Paleoseismicity along the southern Kuril Trench deduced from

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
a Geological
Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Central 7, Higashi 1–1–1, Tsukuba, Ibaraki
305–8567, Japan
b Department 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
above layers of 17th-century volcanic ash. The results of 210 Pbex and 137 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
The long-term prediction of earthquakes is one of the most
important issues in hazard assessment and risk estimation in
tectonically active areas. Recurrence intervals and the timing
of future earthquakes are considered to be predictable provided
that sufficient historical records are available (e.g., Ando, 1975;
Shimazaki and Nakata, 1980; Ishibashi, 1981). In regions
with limited historical data, archaeological and geological
NOTICE: this is the authors’ version of a work that was accepted for
publication in Marine Geology. Changes resulting from peer review are
reflected, but editing, formatting, and pagination from the publishing processes
are not included in this document. A definitive version will be published in
http://dx.doi.org/10.1016/j.margeo.2008.05.015.
∗ Corresponding author. Fax: +81 29 861 3653.
Email address: a.noda@aist.go.jp (Atsushi Noda).
1 Present address: M. T. Brain Corporation, Hayakawa Bld., 2-60-2,
Ikebukuro, Toshima-ku, Tokyo 171-0014, Japan
Article published in Marine Geology (2008) 1–20
approaches are useful in estimating the timing and intensity of
pre-historic earthquakes. In particular, tsunami deposits within
coastal areas and turbidites in deep-sea sediments provide
useful paleoseismic information. Large tsunami waves are able
to transport coastal sands and marine fossils to inland areas,
depositing sediments within lagoons or marshes within which
mud or peat normally accumulate (e.g., Atwater, 1987; Minoura
and Nakaya, 1991; Clarke and Carver, 1992; Dawson and Shi,
2000; Nanayama et al., 2003); however, it must be remembered
that tsunamis are able to traverse entire oceans from their
source regions. For example, tsunami waves associated with
the giant Chilean earthquake of 1960 arrived at the Japanese
coast 22–24 hours after the main shock, with up to 3.8 m of
inundation height (Takahashi and Hatori, 1961). These waves
left tsunami deposits upon marshes (Nanayama et al., 2007). It
is therefore problematic to use tsunami deposits in developing a
long-term earthquake model for a given region, as it is difficult
to determine whether tsunami deposits were derived from local
or distant seismic events.
Turbidites in marine sediments have also been widely applied
in investigations of paleoseismology conducted over the past
two decades, including studies in Cascadia (Adams, 1990;
Goldfinger et al., 2003, 2007), Japan (Inouchi et al., 1996;
Ikehara, 2000; Nakajima and Kanai, 2000; Ikehara, 2001;
Okamura et al., 2005), Canada (Syvitski and Schafer, 1996;
Doig, 1998; St-Onge et al., 2004), and the Mediterranean
(Kastens, 1984; Anastasakis and Piper, 1991; McHugh et al.,
2006); however, a number of points must be kept in mind
when using turbidites as a tool in paleoseismic studies. First,
not all turbidites are generated in association with earthquakes
(e.g., Normark and Piper, 1991). Hyperpycnal flows (Mulder
et al., 2003), storm waves (Hampton et al., 1996), and rapid
sedimentation upon slopes (Mandl and Crans, 1981) can also
lead to slope failure and the generation of turbidity currents.
If turbidites are to be used in studying paleoseismicity, the
selection of coring sites is clearly important in ensuring that
the studied turbidites were likely to have been generated in
association with earthquakes rather than other factors (e.g.,
Nakajima and Kanai, 2000; Goldfinger et al., 2003). Second,
any single sediment core is unlikely to record the entire history
of local seismic events. Submarine slope failures initiated by
earthquakes depend on slope stability, which is controlled in
turn by gravity and seismic loading (Lee and Edward, 1986;
Lee and Baraza, 1999; Lee et al., 1999; Biscontin et al., 2004;
Leynaud et al., 2004; Strasser et al., 2007). The likelihood
of slope failure depends on the sedimentation rate at the site
of potential failure, the recurrence interval of earthquakes in
the area, slope gradient, and the intensity of ground shaking.
For reliable predictions of earthquake recurrence intervals, it
is necessary to correlate turbidite deposits with seismic events
documented in historical records (e.g., Nakajima and Kanai,
2000; Huh et al., 2004; Garcia-Orellan et al., 2006).
Large earthquakes are frequently recorded along the southern
Kuril Trench, eastern Hokkaido, Japan, where the Pacific
Plate is subducting beneath the overriding Okhotsk (North
American) Plate at approximately 8 cm yr−1 (DeMets et al.,
1990; DeMets, 1992; Seno et al., 1996). Six earthquake source
regions have been defined in this area, labeled A to F from west
to east along the northern Japan Trench (A) and the southern
Kuril Trench (B–F), based on a seismic gap hypothesis (Utsu,
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
an earthquake in the area is the 1843 Tokachi-oki earthquake.
This 160-year historical record of seismic events enables us to
estimate an average recurrence interval of 72.2 years for events
along the southern Kuril subduction zone (Earthquake Research
Committee, 2004), although over this period the different
source regions have experienced only two or three events.
For long-term earthquake prediction, we analyze turbidites
deposited upon a submarine fan developed on the forearc
slope. We present new data on the texture, composition, and
depositional age of the studied turbidites. We then discuss
the relationship between turbidite deposition and historical
earthquakes, as well as the recurrence interval of earthquakes
during the Holocene.
2. Geological setting
The Kushiro–Nemuro district of eastern Hokkaido is largely
flat-lying, and contains just one significant river, the Kushiro
River (Fig. 1). Short ephemeral streams of less than 15 km in
length flow into the sea or estuaries. Marine terraces, lagoons,
and estuaries are well developed along coastal areas. Steep
cliffs of the marine terraces are actively eroded by wave action;
coastal erosion is considered to be the main contributor of
sediment to the sea under the present highstand conditions
(Noda and TuZino, 2007). The elevation of uplifted terraces
indicates an average uplift rate of 0.16–0.24 mm yr−1 over
the past 125,000 years (since interglacial stage 5e) (Okumura,
1996).
The average width of the shelf in this area is 20–30 km,
with the shelf margin located at 130–180 m water depth.
Shelf sediments range from muddy to gravelly sand (Noda and
TuZino, 2007; Noda and Katayama, in press). Fine to very fine
sands are widely distributed across the inner–outer shelf, where
the thickness of sediment deposited since the last glacial age
is less than 20 m. Gravels and gravelly sands are distributed
across parts of the inner shelf and along the shelf margin.
The mass accumulation rate of shelf sediments is estimated to
be ∼0.47 Mt yr−1 , representing less than 25% of the material
derived from coastal erosion (Noda and TuZino, 2007).
The forearc slope in this area can be subdivided into three
zones: the upper slope (from the shelf break to 1,000 m water
depth), middle slope (1,000–3,000 m water depth), and lower
slope shallower than the outer high (3,000–3,500 m) (Fig. 2).
The dip of the slope is steepest upon the upper slope (average
5–6◦), reaching 10◦ in places. The middle slope is less steep
(1–3◦), and the lower slope is gentle (< 1◦ ). A middle terrace is
recognized at 2,000–2,200 m water depth (Fig. 2). A number of
gullies incise on the upper slope; some cut through the middle
terrace to the deeper parts of the slope. A submarine fan with
20 km wide and 15 km long is developed on the lower slope.
The seaward margin of the fan is bounded by the outer high
(Fig. 2), which represents a major boundary between the forearc
basin and accretionary prism (e.g., Clift et al., 1998; Dickinson
and Seely, 1979; McNeill et al., 2000).
3. Seismicity
Although there exists no written record of earthquakes
along the southern Kuril Trench prior to the 1843 Tokachi-oki
earthquake, historical literature produced in Honshu indicates
frequent earthquake activity prior to the 19th century (Satake,
2004). During the late 19th and earliest 20th centuries,
earthquakes were recorded in 1843 (M 8.0, Region B), 1856
(M 7.5, Region A), 1893 (M 7.7, Region D), 1894 (M 7.9,
Region C), and 1918 (M 8.0, Region F) (e.g., Hatori, 1973,
1974, 1984; Utsu, 1999). Few events were recorded during the
early 20th century; however, seismic activity increased again
2
60˚N
140˚E
145˚E
40˚N
150˚E
Okhotsk Plate
(North American Plate)
20˚N
100˚E
44˚N
120˚E
140˚E
K
160˚E
Hokkaido
i
ur
Us
l
Kunashiri
Kushiro
Tokachi
plain
C
B
1843 M7.5
A
M7.5
1856 M7.8
2003 M8.0
1952 M8.1
n
1963 M8.1
Etorofu
1918 M8.0
1918 M7.7
D
h
1893 M7.7
1969 M7.8
nc
re
lT
ri
Ku
1894 M7.9
Fig. 2
ca. 8 cm/yr
Japan Trench
1968 M7.9
Honshu
la
F
ds
1973 M7.4
Ko
40˚N
Is
1915
M7.9
Kushiro River
Ma Nemuro
Ta
155˚E
Pacific Plate
300 km
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
Tokachi-oki earthquake (M 8.1, Region B; Geist et al., 2003;
Hirata et al., 2003, 2004; Hamada and Suzuki, 2004), the
1963 Kuril Islands earthquake (M 8.1, Region F; Kanamori,
1970; Beck and Ruff, 1987), the 1968 Tokachi-oki earthquake
(M 7.9, Region A; Fukao and Furumoto, 1975; Schwartz
and Ruff, 1985), the 1969 Kuril Islands earthquake (M 7.8,
Region D; Abe, 1973; Fukao and Furumoto, 1975; Schwartz
and Ruff, 1985, 1987; Kikuchi and Fukao, 1987), and the
1973 Nemuro-oki earthquake (M 7.4, Region C; Sekiya et al.,
1974; Shimazaki, 1974; Aida, 1978). The 2003 Tokachi-oki
earthquake is the most recent event around the studies area
(Yamanaka and Kikuchi, 2003). Although the epicenter of the
2003 earthquake was located at approximately the same site
as that for the 1952 Tokachi-oki earthquake (Fig. 1), tsunami
inversion models indicate that the rupture extent of the 2003
earthquake was restricted to the western half of the rupture
area of the 1952 event (Hirata et al., 2004; Tanioka et al.,
2004; Satake et al., 2006). These historical earthquakes yield
a recurrence interval of ca. 72.2 years for large interplate
earthquakes along the southern Kuril subduction zone.
Large intraslab earthquakes have also occurred within the
subducting plate, including the 1958 Etorofu-oki earthquake
(M 8.1, Region F; Fukao and Furumoto, 1979; Schwartz and
Ruff, 1987; Harada and Ishibashi, 2000) and 1994 Shikotan
earthquake (M 8.2, Region D; Kikuchi and Kanamori, 1995;
Tanioka et al., 1995; Satake and Tanioka, 1999) within the
shallow part of the slab, and the 1924 Etorofu earthquake
(M 7.6, Region C), 1978 Kunashiri Strait earthquake (M 7.7,
Region C; Suzuki, 1979; Kasahara and Sasatani, 1985), and
1993 Kushiro-oki earthquake (M 7.5, Region B; Morikawa and
Sasatani, 2003) within the deep part of the slab. The recurrence
intervals of earthquakes in the subducting plate are estimated
to be 82.8 years for those in the shallow slab and 27.3 years for
those in the deep slab (Earthquake Research Committee, 2004).
Some of the large earthquakes listed above were
accompanied by tsunamis that left characteristic sedimentary
deposits in estuaries, lagoons, and swamps along the coast from
Tokachi to Nemuro (Hirakawa et al., 2000; Nishimura et al.,
2000; Sawai, 2002; Nanayama et al., 2003, 2007). Nanayama
et al. (2007) identified 13 tsunamigenic sand layers within
marsh sediments, and calculated that the corresponding tsunami
events had a recurrence interval of 365–553 years over the past
4,000 years. Turbidites deposited upon the ocean floor also
provide evidence of the recurrence intervals of earthquakes.
Noda et al. (2004, 2008) reported an average recurrence interval
of 68–85 years for the deposition of turbidites off Kushiro over
the past 2400 years.
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
slope
00
−1
−5
00
43˚00'N
0
−2
00
−100
−100
−1
50
−20
−200
−50
Middle
terrace
0
00
0
42˚50'N
−1
0
−2
00
−1
50
50
0
Middle
slope
0
−2
42˚40'N
000
−25
42˚30'N
00
−3
PC06
00
PC02
1037
0
0
Lower
slope
PC01
PC05
−3
00
Outer
high
1038
1036
42˚20'N
Line 17
Line 16
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
sediment, as deduced from the distribution of modern benthic
foraminifers throughout the study region (Matsuo et al., 2004;
Ooi et al., 2005).
Accelerator Mass Spectrometer (AMS) 14 C measurements
of planktonic foraminifers were taken at Beta Analytic Inc.
We picked more than 10 mg of mixed species of planktonic
foraminifers (mainly Globorotalia inflata and Globigerina
bulloides) from six horizons (core PC01, PC05, and PC06) and
mixed benthic foraminifers (mainly Nonionellina labradorica
and Elphidium batialis) from one horizon (core PC02) for the
analysis. The 14 C ages from benthic foraminifers were 820–870
years older than those from planktonic foraminifers of the
same horizon off Kushiro during the late Quaternary (Noda
et al., 2008). We used a reservoir age of 386±16 years for the
14
C ages in this region (Yoneda et al., 2001). The obtained
conventional radiocarbon ages were calibrated to calendar ages
using CALIB rev. 5.0.2 (Stuiver and Braziunas, 1993) and the
dataset marine04.14c (Hughen et al., 2004).
The sediments within the pilot core (core PC05) were split
into 0.5 cm intervals, dried, and powdered for radioactivity
analysis. The powdered samples (1–4 g) were stored in
a capped centrifuge tube for about 1 month to ensure
radioactive equilibrium among daughter nuclides, after which
the radioactivities of 210 Pbex (T1/2 = 22.3 yr) and 137 Cs (T1/2 =
30.1 yr) were measured using a well-type Ge semi-conductor
detector (Kanai, 1993).
Bathymetric data were collected by the Hydrographic and
Oceanographic Department of the Japan Coastal Guard (Japan
Coast Guard (Maritime Safety Agency), 1998). Maps in this
study were created using 10 sec (about 300 m) gridded data.
Seismic reflection profiles were collected using a GI gun
(generator 250 in3 and injector 105 in3 airgun) with a six
channel streamer cable in July and August of 2004 (cruise
GH04) aboard the R/V Hakurei-maru No. 2 of the Japan Oil,
Gas and Metals National Corporation (JOGMEC). The survey
speed was 8 knots (14.8 km/h) and the shooting interval was
6 sec, with a common depth point (CDP) of ∼25 m. The grid
size was 2 miles (3.7 km) E–W and 4.5 miles (8.3 km) N–S
(Fig. 2).
Gravity (cores 1036 and 1037) and piston (core 1038)
sediment cores were collected during cruise GH04 (Table SD1).
Additional sediment cores were obtained using a piston corer
(cores PC01, PC02, PC05, and PC06) in April and May of 2005
(KR0504 cruise) aboard the R/V KAIREI of the Japan Agency
for Marine-Earth Science and Technology (JAMSTEC) (Fig. 2).
Halved cores of samples PC01–PC06 were measured
for γ-ray attenuation at 1 cm intervals using a GEOTEK
Multi-Sensor Core Logger to calculate the wet bulk density.
The wet bulk density of cores obtained during cruise GH04
was analyzed at 2 cm intervals using 7 cm3 plastic cubes.
The water content of sediments was calculated as 100 ×
(wetweight − dryweight)/dryweight. Grain-size analysis was
conducted using a laser particle-analyzer (Cilas 1064) for
selected sandy turbidites at 0.5 cm intervals. This instrument is
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
from selected turbidites were determined from 200–400 points
counted under a stereomicroscope. To study sedimentary
structures and identify sand layers, soft X-radiographs were
taken of slab subsamples (5 × 20 × 1 cm3 ) of the core samples
using a SOFRON TYPE STA-1005 operated at voltage of
45 kV, current of 3 mA, and an irradiation time of 30–90 s. The
thickness of the turbidites was measured on the X-radiographs
by 0.1 cm intervals.
Samples of tephra layers and patches were collected for
petrography and glass chemistry. Description and classification
of shape of glass shards were based on Machida and
Arai (1992). Chemical analysis was performed on a JEOL
JXA-8900R electron probe microanalyzer at the Geological
Survey of Japan. Nine major elements (Si, Ti, Al, Fe, Mn, Mg,
Ca, Na, K) were analyzed with an accelerating voltage of 15 kV
and a beam current of 12 nA. Beam diameter was 10 μm, with
counting times of 20 and 10 sec for peak and background,
respectively. Chemical compositions of glass shards, especially
Ti and K, reflects magma types of source volcanoes and can
be used to identify origin of volcanic ashes (e.g., Westgate
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
et al., 2000; Furukawa and Nanayama, 2006).
5. Results
5.1. Fan physiography
The submarine fan has its apex in the water depth of 3,100 m
and is bounded by the outer fan at the water depth of 3,300 m
(Fig. 3). Several channels from the upper and middle slopes
merge at the top of the fan, and then divide into two channels
on the upper fan (3,100–3,200 m). Cores PC05 and PC06 were
obtained near channels on the upper fan. Sub-bottom profiling
records show strong reflection in the subsurface sediments,
indicating sandy sediments cover on the upper fan (Fig. 4).
The middle–outer fan has a convex-up, lobe-like bathymetry;
the axis is high and the marginal area is low. The gradient
becomes gentler in the distal part. Cores 1037, PC01, and
PC02 were obtained from levees along the channels on the
middle fan (Fig. 3). Several reflections could be recognized
in the sub-bottom profiling records of the middle fan (Fig. 4),
indicating repeated deposition of sands and muds. Cores 1036
and 1038 were recovered from the outer fan; the latter core was
obtained from the most distal part of the lobe.
5.2. Seismic profiles
The shelf and upper slope of the study area are underlain
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
0
A
1000
1500
2000
2500
3000
A
−3100
1.0
42˚30'N
Middle terrace
PC06
−3200
TWT (sec)
2.0
B
−3000
PC01
PC05
PC02
Outer
high
Shelf
3.0
Upper
slope
4.0
−320
0
−3100
Middle slope
5.0
C
42˚25'N
1037
Line 17
1038
3
−3
VE: 11
6.0
20
−3
0
0
2000
2200
Lower slope
ca. 10 km
SSE
2400
2600
2800
3.5
0
−31
00
NNW
B
1036
PC02
4.0
1038
5 km
4.5
42˚20'N
Fig. 3. Detailed bathymetry for the submarine fan in the studied area. Dashed
lines indicate submarine channels on the fan. Solid lines with labels A and
B are for sub-bottom profiling records in Fig. 4.
Water
depth
(m)
5.0
VE: 5.8
Line 16 NNW
2400
3.5
PC06
2600
ca. 5 km
SSE
2800
3000
3200
PC06
PC05
2 km
3150
1036
4.0
3400
C
4.5
5.0
PC05
3200
5.5
VE: 5.8
Line 17 NNW
SSE
ca. 5 km
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
(Fig. 5B and C) (Honza et al., 1978; Klaeschen et al., 1994;
Schnürle et al., 1995). The sediments of the lower slope
represent the formation of a half-graben, suggesting deposition
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
cores are located on the lower slope, where the sediments dip
gently seaward.
A
PC02
Channel
PC01
3270
3300
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
silt, volcanic ash, and turbidites composed of sandy silt
to fine sand (Fig. 6). No debris flow or other mass flow
deposits are recognized in the cores. Hemipelagic mud is
diatomaceous and generally heavily bioturbated. Slightly darker
(10Y3/2–10Y3/1) clayey silt is commonly observed above the
sandy turbidites. The wet bulk density of the hemipelagic mud
slope consists of a gently-folded post-Pliocene sedimentary
succession (Fig. 5A). An anticline is observed in the middle
slope, where a terrace has formed parallel to the shelf margin
(Figs. 2 and 5). The narrow sedimentary basin located between
the upper slope and the middle terrace records the thickest
sediments of the middle slope. The lower slope is bounded by
6
v
v
v
v
1037
(160 cm)
Ta-b
Us-b
v
v
v
v
1038
(468 cm)
PC01
(636 cm)
PC02
(785 cm)
Us-b
v
v
v
v
v
v
v
Ta-b
Us-b
v
v
v
PC05
(594 cm)
Ta-b
Us-b
v
v
v
v
v
v
v
v
v
v
v
~
~
~
Ta-b
Us-b
Ta-a / Ta-b
~
~
~
~
~
~
~
~
~
~
~~
~
~
v
PC06
(243 cm)
~
1036
(224 cm)
v
2357
cal yr BP
4668
cal yr BP
2956
cal yr BP
~~
~~
~~
~ ~
~ ~
~ ~
~ ~
~
~~
~
~
~
Tephra
Bioturbation
~
~
~
~
~
~
vv
~
Olive black (7.5Y3/2)
clayey silt
Olive black (10Y3/2–
10Y3/1) clayey silt
5109
cal yr BP
~
0
~
~
~
Flow-in
~
~
1m
7339
cal yr BP
~
~~
~
~~
Very coarse sand
Coarse sand
Medium sand
Fine sand
Sandy silt–very fine sand
v
v
Ko-g
~
~
~
v
6960
cal yr BP
v
v
v
Ko-g
v
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).
The turbidites are generally coarse silt to fine sand in
grain size, and have sharp basal contacts. The thickness of
the turbidites vary from 0.1 to 13 cm, with coarser-grained
units being thicker. Normal grading or parallel lamination is
general; some contain basal layers of fining-up coarse sand.
Amalgamation is commonly observed in thick turbidites, where
two to four turbidites join without any intervening hemipelagic
mud (Fig. 8). In the amalgamated turbidites, sedimentary
structures (e.g., parallel or cross laminations) in the lower layers
7
Historical large earthquakes
are sometimes truncated by overlying turbidites. The densities
of the turbidites vary from 1.6 to 2.4 g cm−3 (Figs. 7 and 8).
The upper parts of the cores contain fewer and thinner
turbidites than the lower parts (Fig. 7). Cores 1036 and 1038,
sampled from the outer fan, contain fewer and thinner turbidites
(1.8–2.6 turbidites per 100 cm) relative to other samples. In
contrast, core PC06 from the upper fan records the highest
frequency (11.1 per 100 cm). The density profile and turbidite
thickness distributions for PC01 are comparable to those for
PC02 (Fig. 7); turbidites are more commonly observed and
thicker in the lower sections of both cores.
ka
0
Tephra
Ta-a (AD 1739)
Tsunami events
Region B
KS1 (AD 1952 or 1960)
AD 1952
KS2 (AD 1843)
AD 1843
Ko-c2 (AD 1694)
Ta-b (AD 1667)
Us-b (AD 1663)
Region C
AD 1973
AD 1894
No historical records
of earthquakes
KS3 (AD 1635?)
KS4 (AD 1290–1391?)
1
B-Tm, Ma-b
(1.0 ka)
2
3
5.4. Composition of sand grains
KS5–KS10 (recurrence
intervals of 372–422 yrs)
Ta-c2
(2.5–2.7 ka)
4
KS11– (recurrence
intervals of 406–553 yrs)
5
The sand fractions of the turbidites are predominantly made
up of volcanic glass and diatoms (Fig. 9). The volcanic glass
is mainly pumice-type glass, with lesser bubble wall-type
and massive-type glass. Almost all of the volcanic glass is
fresh, but some is stained brown. Light minerals are composed
of quartz and feldspar, with both minerals being generally
fresh and euhedral, suggesting a volcaniclastic origin. Among
the heavy minerals, ortho- and clinopyroxene and opaque
minerals are common, with lesser hornblende and biotite.
The proportion of heavy minerals in the sampled turbidites
is generally low (< 3%), but they are highly concentrated in
core PC02 at 597–599 cmbsf and PC06 at 156–160 cmbsf
(Table SD2; Fig. 9). Benthic foraminifers make up 10–40%
of the sandy turbidites (Table SD2); planktonic foraminifers
make up 10–20%, although they are rare in the surrounding
hemipelagic mud. The low density of foraminifers means that
they might have behaved as coarser grains than the same-sized
minerals and rock fragments during deposition (Fig. 8C and
D). Diatoms constitute as much as 83% of sand grains, being
mainly observed in relatively fine and thin turbidites. Indicators
of shallow water, such as bivalves, glauconite, and plant
fragments, are rarely observed.
The composition of sand grains in PC01 is similar to that in
PC02. The upper and middle parts of the cores record a high
percentage of diatoms, while the lower parts are dominated by
minerals and rock fragments. Turbidites within core PC01 at
200–250 cmbsf contain relatively few biogenic tests, as with
PC02. Turbidites in the lower parts of cores PC05 and PC06
also show high concentrations of minerals and rock fragments.
These layers are traceable among the cores.
6
7
Ko-g (6.5 ka)
Fig. 10. Summary of tephrochronology (Furukawa and Nanayama, 2006) with
tsunami events (Nanayama et al., 2007) and large historical earthquakes in
the eastern Hokkaido. Source volcanoes: B, Baitoushan; Ko, Komagatake;
Ma, Mashu; Ta, Tarumai; Us, Usu. Locations of the volcanoes are shown in
Fig. 1.
(1,000–2,000 m water depth) (Abe and Hasegawa, 2003;
Matsuo et al., 2004; Uchida, 2006). Few shelf or upper
slope assemblages are recognized in the sampled turbidites.
Although Elphidium batialis found in PC06 is lightly
dissolved, the occurrence of species with thin tests, such as
Nonionellina labradorica, indicates only minor dissolution
effects. Planktonic foraminifers within the turbidites are
generally well sorted.
5.6. Volcanic ashes
The most conspicuous volcanic ashes are observed at
30–100 cmbsf within cores 1036, 1037, 1038, PC01, PC02,
and PC05 (Fig. 6). The ashes are present as small patches in
PC05, but are absent in PC06. The identified ash layers are
up to 8 cm thick, and commonly contain beds of two distinct
colors: a light brownish gray (5YR7/1) lower bed and a reddish
gray (2.5YR5/1–10R5/1) upper bed. Both contain ortho- and
clinopyroxene in addition to plagioclase and opaque minerals;
hornblende grains are only included in the lower tephra
(Table 2). Glass chemistry is characterized by low TiO2 and
K2 O values for the lower tephra and high K2 O/TiO2 ratio for the
upper tephra. The petrographic and geochemical characteristics
indicate that the lower and upper ashes are correlated with Us-b
(A.D. 1663) from the Usu volcano and Ta-b (A.D. 1667) from
the Tarumai volcano, respectively (Figs. 10 and 11; Table 2).
Cores PC01 and PC02 contain small patches of volcanic
ashes at the lower part of the cores. The glass chemistry
in the ashes indicates that they are prehistoric tephras from
Komagatake volcano (Fig. 11). We know only the Ko-g tephra
as prehistoric volcanic ashes sourced from the Komagatake
onland and offshore of the eastern Hokkaido (Fig. 10)
(Furukawa and Nanayama, 2006). They can be, therefore,
5.5. Foraminifers
Foraminiferal tests in the turbidites showed small size (fineto very fine-grained) and relatively good preservation enabled
us to identify species. Benthic foraminiferal assemblages in
selected turbidites from cores PC05 and PC06 are dominated
by Cassidulina norvangi, Islandiella norcrossi, Elphidium
batialis, and Uvigerina akitaensis (Table 1), with lesser
species of Bolivina spissa, Epistominella pacifica, Nonionellina
labradorica, and Takayanagia delicata. The dominant species
are characteristic of the upper–middle slope environments
8
Thickness (cm)
0
5
10
15 0
5
10
15 0
5
10
15 0
5
10
15 0
5
10
15 0
5
10
15 0
5
10
15
0
?
Depth (cmbsf)
200
?
2357 yBP
?
7339 yBP
4668 yBP
2956 yBP
5109 yBP
400
Ta-b (AD1667)
Us-b (AD1663)
Ko-g (6.5 ka)
6960 yBP
600
1036
800
1
2
WBD (g/cm3)
1037
1
2
1038
1
PC01
2
1
2
PC02
1
2
PC05
1
2
PC06
1
2
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.
cm below sea floor (top)
cm below sea floor (bottom)
Angulogerina ikebei
Bolivina decussata
Bolivina spissa
Bolivina sp. A
Buccella spp.
Bulimina aculeata
Bulimina striata
Bulimina tenuata
Cassidulina norvangi
Cibicides lobatulus
Cibicides spp.
Cibicidoides sp.
Cornuspiroides sp.
Cribroelphidium sp.
Dentalina sp.
Eilohedra nipponica
Elphidium batialis
Elphidium spp.
Epistominella pacifica
Epistominella sp.
Epistominella spp.
Fissulina spp.
Fursenkoina cf. rotundata
Fursenkoina sp.
Globobulimina auricurata
Globocassidulina spp.
Gyroidina sp.
Gyroidina spp.
Islandiella norcrossi
Melonis pompilioides
Melonis sp.
Lagena spp.
Nonionella globosa
Nonionellina labradorica
Oridorsalis umbonatus
Pseudoparrella takayanagii
Pseudoparrella sp.
Pullenia salisburyi
Pullenia bulloides
Pullenia spp.
Pyrgo sp.
Takayanagia delicata
Uvigerina akitaensis
Uvigerina senticosa
Valvulineria spp.
Vaginulina sp.
Others
PC05–1 PC05–2 PC05–3 PC05–4 PC06–1 PC06–2 PC06–3 PC06–4 Environment
390.5 397.5 469.5 522.5
88
103
136
141
392.5 399.5 472.5 525.5
94
107
138
145
1.3
0.6
2.3
1.3
0.6
1.3
8.7
1.0
1.0
2.0
0.5
0.5
7.0
0.5
0.3
0.7
1.0
2.8
1.0
0.7
9.1
0.3
3.5
13.1
11.1
4.0
0.5
0.5
0.3
1.0
0.6
0.5
0.5
2.0
22.6
5.0
1.5
0.3
6.8
1.3
0.6
0.6
0.3
0.6
0.3
5.2
4.2
1.0
0.5
0.5
5.5
1.6
1.6
14.4
0.7
18.1
7.0
9.8
6.5
4.2
2.8
0.5
1.0
0.3
2.1
1.4
1.5
5.5
2.5
0.5
0.5
4.5
0.8
9.9
1.5
0.5
8.8
1.2
3.0
0.6
US
US
UMS
3.6
1.2
1.2
1.8
11.4
UMS
0.5
4.0
13.6
12.0
4.0
1.4
3.8
10.8
3.8
5.2
3.1
1.6
0.8
2.3
3.8
0.8
0.7
1.0
25.5
8.7
6.1
1.0
0.3
0.3
1.3
0.3
7.4
1.6
1.6
0.6
2.4
0.8
0.5
0.5
0.5
31.5
4.0
10.5
1.5
26.7
8.4
3.8
1.0
0.8
Shelf
Shelf
UMS
3.6
14.5
7.8
5.2
0.5
1.8
12.0
10.8
8.4
MS
MS
1.2
1.8
UMS
1.5
14.5
2.1
4.1
3.1
1.0
6.7
1.0
2.6
0.6
13.8
UMS
0.5
3.1
3.6
2.4
4.8
2.1
1.0
1.6
4.8
MS
0.6
1.2
0.8
0.8
4.8
8.8
2.4
1.6
1.5
1.5
8.5
0.8
0.5
6.4
0.8
1.6
4.8
4.5
1.5
2.3
2.3
0.5
3.1
1.6
3.2
1.0
0.5
0.5
3.5
0.5
3.2
0.3
4.8
2.4
2.8
3.5
0.3
15.0
0.5
0.5
4.5
1.4
4.8
1.0
Total benthic foram. number
Total planktonic foram. number
P/T ratio
294
457
60.9
190
467
71.1
282
699
71.3
119
172
59.1
197
100
33.7
Shelf
Upper slope
Upper-middle slope
Middle slope
Others
0.0
1.9
19.7
32.6
45.8
0.5
1.0
27.6
20.6
50.3
0.4
0.7
18.5
28.6
51.9
0
2.4
12.8
21.6
63.2
0.0
1.0
29.0
42.5
27.5
3.8
9.9
0.8
0.5
4.1
7.8
6.2
0.5
0.6
4.2
3.6
0.6
1.6
3.0
131
94
41.8
189
336
64.0
161
419
72.2
1.5
0.0
29.0
32.1
37.4
0.0
3.1
20.7
23.3
52.8
0.0
4.2
23.4
22.2
50.3
9
UMS
UMS
A (PC01)
Gray scale
200
100
Density
0
1.5
Gray scale
B (PC02)
2.0
200
2.5
100
Density
0
1.5
2.0
2.5
575
500
580
505
585
Depth (cmbsf)
Depth (cmbsf)
510
515
520
590
595
600
525
605
530
610
535
C (PC05)
Gray scale
200
100
Mean grain size (µm)
Density
0 1.0
1.5
2.0
0
100
200
300
200
Depth (cmbsf)
205
Mean grain size
210
Section
boundary
215
Sand
220
Foraminiferasrich zone
Silt
225
0
50
100
% grain size
D (PC05)
Density
Gray scale
200
100
0 1.0
1.5
Mean grain size (µm)
2.0
0
100
200
300
270
275
Depth (cmbsf)
Mean grain size
280
Silt
285
Sand
Foraminiferasrich zone
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
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
for core PC06 is the lowest among the cores, being 31 cm ky−1
5.7. Sedimentation rate
Age models for the cored sediments were established
using tephrochronology and AMS-derived 14 C ages (Table 3;
10
Content (%)
0
50
100 0
50
100 0
50
100 0
50
100 0
50
100 0
50
100
0
Depth (cmbsf)
200
400
Ta-b (AD1667)
Us-b (AD1663)
600
Ko-g (6.5 ka)
800
1037
1038
PC01
PC02
PC05
Light Minerals
Rock fragments
Benthic foraminifers
Heavy Minerals
Volcanic glasses
Planktonic foraminifers
PC06
Diatoms
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
By
Ta-b (1667)
GH04-13 1036 38–43
fine–medium ash
opx, cpx
GH04-14 1036 45–46
coarse ash
cpx, opx, hbl Pf, Ps
Ta , Us
GH04-17 1037 30–32
medium–coarse ash opx, cpx
Pf
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)
Us, Ta
Us-b (1663)
Us
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
KR05-6 PC02 38–39.5
medium–fine asho px, cpx
By, Pf
Ko-ph, Ta
Ko-g (6.5 ka) or older
By, Pf
Ta, Ko-h, Us
Ta-b (1667) including reworked Us-b and Ko-c2
Us-b (1663)
KR05-7 PC02 39.5–41
coarse ash
opx, hbl, cpx Ps, Pf
Us
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 :
∗2 :
Listed in order of abundance. Minerals: cpx, crynopyroxene; opx, orthopyroxene; hbl, hornblende. Plagioclase and opaque minerals occur in all samples.
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
(Ta-b and Us-b), recent sedimentation rates are 114 cm yr−1 for
PC01, 112 cm yr−1 for PC02, and 92 cm yr−1 for PC05 over
the past ca. 0.34 kyrs.
Recurrence intervals of turbidite deposition are calculated
from sedimentation rates and turbidite numbers (Fig. 12B).
Core PC05 has the smallest value of 113 yrs during 0–0.34 kyrs,
although the older part (0.34–5.1 kyrs) shows the largest vale of
439 years. The intervals are 153–169 yrs for PC02, 230–345 yrs
for PC01, and 285 yrs for PC06.
5.8.
210
Pbex and 137 Cs geochronology
Radioactivity analysis of 210 Pbex and 137 Cs were performed
for the sediment within the pilot core of PC05 (Fig. 13;
Table SD3). The values of 210 Pbex are approximately uniform in
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
5
A
B
GH04-13
(1036)
n=39
C
GH04-14
(1036)
n=23
D
GH04-17
(1037)
n=29
GH04-19
(1038)
n=29
F
GH04-21
(1038)
n=59
G
KR05-1
(PC01)
n=60
H
KR05-3
(PC01)
n=38
KR05-4
(PC01)
n=21
J
KR05-6
(PC02)
n=19
K
KR05-7
(PC02)
n=30
L
KR05-8
(PC02)
n=25
KR05-12
(PC05)
n=32
N
Baitoushan
K2O
4
3
2
1
0
6
5
Tarumai
Komagatake
historic
prehistoric
Usu
Mashu
E
K2O
4
3
2
1
0
6
5
I
K2O
4
3
2
1
0
6
M
5
KR05-13
(PC05)
n=31
0
0.2
0.4 0.6
TiO2
0
0.2
0.4 0.6 0.8
TiO2
K2O
4
3
2
1
0
0
0.2
0.4 0.6
TiO2
0
0.2
0.4 0.6
TiO2
Fig. 11. K2 O–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
14
Conventional
δ13 C
Calibrated age
Calibrated age
Median
C age (yr BP) 14 C 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
7200±50
−1.1
7292–7398
7235–7430
7339
Mixed planktonic 6810±50
part of the core (12–28 cmbsf), the ln(210 Pbex ) values linearly
decrease; the apparent sedimentation rate is estimated to be
0.26 cm yr−1 to the exclusion of turbidite thickness.
As the result of atmospheric nuclear tests, 137 Cs began
to appear in environmental samples at measurable levels
from A.D. 1954. Atmospheric fluxes of these fallout nuclides
12
(A)
(B)
Depth (cm)
0
200
400
Recurrence interval (yrs)
0
600
200
400
600
0
PC01
PC01 (0−2.4 kyr)
PC02
PC01 (2.4−7.0 kyr)
Age (kyr BP)
2
PC05
PC02 (0−0.34 kyr)
PC06
PC02 (0.34−6.5 kyr)
4
PC05 (0−0.34 kyr)
PC05 (0.34–5.1 kyr)
6
PC06 (0−7.3 kyr)
8
Fig. 12. (A) Age–depth profiles for selected cores based on
of turbidites.
WC (%)
0
200
14 C
MGS (μm)
400 0
20
40
ages of planktonic foraminifers and tephrostratigraphy. (B) Recurrence intervals of deposition
ln(210 Pbex) (dpm/g)
60
3
4
137Cs
5 0
0
(dpm/g)
0.5
1.0 X-radiograph
Depth (cmbsf)
Earthquake
10
1973
Nemuro-oki
20
1961
Nemuro-oki
1952
Tokachi-oki
30
Fig. 13. X-ray image and depth profiles of water content (WC), mean grain size (MGS), 210 Pbex , and 137 Cs for the pilot core (PC05). The error bars for
210 Pb
137 Cs data represent ±1s about the means, as calculated using counting statistics. Earthquakes are possible triggers of the turbidite deposition.
ex and
Results of the radioactivity analysis are presented in Table SD3.
then followed the pattern of activities released from nuclear
detonations, which peaked in 1963 and decreased after the
enactment of the Test-Ban Treaty in the same year. The depth
profile of 137 Cs conforms to the history of nuclear fallout,
beginning of the detection at 23 cmbsf, showing high values
in the middle part (15–20 cmbsf), and then decreasing to the
top (Fig. 13; Table SD3). Given that 137 Cs is undetectable in
the sample of deeper than 25 cmbsf, the detection limit lies
between the second and third turbidites. The 137 Cs data indicate
the sedimentation rate of 0.39–0.43 cm yr−1 with consideration
for turbidite deposition.
6. Discussion
6.1. Origin of the turbidites
Turbidity currents can be triggered by a number of natural
causes in addition to earthquakes, including floods, storms, and
rapid sedimentation (e.g., Normark and Piper, 1991; Locat and
Lee, 2002). The fact that the study area is not fed by large
rivers and contains a wide shelf (20–30 km) probably precludes
the direct input of terrestrial material into the forearc slope
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
the turbidites were derived from upper–middle slope sediments
under the influence of gravity. In addition, seaward thinning of
surface seismic reflections in the lower slope (Fig. 5) supports
turbidites were derived from the upper–middle slope rather than
the outer high.
Based on small volume of each turbidite bed (less than
0.003–0.03 km3 for 1–10 cm thick turbidites that would cover
15×20 km of the fan), lack of deposits derived from slides
or debris flows in the proximal core (PC06), and fewer and
thinner turbidites in more distal part of the fan, slope failures
on the slope were considered to be small-scale or thin-skinned.
A relatively good preservation of benthic foraminiferal test
in the turbidites also infer that they were derived from not
deeply-buried sediments but very surface sediments without
diagenesis. The small-scale or thin-skinned failures may be
because (i) the steep forearc slope prohibits settlement of
sufficient sediments for large-scale slope failures, (ii) repeated
earthquakes remove unstable hemipelagic muds on slopes,
or (iii) insufficient sediment inputs due to no large rivers
and highstand sealevel. The small volumes of turbidites may
be an additional evidence that turbidites were generated by
earthquakes (cf. Goldfinger et al., 2003).
The common occurrence of amalgamation (multiple coarse
fraction pulses) within thick turbidites infers deposition from a
flow with multiple pulses or multiple flows that occurred over
a short time period. Such amalgamated turbidites have been
reported previously from seismically active regions of Japan
(Nakajima and Kanai, 2000; Noda et al., 2008) and Cascadia
(Goldfinger et al., 2007). It is not possible to produce an
amalgamated turbidite from a simple waning turbidity current
of the type that produces a typical turbidite represented by the
Bouma sequence. The occurrence of multiple slope failures
over a short period can be attributed to strong ground shaking
associated with a large earthquake. Multiple failures upon a
slope have the potential to flow downslope and transform into
turbidity currents, converging at the apex of the submarine fan;
in this way, multiple flows pass over the fan.
Peak ground acceleration (PGA) at a given site can be
calculated using an empirical attenuation relationship (Boore
and Joyner, 1982; Campbell, 1985; Fukushima and Tanaka,
1990; Si and Midoriwaka, 1999). We calculated PGA at the
assumed source point for the major (> M 7) earthquakes using
the relationship proposed by Fukushima and Tanaka (1990):
log10 PGA = 0.41M−log10 (R+0.032×100.41M )−0.0034R+1.30,
(1)
where PGA is the peak ground acceleration (cm s−2 ), R
is the shortest distance from a fault plane (if available) or
hypocenter (km), and M is magnitude from Utsu (1999) and
Japan Meteorological Agency (2006). The distance (R) between
the points is approximated by a 3D application of Pythagoras’s
theorem:
R2 = D2 + (p(Ax − Bx))2 + (p(Ay − By))2 ,
(2)
where D is the hypocenter depth (km), Ax and Ay represent
the longitude and latitude of the source point in degree, Bx and
By represent the longitude and latitude of the fault rupture or
hypocenter, and p is a constant (111.32 km). Fukushima and
Tanaka (1990) reported that predicted PGA values are similar
to observed values at hard soil sites, but underestimated by
about 40% at soft soil sites. Therefore, we multiplied the value
of PGA calculated using Eq. (1) by 1.4 as a site effect.
Peak ground acceleration (PGA) can also be estimated using
the following empirical relationship (Si and Midoriwaka, 1999):
log10 PGA = b − log10 (R + c) − 0.003R
(3)
where
b = 0.53M + 0.0044D + d + 0.38
c = 0.0055 × 10
0.50M
.
(4)
(5)
The value of d depends on the type of earthquake, whether
shallow (0.00), interplate (−0.04), or intraplate (0.17). Eq. (3)
was optimized for soil with a shear-wave velocity (V s ) of
400 m s−1 (Si and Midoriwaka, 1999), suggesting it could
apply to relatively soft basement. Although there were few
reports about shear-wave velocity of forearc slope sediments,
Goldberg (2003) reported a nearly constant V s (∼300 m s−1 ) in
the forearc accretionary sediments above 100 mbsf, off Nankai,
southwest Japan. A similar shear-wave velocity for the forearc
slope sediments could be assumed, we did not consider a site
effect for Eq. (3).
The calculated PGA of historical earthquakes (Table 4)
show that the 1894, 1952, 1961, 1973, and 2004 earthquakes
could have large PGA (> 150 cm s−2 ) near the study area
(Table 4; Fig. 15). Which earthquakes could trigger deposition
of turbidites? The depositional ages of the recent three turbidites
in the pilot core of PC05 (Fig. 13) are estimated as 1946–1971,
1930–1962, and 1910–1950, based on the sedimentation rate
of 0.26–0.43 cm s−1 derived from 210 Pbex and 137 Cs analysis.
The detection of 137 Cs radioactivity below the second turbidite
indicates that it was deposited after A.D. 1954. The first and
6.2. Correlation with historical earthquakes
The initiation of slope failure generally depends on
excess pore-pressure generated by earthquake-induced ground
acceleration (e.g., Seed and Idriss, 1971). Critical earthquake
horizontal accelerations of 80–190 and 80–280 cm s−2 have
been reported for the Eel margin of California (Lee and Edward,
1986; Lee et al., 1999) and the Japan Sea (Lee et al., 1993,
1996), respectively. Here, we seek to correlate the turbidites
identified in the pilot core (PC05) with known historical
earthquakes, based on calculated ground accelerations. Our
calculations assumed a point source of slope failures centered
on the upper slope off Nemuro (145◦ 32’E, 42◦ 52’N). The oldest
historical earthquake recorded in eastern Hokkaido is the 1843
Nemuro-oki earthquake. Many large earthquakes have been
recorded since this time, including shallow, interplate, and deep
(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) and Japan 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) and Si 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
180.9
149.5
91.3 (SD)
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
26.0
9
B
23-Apr-1962 7.1
143.92
42.23
60 Is 202.2 (HC)
20.5
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
C
29-Nov-2004 7.1
145.30
42.90
48 Is
54.7 (HC)
183.7
190.4
18
Units: Depth and distance, km; PGA, cm s−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
43˚N
Kushiro 11
8
14
18
M7.5
M7
Source
Target
1
15
2
16
12
10
5
ch
3
2003
17
41˚N
1969
C
1973
9
42˚N
D
ril
n
Tre
Ku
6 1952
B
A
Pacific Plate
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
and 1961 Nemuro-oki earthquakes (Figs. 13 and 15). The
third turbidite may be associated with the 1952 Tokachi-oki
earthquake. Although it occurred in the region B and its PGA
15
is lower than the 1961 and 1973 earthquakes, relatively long
duration after the 1894 and 1900 earthquakes (Fig. 15) might
enable to accumulate between 2 and 5 cm of surface sediment
upon the upper–middle slope, consisting of pelagic fallout
where the rate of pelagic sedimentation is 0.032–0.088 cm yr−1
(Noda and TuZino, 2007).
The turbidites in the pilot core (PC05) were possibly
deposited in association with earthquakes that generated
strong (> 150 cm s−2 ) ground shaking; however, not all
large earthquakes are recorded in the sedimentary record as
turbidites. We could identified only three turbidites above the
17th-century tephra (Ta-b) in core PC05; the 1894 and 2004
earthquakes or any historical earthquakes were not recorded
in the core. The pilot core might record about half of the
strong earthquakes that occurred in the source region of the
slope failures. It must be remembered that the initiation of
slope failure requires sufficient unconsolidated sediment: strong
earthquakes are unlikely to generate turbidites if the source area
contains insufficient soft sediment. In addition to earthquake
magnitude, the recurrence interval is possible another factor
that influences the triggering mechanisms of slope failure.
of the recurrence interval of turbidite deposition in PC02 are
twice the interval of interplate earthquakes in the area over the
past 160 years, as deduced from historical records (72.2 years;
Earthquake Research Committee, 2004).
Nanayama et al. (2007) reported recurrence intervals of
∼550 years for large tsunamis in the Kushiro–Nemuro region
over the past 4,000 years (Fig. 10). The turbidites analyzed
in the present study record a greater number of events than
that indicated by tsunami deposits. Nanayama et al. (2007)
identified an unusually large tsunami that inundated the area
during the 16th century (KS3 in Fig. 10). This tsunami was
potentially associated with a giant earthquake related to rupture
along a multi-segment fault (Regions B and C) (Nanayama
et al., 2003). None of the cores obtained from the submarine fan,
however, contain conspicuous turbidites immediately below
the 17th-century volcanic ash. Thin (2 cm thick) turbidites
identified below the Us-b tephra in cores PC02 and 1038
(Fig. 7) are possibly related to the 16th-century event. It remains
uncertain as to whether submarine deposition accompanied this
unusually large-scale event.
7. Conclusions
6.3. Recurrence intervals
With the aim of estimating the long-term recurrence interval
of earthquakes off eastern Hokkaido, seven sediment cores
were obtained from a submarine fan on the forearc slope.
The upper slope is steep (3–10◦) and incised by a series of
gullies, some of which cut through the middle slope to the
lower slope where the submarine fan is developed. The retrieved
cores contain a number of turbidites that probably originated
from the upper–middle slope (> 1,000 m water depth), as
indicated by benthic foraminiferal assemblages. The deep-sea
origin of the turbidites suggests that the turbidity currents were
triggered by earthquakes. 210 Pbex and 137 Cs geochronology, in
combination with the calculated peak ground accelerations of
historical earthquakes, indicate that the three recent turbidites
are correlated with the 1952, 1961, and 1973 earthquakes. The
identified turbidites possibly record half of the earthquakes with
sufficient strength of ground shaking (∼150 cm s−2 ) around the
source area of slope failure, possibly due to changes of the
course of turbidity currents on the fan over time or frequent
removal of unstable sediments on the slope. The depositional
intervals of the analyzed turbidites are 113–439 years over the
past 7 kyrs. Given that not every seismic event is recorded in
any single core, the recurrence interval of earthquakes in this
region is estimated to be less than 113 years.
The recurrence intervals of the turbidite deposition in the
lower part of PC05 and PC06 were longer than PC01 and
PC02 obtained from the middle fan (Fig. 12B). Relatively thick
turbidites observed in the cores of the upper fan infer that more
erosive currents on the upper fan passed than those on the
middle fan. The steepness of the slope upon which the currents
passed is another interpretation of less turbidite numbers in the
upper fan. Because the slope is one of the variables for velocity
of body of turbidity currents (e.g., Middleton and Hampton,
1976), deposition will not occur upon which currents are too
fast to settle the suspended particles.
The change in recurrence intervals in PC05 (Fig. 12B)
indicates that changes of the course of turbidity currents
over time, thereby failing to transport detritus through the
site of PC05. This type of channel avulsion has been
reported previously from other submarine fans (Normark, 1970;
Normark et al., 1979). Several channels merge at the apex of
the fan divided into two tributaries (Fig. 3); one has the course
toward south along the western boundary of the fan through
the site of PC05, the other flows toward southeast near the
sites of PC01 and PC02. Not all currents deposit turbidites over
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
turbidites is attributed to its location on levees in the center
of the fan, where turbidites are more likely to deposit than in
the channels of the upper fan and on the marginal fan. Erosion
by currents could be less effective on levees of the middle fan
than in channels. Nevertheless, it is unlikely that PC02 provides
a complete record of seismogenic turbidites. About 150 years
Acknowledgements
We are greatly indebted to the officers, crew, and research
staff of cruises GH04 and KR0504 for the collection of
data. We also thank Hajime Katayama for data collected on
cruise GH04, and Yukinobu Okamura, Kenji Satake, Ken
Ikehara, Kohsaku Arai, and Tomoyuki Sasaki for data collected
on cruise KR0504. The sea-beam data used for compiling
16
M8
M7.5
PGA (cm s−2)
300
M7
1952
Tokachi-oki
(M8.1)
M6.5
200
1973 Nemuro-oki (M7.4)
1894 Nemuro-oki
(M7.9)
1961
Nemuro-oki
(M7.2)
2004 Kushiro-oki (M7.1)
1993 Kushiro-oki (M7.5)
100
0
1850
1900
1950
2000
Calendar year
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 ) of PGA for turbidite deposition.
undersea topography were collected by the Hydrographic and
Oceanographic Department of the Japan Coastal Guard. We
are grateful to Azusa Nishizawa for providing bathymetric
data, Ken’ichi Ohkushi for picking foraminifers, and Masayuki
Yoshimi for calculations of PGA. Constructive comments
by anonymous reviewers and D. J. W. Piper prompted a
significant revision of this paper. This study was part of
the “Marine Geological Mapping Project of the Continental
Shelves Around Japan” program supported by the Geological
Survey of Japan/AIST. Financial support for this research was
also provided by the Japan Nuclear Energy Safety Organization
(JNES).
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