Tsunamis

HOLOCENE TSUNAMI TRACES ON KUNASHIR ISLAND, KURILE SUBDUCTION ZONE (p. 171-172)

A.YA. ILIEV1, V.M. KAISTRENKO1, E.V. GRETSKAYA1, E.A. TIKHONCHUK1, N.G. RAZJIGAEVA2, T.A. GREBENNIKOVA2, L.A. GANZEY2 AND A.A. KHARLAMOV3

1 Institute of Marine Geology and Geophysics FEB RAS, Yuzhno-Sakhalinsk, Russia
2 Pacific Institute of Geography FEB RAS, Vladivostok, Russia y
3 P.P. Shirshov Institute of Oceanology RAS, Moscow, Russia

The paper presents the detailed study of sediments deposited by middle-late Holocene tsunami in the Pacific Ocean at the Okhotsk Sea area and Izmena Bay coast of Kunashir Island. Seventeen thin sand-layers were found to be intercalated within peat of lacustrine deposits. Field data, grain-size composition, and biostratigraphical data allow interpretation of them as paleotsunami traces. Age of the sand-sheet was based a on radiocarbon dating and tephrostratigraphy. Diatoms helped identify the tsunami deposits’ origins and confirm that the sands had a marine source, and establish the landward extent of tsunami inundation. Tsunami deposits contain more contents of neritic and oceanic diatoms than marine units deposited during Holocene transgressions. The sand layers were deposited by tsunami with a maximal run-up more than 7 m, and penetration inland more the 2.5 km. The time period relating to the found tsunami deposits is 6,000–7,000 years. Correlation of tsunami events of Kunashir, Iturup Island, and Eastern Hokkaido have been done.

Key words:
tsunami deposits, diatoms, inundation limit, Holocene, Southern Kuriles.

1 Introduction

Located in one of the most active seismic regions of North-Western Pacific, Kunashir Island is known to have several great earthquakes during historical times, some of which were accompanied by tsunami. Tsunami waves produced by the 4 October 1994 earthquake, the epicenter of which was located near Shikotan Island, had the heights up to 8.7 m above mean sea level on the eastern coast of the Kunashir Island (Korolyov et al., 1997). Catastrophic tsunami are rare events, and historical data does not include reliable estimates of their frequency, impact on natural environments, and their effect on coastal development and tsunami hazard prognoses. The modern approach to such problems supposes a search and analysis of geological traces of paleotsumani that allows a reconstruction of the chronology of catastrophic tsunami at the Holocene, estimate their frequency and intensity, and spatial distribution on the basis of the correlation of tsunami deposits of contiguous areas. Evidence of strong tsunamis and pre-historic earthquakes in this region has been found in Iturup Island (Bulgakov et al., 1995) and Eastern Hokkaido (Nanayama et al., 2000, 2003, Sawai, 2002, Hirakawa et al., 2003).

Existing tsunami catalogues contain the tsunami data for the Kunashir Island coast only since 1958. Such a short tsunami history of this region doesn’t allow the creation of a good model for the description of the tsunami activity. However the needed tsunami data can be found in Holocene coastal sequences. Holocene peatlands, widespread along the Pacific coast of Kunashir and within low isthmuses are informative objects for paleotsunami study. Other useful objects for examination are numerous modern, and ancient, coastal lakes in the region. The paleotsunami study includes searches of their traces in Holocene sequences and, afterwards, the identification of their origins by diatom analysis and sedimentological methods.

Diatoms are particularly useful in studies of paleotsunami deposits because different species are found in freshwater than in brackish-marine environments, and, therefore, can be used to identify past marine incursion, including those associated with tsunami (Minoura and Nakata, 1994, Hemphill-Haley, 1996, Nanayama et al., 2000, 2003, Nishimura et al., 2000, Sawai, 2002). Frequent volcanic activity on Kunashir Island are main factors for favorable diatoms development in different freshwater and marine environments (Grebennikova, 2000). The presence of marine and brackish diatoms among freshwater assemblages in thin sand layers indicates short-time marine influence, which can be connected to storm-surge or tsunami. Marine diatoms from these deposits are rare and most valves are fragmented (Sawai, 2001, 2002). Diatom assemblages from marine deposits that formed during transgressions are characterized by high abundance and high diversity of marine species and well-preserved valves. Ecological parameters of such diatom assemblages are typical for climatic warming. Thickness of transgressive sequences is, as a rule, significant. This paper presents the result of reconnaissance work, some stratigraphical data, and other evidence, for a several tsunami that occurred about 6,000-7,000 years BP on the South Kurile region.