Chaotic sediments composed of volcaniclastic materials are well exposed in non-metamorphosed and highly porous acretionary prism sediments in the southern part of the Miura and Boso Peninsulas, central Japan. The southern part of the Miura Peninsula, central Japan, exposes the Misaki Formation, late Miocene to early Pliocene deep-marine forearc sediments of the Izu arc that have been accreted to another forearc during the Izu arc collision against the Honshu arc. Studying these widely distributed chaotic sediments is one of key ways to understanding the complex tectonic settings and their implication for forearc accretion and Izu arc collision. These chaotic sedimentary units are classified into four types based on grain size contrast between the blocks and matrices: type 1 is a layered chaotic body composed of siltstone blocks and sand-pebble matrix; type 2 is a massive body of siltstone blocks and sandstone-siltstone alternating blocks with sandy and partly silty matrix; type 3 is a layered body of folded sandstone blocks and siltstone-dominant matrix with lenticular sandstones which include bioturbation and cross lamination; type 4 is a layered body of intraformational folded siltstone and thin beds of sandstone without any blocks. These units were formed by two processes: for types 3 and 4, gravity-driven slumping at the surface of the sea floor; and the other, for types 1 and 2, is subsurface liquefaction and injection. Units deposited by the latter mechanism have various kinds of blocks, including slump deposits, conjugate thrust system, vein structure, duplex structure, etc. The surface slumping occurred simultaneously with the Misaki Formation sedimentation on the Izu forearc, whereas the liquefaction and injection was driven by large earthquakes on the Honshu forearc while the sediments were being undergoing accretion and even afterward. Thus, these chaotic sediments reflect gravitational instablities and earthquakes just before and after the Misaki Formation accretion.