Activity of the Yoro fault system determined from coseismic subsidence events recorded in the Holocene delta sequence of the Nobi plain, central Japan

The Holocene delta sequence of the Nobi Plain, central Japan, records coseismic subsidence in the western part of the plain caused during the past 6000 years by movement on the Yoro fault. We identified four episodes of upward-fining deposition and increasing electrical conductivity in delta front deposits that occurred 600- 200, 1300-900, 4200-3800, and 5600-4700 years ago, suggesting that episodes of sea-level rise interrupted progradation of the Kiso River delta. Considering the general trend of relative sea-level fall during the middle to late Holocene under the influence of eustasy and hydroisostasy, the temporal correspondence of rises of relative sea level with activity on the Yoro fault suggests they reflect coseismic subsidence due to Yoro fault activity. The timing of the coseismic subsidence events identified in this study also corresponds with that of previously reported faulting events at the Kuwana fault to the south of the Yoro fault. These results are consistent with the previous interpretation that the Yoro and Kuwana faults comprise a behavioral segment within the Yoro fault system. Our off-fault paleoseismic study shows that coseismic subsidence events detected in Holocene delta sequences can provide further evidence of faulting events inferred from the on-fault paleoseismic studies.