HOLOCENE RELATIVE SEA-LEVEL CHANGES ASSOCIATED WITH COASTAL UPLIFT EVENTS IN EASTERN HOKKAIDO ALONG KURIL SUBDUCTION ZONE

Litho- and biostratigraphy beneath coastal salt marshes in eastern Hokkaido record six relative sea-level falls during the past 4000 years, probably resulted from deep postseismic slips following unusual earthquakes along the Kuril subduction zone. Eastern Pacific coast of Hokkaido has been steadily submerging at fast rate of 8-9 mm/yr in the 20th century, as recorded by tide gauges. Pleistocene marine terraces, however, imply 0.3-0.5 mm/yr of net uplift during the past 125,000 years. To balance these opposite coastal movements, emergence events were needed. Repetition of six relative sea-level fall events in four coastal marshes was marked by estuary-wide stratigraphic contacts and by contrasts of diatom and plant macrofossil assemblages in the deposits. Sharp peat-over-mud contacts clearly separate the diatom and the plant macrofossil dominance: salt-tolerant taxa in lower mud and freshwater taxa in upper peat. Radiocarbon ages of plant macrofossils and tephra show the dates of these six events are 3900-3600, 2700-2400, 2000-1700, 1400-1000, 700-500, and 300 cal yr B.P. Diatom-based transfer function for land-level reconstruction calculated the amplitude of four of the emergence events to be at least 1.0m. The coastal uplift, the most likely cause of the relative sea-level falls in this area, may be generated by infrequent seismic slip in the Kuril subduction zone, although such large-scale events have not been recorded in interplate earthquake cycle of 19-20th century. Recurrence intervals of unusual tsunami deposits, recently found along the Pacific coast of eastern Hokkaido (Nanayama et al., 2003), is about 500 years. This recurrence is not inconsistent with timings of relative sea-level falls.