Matsushima Bay as an Early Holocene coastal mega-landslide,
Northeast Japan
Shuichi Hasegawa (Kagawa University, Japan) Timihiro Sawada (Sawa Soft Science, Japan)
Ranjan Kumar Dahal (Kagawa University, Japan and Tribhuvan University, Nepal)
Atsuko Nonomura (Kagawa University, Japan) Minoru Yamanaka (Kagawa University, Japan)
visited both Matsushima and Kisakata. He composed three
haiku poems for Kisakata, but he could not express his
excitement in a haiku poem for Matsushima.
Matsushima and Matsushima Bay have long been
considered as a typical submerged coast, but they are inferred
to have been formed by a coastal mega-landslide in middle
Holocene age from geological and topographical inferences.
Abstract. Matsushima, a group of island at Matsushima Bay
in Miyagi Prefecture, northeast Japan, is one of the three
famous scenic spots of Japan. It is composed of more than
200 islands in Matsushima Bay and the islands just out into
the sea. Topographically Matsushima Bay suddenly breaks
the gently concaved coastline from Sendai Bay to Ishinomaki
Bay. Matsushima and Matsushima Bay have been considered
as a typical submerged coast, but they are inferred to have
been formed by a coastal mega-landslide in middle Holocene
age from geological and topographical evidences.
Keywords. Mega-landslide, Holocene, Jomon transgression,
Active fault, topography
1. Introduction
Mega-landslides due to volcanic activities and
earthquakes have caused severe damage to the surrounding
areas. Sector collapse of volcanoes is one of the most
destructive landslides. Debris avalanche deposits from a
sector collapse generally form strange topography punctuated
by hundreds of small hills, ridges and closed depressions.
The 1792 Mayuyama sector collapse of Unzen volcano in
Kyushu, southern Japan, caused debris avalanche which
flowed through ancient Shimabara City and entered the sea.
As a result, a giant tsunami was generated (Inokuchi, 2006)
and more than 15,000 people were killed by the landslide and
tsunami. The debris avalanches which entered the sea are
characterized hundreds of many small islands and sunken
rocks in Shimabara Bay. The locality is now called as
Tsukumo-jima and it means 99 islands in Japanese. Usually,
with the help of topography and geology, pre-historic sector
collapses are recognized. Another example of sector collapses
is found in Kisakata, western Tohoku, northern Japan and it is
also characterized by a hundred small islands (Tsukumo-jima)
scattered in the sea. They are debris avalanche deposits from
Mt. Chokai volcano occurred about 2,600 years ago (Inokuchi,
2006).
Mega-landslides in costal area sometimes generate
mega-tsunamis. In 1958, a rockslide at Lituya Bay in Alaska,
caused by an earthquake of magnitude 7.7, produced a
mega-tsunami measuring 524 m high (Miller, 1960). In 1963,
mega-landslide at Vajont Dam reservoir in Italy generated a
250 m high mega-tsunami and killed almost 2,000 people
(Kiersch, 1964). Although mega-Landslides are rare
phenomena, evidence for prehistoric mega-landslides has
been found both from land and underwater.
Matsushima in eastern Tohoku is one of the three famous
scenic spots of Japan. It is composed of more than 200 islands
in Matsushima Bay, which faces the Pacific Ocean (Fig. 1).
Matsushima often praised as eastern Matsushima and western
Kisakata which faces the Japan Sea. Matsuo Basho, the most
famous haiku poet of Edo period in Japan, praised
Matsushima as the most beautiful landscape in Japan. He
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Fig. 1 Locality of Matsushima Bay (modified after
Editorial Group for Computer Graphics, Geology of
Japanese Islands, 2002)
2. Topography and geology of Matsushima
2.1 Topography
Matsushima is composed of more than 200 islands.
Matsushima Bay is a semicircular in shape, with 10 km in
EN-SW direction and 5 km in NW-SE direction (Fig. 2).
Along the southeastern margin of Matsushima Bay, a number
of islands are concentrated from Miyato Island to
Shichigahama Peninsula. These islands and peninsula, named
as Matsushima Islands, are located between the beach ridges
along Sendai and Ishinomaki Bays. It looks like they
suddenly interrupt the gently concaved coastline from Sendai
Bay to Ishinomaki Bay (Fig. 2). The area of Matsushima
Islands is roughly equal to those of Matsushima Bay.
1:25,000 scale topographic map of coastal areas
“Matsushima” (Geographical Survey Institute of Japan, 1982)
and 1:25,000 scale land condition map of coastal areas
“Matsushima” (Geographical Survey Institute of Japan, 1984)
have mapped out that Matsushima Bay is flat and less than
4m in depth and a number of sunken rocks are distributed in
and to the south of Matsushima Islands (Fig.3).
Summit level of Matsushima Islands are usually about
50m high. On the contrary, inland Matsushima Hill is about
100m high and is about 50m higher than Matsushima Islands.
2.2 Geology of Matsushima
Matsushima Islands mainly consist of the Miocene
Matsushimawan Group which strike NW-SE direction (Ishii
et al., 1982; Ishii et al., 1983). Similarly Matsushima Hill is
mainly composed of the the Miocene Matsushimawan Group
which strike NW-SE direction. The eastern extension of the
active Nagamachi-Rifu Fault Zone, which is trending NE-SW,
is inferred in Matsushima Hill, but fault topography is not
recognized.
The Matsushimawan Group in Matsushima and
Matsushima Hill forms similar gentle anticlines and synclines
trending from northwest to southeast. The difference of
geologic structure of Matsushima Islands from Matsushima
Hill is that strike faults are distinct in the Matsushimawan
Group in Matsushima Islands (Ishii et al., 1982; Ishii et al.,
1983).
Matsushima Bay is underlain mainly by Holocene marine
clay sediments which directly cover the Miocene bedrocks
(Ishii et al., 1983). The Pleistocene sediments have not been
reported.
Fig. 2 Topography of Matsushima (Illustrated by
Kashimir3D), inferred beach between Sendai Bay to
Ishinomaki Bay is also shown in the map
3. Mega-landslide as the origin of Matsushima
3.1 Topographical restoration
Topography of Matsushima Bay and Matsushima Islands
indicate that Matsushima Bay is the source area of a
mega-landslide and Matsushima Islands are slide masses.
This interpretation indicates that previously, Matsushima
Islands including Shichigahama Peninsula were situated on
the area of Matsushima Bay and beaches and sand ridges
continue from Sendai Bay to Ishinomaki Bay. This restoration
suggests that hills which had been located on the area of
Matsushima Bay slid about 5km toward southeast direction
and have decrease there elevation about 50 m. The sunken
rocks also indicate the subsequent secondary slides and third
slides (Fig.3)
Fig. 3 Topography of Matsushima mega-slide (Modified
after Geographical Survey Institute of Japan (1984)
Fig. 4 Geology of Matsushima mega-landslide (modified after Ishii et al.(1982) and Ishii et al.(1983) )
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3.2 Geological restoration
The topographical restoration coincides with the
geological restoration by lithology, successions and geologic
structures (Fig. 4). This restoration suggests that hills which
had been located on the area of Matsushima Bay slid toward
southeast direction parallel to the strike of bedrocks without
separated into small fragments. Some of strike-slip faults
distributed in Matsushima Islands are inferred to have formed
by sliding and the sliding hills might be separated into several
large masses. The sliding surface is not exposed, but the
restoration indicates that the sliding surface is almost
horizontal (Fig.5).
Fig. 6 History of Satomaha shell mound in Miyato Island
(modified after Historical Museum of Jomon Village
Okumatsushima (2008) )
Fig. 5 Schematic profiles of Matsushima mega-slide
(Section lines are shown in Fig. 4.)
4. Cause and result of mega-landslide
4.1 Tigger of mega-landslide
As Matsushima is not a volcano, a strong earthquake from
nearby active fault was the most probable trigger of
mega-landslide. Matsushima area has suffered the 1978
Mitagikenoki subduction earthquake (M7.4) and the 2003
Miyagikenhokubu earthquake (M6.4), but no large-scale
deep-seated landslide occurred. Recent earthquake-induced
large-scale deep-seated landslides support big earthquake
caused by nearby active fault is the most probable trigger.
Matsushima Hill is located at the eastern extension of the
active Nagamachi-Rifu fault zone whose estimated magnitude
is from7.0 to 7.5. Moreover the fault extension coincides with
the scar area of the mega-landslide (Fig. 7). Therefore the
Nagamachi-Rifu fault is the most probable trigger. The last
faulting is estimated after about 16,000 yrs. B.P.
(Headquarters of Earthquake Research Promotion, 2002).
3.3 Age of Matsushima mega-landslide
Matsushima Bay is underlain mainly by Holocene marine
clay sediments which directly cover the Miocene bedrocks
(Ishii et al., 1983). This indicates that beginning of deposition
of marine clay in Matsushima Bay was just after the
occurrence of mega-landslide. As the marine clay attains 20
m at Shiogama, the age of the sediments can be traced back to
Early to Middle Holocene. Drilling core of Matsushima Bay
is essential to determine the exact time of beginning of
sedimentation.
In Matsushima area, shell mounds of Jomon age (early to
late Holocene) are found on the hills of Matsushima Island
(see Fig. 1). A carbon-14 dating from Daigigakoi shell mound
located at the highland (about 40m in height) of
Shichigahama Peninsula is 5680±120yrsB.P. (Ito, 2002).
At Satohama shell mounds in Miyato Island, early Jomon
settlements had started from highlands and had descended
elevation during Jomon age (Fig. 6). Interesting fact is that
the percentage of shells on rocky coasts was high at early
Jomon age and shells in sandy and silty beaches has gradually
increased as dating up to latest Jomon age (Fig. 5). These
histories of the shell mounds indicate that the early Jomon
people had to start settling down on safe but inconvenient
highlands. At that time, rocky coasts were widely distributed
in Matsushima Islands. Probably early Jomon people had
tried to prevent disasters caused by mega-landslide.
Theses geological and archeological information indicates
that the mega-landslide occurred about 6000 yrs B.P. in
middle Holocene.
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Fig. 7 Active faults of Matsushima area (Compiled after
Modified after Headquarters of Earthquake Research
Promotion (2002) and Japan Coast Guard (2002))
Another active fault is located in Sendai Bay (Japan Coast
Guard, 2002). The submarine active fault is about 33 km in
length and is trending NNW-SSE. Matsushima Bay is located
northern extension of the submarine fault and the distance is
about 10 km.
At present we do not have the precise age of the last
faulting and the exact age of mega-landslide.
Ishii, T., Yanagisawa, Y., Yamaguchi, S., Sangawa, A. and
Matsuno, K. (1982) Geology of Matsushima District.
Quardrangle Series, Scale 1:50,000, Geol. Surv.
Japan,121p. (in Japanese with English abstract, 9p.)
Ishii, T., Yanagisawa, Y. and Yamaguchi, S. (1983) Geology
of Shiogama District. Quardrangle Series, Scale 1:50,000,
Geol. Surv. Japan,112p. (in Japanese with English
abstract, 6p.)
Inokuchi T. (2006) properties of sector-collapse and debris
avalanche on Quaternary volcanoes in Japan. Jour. Japan
Landslide Society, Vol.42, pp.409-420. (in Japanese with
English abstract)
Ito, A. (2002) Pottery chronology and C-14 dates in Jomn and
Yayoi Period in Miyagi Prefecture, Miyagi Archaeology,
No.4, pp.121-126. (in Japanese)
Japan Coast Guard (2002) Distribution of the faults in Sendai
Bay,
http://cais.gsi.go.jp/KAIHOU/report/kaihou71/02-17.pdf. .
(in Japanese)
Kiersch, G. A. (1964) Vaiont reservoir disaster. Civil
Engineering, Vol. 34, pp. 32-39.
Miller, D. J. (1960) Giant waves in Lituya Bay, Alaska, Geol.
Surv. Professional Paper 354-C, U. S. Government
Printing Office, Washinton.
Minoura, K., Imamura, F., Sugawara, D., Kono, Y. and
Iwashita, T. (2001) The 869 Jogan tsunami deposit and
recurrence interval of large-scale tsunami on the Pacific
coast of northeast Japan. Journal of Natural Disaster
Science, 23, pp. 83-88.
Sugimoto,
T.,
(2000)
KASHIMIR
3D,
http://www.kashmir3d.com/index-e.html
4.2 Origin of sliding surface
As the sliding surface is inferred to be almost horizontal
(see Fig. 5), it must be originated from fault-related plane. At
present we do not obtain the material from the sliding surface
and further research is necessary for mechanism of the
mega-landslide.
4.3 Possibility of landslide-triggered mega-tsunami
The estimated age of the mega-landslide is around 6000
yrs B.P. during middle Holocene age, when the sea-level was
nearly or about 1 m above the present level. This suggests that
the mega-landslide entered the sea and generated a
maga-tsunami, such as Lituya Bay tsunami in Alaska. In
Sendai region, the large-scale 869 Jogan tsunami deposits are
widely distributed (Minoura et al., 2002), but the large-scale
Jomon tsunami deposits remain as future investigation.
Conclusions
The major findings of the investigation are:
1. Matsushima Bay is the source area of a mega-landslide and
Matsushima Islands are slide masses. Hills which had been
located on the area of Matsushima Bay slid toward southeast
direction parallel to the strike of bedrocks without separated
into small fragments. The sliding surface is estimated to be
almost horizontal (Fig. 4).
2. The mega-landslide is inferred to have occurred around
6000 yrs B.P. in middle Holocene.
3. A strong earthquake from nearby active fault was the most
probable trigger of mega-landslide.
4. The mega-landslide must have entered the sea and
generated a maga-tsunami.
Acknowledgments
The authors would like to express gratitude to Ms. Seiko
Tsuruta for drawing figures.
References
Editorial Group for Computer Graphics, Geology of Japanese
Islands (2002) Computer Graphics, Geology of Japanese
Islands CD-ROM Version, Maruzen Co. Ltd.
Geographical Survey Institute of Japan (1982) 1:25,000 scale
topographic map of coastal areas “Matsushima”,
Geographical Survey Institute of Japan.
Geographical Survey Institute of Japan (1984) 1:25,000 scale
land condition map of coastal areas “Matsushima”,
Geographical Survey Institute of Japan.
Headquarters of Earthquake Research Promotion (2002)
Evaluation of the Nagamachi-Rifu fault zone,
http://www.jishin.go.jp/main/chousa/02feb_rifu/index.ht
m. (in Japanese)
Historical Museum of Jomon Village Okumatsushima (2008)
Satohama
Shell-mound,
http://www.city.higashimatsushima.miyagi.jp/02_jomon/s
atohama/page04.jpg. (in Japanese)
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