What we have discovered in outer-rise – trench – volcanic arc systems: results from JAMSTEC’s marine geophysical studies
Recent advancement of observational, experimental and theoretical studies of seismogenic zones have allowed us to develop a conceptual model of a subduction zone megathrust. The model shows depth-variant slip behaviors, changing from creep/aseismic slip, to seismogenic slip then to aseismic slip through slow slip, with increasing depth along the subduction interface. However, recent striking observations form the 2011 Tohoku-Oki earthquake strongly indicates need to revise the conceptual model. For examples, rapid response marine geophysical studies after the Tohoku-Oki earthquake obtained direct evidences showing large co-seismic slips reaching the trench axis. Another fundamentally important observation we found, as an input to the subduction zones, is structural evidence indicating hydration of oceanic plates entering subduction zones. A series of large-scale seismic profiles, including three 600-km-long profiles extending from the Pacific basin across the outer rise and southern Kuril and Japan trenches, show that Vp is markedly lower and Vp/Vs markedly higher from about 150 km seaward of the trench to the trench axis. These structural variations are stronger along the profiles into the Japan trench, where throws of bending-related extensional faults of an oceanic plate are greater and seismicity in the upper plane of the double seismic zone is remarkably higher than in the Kuril trench. I will also give a review of JAMSTEC’s seismic studies in the Izu-Bonin volcanic island arc, as an out-put form the subduction zone. Resultant seismic images of the volcanic arc show strong along-arc variation which is related to volcanic activities and history.