISGREECE, THALES WAS RIGHT). The unexpected giga Japan earthquake and tsunami illustrated that the understanding of seismic hazard in subduction environments is inadequate. Subduction zones are tectonic boundaries where an oceanic plate slides beneath another plate causing the largest earthquakes on Earth (Magnitudes larger than 8). Because these events happen in the coastal domain they often generate destructive tsunami waves.
The subduction zone of the Hellenic Arc is responsible of the highest seismic activity of Europe and Mediterranean area. Its 400 km long south-western segment has been responsible for the largest known earthquake in Europe: the AD 365 ~M 8 earthquake which occurred offshore South of Peloponnesus near SW Crete Island and generated a devastating tsunami that spread into the Nile Delta region. As for all major earthquakes (in Japan the last earthquake larger than M 8 preceding that of Tohoku occurred in 869 AD ) it will occur again. However, we miss reliable and accurate structural information on the location, size and geometry of the seismogenic portion of the subduction mega-thrust fault plane ,that is offshore.
To achieve our targets and image the deep structure of the subduction mega-thrust fault the Institute of Geodynamics with Dr. Maria Sachpazi performed a seismic marine campaign with reflection and refraction legs between South Peloponnesus and SW of Crete (November 2012) . The methodology uses a source that discharges air under high pressure into the water. The propagated energy is reflected at depth and recorded by the hydrophones deployed along the seismic streamer, a long cable towed behind the vessel. Unfortunately such infrastructure is not available in Greece. The campaign could be realized by using the French academic research vessel POURQUOI PAS of IFREMER thanks to collaboration with the scientific group of the University of Nice-Sofia Antipolis (Dr. Mireille Laigle). Coincident multi-channel reflection and refraction seismics including two 250 km long transects crossing the subduction SW Arc were carried out. For the refraction investigation, 40 sites were occupied by Ocean-Bottom Hydrophones (OBS) provided by GEOMAR (Kiel University).
The Institute of Geodynamics with Dr. Maria Sachpazi has recently surveyed the same area during one year by installing 40 marine seismographs (OBS) provided by Universities of Kiel and of Sofia Antipolis in order to detect the offshore seismicity. First results validate our research efforts by revealing “irregular” seismic patterns and confirm that seismic hazard approach by merely monitoring on land the seismic activity of an offshore seismogenic zone is not enough.
Some of the key questions attempted to be addressed by the joint interpretation of marine profiles and accurate location of offshore seismic events thanks to the OBS monitoring are :Where is the mega-thrust fault in map and depth, and its potentially seismogenic part that could generate a major earthquake, with respect to the coastal region where people are living? What is the maximum possible magnitude of such an event?