Recent progress of seismic studies on plate boundary structures gives us impressive features with sporadic seismic reflectors and splay-type large faults found near and on the subducting plate boundary, which show the complicated geomorphology derived from the faults and subducting oceanic island and ridge. Seismic activity within the slab together with overriding sedimentary and metamorphic rocks also strongly suggests fractures due to dehydration reactions. Further, the finding of incompatibility among strong seismic reflections and strong seismic activity and/or the asperity along the plate boundary leads us to imagine that the free fluid occupying many cracks might control differences between the asperity and the non-asperity of the plate boundary. In this paper, we briefly review this recent progress in seismic and material sciences from field research on plate boundary structure and dynamics.