Re-examination of coastal submergence events during the last 4000 years in the Ukishima-ga-hara lowland, central Japan: an aid for the long-term evaluation of the Fujikawa-kako fault zone

Abstract As many as twenty submergence events over the past 4000 years have been identified in Ukishima-ga-hara, a lowland facing the Suruga Trough in central Japan. We obtained 17 cores 5–25 m long from the reclaimed area, along with a 15 m core and two 2 m slice samples from the preserved natural marsh. The cores contain both local (Obuchi scoria, ~ 1500 BP) and regional (Kawagodaira pumice layer, ~ 3000 BP) tephra layers useful for correlating the collected cores within the study site. One core continuously records paleoenvironments from approximately 4000 BP to the present; stratigraphic observations and changes in both diatom and plant macrofossil assemblages were employed to identify 20 coastal submergence events during the period recorded in that core. We applied a Bayesian age–depth model to that core using 26 radiocarbon ages to estimate the depositional ages of submergence events (events T–A, from oldest to youngest). Two of these events (events R and H) were identified based on both stratigraphic and diatom/plan macrofossil assemblage changes, and two others were coeval with laminae and scoria layers (events J and G); the remaining events were only discernable based on changes in diatom and plant macrofossil assemblages. Across the contacts indicating submergence events, diatom assemblages showed transitions from terrestrial to planktonic species, and plant macrofossils indicated increases in floating-leaved and submerged species. Based on correlations with previous reports of historical earthquakes and geological records, events C–G are potentially linked to historical earthquakes along the Nankai Trough. If events H and R indeed represent relatively large submergence events, they might be related to the last two slip events along the Fujikawa-kako fault zone. Graphical Abstract