Paleoseismicity along the southern Kuril Trench deduced from submarine-fan turbidites ∗ , Atsushi Noda a, Taqumi TuZino a Yutaka Kanai a Ryuta Furukawa a Jun-ichi Uchida b 1 aGeological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Central 7, Higashi 1–1–1, Tsukuba, Ibaraki 305–8567, Japan bDepartment of Earth Science, Faculty of Science, Kumamoto University, 39-1, Kurokami 2-chome, Kumamoto 860-8555, Japan Received 24 August 2007; revised 22 May 2008; accepted 27 May 2008 Abstract Large (> M 8), damaging interplate earthquakes occur frequently in the eastern Hokkaido region, northern Japan, where the Pacific Plate is subducting rapidly beneath the Okhotsk (North American) Plate at approximately 8 cm yr−1. With the aim of estimating the long-term recurrence intervals of earthquakes in this region, seven sediment cores were obtained from a submarine fan located on the forearc slope along the southern Kuril Trench, Japan. The cores contain a number of turbidites, some of which can be correlated among the cores on the basis of the analysis of lithology, chronology, and the composition of sand grains. Foraminiferal assemblages and the composition of sand grains indicate that the upper–middle slope (> 1,000 m water depth) is the source of the turbidites. The deep-sea origin of the turbidites is consistent with the hypothesis that they were derived from slope failures initiated by strong shaking associated with earthquake events. The recurrence intervals of turbidite deposition are 113–439 years for events that occurred over the past 7 kyrs; the short intervals are recorded in the cores obtained from levees on the middle fan. Although many large earthquakes (> 150 cm s−2 of peak ground acceleration at the inferred slump sources) occurred during the 19th and 20th centuries, the pilot core from the upper fan contains only three turbidites located stratigraphically 210 137 above layers of 17th-century volcanic ash. The results of Pbex and Cs dating, combined with simulations of the ground accelerations of historical earthquakes, enable correlation of the three turbidites with known historical earthquakes: the 1952 Tokachi-oki and the 1961 and 1973 Nemuro-oki earthquakes. The turbidites within the sampled cores potentially record about half of the large earthquakes known to have occurred over the interval covered by the cores. The fact that any single core records only a portion of the known seismic events suggests that the recurrence interval of earthquakes in this region is less than 113 years. Key words: Turbidite, Submarine fan, Paleoseismicity, Hokkaido, Japan, Kuril Trench 1. Introduction approaches are useful in estimating the timing and intensity of pre-historic earthquakes. In particular, tsunami deposits within The long-term prediction of earthquakes is one of the most coastal areas and turbidites in deep-sea sediments provide important issues in hazard assessment and risk estimation in useful paleoseismic information. Large tsunami waves are able tectonically active areas. Recurrence intervals and the timing to transport coastal sands and marine fossils to inland areas, of future earthquakes are considered to be predictable provided depositing sediments within lagoons or marshes within which that sufficient historical records are available (e.g., Ando, 1975; mud or peat normally accumulate (e.g., Atwater, 1987; Minoura Shimazaki and Nakata, 1980; Ishibashi, 1981). In regions and Nakaya, 1991; Clarke and Carver, 1992; Dawson and Shi, with limited historical data, archaeological and geological 2000; Nanayama et al., 2003); however, it must be remembered that tsunamis are able to traverse entire oceans from their NOTICE: this is the authors’ version of a work that was accepted for source regions. For example, tsunami waves associated with publication in Marine Geology. Changes resulting from peer review are the giant Chilean earthquake of 1960 arrived at the Japanese reflected, but editing, formatting, and pagination from the publishing processes coast 22–24 hours after the main shock, with up to 3.8 m of are not included in this document. A definitive version will be published in inundation height (Takahashi and Hatori, 1961). These waves http://dx.doi.org/10.1016/j.margeo.2008.05.015. ∗ Corresponding author. Fax: +81 29 861 3653. left tsunami deposits upon marshes (Nanayama et al., 2007). It Email address: [email protected] (Atsushi Noda). is therefore problematic to use tsunami deposits in developing a 1 Present address: M. T. Brain Corporation, Hayakawa Bld., 2-60-2, long-term earthquake model for a given region, as it is difficult Ikebukuro, Toshima-ku, Tokyo 171-0014, Japan Article published in Marine Geology (2008) 1–20 to determine whether tsunami deposits were derived from local earthquakes, as well as the recurrence interval of earthquakes or distant seismic events. during the Holocene. Turbidites in marine sediments have also been widely applied in investigations of paleoseismology conducted over the past 2. Geological setting two decades, including studies in Cascadia (Adams, 1990; Goldfinger et al., 2003, 2007), Japan (Inouchi et al., 1996; The Kushiro–Nemuro district of eastern Hokkaido is largely Ikehara, 2000; Nakajima and Kanai, 2000; Ikehara, 2001; flat-lying, and contains just one significant river, the Kushiro Okamura et al., 2005), Canada (Syvitski and Schafer, 1996; River (Fig. 1). Short ephemeral streams of less than 15 km in Doig, 1998; St-Onge et al., 2004), and the Mediterranean length flow into the sea or estuaries. Marine terraces, lagoons, (Kastens, 1984; Anastasakis and Piper, 1991; McHugh et al., and estuaries are well developed along coastal areas. Steep 2006); however, a number of points must be kept in mind cliffs of the marine terraces are actively eroded by wave action; when using turbidites as a tool in paleoseismic studies. First, coastal erosion is considered to be the main contributor of not all turbidites are generated in association with earthquakes sediment to the sea under the present highstand conditions (e.g., Normark and Piper, 1991). Hyperpycnal flows (Mulder (Noda and TuZino, 2007). The elevation of uplifted terraces et al., 2003), storm waves (Hampton et al., 1996), and rapid − indicates an average uplift rate of 0.16–0.24 mm yr 1 over sedimentation upon slopes (Mandl and Crans, 1981) can also the past 125,000 years (since interglacial stage 5e) (Okumura, lead to slope failure and the generation of turbidity currents. 1996). If turbidites are to be used in studying paleoseismicity, the The average width of the shelf in this area is 20–30 km, selection of coring sites is clearly important in ensuring that with the shelf margin located at 130–180 m water depth. the studied turbidites were likely to have been generated in Shelf sediments range from muddy to gravelly sand (Noda and association with earthquakes rather than other factors (e.g., TuZino, 2007; Noda and Katayama, in press). Fine to very fine Nakajima and Kanai, 2000; Goldfinger et al., 2003). Second, sands are widely distributed across the inner–outer shelf, where any single sediment core is unlikely to record the entire history the thickness of sediment deposited since the last glacial age of local seismic events. Submarine slope failures initiated by is less than 20 m. Gravels and gravelly sands are distributed earthquakes depend on slope stability, which is controlled in across parts of the inner shelf and along the shelf margin. turn by gravity and seismic loading (Lee and Edward, 1986; The mass accumulation rate of shelf sediments is estimated to Lee and Baraza, 1999; Lee et al., 1999; Biscontin et al., 2004; − be ∼0.47 Mt yr 1, representing less than 25% of the material Leynaud et al., 2004; Strasser et al., 2007). The likelihood derived from coastal erosion (Noda and TuZino, 2007). of slope failure depends on the sedimentation rate at the site The forearc slope in this area can be subdivided into three of potential failure, the recurrence interval of earthquakes in zones: the upper slope (from the shelf break to 1,000 m water the area, slope gradient, and the intensity of ground shaking. depth), middle slope (1,000–3,000 m water depth), and lower For reliable predictions of earthquake recurrence intervals, it slope shallower than the outer high (3,000–3,500 m) (Fig. 2). is necessary to correlate turbidite deposits with seismic events The dip of the slope is steepest upon the upper slope (average documented in historical records (e.g., Nakajima and Kanai, ◦ ◦ 5–6 ), reaching 10 in places. The middle slope is less steep 2000; Huh et al., 2004; Garcia-Orellan et al., 2006). ◦ ◦ (1–3 ), and the lower slope is gentle (< 1 ). A middle terrace is Large earthquakes are frequently recorded along the southern recognized at 2,000–2,200 m water depth (Fig. 2). A number of Kuril Trench, eastern Hokkaido, Japan, where the Pacific gullies incise on the upper slope; some cut through the middle Plate is subducting beneath the overriding Okhotsk (North − terrace to the deeper parts of the slope. A submarine fan with American) Plate at approximately 8 cm yr 1 (DeMets et al., 20 km wide and 15 km long is developed on the lower slope. 1990; DeMets, 1992; Seno et al., 1996). Six earthquake source The seaward margin of the fan is bounded by the outer high regions have been defined in this area, labeled A to F from west (Fig. 2), which represents a major boundary between the forearc to east along the northern Japan Trench (A) and the southern basin and accretionary prism (e.g., Clift et al., 1998; Dickinson Kuril Trench (B–F), based on a seismic gap hypothesis (Utsu, and Seely, 1979; McNeill et al., 2000). 1972, 1979, 1995)(Fig.1). The hypothesis is explained in terms of large interplate earthquakes that occur periodically in each of the source regions.