Paleoseismicity Along the Southern Kuril Trench Deduced From

Home , 1993 Kushiro earthquake, Submarine earthquake

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. The oldest historical record of 3. Seismicity an earthquake in the area is the 1843 Tokachi-oki earthquake. This 160-year historical record of seismic events enables us to Although there exists no written record of earthquakes estimate an average recurrence interval of 72.2 years for events along the southern Kuril Trench prior to the 1843 Tokachi-oki along the southern Kuril subduction zone (Earthquake Research earthquake, historical literature produced in Honshu indicates Committee, 2004), although over this period the different frequent earthquake activity prior to the 19th century (Satake, source regions have experienced only two or three events. 2004). During the late 19th and earliest 20th centuries, For long-term earthquake prediction, we analyze turbidites earthquakes were recorded in 1843 (M 8.0, Region B), 1856 deposited upon a submarine fan developed on the forearc (M 7.5, Region A), 1893 (M 7.7, Region D), 1894 (M 7.9, slope. We present new data on the texture, composition, and Region C), and 1918 (M 8.0, Region F) (e.g., Hatori, 1973, depositional age of the studied turbidites. We then discuss 1974, 1984; Utsu, 1999). Few events were recorded during the the relationship between turbidite deposition and historical early 20th century; however, seismic activity increased again

2 60˚N 140˚E 145˚E 150˚E 155˚E

OkhotskOkhotsk PlatePlate 19151915 M7.9M7.9 40˚N (North(North AmericanAmerican Plate)Plate) F

11963963 M8.1M8.1 19181918 MM8.08.0 EtorofuEtorofu 20˚N Kuril Islands 19181918 MM7.77.7 100˚E 120˚E 140˚E 160˚E D KunashiriKunashiri 44˚N HokkaidoHokkaido KushiroKushiro RRiveriver MaMa 18931893 M7.7M7.7 NemuroNemuro C KKushiroushiro 19691969 M7.8M7.8 TokachiTokachi TaTa B UsUs plainplain 19731973 M7.4M7.4 Kuril Trench KoKo 11894894 MM7.97.9 18431843 MM7.57.5 A 20032003 M8.0M8.0 Fig. 2 18561856 MM7.8M7.57.8 19521952 M8.1M8.1 11968968 M7.9M7.9 ca. 8 cm/yr 40˚N Pacific Plate HonshuHonshu

300 km Japan Trench

Fig. 1. Tectonic setting and location of the study area. Solid and open circles are epicenters of the historical interplate earthquakes. Labels A–F represent source regions of the earthquake. Abbreviations of volcanoes: Ko, Komagatake; Us, Usu; Ta, Tarumai; Ma, Mashu. during the middle and late 20th century, including the 1952 earthquake (M 8.2, Region D; Kikuchi and Kanamori, 1995; Tokachi-oki earthquake (M 8.1, Region B; Geistetal., 2003; Tanioka et al., 1995; Satake and Tanioka, 1999) within the Hirata et al., 2003, 2004; Hamada and Suzuki, 2004), the shallow part of the slab, and the 1924 Etorofu earthquake 1963 Kuril Islands earthquake (M 8.1, Region F; Kanamori, (M 7.6, Region C), 1978 Kunashiri Strait earthquake (M 7.7, 1970; Beck and Ruff, 1987), the 1968 Tokachi-oki earthquake Region C; Suzuki, 1979; Kasahara and Sasatani, 1985), and (M 7.9, Region A; Fukao and Furumoto, 1975; Schwartz 1993 Kushiro-oki earthquake (M 7.5, Region B; Morikawa and and Ruff, 1985), the 1969 Kuril Islands earthquake (M 7.8, Sasatani, 2003) within the deep part of the slab. The recurrence Region D; Abe, 1973; Fukao and Furumoto, 1975; Schwartz intervals of earthquakes in the subducting plate are estimated and Ruff, 1985, 1987; Kikuchi and Fukao, 1987), and the to be 82.8 years for those in the shallow slab and 27.3 years for 1973 Nemuro-oki earthquake (M 7.4, Region C; Sekiya et al., those in the deep slab (Earthquake Research Committee, 2004). 1974; Shimazaki, 1974; Aida, 1978). The 2003 Tokachi-oki Some of the large earthquakes listed above were earthquake is the most recent event around the studies area accompanied by tsunamis that left characteristic sedimentary (Yamanaka and Kikuchi, 2003). Although the epicenter of the deposits in estuaries, lagoons, and swamps along the coast from 2003 earthquake was located at approximately the same site Tokachi to Nemuro (Hirakawa et al., 2000; Nishimura et al., as that for the 1952 Tokachi-oki earthquake (Fig. 1), tsunami 2000; Sawai, 2002; Nanayama et al., 2003, 2007). Nanayama inversion models indicate that the rupture extent of the 2003 et al. (2007) identified 13 tsunamigenic sand layers within earthquake was restricted to the western half of the rupture marsh sediments, and calculated that the corresponding tsunami area of the 1952 event (Hirata et al., 2004; Tanioka et al., events had a recurrence interval of 365–553 years over the past 2004; Satake et al., 2006). These historical earthquakes yield 4,000 years. Turbidites deposited upon the ocean floor also a recurrence interval of ca. 72.2 years for large interplate provide evidence of the recurrence intervals of earthquakes. earthquakes along the southern Kuril subduction zone. Noda et al. (2004, 2008) reported an average recurrence interval Large intraslab earthquakes have also occurred within the of 68–85 years for the deposition of turbidites off Kushiro over subducting plate, including the 1958 Etorofu-oki earthquake the past 2400 years. (M 8.1, Region F; Fukao and Furumoto, 1979; Schwartz and Ruff, 1987; Harada and Ishibashi, 2000) and 1994 Shikotan

3 145˚20'E 145˚30'E 145˚40'E 145˚50'E 146˚00'E 43˚20'N

Nemuro 10 km

Shelf

43˚10'N Upper −100 slope

−200 43˚00'N −500 −100 −1000 Middle

−1500 terrace

−200 −2000 −500

42˚50'N

−2500

−1000 Middle slope −1500

−2000 42˚40'N

−2500 −3000 Lower

42˚30'N PC06 slope PC01 PC05 PC02 Outer 1037 high −3000 1038 1036

42˚20'N LineLine 1717 LineLine 1616

Fig. 2. Bathymetry, sampling localities, and seismic recording lines. A detailed bathymetrical map of the surrounded area is shown in Fig. 3.

4 4. Data and methods Benthic foraminifers were extracted from sandy turbidites in samples PC05 and PC06 to determine the sources of the Bathymetric data were collected by the Hydrographic and sediment, as deduced from the distribution of modern benthic Oceanographic Department of the Japan Coastal Guard (Japan foraminifers throughout the study region (Matsuo et al., 2004; Coast Guard (Maritime Safety Agency), 1998). Maps in this Ooi et al., 2005). 14 study were created using 10 sec (about 300 m) gridded data. Accelerator Mass Spectrometer (AMS) C measurements Seismic reflection profiles were collected using a GI gun of planktonic foraminifers were taken at Beta Analytic Inc. (generator 250 in3 and injector 105 in3 airgun) with a six We picked more than 10 mg of mixed species of planktonic channel streamer cable in July and August of 2004 (cruise foraminifers (mainly Globorotalia inflata and Globigerina GH04) aboard the R/V Hakurei-maru No. 2 of the Japan Oil, bulloides) from six horizons (core PC01, PC05, and PC06) and Gas and Metals National Corporation (JOGMEC). The survey mixed benthic foraminifers (mainly Nonionellina labradorica speed was 8 knots (14.8 km/h) and the shooting interval was and Elphidium batialis) from one horizon (core PC02) for the 14 6 sec, with a common depth point (CDP) of ∼25 m. The grid analysis. The C ages from benthic foraminifers were 820–870 size was 2 miles (3.7 km) E–W and 4.5 miles (8.3 km) N–S years older than those from planktonic foraminifers of the ff (Fig. 2). same horizon o Kushiro during the late Quaternary (Noda ± Gravity (cores 1036 and 1037) and piston (core 1038) et al., 2008). We used a reservoir age of 386 16 years for the 14 sediment cores were collected during cruise GH04 (Table SD1). C ages in this region (Yoneda et al., 2001). The obtained Additional sediment cores were obtained using a piston corer conventional radiocarbon ages were calibrated to calendar ages (cores PC01, PC02, PC05, and PC06) in April and May of 2005 using CALIB rev. 5.0.2 (Stuiver and Braziunas, 1993)andthe (KR0504 cruise) aboard the R/V KAIREI of the Japan Agency dataset marine04.14c (Hughen et al., 2004). for Marine-Earth Science and Technology (JAMSTEC) (Fig. 2). The sediments within the pilot core (core PC05) were split Halved cores of samples PC01–PC06 were measured into 0.5 cm intervals, dried, and powdered for radioactivity for γ-ray attenuation at 1 cm intervals using a GEOTEK analysis. The powdered samples (1–4 g) were stored in Multi-Sensor Core Logger to calculate the wet bulk density. a capped centrifuge tube for about 1 month to ensure The wet bulk density of cores obtained during cruise GH04 radioactive equilibrium among daughter nuclides, after which 210 = 137 = was analyzed at 2 cm intervals using 7 cm3 plastic cubes. the radioactivities of Pbex (T1/2 22.3 yr) and Cs (T1/2 The water content of sediments was calculated as 100 × 30.1 yr) were measured using a well-type Ge semi-conductor (wetweight − dryweight)/dryweight. Grain-size analysis was detector (Kanai, 1993). conducted using a laser particle-analyzer (Cilas 1064) for selected sandy turbidites at 0.5 cm intervals. This instrument is 5. Results able to determine grain sizes in the range between 0.4 and 500 μm. The compositions of medium sand (0.25–0.5 mm) fractions 5.1. Fan physiography from selected turbidites were determined from 200–400 points counted under a stereomicroscope. To study sedimentary The submarine fan has its apex in the water depth of 3,100 m structures and identify sand layers, soft X-radiographs were and is bounded by the outer fan at the water depth of 3,300 m × × 3 taken of slab subsamples (5 20 1cm )ofthecoresamples (Fig. 3). Several channels from the upper and middle slopes using a SOFRON TYPE STA-1005 operated at voltage of merge at the top of the fan, and then divide into two channels 45 kV, current of 3 mA, and an irradiation time of 30–90 s. The on the upper fan (3,100–3,200 m). Cores PC05 and PC06 were thickness of the turbidites was measured on the X-radiographs obtained near channels on the upper fan. Sub-bottom profiling by 0.1 cm intervals. records show strong reflection in the subsurface sediments, Samples of tephra layers and patches were collected for indicating sandy sediments cover on the upper fan (Fig. 4). petrography and glass chemistry. Description and classification The middle–outer fan has a convex-up, lobe-like bathymetry; of shape of glass shards were based on Machida and the axis is high and the marginal area is low. The gradient Arai (1992). Chemical analysis was performed on a JEOL becomes gentler in the distal part. Cores 1037, PC01, and JXA-8900R electron probe microanalyzer at the Geological PC02 were obtained from levees along the channels on the Survey of Japan. Nine major elements (Si, Ti, Al, Fe, Mn, Mg, middle fan (Fig. 3). Several reflections could be recognized Ca, Na, K) were analyzed with an accelerating voltage of 15 kV μ in the sub-bottom profiling records of the middle fan (Fig. 4), and a beam current of 12 nA. Beam diameter was 10 m, with indicating repeated deposition of sands and muds. Cores 1036 counting times of 20 and 10 sec for peak and background, and 1038 were recovered from the outer fan; the latter core was respectively. Chemical compositions of glass shards, especially obtained from the most distal part of the lobe. Ti and K, reflects magma types of source volcanoes and can be used to identify origin of volcanic ashes (e.g., Westgate 5.2. Seismic profiles and Evans, 1978; Larsen, 1981). The results were compared to volcanic ashes whose chemical compositions were previously reported (Katsui et al., 1978; Furukawa et al., 1997; Shimada The shelf and upper slope of the study area are underlain et al., 2000; Furukawa and Nanayama, 2006). by Cretaceous–Pliocene sedimentary rocks (Honza et al., 1978; TuZino et al., 2004, 2005), while the middle–lower

5 145˚45'E 145˚50'E 145˚55'E 146˚00'E CDP

500 1000 1500 2000 2500 3000 A 0 −3100 A 1.0 42˚30'N PC06 MMiddleiddle tterraceerrace

2.0 −3200 ShelfShelf OuterOuter B highhigh 3.0 −3000 PC01 PC05 UpperUpper

PC02 TWT (sec) slopeslope 4.0 −3100 MiddleMiddle sslopelope −3200 5.0 C 42˚25'N 6.0 VE:VE: 1111 1037 ca.ca. 1010 kmkm LLowerower sslopelope 1038 LineLine 1177 NNWNNW SSESSE

−3200 2000 2200 2400 2600 2800 3.5 −3100 −3300 B 1036 PC02 4.0 1038 5 kmkm

42˚20'N 4.5

Fig. 3. Detailed bathymetry for the submarine fan in the studied area. Dashed 5.0 VE:VE: 5.85.8 lines indicate submarine channels on the fan. Solid lines with labels A and cca.a. 5 kmkm B are for sub-bottom proﬁling records in Fig. 4. LineLine 1166 NNWNNW SSESSE

24002600 2800 3000 3200 3400 Water 3.5 depth PC06 PC06PC06 C (m) PC05 1036 4.0 3150 2 km

4.5

5.0 VE:VE: 5.85.8 ca.ca. 5 kmkm PC05 LLineine 1177 NNWNNW SSESSE 3200 5.5

Fig. 5. Seismic proﬁles across the forearc slope and submarine fans. Horizontal axis: 1 CDP = ∼25 m; Vertical axis: 1 sec TWT (two-way travel time) = ∼750 m in seawater.

3250 the outer high that consists of non-layered acoustic basements A (Fig. 5BandC)(Honza et al., 1978; Klaeschen et al., 1994; Schn¨urle et al., 1995). The sediments of the lower slope PC02 Channel represent the formation of a half-graben, suggesting deposition 3270 PC01 associated with normal faulting (Fig. 5B and C). Vertical displacement upon the normal fault ranges from 1 to 1.5 sec TWT (two-way travel time). The sampling sites of the sediment 3300 cores are located on the lower slope, where the sediments dip gently seaward. B 5.3. Lithology

Fig. 4. Sub-bottom profiling records for selected coring sites. Locations for survey lines are in Fig. 3. All cores consist of hemipelagic mud that consists of olive black (7.5Y3/2–10Y3/1 in Munsell color value) clayey slope consists of a gently-folded post-Pliocene sedimentary silt, volcanic ash, and turbidites composed of sandy silt succession (Fig. 5A). An anticline is observed in the middle to fine sand (Fig. 6). No debris flow or other mass flow slope, where a terrace has formed parallel to the shelf margin deposits are recognized in the cores. Hemipelagic mud is (Figs. 2 and 5). The narrow sedimentary basin located between diatomaceous and generally heavily bioturbated. Slightly darker the upper slope and the middle terrace records the thickest (10Y3/2–10Y3/1) clayey silt is commonly observed above the sediments of the middle slope. The lower slope is bounded by sandy turbidites. The wet bulk density of the hemipelagic mud

6 1036 1037 1038 PC01 PC02 PC05 PC06 (224 cm) (160 cm) (468 cm) (636 cm) (785 cm) (594 cm) (243 cm)

vv vv Us-b v v vv Ta-b v v Ta-b vv vv Ta-b Us-b vv v Us-b vv Us-b Ta-a / Ta-b v ~ ~ Ta-b ~ vv vv vv Us-b

vv ~ ~ ~ ~ ~ ~ vv ~ ~ ~ ~ ~ ~ ~ ~ ~ v

2357 cal yr BP 7339 4668 cal yr BP cal yr BP 1 m ~ ~ 2956 cal yr BP ~ ~ ~ ~ ~ 5109 Flow-in ~ ~ ~ ~ ~ cal yr BP

0 ~

Olive black (7.5Y3/2) clayey silt

Olive black (10Y3/2– ~ ~ ~ ~ ~ 10Y3/1) clayey silt ~ ~ ~ ~ ~ ~ ~ ~ v v Tephra ~ ~ ~

~ Bioturbation ~ ~ ~ Very coarse sand Coarse sand Medium sand Fine sand Sandy silt–very fine sand ~ ~ ~ ~ ~ v ~ vv Ko-g ~ ~ ~ 6960 cal yr BP

v v Ko-g

v v

9830 cal yr BP (Benthic Foram.)

Fig. 6. Descriptions of the sediment cores. ranges from 1.1 to 1.4 g cm−3 (Fig. 7). general; some contain basal layers of ﬁning-up coarse sand. The turbidites are generally coarse silt to ﬁne sand in Amalgamation is commonly observed in thick turbidites, where grain size, and have sharp basal contacts. The thickness of two to four turbidites join without any intervening hemipelagic the turbidites vary from 0.1 to 13 cm, with coarser-grained mud (Fig. 8). In the amalgamated turbidites, sedimentary units being thicker. Normal grading or parallel lamination is structures (e.g., parallel or cross laminations) in the lower layers

7 are sometimes truncated by overlying turbidites. The densities Historical large earthquakes −3 ka Tephra Tsunami events Region B Region C of the turbidites vary from 1.6 to 2.4 g cm (Figs. 7 and 8). 0 AD 1973 The upper parts of the cores contain fewer and thinner KS1 (AD 1952 or 1960) AD 1952 AD 1894 turbidites than the lower parts (Fig. 7). Cores 1036 and 1038, KS2 (AD 1843) AD 1843 Ta-a (AD 1739) sampled from the outer fan, contain fewer and thinner turbidites Ko-c2 (AD 1694) No historical records (1.8–2.6 turbidites per 100 cm) relative to other samples. In Ta-b (AD 1667) of earthquakes contrast, core PC06 from the upper fan records the highest Us-b (AD 1663) KS3 (AD 1635?) frequency (11.1 per 100 cm). The density profile and turbidite KS4 (AD 1290–1391?) thickness distributions for PC01 are comparable to those for 1 B-Tm, Ma-b PC02 (Fig. 7); turbidites are more commonly observed and (1.0 ka) KS5–KS10 (recurrence 2 intervals of 372–422 yrs) thicker in the lower sections of both cores. Ta-c2 3 (2.5–2.7 ka) 5.4. Composition of sand grains 4 KS11– (recurrence 5 intervals of 406–553 yrs) The sand fractions of the turbidites are predominantly made 6 up of volcanic glass and diatoms (Fig. 9). The volcanic glass is mainly pumice-type glass, with lesser bubble wall-type 7 Ko-g (6.5 ka) and massive-type glass. Almost all of the volcanic glass is fresh, but some is stained brown. Light minerals are composed Fig. 10. Summary of tephrochronology (Furukawa and Nanayama, 2006) with tsunami events (Nanayama et al., 2007) and large historical earthquakes in of quartz and feldspar, with both minerals being generally the eastern Hokkaido. Source volcanoes: B, Baitoushan; Ko, Komagatake; fresh and euhedral, suggesting a volcaniclastic origin. Among Ma, Mashu; Ta, Tarumai; Us, Usu. Locations of the volcanoes are shown in the heavy minerals, ortho- and clinopyroxene and opaque Fig. 1. minerals are common, with lesser hornblende and biotite. The proportion of heavy minerals in the sampled turbidites (1,000–2,000 m water depth) (Abe and Hasegawa, 2003; is generally low (< 3%), but they are highly concentrated in Matsuo et al., 2004; Uchida, 2006). Few shelf or upper core PC02 at 597–599 cmbsf and PC06 at 156–160 cmbsf slope assemblages are recognized in the sampled turbidites. (Table SD2; Fig. 9). Benthic foraminifers make up 10–40% Although Elphidium batialis found in PC06 is lightly of the sandy turbidites (Table SD2); planktonic foraminifers dissolved, the occurrence of species with thin tests, such as make up 10–20%, although they are rare in the surrounding Nonionellina labradorica, indicates only minor dissolution hemipelagic mud. The low density of foraminifers means that effects. Planktonic foraminifers within the turbidites are they might have behaved as coarser grains than the same-sized generally well sorted. minerals and rock fragments during deposition (Fig. 8Cand D). Diatoms constitute as much as 83% of sand grains, being 5.6. Volcanic ashes mainly observed in relatively fine and thin turbidites. Indicators of shallow water, such as bivalves, glauconite, and plant The most conspicuous volcanic ashes are observed at fragments, are rarely observed. 30–100 cmbsf within cores 1036, 1037, 1038, PC01, PC02, The composition of sand grains in PC01 is similar to that in and PC05 (Fig. 6). The ashes are present as small patches in PC02. The upper and middle parts of the cores record a high PC05, but are absent in PC06. The identified ash layers are percentage of diatoms, while the lower parts are dominated by up to 8 cm thick, and commonly contain beds of two distinct minerals and rock fragments. Turbidites within core PC01 at colors: a light brownish gray (5YR7/1) lower bed and a reddish 200–250 cmbsf contain relatively few biogenic tests, as with gray (2.5YR5/1–10R5/1) upper bed. Both contain ortho- and PC02. Turbidites in the lower parts of cores PC05 and PC06 clinopyroxene in addition to plagioclase and opaque minerals; also show high concentrations of minerals and rock fragments. hornblende grains are only included in the lower tephra These layers are traceable among the cores. (Table 2). Glass chemistry is characterized by low TiO2 and K2O values for the lower tephra and high K2O/TiO2 ratio for the 5.5. Foraminifers upper tephra. The petrographic and geochemical characteristics indicate that the lower and upper ashes are correlated with Us-b Foraminiferal tests in the turbidites showed small size (fine- (A.D. 1663) from the Usu volcano and Ta-b (A.D. 1667) from to very fine-grained) and relatively good preservation enabled the Tarumai volcano, respectively (Figs. 10 and 11;Table2). us to identify species. Benthic foraminiferal assemblages in Cores PC01 and PC02 contain small patches of volcanic selected turbidites from cores PC05 and PC06 are dominated ashes at the lower part of the cores. The glass chemistry by Cassidulina norvangi, Islandiella norcrossi, Elphidium in the ashes indicates that they are prehistoric tephras from batialis,andUvigerina akitaensis (Table 1), with lesser Komagatake volcano (Fig. 11). We know only the Ko-g tephra species of Bolivina spissa, Epistominella pacifica, Nonionellina as prehistoric volcanic ashes sourced from the Komagatake labradorica,andTakayanagia delicata. The dominant species onland and offshore of the eastern Hokkaido (Fig. 10) are characteristic of the upper–middle slope environments (Furukawa and Nanayama, 2006). They can be, therefore,

8 Thickness(cm) 0 5 10 15 0 5 10 15 0510150 5 10 15 0 5 10 15 0 5 10 15 0 5 10 15 0

200 ? ? 23572357 yBPyBP 73397339 yBPyBP ? 46684668 yBPyBP 29562956 yBPyBP

51095109 yBPyBP (cmbsf) 400 Ta-b (AD1667) Us-b (AD1663)

Depth Ko-g (6.5 ka) 600 69606960 yBPyBP

1036 1037 1038 PC01 PC02 PC05 PC06 800 12121212121212 WBD (g/cm3)

Fig. 7. Thicknesses of turbidites (solid bars) and wet bulk density (WBD) of the sediments (gray lines). Identified volcanic ash layers and dated horizons are also indicated. Table 1 Occurrence (%) of benthic foraminifers within the sampled turbidites. Abbreviations: US, Upper slope; UMS, Upper–middle slope; MS, Middle slope. Sample no. PC05–1 PC05–2 PC05–3 PC05–4 PC06–1 PC06–2 PC06–3 PC06–4 Environment cm below sea floor (top) 390.5 397.5 469.5 522.5 88 103 136 141 cm below sea floor (bottom) 392.5 399.5 472.5 525.5 94 107 138 145 Angulogerina ikebei 1.3 0.7 0.5 1.2 US Bolivina decussata 0.6 1.0 2.4 0.5 3.1 3.0 US Bolivina spissa 2.3 1.0 1.0 0.8 5.5 0.8 1.6 0.6 UMS Bolivina sp. A 2.3 Buccella spp. 2.0 2.8 1.6 2.5 3.8 0.5 3.6 Bulimina aculeata 1.3 1.0 0.8 1.2 Bulimina striata 0.6 0.5 0.7 1.2 UMS Bulimina tenuata 1.3 0.5 1.6 0.5 0.8 0.5 1.8 Cassidulina norvangi 8.7 7.0 9.1 14.4 4.5 9.9 8.8 11.4 Cibicides lobatulus 0.3 1.5 Shelf Cibicides spp. 0.5 Shelf Cibicidoides sp. 0.7 0.5 UMS Cornuspiroides sp. 0.3 Cribroelphidium sp. 0.5 Dentalina sp. 0.5 Eilohedra nipponica 1.0 3.5 0.7 4.0 0.5 1.5 3.6 1.8 MS Elphidium batialis 25.5 13.1 18.1 13.6 31.5 26.7 14.5 12.0 MS Elphidium spp. 8.7 11.1 7.0 12.0 4.0 8.4 7.8 10.8 Epistominella pacifica 6.1 4.0 9.8 4.0 10.5 3.8 5.2 8.4 MS Epistominella sp. 1.0 0.5 Epistominella spp. 0.3 1.4 1.0 0.8 0.6 Fissulina spp. 0.3 Fursenkoina cf. rotundata 1.3 0.5 0.3 1.2 Fursenkoina sp. 0.3 0.5 Globobulimina auricurata 1.5 2.1 1.2 UMS Globocassidulina spp. 1.0 0.5 0.3 0.8 0.8 4.1 1.8 Gyroidina sp. 0.6 0.5 0.8 3.1 Gyroidina spp. 2.0 3.8 4.8 1.5 1.5 1.0 0.6 Islandiella norcrossi 7.4 22.6 10.8 8.8 8.5 14.5 6.7 13.8 UMS Melonis pompilioides 1.6 5.0 3.8 2.4 1.0 Melonis sp. 1.6 1.5 5.2 1.6 0.5 2.6 Lagena spp. 0.6 Nonionella globosa 0.5 2.4 Nonionellina labradorica 6.8 6.5 4.2 6.4 4.5 2.3 3.1 4.8 Oridorsalis umbonatus 1.3 2.8 0.8 1.5 2.3 3.6 Pseudoparrella takayanagii 1.6 Pseudoparrella sp. 0.6 0.5 1.0 4.8 0.5 3.1 2.1 4.8 Pullenia salisburyi 0.6 0.3 1.0 Pullenia bulloides 0.3 1.5 2.1 1.6 1.0 1.6 Pullenia spp. 0.6 5.5 1.4 3.2 0.5 0.6 Pyrgo sp. 0.3 0.5 0.5 Takayanagia delicata 5.2 2.4 3.8 4.1 4.2 UMS Uvigerina akitaensis 4.2 3.5 2.8 3.2 15.0 9.9 7.8 3.6 UMS Uvigerina senticosa 1.0 0.5 3.5 0.5 0.8 6.2 Valvulineria spp. 0.3 0.5 0.5 0.6 Vaginulina sp. 0.3 Others 4.8 4.5 1.4 4.8 1.0 1.6 3.0 Total benthic foram. number 294 190 282 119 197 131 189 161 Total planktonic foram. number 457 467 699 172 100 94 336 419 P/T ratio 60.9 71.1 71.3 59.1 33.7 41.8 64.0 72.2

Shelf 0.0 0.5 0.4 0 0.0 1.5 0.0 0.0 Upper slope 1.9 1.0 0.7 2.4 1.0 0.0 3.1 4.2 Upper-middle slope 19.7 27.6 18.5 12.8 29.0 29.0 20.7 23.4 Middle slope 32.6 20.6 28.6 21.6 42.5 32.1 23.3 22.2 Others 45.8 50.3 51.9 63.2 27.5 37.4 52.8 50.3

9 A (PC01) Gray scale Density B (PC02) Gray scale Density 200 100 0 1.5 2.0 2.5 200 100 0 1.5 2.0 2.5

575 500

580 505

585 510

590 515 (cmbsf) 595 Depth (cmbsf) Depth 520

600 525

605 530

610 535

µ C (PC05) Gray scale Density Mean grain size ( m) 200 100 0 1.0 1.5 2.0 0 100 200 300

200

205 Mean grain size

210 Section boundary (cmbsf) 215

Depth SandSand 220 Foraminiferas- Silt rich zone 225 0 50 100 % grain size µ D (PC05) Gray scale Density Mean grain size ( m) 200 100 0 1.0 1.5 2.0 0 100 200 300 270

275

Mean grain size 280 Silt (cmbsf) 285 Foraminiferas- SandSand rich zone Depth

290

0 50 100 295 % grain size

Fig. 8. X-radiographs of selected cores, along with gray-scale, density, and grain-size data. correlated to the Ko-g (ca. 6.5 ka) tephra. Fig. 12A). The sedimentation rates for cores PC01 and PC02 are approximately constant throughout the entire cores (84 and 94 cm ky−1, respectively). The rate for core PC05 is estimated 5.7. Sedimentation rate to be 54 cm ky−1 from the top to 257 cmbsf, and 238 cm ky−1 between 257 and 362 cmbsf. The estimated sedimentation rate Age models for the cored sediments were established for core PC06 is the lowest among the cores, being 31 cm ky−1 using tephrochronology and AMS-derived 14C ages (Table 3;

10 Content (%) 0 50 100 0 50 100 0 50 100 0501000 50 100 050100 0

200

(cmbsf) 400

Ta-b (AD1667) Depth Us-b (AD1663) 600 Ko-g (6.5 ka)

800 1037 1038 PC01 PC02 PC05 PC06 Light Minerals Rock fragments Benthic foraminifers Diatoms

Heavy Minerals Volcanic glasses Planktonic foraminifers

Fig. 9. Composition of sand grains in turbidites. Table 2 Petrographical characteristics and deduced source volcanoes of tephras in the sediments. Sample Core Depth (cm bsf) Components Minerals∗1 Glass shards∗2 Source volcano∗3 Interpretation∗4 GH04-13 1036 38–43 fine–medium ash opx, cpx By Ta , Us Ta-b (1667) GH04-14 1036 45–46 coarse ash cpx, opx, hbl Pf, Ps Us, Ta Us-b (1663) GH04-17 1037 30–32 medium–coarse ash opx, cpx Pf Us Us-b (1663) GH04-19 1038 92–93.8 fine–medium ash opx, cpx Pf, Ps Ta, Us Ta-b (1667) GH04-21 1038 96.6–97.4 fine–medium ash opx, cpx Pf Us, Ta Us-b (1663) KR05-1 PC01 38.5–45 medium–coarse ash opx, cpx Ps, Pf Ta, Us Ta-b (1667) including reworked Us-b (1663) KR05-3 PC01 45–48 coarse ash hbl, opx Ps, Pf Us Us-b (1663) KR05-4 PC01 557–557.5 fine ash opx, cpx By, Pf Ko-ph, Ta Ko-g (6.5 ka) or older KR05-6 PC02 38–39.5 medium–fine asho px, cpx By, Pf Ta, Ko-h, Us Ta-b (1667) including reworked Us-b and Ko-c2 KR05-7 PC02 39.5–41 coarse ash opx, hbl, cpx Ps, Pf Us Us-b (1663) KR05-8 PC02 617.5 fine ash opx, cpx By Ko-ph Ko-g (6.5 ka) or older KR05-12 PC05 17–17.5 fine ash opx, cpx By Ta, Ko-h Ta-a (1739) or Ta-b (1667) including reworked Ko-c2 KR05-13 PC05 31–31.5 fine ash opx, cpx By, Ps Ta, Ko-h, Ma, Us, B Ta-b (1667) or later, including reworked Ko-c2 (1694), Ma-b (1 ka), B-Tm (1 ka) ∗1: Listed in order of abundance. Minerals: cpx, crynopyroxene; opx, orthopyroxene; hbl, hornblende. Plagioclase and opaque minerals occur in all samples. ∗2: Listed in order of abundance. Glass shards: Ps, spongy pumice type; Pf, fibrous pumice type; By, Y-shaped bubble type. ∗3: Listed in order of abundance. Source volcanoes: B, Baitoushan; Ko-h, Komagatake (historic); Ko-ph, Komagatake (prehistoric); Ma, Mashu; Ta, Tarumai; Us, Usu. ∗4: Based on characteristics of morphology of glass shards, mineralogical components, major elements of glass shards, and stratigraphic positions. from the top to 227 cmbsf. Based on the 17th-century tephras for PC01, and 285 yrs for PC06. (Ta-b and Us-b), recent sedimentation rates are 114 cm yr−1 for PC01, 112 cm yr−1 forPC02,and92cmyr−1 for PC05 over 210 137 the past ca. 0.34 kyrs. 5.8. Pbex and Cs geochronology Recurrence intervals of turbidite deposition are calculated 210 137 from sedimentation rates and turbidite numbers (Fig. 12B). Radioactivity analysis of Pbex and Cs were performed Core PC05 has the smallest value of 113 yrs during 0–0.34 kyrs, for the sediment within the pilot core of PC05 (Fig. 13; 210 although the older part (0.34–5.1 kyrs) shows the largest vale of TableSD3).Thevaluesof Pbex are approximately uniform in 439 years. The intervals are 153–169 yrs for PC02, 230–345 yrs the top 12 cm of the sediment, within which the water content and mean grain size are also constant (Fig. 13). In the lower

11 6 ABCDGH04-13 GH04-14 GH04-17 5 BaitoushanBaitoushan (1036) (1036) (1037) n=39 n=23 n=29 4

3 K2O TarumaiTarumai KomagatakeKomagatake 2 hhistoricistoric Usu prehistoricprehistoric 1 MMashuashu

0 6 GH04-19 GH04-21 KR05-1 KR05-3 EFGH (PC01) 5 (1038) (1038) (PC01) n=29 n=59 n=60 n=38 4

3 K2O

0 6 I KR05-4 J KR05-6 KLKR05-7 KR05-8 5 (PC01) (PC02) (PC02) (PC02) n=21 n=19 n=30 n=25 4

3 K2O

0 6 KR05-12 KR05-13 0 0.2 0.4 0.6 0 0.2 0.4 0.6 0.8 MN TiO2 TiO2 5 (PC05) (PC05) n=32 n=31 4

3 K2O

0 0 0.2 0.4 0.6 0 0.2 0.4 0.6 TiO2 TiO2

Fig. 11. K2O–TiO2 diagrams for volcanic glasses from probable source volcanoes (A) and ashes in the sediment cores (B–N). Petrographical descriptions are shown in Table 2. Table 3 Radiocarbon dating of foraminifers from samples of hemipelagic mud. Calibrated ages were calculated based on a local reservoir correction of 386±16 years (Yoneda et al., 2001). Lab code Core Depth Sample type Measured Conventional δ13C Calibrated age Calibrated age Median 14Cage(yrBP)14C age (yr BP) (permil) (1σ) (cal yr BP) (2σ) (cal yr BP) probability Beta-221966 PC01 221–236 Mixed planktonic 2660±40 3050±40 −1.4 2302–2413 2246–2504 2357 Beta-221967 PC01 296–311 Mixed planktonic 3140±40 3550±40 −0.3 3309–3414 3241–3458 3357 Beta-237746 PC01 581–601 Mixed planktonic 6440±40 6850±40 −0.2 6884–7018 6838–7118 6960 Beta-237747 PC02 783–793 Mixed Benthic 9480±40 9830±40 −3.5 Beta-221968 PC05 249.5–264.5 Mixed planktonic 4550±50 4860±40 −5.8 4600–4735 4527–4798 4668 Beta-221969 PC05 354.5–369.5 Mixed planktonic 4800±40 5200±40 −0.4 5030–5205 4956–5262 5109 Beta-221970 PC06 224–231 Mixed planktonic 6810±50 7200±50 −1.1 7292–7398 7235–7430 7339

210 137 part of the core (12–28 cmbsf), the ln( Pbex) values linearly As the result of atmospheric nuclear tests, Cs began decrease; the apparent sedimentation rate is estimated to be to appear in environmental samples at measurable levels 0.26 cm yr−1 to the exclusion of turbidite thickness. from A.D. 1954. Atmospheric ﬂuxes of these fallout nuclides

12 (A) Depth (cm) (B) Recurrence interval (yrs) 0 200 400 600 0 200 400 600 0 PC01 PC01 (0−2.4 kyr)

PC02 PC01 (2.4−7.0 kyr) 2 PC05 PC02 (0−0.34 kyr) PC06 PC02 (0.34−6.5 kyr) 4

Age (kyr BP) PC05 (0−0.34 kyr)

6 PC05 (0.34–5.1 kyr)

PC06 (0−7.3 kyr) 8

Fig. 12. (A) Age–depth proﬁles for selected cores based on 14C ages of planktonic foraminifers and tephrostratigraphy. (B) Recurrence intervals of deposition of turbidites. 210 137 WC (%) MGS (μm) ln( Pbex) (dpm/g) Cs (dpm/g) 0 200 4000 20 40 60 3 4 5 0 0.5 1.0 X-radio- graph 0

Earthquake

1973 Nemuro-oki

Depth (cmbsf) 20 1961 Nemuro-oki

1952 30 Tokachi-oki

210 137 Fig. 13. X-ray image and depth profiles of water content (WC), mean grain size (MGS), Pbex,and Cs for the pilot core (PC05). The error bars for 210 137 Pbex and Cs data represent ±1s about the means, as calculated using counting statistics. Earthquakes are possible triggers of the turbidite deposition. Results of the radioactivity analysis are presented in Table SD3. then followed the pattern of activities released from nuclear 6. Discussion detonations, which peaked in 1963 and decreased after the enactment of the Test-Ban Treaty in the same year. The depth 6.1. Origin of the turbidites profile of 137Cs conforms to the history of nuclear fallout, beginning of the detection at 23 cmbsf, showing high values Turbidity currents can be triggered by a number of natural in the middle part (15–20 cmbsf), and then decreasing to the causes in addition to earthquakes, including floods, storms, and top (Fig. 13; Table SD3). Given that 137Cs is undetectable in rapid sedimentation (e.g., Normark and Piper, 1991; Locat and the sample of deeper than 25 cmbsf, the detection limit lies Lee, 2002). The fact that the study area is not fed by large between the second and third turbidites. The 137Cs data indicate rivers and contains a wide shelf (20–30 km) probably precludes the sedimentation rate of 0.39–0.43 cm yr−1 with consideration the direct input of terrestrial material into the forearc slope for turbidite deposition. by flooding or storms. Benthic foraminifers in the turbidites suggest that the sands were derived from the upper–middle slope (deeper than 1,000 m water depth) rather than from the shelf. The steep gradient of the slope (5–10◦) and the presence

13 of numerous gullies are consistent with this hypothesis that Peak ground acceleration (PGA) at a given site can be the turbidites were derived from upper–middle slope sediments calculated using an empirical attenuation relationship (Boore under the influence of gravity. In addition, seaward thinning of and Joyner, 1982; Campbell, 1985; Fukushima and Tanaka, surface seismic reflections in the lower slope (Fig. 5) supports 1990; Si and Midoriwaka, 1999). We calculated PGA at the turbidites were derived from the upper–middle slope rather than assumed source point for the major (> M 7) earthquakes using the outer high. the relationship proposed by Fukushima and Tanaka (1990): Based on small volume of each turbidite bed (less than 0.003–0.03 km3 for 1–10 cm thick turbidites that would cover . log PGA = 0.41M−log (R+0.032×100 41M)−0.0034R+1.30, 15×20 km of the fan), lack of deposits derived from slides 10 10 (1) or debris flows in the proximal core (PC06), and fewer and − where PGA is the peak ground acceleration (cm s 2), R thinner turbidites in more distal part of the fan, slope failures is the shortest distance from a fault plane (if available) or on the slope were considered to be small-scale or thin-skinned. hypocenter (km), and M is magnitude from Utsu (1999)and A relatively good preservation of benthic foraminiferal test Japan Meteorological Agency (2006). The distance (R) between in the turbidites also infer that they were derived from not the points is approximated by a 3D application of Pythagoras’s deeply-buried sediments but very surface sediments without theorem: diagenesis. The small-scale or thin-skinned failures may be because (i) the steep forearc slope prohibits settlement of R2 = D2 + (p(Ax − Bx))2 + (p(Ay − By))2 , (2) sufficient sediments for large-scale slope failures, (ii) repeated earthquakes remove unstable hemipelagic muds on slopes, where D is the hypocenter depth (km), Ax and Ay represent or (iii) insufficient sediment inputs due to no large rivers the longitude and latitude of the source point in degree, Bx and and highstand sealevel. The small volumes of turbidites may By represent the longitude and latitude of the fault rupture or be an additional evidence that turbidites were generated by hypocenter, and p is a constant (111.32 km). Fukushima and earthquakes (cf. Goldfinger et al., 2003). Tanaka (1990) reported that predicted PGA values are similar The common occurrence of amalgamation (multiple coarse to observed values at hard soil sites, but underestimated by fraction pulses) within thick turbidites infers deposition from a about 40% at soft soil sites. Therefore, we multiplied the value ff flow with multiple pulses or multiple flows that occurred over of PGA calculated using Eq. (1) by 1.4 as a site e ect. a short time period. Such amalgamated turbidites have been Peak ground acceleration (PGA) can also be estimated using reported previously from seismically active regions of Japan the following empirical relationship (Si and Midoriwaka, 1999): (Nakajima and Kanai, 2000; Noda et al., 2008) and Cascadia (Goldfinger et al., 2007). It is not possible to produce an = − + − . log10 PGA b log10(R c) 0 003R (3) amalgamated turbidite from a simple waning turbidity current of the type that produces a typical turbidite represented by the where Bouma sequence. The occurrence of multiple slope failures b = 0.53M + 0.0044D + d + 0.38 (4) over a short period can be attributed to strong ground shaking 0.50M associated with a large earthquake. Multiple failures upon a c = 0.0055 × 10 . (5) slope have the potential to flow downslope and transform into The value of d depends on the type of earthquake, whether turbidity currents, converging at the apex of the submarine fan; shallow (0.00), interplate (−0.04), or intraplate (0.17). Eq. (3) in this way, multiple flows pass over the fan. was optimized for soil with a shear-wave velocity (Vs)of 400 m s−1 (Si and Midoriwaka, 1999), suggesting it could apply to relatively soft basement. Although there were few 6.2. Correlation with historical earthquakes reports about shear-wave velocity of forearc slope sediments, −1 Goldberg (2003) reported a nearly constant Vs (∼300 m s )in The initiation of slope failure generally depends on the forearc accretionary sediments above 100 mbsf, off Nankai, excess pore-pressure generated by earthquake-induced ground southwest Japan. A similar shear-wave velocity for the forearc acceleration (e.g., Seed and Idriss, 1971). Critical earthquake slope sediments could be assumed, we did not consider a site −2 horizontal accelerations of 80–190 and 80–280 cm s have effect for Eq. (3). been reported for the Eel margin of California (Lee and Edward, The calculated PGA of historical earthquakes (Table 4) 1986; Lee et al., 1999) and the Japan Sea (Lee et al., 1993, show that the 1894, 1952, 1961, 1973, and 2004 earthquakes 1996), respectively. Here, we seek to correlate the turbidites could have large PGA (> 150 cm s−2) near the study area identified in the pilot core (PC05) with known historical (Table 4;Fig.15). Which earthquakes could trigger deposition earthquakes, based on calculated ground accelerations. Our of turbidites? The depositional ages of the recent three turbidites calculations assumed a point source of slope failures centered in the pilot core of PC05 (Fig. 13) are estimated as 1946–1971, ff ◦ ◦ on the upper slope o Nemuro (145 32’E, 42 52’N). The oldest 1930–1962, and 1910–1950, based on the sedimentation rate historical earthquake recorded in eastern Hokkaido is the 1843 −1 210 137 of 0.26–0.43 cm s derived from Pbex and Cs analysis. Nemuro-oki earthquake. Many large earthquakes have been The detection of 137Cs radioactivity below the second turbidite recorded since this time, including shallow, interplate, and deep indicates that it was deposited after A.D. 1954. The first and (intraplate) earthquakes (Table 4;Fig.14).

14 Table 4 Simulated peak ground acceleration (PGA) for interplate, shallow, and deep earthquakes in the area oﬀ Nemuro. The magnitudes and locations of hypocenters are from Utsu (1999)andJapan Meteorological Agency (2006). The distance values represent the distance between the assumed source point (145◦32’E, 42◦52’N) and the hypocenters (HC) or the nearest fault plane (SD). PGA (Fu) and PGA (Si) were determined using equations of Fukushima and Tanaka (1990)andSi and Midoriwaka (1999), respectively. Num Region Date M Longitude Latitude Depth Type Distance PGA (Fu) PGA (Si) 1 C 22-Mar-1894 7.9 146.00 42.50 0 Il 66.1 (HC) 237.4 185.1 2 C 25-Dec-1900 7.1 146.00 43.00 0 Is 54.1 (HC) 186.0 118.6 3 B 18-Mar-1915 7.0 143.60 42.10 0 Is 231.5 (HC) 13.2 9.1 4 C 01-Jul-1924 7.6 147.50 45.00 0 D 323.0 (HC) 8.0 11.4 5 C 27-Dec-1924 7.0 147.00 43.00 0 Is 164.0 (HC) 30.6 19.9 6 B 04-Mar-1952 8.1 144.13 41.80 0 Il 196.0 (HC) 48.0 43.1 B 04-Mar-1952 8.1 144.80 42.50 0 Il 91.3 (SD) 180.9 149.5 7 E 07-Nov-1958 8.1 148.58 44.38 32 S 380.1 (HC) 6.7 10.7 8 C 12-Aug-1961 7.2 145.57 42.85 80 Is 80.1 (HC) 122.8 181.5 9 B 23-Apr-1962 7.1 143.92 42.23 60 Is 202.2 (HC) 20.5 26.0 . 10 C 23-Jun-1964 7.1 146.47 42.98 80 S 132.1 (HC) 51.2 82.8 11 D 12-Aug-1969 7.8 147.82 43.44 41 Il 265.6 (HC) 17.4 23.3 D 12-Aug-1969 7.8 147.00 43.00 41 Il 169.0 (SD) 53.5 66.0 12 C 17-Jun-1973 7.4 145.95 42.97 40 Il 62.3 (HC) 191.3 198.2 C 17-Jun-1973 7.4 145.55 42.85 40 Il 40.1 (SD) 295.5 307.8 13 D 06-Dec-1978 7.7 146.67 44.55 118 D 255.0 (HC) 18.1 82.6 14 B 15-Jan-1993 7.5 144.36 42.92 101 D 165.2 (HC) 44.8 151.8 15 D 04-Oct-1994 8.2 147.71 43.37 23 S 249.8 (HC) 28.1 37.5 16 C 28-Jan-2000 6.8 146.90 43.00 6.8 S 153.0 (HC) 30.0 21.5 17 B 25-Sep-2003 8.0 144.08 41.78 42 Il 206.4 (HC) 39.6 53.5 B 25-Sep-2003 8.0 144.35 42.25 42 Il 154.4 (SD) 73.8 95.7 18 C 29-Nov-2004 7.1 145.30 42.90 48 Is 54.7 (HC) 183.7 190.4 Units: Depth and distance, km; PGA,cms−2. Abbreviations in Type: S, shallow; D, deep; Is, small interplate; Il large interplate earthquakes. 142˚E 143˚E 144˚E 145˚E 146˚E 147˚E 148˚E 149˚E 44˚N

50 km 11 Nemuro M8 1515 2 16 5 1969 M7.5 Kushiro 11118 43˚N 1414 D M7 18 12 10 TargetTSourceSaorugrecte 1 1973 C 9 3 2003 42˚N 1717 Kuril Trench 6 1952 B A Pacific Plate 41˚N

Fig. 14. Epicenters of earthquakes (> M 7.0) and source areas of large interplate earthquakes in the area oﬀ Nemuro. Source areas are from Kasahara (1975), Aida (1978), and Hirata et al. (2003) for the 1952 Tokachi-oki earthquake, and Hatori (1974), Japan Meteorological Agency (1974), Shimazaki (1974), and Kasahara (1983) for the 1973 Nemuro-oki earthquake. second turbidites, therefore, can be correlated with the 1973 third turbidite may be associated with the 1952 Tokachi-oki and 1961 Nemuro-oki earthquakes (Figs. 13 and 15). The earthquake. Although it occurred in the region B and its PGA

15 is lower than the 1961 and 1973 earthquakes, relatively long of the recurrence interval of turbidite deposition in PC02 are duration after the 1894 and 1900 earthquakes (Fig. 15) might twice the interval of interplate earthquakes in the area over the enable to accumulate between 2 and 5 cm of surface sediment past 160 years, as deduced from historical records (72.2 years; upon the upper–middle slope, consisting of pelagic fallout Earthquake Research Committee, 2004). where the rate of pelagic sedimentation is 0.032–0.088 cm yr−1 Nanayama et al. (2007) reported recurrence intervals of (Noda and TuZino, 2007). ∼550 years for large tsunamis in the Kushiro–Nemuro region The turbidites in the pilot core (PC05) were possibly over the past 4,000 years (Fig. 10). The turbidites analyzed deposited in association with earthquakes that generated in the present study record a greater number of events than strong (> 150 cm s−2) ground shaking; however, not all that indicated by tsunami deposits. Nanayama et al. (2007) large earthquakes are recorded in the sedimentary record as identified an unusually large tsunami that inundated the area turbidites. We could identified only three turbidites above the during the 16th century (KS3 in Fig. 10). This tsunami was 17th-century tephra (Ta-b) in core PC05; the 1894 and 2004 potentially associated with a giant earthquake related to rupture earthquakes or any historical earthquakes were not recorded along a multi-segment fault (Regions B and C) (Nanayama in the core. The pilot core might record about half of the et al., 2003). None of the cores obtained from the submarine fan, strong earthquakes that occurred in the source region of the however, contain conspicuous turbidites immediately below slope failures. It must be remembered that the initiation of the 17th-century volcanic ash. Thin (2 cm thick) turbidites slope failure requires sufficient unconsolidated sediment: strong identified below the Us-b tephra in cores PC02 and 1038 earthquakes are unlikely to generate turbidites if the source area (Fig. 7) are possibly related to the 16th-century event. It remains contains insufficient soft sediment. In addition to earthquake uncertain as to whether submarine deposition accompanied this magnitude, the recurrence interval is possible another factor unusually large-scale event. that influences the triggering mechanisms of slope failure. 7. Conclusions 6.3. Recurrence intervals With the aim of estimating the long-term recurrence interval of earthquakes off eastern Hokkaido, seven sediment cores The recurrence intervals of the turbidite deposition in the were obtained from a submarine fan on the forearc slope. lower part of PC05 and PC06 were longer than PC01 and ◦ The upper slope is steep (3–10 )andincisedbyaseriesof PC02 obtained from the middle fan (Fig. 12B). Relatively thick gullies, some of which cut through the middle slope to the turbidites observed in the cores of the upper fan infer that more lower slope where the submarine fan is developed. The retrieved erosive currents on the upper fan passed than those on the cores contain a number of turbidites that probably originated middle fan. The steepness of the slope upon which the currents from the upper–middle slope (> 1,000 m water depth), as passed is another interpretation of less turbidite numbers in the indicated by benthic foraminiferal assemblages. The deep-sea upper fan. Because the slope is one of the variables for velocity origin of the turbidites suggests that the turbidity currents were of body of turbidity currents (e.g., Middleton and Hampton, triggered by earthquakes. 210Pb and 137Cs geochronology, in 1976), deposition will not occur upon which currents are too ex combination with the calculated peak ground accelerations of fast to settle the suspended particles. historical earthquakes, indicate that the three recent turbidites The change in recurrence intervals in PC05 (Fig. 12B) are correlated with the 1952, 1961, and 1973 earthquakes. The indicates that changes of the course of turbidity currents identified turbidites possibly record half of the earthquakes with over time, thereby failing to transport detritus through the − sufficient strength of ground shaking (∼150 cm s 2) around the site of PC05. This type of channel avulsion has been source area of slope failure, possibly due to changes of the reported previously from other submarine fans (Normark, 1970; course of turbidity currents on the fan over time or frequent Normark et al., 1979). Several channels merge at the apex of removal of unstable sediments on the slope. The depositional the fan divided into two tributaries (Fig. 3); one has the course intervals of the analyzed turbidites are 113–439 years over the toward south along the western boundary of the fan through past 7 kyrs. Given that not every seismic event is recorded in the site of PC05, the other flows toward southeast near the any single core, the recurrence interval of earthquakes in this sites of PC01 and PC02. Not all currents deposit turbidites over region is estimated to be less than 113 years. the entire fan; one may deposit turbidites through the western channel, another may flow over the eastern fan. Three turbidites within the half century in the pilot core of PC05 suggest recent turbidity currents flow in the western channel. The fact that core PC02 records the largest number of Acknowledgements turbidites is attributed to its location on levees in the center We are greatly indebted to the officers, crew, and research of the fan, where turbidites are more likely to deposit than in staff of cruises GH04 and KR0504 for the collection of the channels of the upper fan and on the marginal fan. Erosion data. We also thank Hajime Katayama for data collected on by currents could be less effective on levees of the middle fan cruise GH04, and Yukinobu Okamura, Kenji Satake, Ken than in channels. Nevertheless, it is unlikely that PC02 provides Ikehara, Kohsaku Arai, and Tomoyuki Sasaki for data collected a complete record of seismogenic turbidites. About 150 years on cruise KR0504. The sea-beam data used for compiling

16 M8 1894 Nemuro-oki 1973 Nemuro-oki (M7.4) 300 M7.5 (M7.9) 1961 2004 Kushiro-oki (M7.1)

) M7 Nemuro-oki −2 1952 (M7.2) 19931993 Kushiro-okiKushiro-oki ((M7.5)M7.5) s

M6.5 Tokachi-oki 200 (M8.1) (cm

100 PGA

0 1850 1900 1950 2000 Calendaryear

Fig. 15. Calendar year of earthquakes and simulated PGA at the probable source region of turbidity currents. The dashed line is an estimated critical value − (∼150 cm s 2)ofPGA for turbidite deposition. undersea topography were collected by the Hydrographic and Journal of Geophysical Research 92 (B13), 14123–14138. Oceanographic Department of the Japan Coastal Guard. We Biscontin, G., Pestana, J. M., Nadim, F., 2004. Seismic triggering of submarine are grateful to Azusa Nishizawa for providing bathymetric slides in soft cohesive soil deposits. Marine Geology 203 (3–4), 341–354. Boore, D. M., Joyner, W. B., 1982. The empirical prediction of ground motion. data, Ken’ichi Ohkushi for picking foraminifers, and Masayuki Bulletin of the Seismological Society of America 72 (6B), S43–60. Yoshimi for calculations of PGA. Constructive comments Campbell, K. W., 1985. Strong motion attenuation relations: a ten-year by anonymous reviewers and D. J. W. Piper prompted a perspective. Earthquake Spectra 1, 750–804. significant revision of this paper. This study was part of Clarke, Samuel H, J., Carver, G. A., 1992. Late Holocene tectonics and the “Marine Geological Mapping Project of the Continental paleoseismicity, southern Cascadia subduction zone. Science 255 (5041), 188–192. Shelves Around Japan” program supported by the Geological / Clift, P. D., MacLeod, C. J., Tappin, D. R., Wright, D. J., Bloomer, S. H., Survey of Japan AIST. Financial support for this research was 1998. Tectonic controls on sedimentation and diagenesis in the Tonga also provided by the Japan Nuclear Energy Safety Organization Trench and forearc, Southwest Pacific. Geological Society of America (JNES). Bulletin 110 (4), 483–496. Dawson, A. G., Shi, S., 2000. Tsunami deposits. Pure and Applied Geophysics 157, 875–897. DeMets, C., 1992. Oblique convergence and deformation along the Kuril and Japan trenches. Journal of Geophysical Research 97 (B12), 17615–17625. DeMets, C., Gordon, R. G., Argus, D. F., Stein, S., 1990. Current plate References motions. Geophysical Journal International 101 (2), 425–478. Abe, K., 1973. Tsunami and mechanism of great earthquakes. Physics of the Dickinson, W. R., Seely, D. R., 1979. Structure and stratigraphy of forearc Earth and Planetary Interiors 7, 143–153. regions. American Association of Petroleum Geologists Bulletin 63 (1), Abe, K., Hasegawa, S., 2003. Distribution of benthic foraminifers off Tokachi, 2–31. Hokkaido. In: Okamura, Y. (Ed.), Marine Geological and Geophysical Doig, R., 1998. 3000-year paleoseismological record from the region of the Studies on the Collision Zone of Kuril and Northeast Japan Arc—Off 1988 Saguenay, Quebec, earthquake. Bulletin of the Seismological Society Tokachi Area—. Preliminary Reports on Researches in the 2002 Fiscal of America 88 (5), 1198–1203. Year, GSJ Interim Report, no. 26. Geological Survey of Japan, AIST, Earthquake Research Committee (Ed.), 2004. Long-term evaluation of Tsukuba, pp. 114–121, (in Japanese). earthquakes in the Kuril Trench. The Headquarters for Earthquake Adams, J., 1990. Paleoseismicity of the Cascadia subduction zone: evidence Research Promotion, Tokyo, 35pp., (in Japanese). from turbidites off the Oregon-Washington margin. Tectonics 9 (4), URL http://www.jishin.go.jp/main/chousa/04dec 569–583. chishima2/index.htm Aida, I., 1978. Reliability of a tsunami source model derived from fault Fukao, Y., Furumoto, M., 1975. Foreshocks and multiple shocks of large parameters. Journal of Physics of the Earth 26, 57–73. earthquakes. Physics of the Earth and Planetary Interiors 10 (4), 355–368. Anastasakis, G. C., Piper, D. J. W., 1991. The character of seismo-turbidites in Fukao, Y., Furumoto, M., 1979. Stress drops, wave spectra and recurrence the S-1 sapropel, Zakinthos and Strofadhes basins, Greece. Sedimentology intervals of great earthquakes; implications of the Etorofu earthquake of 38 (4), 717–733. 1958 November 6. Geophysical Journal of the Royal Astronomical Society Ando, M., 1975. Possibility of a major earthquake in the Tokai District, Japan 57 (1), 23–40. and its pre-estimated seismotectonic effects. Tectonophysics 25, 69–85. Fukushima, Y., Tanaka, T., 1990. A new attenuation relation for peak Atwater, B. F., 1987. Evidence for great Holocene earthquakes along the horizontal acceleration of strong earthquake ground motion in Japan. outer coast of Washington State. Science 236 (4804), 942–944. Bulletin of the Seismological Society of America 80 (4), 757–783. Beck, S. L., Ruff, L. J., 1987. Rupture process of the great 1963 Kurile Islands Furukawa, R., Nanayama, F., 2006. Holocene pyroclastic fall deposits earthquake sequence: asperity interaction and multiple event rupture. along the Pacific coastal region of eastern Hokkaido. Bulletin of the

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