Abstract
A small mound of sediment was found on the deep ocean floor by a swath mapping survey associated with 3.5 kHz subbottom profiling on the outer swell east off the Japan Trench at a depth of 5220 m. It is 130 m high, oblately elongated and situated on a NE-SW trending normal fault. No evidence of active seepage of deep methane-rich water such as biological communities and an anomalous sediment color has been found by direct observation of the bottom using a deep-towed television system. The superficial and surrounding characteristics suggest that this mound is a type of mud volcano, and be best called a mud ridge. It would be the first diapir discovered in the deep floor oceanward of a trench. The origin of this mound may be the injection of fluidized sediment expelled by highly compressed pore water through a normal fault plane associated with an earthquake shock generated by extensional forces caused by downward bending of the oceanic lithosphere near the trench.
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