Although there are many active faults in Northeast Japan, only a few of the faults in the inner zone have been investigated in detail. The Kitakami Lowland, running parallel to the volcanic front of Northeast Japan, is a typical tectonic depression delineated by active faults. In this paper, the past large seismic activities and fault structure of the Uwandaira Fault Group (central part of the western boundary faults), west of Hanamaki City, are discussed mainly based on trench excavation across the southern part of the fault group. The results are summarized as follows:
(1) Two events are inferred along the Uwandaira Fault Group since the later stadial of the Last Glacial; the latest one occurred ca. 4, 000 to 6, 000, and the one before that, later than ca. 20, 000 years ago.
(2) The time interval of the events is rather long, and is estimated as several to ten thousand years. It is, however, possible that the fault will generate a large seismic event in the near future.
(3) A series of sediments on the trench walls are cut by low-angle thrust faults, which caused large horizontal shortening. The structure of the sediments suggests that the fault plane dips even less beneath the trench floor.
(4) The net slip on the fault plane at each event was about 2m, and the average slip rate of the fault is estimated to be on the order of 10
-1m/1, 000 years.
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