New features revealed by multi-method seismic imaging of the seismic structure and activity of the Lesser Antilles and Hellenic subduction zones, and their comparison with Tohoku

The densely populated two subduction zones on the European Union territory, in the western Hellenic and the Lesser Antilles regions have been currently considered as having a low seismic coupling, that is aseismic from the point of view of megathrust earthquakes which have been commonly unexpected there. The occurrence of the 2004 megathrust event in Sumatra, also unexpected, prompted support to our research projects submitted before. The unexpected occurrence of the Tohoku 2011 event since, provides terms of comparison for our results. We applied to both subduction zones specific approaches to the seismic structure and activity by a cluster of active and passive offshore-onshore seismic experiments with Research Vessels, OBS (Ocean Bottom Seismometers) and land seismometer arrays and funding from several countries, coordinated within the ¿Thales was right¿ proposal to the European Union action (see also Laigle et al. in session T02, and Sachpazi et al. in the present session). Each of the two studies allows comparing different seismic methods, in their resolution-penetration, and building on their joint use: i) MCS multichannel reflection seismics, ii) OBS and offshore-onshore refraction seismics and shot tomography, iii) Teleseismic converted waves imaging of deep and steep interfaces (e.g. RF, receiver-function), iv) local earthquakes high-resolution location by OBS and land arrays, v) search for non-volcanic tremor and low-frequency earthquakes. A number of features revealed in spite of the much lower level of current seismic activity share a similarity with those originally interpreted in the frame of a largely aseismic subduction zone in Tohoku, for which the occurrence of the 2011 event imposed another view. Such features are: i) the extent of the crust on crust subduction thrust, proxy to the downdip extent of the seismogenic, could be identified and mapped much landward than commonly considered, ii) furthermore, the Tohoku rupture showed the seismogenic zone to extend even much farther, to under the mantle of the upper plate down to the so-called ¿deep flat-thrust repeating earthquake¿ previously interpreted as small asperities in a stable gliding environment, and we have now been able to identify deep flat-thrust earthquakes in our study regions, which raises the question of possible downdip extent of rupture; iii) resolving thus seismic activity and structure we can also for the first time document ¿Double Seismic Zones¿ of the slabs as originally discovered in Tohoku decades ago; iv) we resolve also clearly ¿supraslab earthquakes¿ that were only fully described in Tohoku some time before the mega-earthquake, but not readily explained in a mantle corner that was commonly considered as being serpentinized above the subduction thrust, the latter being hence aseismic, which indeed the 2011 event rupturing down there showed not to be the case. Finer comparison among the Lesser Antilles and Western Hellenic for the seismicity, though much lower than in Tohoku, but with high-resolution images of the deep structure around the subduction thrust under the forearc sheds light on the structural environment of the diverse types of seismicity and the deep mechanisms promoting the notable surface extension in the upper plates.Peer Reviewe