Paleoseismology along the Southern Kuril Trench, Inferred from Diatom-based Sea-level Reconstructions

Abstract

This paper reviews the late Holocene sea-level changes associated with plate-boundary earthquakes along the Pacific coast of eastern Hokkaido. This tectonically active area, where the Pacific plate subducts at 8 cm/yr, has been steadily subsiding at the 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. In addition, an oyster bank which is indicative of the past mean tidal level shows nearly null net vertical movement during the past 6,000 years. To reconcile these opposite coastal movements, coastal uplift events were Needed. Repetition of coastal uplift was marked by emergence events (sea-level fall) in three coastal marshes (Akkeshi, Hichirippu, and Onnetoh). The sea-level fall events provided estuary-wide stratigraphic contacts, and the stratigraphic contacts clearly separate the diatom and the plant macrofossil dominance: marine-brackish taxa in lower mud and freshwater taxa in upper peat. Three of the sea-level fall events were attributed to coastal uplift based on the following three criteria ; (1) widespread peat-over-mud contact, (2) coincidence of tsunami deposits, and (3) regional synchroneity of sea-level fall events. Radiocarbon ages tephrochronology show the three events occurred approximately 1,400-1,000, approximately 800-300, and approximately 300 cal yrs BP. The coastal uplift is probably generated by infrequent decadal seismic slip, although such large-scale slips have not been recorded in the plate-boundary earthquake cycle of the 19th and 20th centuries in the Kuril trench. Diatom-based transfer functions for sea-level reconstructions, which are applied to deposits just below and above the 17th-century tsunami deposits, show several decimeters of preseismic subsidence, no coseismic change, and at least 1 m of postseismic uplift. The amplitude of postseismic uplift is consistent with 5 m of aseismic slip of a deeper part than the seismogenic zone in the Kuril trench. Such decadal postseismic slips in a deeper part than the seismogenic zone have been observed in unusually large plate-boundary earthquakes in Chile and Alaska. I concluded that Holocene coastal uplift along the Pacific coast of eastern Hokkaido was triggered by an unusually large plate-boundary earthquake in the Kuril trench.