Holocene Tsunami Deposits From Large Tsunamis Along the Kuril Subduction Zone, Northeast Japan

Holocene tsunami deposits in eastern Hokkaido between Nemuro and Tokachi show that the Kuril subduction zone repeatedly produced earthquakes and tsunamis larger than those recorded in this region since AD 1804 (Nanayama et al., Nature, 424, 660-663, 2003). Twenty-two postulated tsunami sand layers from the past 9500 years are preserved on lake bottom near Kushiro City, and about ten postulated tsunami sand layers from the past 3000 years are preserved in peat layers on the coastal marsh of Kiritappu. We dated these ten tsunami deposits (named Ts1 to Ts10 from shallower to deeper) in peat layers by radiocarbon and tephrochronology, correlated them with historical earthquakes and tsunamis, and surveyed their spatial distribution to estimate the tsunamisO inland inundation limits. Ts10 and Ts9 are under regional tephra Ta-c2 (ca. 2.5 ka) and represent prehistorical events. Ts8 to Ts5 are between two regional tephra layers Ta-c2 and B-Tm (ca. 9th century). In particular, Ts5 is found just below B-Tm, so it is dated 9th century (Heian era). Ts4 is dated ca 13th century (Kamakura era), while Ts3, found just below Us-b and Ta-b (AD 1667-1663), is dated 17th century (Edo era). Ts2 is dated 19th century (Edo era) and may correspond to the AD 1843 Tempo Tokachi-oki earthquake (Mt 8.0) recorded in a historical document Nikkanki of Kokutai-ji temple at Akkeshi. Ts1 is inferred 20th century and may correspond to the tsunami from the AD 1960 Chilean earthquake (M 9.5) or the AD 1952 Tokachi-oki earthquake (Mt 8.2). Our detailed surveys indicate that Ts3 and Ts4 can be traced more than 3 km from the present coast line in Kirittapu marsh, much longer than the limits (< 1 km) of recent deposits Ts1 and Ts2 or documented inundation of the 19th and 20th century tsunamis. The recurrence intervals of great tsunami inundation are about 400 to 500 years, longer than that of typical interplate earthquakes along the Kuril subduction zone. The longer interval and the apparent large tsunami inundation indicate unusual origin of these tsunamis.