1992 年 101 巻 1 号 p. 1-18
The Nobi Active Fault System, one of the most active fault systems in Japan, is composed of a series of active faults extending 80 km in the NW-SE direction with en echelon arrange-ment. These faults display predominantly left-lateral displacement. In 1891, major strands within the system were ruptured during the great (M=a8.0) seismic event (the Nobi earthquake) of 1891. The magnitude of this event was extraordinarily large for normal intraplate earth-quakes.
In order to clarify the long-term behavior of the fault system, the activity of each segment or strand has been examined by geological and geomorphological methods. A section of the Late Pleistocene-Holocene sediments broken by one of the segmented faults (Umehara Fault) was studied to establish the timing of faulting mainly by trench excavation The results are summarized as follows:
(1) An accumulated displacement ensures that the Neodani Fault lying at the middle of whole length of the Nobi Fault System is the most active and longest strand. The Neodani Fault has ruptured at least, once every few thousand years with higher frequency than other strands.
(2) The entrenchment across the Umehara Fault provided recognition of the horizons on which paleo-seismic events had taken place on the fault zone. The six events were inferred within trench logs. The ages are 1891 A. D.(Event 1), 20, 000 y. B. P.(Event 2), 28, 000 y. B. P.(Event 3) and prior to 30. 000 y. B. P.(Event 4-6). The ages of Events 2, 3 and 6 are not conclusive.
(3) The recurrence intervals of the Umehara Fault between the last three events are over 10, 000 years, which are ten times longer than that of the Neodani Fault.
(4) Thus, other strands in this fault system are inferred to have been dormant even when the Neodani Fault ruptured. It is likely that very large earthquake seldom occurred in association with faulting not only along the Neodani Fault but also along the other less active strand such as the Umehara Fault.
(5) The 1891 Nobi earthquake was an extraordinarily large one. However, the record of the great Nobi earthquake should be used in evaluating future seismic hazards. especially in areas where long active faults are densely distributed.