| Research Abstract |
Results of the two years project for submarine seismogeology are summarized methodologically and tactically as follows :
Since most active faults surrounding JapaneseIslands locate along boundaries between submarine ridges and basins similarly as on land, they can be roughly located if surface ship data of seismic profiling surveys are available. Direct observation of active faults on the bottom, however, is rather difficult because the more active faults tends to be buried easily by turbulent bottom current due to their own seismic activities. Not almighty but effective indicators used as markers in searching for submarine active faults are altered-color mats, including authigenic chemical deposits, bacterial mats, and fields of clam colonies. In addition, zones of fresh debris, ground fissures, brand-new fault scarplets which cut the recent sediments, fresh slickenlines on fault planes, and so on, are also available.
Submarine co-seismic diastrophism in the northern epicenter area of t
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he 1993 South-western off Hokkaido earthquake was reported for the first time in the world. The report includes the new identifications of 1) "quake-rubble" characterized as autochtonous brecciation of unconsolidated mud layr and 2) "jump-fall rubble" scarcely distributed on the muddy flat-floor in front of steep scarps due to slope failure and/or landslide, which denote the strong motions during the main-shock and were visibly observed by submersible investigations.
Seismotectonics along the eastern Nankai Trough werere-investigated on the basis of photographic analyzes of still-photos and video films recorded during the KAIKO and KAIKO-NANKAI programs, using structural indicators such as slip sense of fault, clastic dikes and joints. Tectonic stress orientation differs among the inner and outer tough slopes, indirectly indicating eastern motion of the Amurian plate (Enshu-nada deepsea terrace) and northward motion of the Philippine Sea plate (Zenisu Ridge). On the contrary, no such kind of stress zonation is found along the eastern maegin of Japan Sea, possibly because the plate boundary between the Amur and the Okhotsk plates are too immature. Less
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