Many active faults trending N-S along basin-mountain boundaries are recognized in Northeast Japan, but only a few of them have experienced surface faulting in historical time; most of them seem to have been quiescent in the past several hundred years or more. Thus earthquakes are anticipated to occur from these active faults in the near future. To detect the recurrence intervals of faulting, which can be obtained by the excavation study, is indispensable for the long term prediction of earthquakes.
We excavated a trench at Kitasakai, Sakata City, across the Kannonji fault, one of the eastern boundary faults of the Shonai plain, Northeast Japan, in order to reveal its late Holocene activity including a possible faulting event associated with the Shonai earthquake (M=7.0) of 1894 A. D., which caused severe damage along this fault.
Our excavation has revealed that (1) the last surface faulting event on the Kannonji fault occurred in a period from 2, 500 years B. P. to 1894 A. D., and that (2) no surface faulting occurred (at least at the trenching site) in association with the Shonai earthquake of 1894. Careful examination of historical records, however, strongly suggests that the earthquake of 1894 was also generated from this fault; it is likely that thick, unconsolidated sediments prevented the rupture from propagating up-dip to the surface. These results indicate that the interval between the last two earthquakes originating from the Kannonji fault is less than 2, 500 years. It could be 1, 000 years, because the event revealed by excavation is possibly correlated to the historically-documented earthquake of 850 A. D..
