Four trenches (Trenches A-D) were explored across the Yasutomi fault (a strand of the Yamasaki fault system) to date recent past faultings. Trench A was 3 m deep and 28 m long across the fault (Upper in Fig. 3 and Fig. 4), and the additional excavation was made down to 5 m deep (Fig. 5) from the western wall of trench A. The sizes of other tenches are similar to that of trench A. Since this site was being developed after this trenching for the building lot of a factory, many pieces of important geological evidence were gradually exposed with progress of the construction. This enabled us to make a further detailed geological and geomorphological study of the Yasutomi fault. The results are summarized as follows : 1) Yasutomi fault, which has been considered to be predominantly left-slip active one estimated from tectonic morphologies, was geologically confirmed that this had dislocated with predominantly lateral-slip component at least since a few tens of thousand years. 2) Widely sheared zones appeared along the north side of the active trace do not accompany any tectonic features. Therefore, this straightly trending depressional zone is to be recognized as a fault-line valley. A new fault was originated along the southern rim of pre-existed weak zone probably since the late Quaternary. 3) The valley-filling deposits are disturbed at the lower part of the trench but not at the upper part this suggests that the fault has not moved since the deposition of the upper horizon although small earthquakes have been reported to occur frequently around the fault. Sense and amount of vertical offset, drugged structure and other fault features vary laterally along this, as common in high angle strike-slip fault. 4) The latest displacement occurred between late 7 th and 12 th Centuries, probably associated with the 868 Harima Earthquake (M=7.1). Two more faultings were also inferred from C-14 dates of disturbed and undisturbed strata within a deformed zone of the fault, although they are less reliable. The recurrence interval of earthquakes as large as the 868 event is estimated to be at least 1000 or possibly a few thousand years along this strand of the Yamasaki fault system.
In this part of the article the Cambro-Ordovician stratigraphy is successively described of the 3 N Arc comprizing New Guinea, New Caledonia and New Zealand (1), Australian continent (2), Tasmania (3) and Antarctica (4). In summary it is noted that (1) in the Cambrian period the Adelaide geosyncline extended into the Transantarctic mountains along the Pacific margin, (2) in the Tasman geosyncline to the east volcanism took place in the older Palaeozoic periods, (3) the excellent graptolite zonation is seen in Victoria, while the Ordovician shelly facies is wide spread on Central and Northwest Australia, (4) in Tasmania the Cambrian system is allied to that of Victoria in facies, but the non-graptolite facies is well developed in the Ordovician system, (5) the Tasman geosyncline was strongly deformed by the Devonian Tabberabberan and later orogenies till at length it turned out the folded mountains by the end of the Permo-Triassic age, while the so-called Hokonui or Rangitata orogeny which corresponds to the late Mesozoic Sakawa orogeny in Japan has taken place in New Zealand, and (6) the Cambro-Ordovician faunas of these Southwestern Pacific areas are intimately related to those of Eastern Asia.
A terminal moraine of the Yoshihara glacial stage was considered as the oldest glacial advance in the Matsukawa Kitamata Valley (KOAZE et al. 1974). By trenching the top of the terminal moraine (Figs. 1 and 2), the authors discovered the Aso-4 ash fall and Epm pumice fall in the tephra layer covering the glacial till. The total thickness of the tephra layer overlying the till is 270 cm, and the horizons of the Aso-4 ash fall and the EPm pumice fall are about 245 cm and 175 cm deep from the ground surface respectively (Fig. 3). Both marker tephra were identified by the examination of their petrographic characteristics such as heavy mineral composition and reflaction index of glass, orthopyroxene and hornblende (Table 1 and Fig. 3). According to the inferred age of these marker tephra (MACHIDA et al. 1985, MACHIDA and ARAI 1979), the age of the Yoshihara glacial stage is, at least, before 70, 000 y. B. P.