Seismological observation in outer-rise of the Japan Trench toward exploring interaction between megathrust and incoming plate

Abstract:

Previous geophysical, geological and petrological studies suggest a wide range of interactions between tectonic/magmatic processes in a subduction zone and incoming oceanic plate. For example, following a great megathrust earthquake, a large normal-faulting earthquake often occur in an outer- rise region of an incoming plate; the 1933 Meiji-sanriku earthquake (Mw8.1) following the 1896 Meiji-sanriku earthquake (M~8.2) are recognized as typical examples of those doublets in the Japan Trench. Another tectonic process which may control an interaction between processes in subduction interface and incoming plate is hydration/dehydration process of the oceanic plate; i.e., a dehydration process from the subducting oceanic plate is believed to affect various subduction zone processes, including arc volcanism, generation of earthquakes and non-volcanic tremor and inter-plate slow slip. In order to obtain fundamental data for exploring an interaction between a subduction process and an incoming plate, we started a new geophysical project in the outer-rise of the Japan Trench by using an ultra-deep OBS and other active-passive seismological tools. Preliminary results from the outer-rise project show that 1) reduction of Vp and increase of Vp/Vs in the crust and the uppermost mantle from ~150 km seaward of the trench, which is interpreted to be attributed to fluid percolation through bend-related faults, 2) clear Moho reflection of the incoming plate is observed to the trench from the outer-rise with partially obscure Moho reflections, 3) some of clusters of the aftershocks of the 2011 Tohoku-oki earthquake, predominantly normal fault aftershocks, extend to deeper in the mantle (~40 km deep) in an area where the Moho reflection is obscure, and the normal fault earthquake became deeper than earthquakes occurred before the 2011 earthquake. Those observations suggest that growing bend-related normal faults can be a primal mechanism to bring water deeper into the oceanic plate and this process may be enhanced by stress change in the incoming/subducting oceanic plate during a large megathrust earthquake. As one of final goals of this project, we expect to obtain an along-trench variation of hydration of the oceanic plate, which may be a factor controlling a variation of plate coupling in a subduction interface.