Along Hokkaido's Pacific coast near the town of Kiritappu, sandy deposits in a muddy lagoon and on a nearby beach‐ridge plain provide evidence for 15 tsunamis between 200 and 6000 years ago. Additional sand beds at the lagoon probably represent the historical tsunamis of A.D. 1843 and 1894. We observed the sequences of sandy deposits in continuous slices 2 to 4 m deep. Some of the deposits consist of just a single sand bed, whereas others contain multiple units of sand, muddy sand (or sandy mud), and mud caps including plant detritus. We also found at the lagoon a 17th century tsunami deposit that thickens and thins regardless of elevation or distance inland. We bracketed the ages of most of the inferred tsunamis by radiocarbon dating of detritus, mainly seeds and leaves at the lagoon and charcoal at the beach‐ridge plain, from pretsunami and posttsunami beds. Tsunami dates computed from the bracketing ages commonly have uncertainties spanning 2 to 4 centuries. Within these uncertainties, the inferred sequence of 15 prehistoric tsunamis at the lagoon, beginning almost 6000 years ago, can be matched tsunami by tsunami with the inferred history at the beach‐ridge plain, 15 km away. The sand sheet extents suggest that most of these tsunamis were larger than any generated at Hokkaido in the last 200 years. The intervals between these inferred outsized tsunamis average nearly 400 years but range widely from about 100 to about 800 years.
Sandy deposits of marine origin underlie the floor of Suijin-numa, a coastal lake midway along the subduction zone marked by the Japan Trench. The deposits form three units that are interbedded with lacustrine peat and mud above a foundation of marine, probably littoral sand. Unlike the lacustrine deposits, all three sandy units contain marine and brackish diatoms. The middle unit (B) contains, in addition, graded beds suggestive of multiple waves of long wavelength and period. The uppermost unit (C) probably dates to a time in the area's written history when the lake was separated from the sea by a beach-ridge plain at least 0.5 km wide and several metres high. Units A and B postdate AD 540—870, and unit C postdates AD 1030—1640 as judged from radiocarbon dating of leaves and seeds. Unit B pre-dates AD 915 and unit C postdates that year as judged from a tephra within the peat that separates units B and C. The age constraints permit correlation of unit B with a tsunami in AD 869 that reportedly devastated at least 100 km of coast approximately centred on Sendai. Unit C may represent a later catastrophic tsunami in 1611, or perhaps a storm surge that inundated much of Sendai. The lake lacks obvious signs of tsunamis from the region's largest twentieth-century earthquakes, which were centred to the north in 1933 (M 8.1) and directly offshore in 1936 (M 7.5), and 1978 (Mw 7.6).
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