Along-trench structural variation and seismic coupling in the northern Japan subduction zone

Large destructive interplate earthquakes, such as the 2011 M_w 9.0 Tohoku-oki earthquake, have occurred repeatedly in the northern Japan subduction zone. The spatial distribution of large interplate earthquakes shows distinct along-trench variations, implying regional variations in interplate coupling. We conducted an extensive wide-angle seismic survey to elucidate the along-trench variation in the seismic structure of the forearc and to examine structural factors affecting the interplate coupling beneath the forearc mantle wedge. Seismic structure models derived from wide-angle traveltimes showed significant along-trench variation within the overlying plate. In a weakly coupled segment, (i) the sediment layer was thick and flat, (ii) the forearc upper crust was extremely thin, (iii) the forearc Moho was remarkably shallow (about 5 km), and (iv) the P-wave velocity within the forearc mantle wedge was low, whereas in the strongly coupled segments, opposite conditions were found. The good correlation between the seismic structure and the segmentation of the interplate coupling implies that variations in the forearc structure are closely related to those in the interplate coupling.