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Influence of tsunamis on coastal lowlands : case of the 2011 Tohoku earthquake
津波の海岸低地への影響 : 2011年東北地方太平洋沖地震を例にして
松原, 彰子(Matsubara, Akiko)
慶應義塾大学日吉紀要刊行委員会
2014
慶應義塾大学日吉紀要. 社会科学 (The Hiyoshi review of the social
sciences). No.25 (2014. ) ,p.1- 13

挿図
Departmental Bulletin Paper
https://koara.lib.keio.ac.jp/xoonips/modules/xoonips/detail.php?koara_id=AN10425
830-20150331-0001

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Influence of Tsunamis on Coastal Lowlands

Influence of Tsunamis on Coastal Lowlands:
Case of the 2011 Tohoku Earthquake

Akiko Matsubara

I. Introduction
The Japanese Islands are composed of arcs and trench systems and were tectonically
active during the Quaternary. In the islands, coastal lowlands are distributed mainly in thesubsiding regions. Rivers supply large amounts of sediment to the coastal lowlands; thismaterial is derived from volcanoes or the upper reaches of uplifting mountains. Thisdeposition has been ongoing since the last glacial stage in the Late Pleistocene, resulting
in the accumulation of thick unconsolidated deposits.
The coastal lowlands of the Japanese Islands can be classified into three types: (1)alluvial fans that develop at river mouths with a large supply of coarse sediments, whichface the steep sea bottom; (2) alluvial deltas, which dominantly occur in the inner parts ofbays and are supplied with large amounts of fine sediment; and (3) sand and gravel ridgebackmarsh complexes, in which the ridges are usually parallel to the shore and represent
former coastal barriers and beach ridges. Of the three types of coastal lowlands, the ridgebackmarsh complexes are most extensive.
Coastal ridges such as coastal barriers and beach ridges are widely distributed along
stable trailing-edge coasts across the globe. The primary requisites for the formation ofcoastal ridges are sufficient sediment supply, processes allowing the development andmaintenance of the ridges and an appropriate geomorphological setting. Although theJapanese Islands are in a tectonically active area that includes island arcs and trench systems,
coastal ridge landforms are generally found in the coastal lowlands of this area (Fig. 1).
―

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140°

145°E

Sea of Okhotsk

45°N

135°

40°

Sea of Japan

I
S

130°

a

e
35°

K
c

Paciﬁc Ocean

b

d

30°

coastal ridges

0100

200 300 km

Fig. 1 Distribution of coastal ridges and plate boundaries in Japan
I: Ishinomaki Lowland S: Sendai Lowland K: Kujukurihama Lowland
a. Japan Trench b. Izu-Ogasawara Trench c. Sagami Troughd. Suruga-Nankai Trough e. Fossa Magna―

2―

Influence of Tsunamis on Coastal Lowlands

Matsubara (1988; 2000) reconstructed the palaeoenvironmental changes that had
occurred within the embayments along Suruga Bay, on the basis of analysis of fossil
foraminiferal assemblages from bore-hole cores; Matsubara (1988; 2000) used these todescribe the geomorphological development of the coastal ridges in this area during the
Holocene. These case studies served to indicate the processes involved in thedevelopment of coastal barriers while the sea level rose during the early to mid-Holocene,in addition to those involved in the development of beach ridges when the sea levelstabilized or was slightly lowered during the late Holocene.
From the perspective of human activities in coastal lowlands, archaeological sites datingfrom the later Jomon period, around 3,000 BP, are widely distributed on the coastal ridges.
This suggests that the coastal ridges in the lowlands were important landforms for humansettlement during the Holocene. Furthermore, natural disasters such as tsunamis or
typhoons may have affected the coastal lowlands. The Tohoku Earthquake was a massive
earthquake (epicentre: 38°06.2’ N, 142°51.6’ E, Mw 9.0), which occurred on 11 March
2011. The source region of the earthquake was 450 km in length from north to south and
200 km in width from east to west along the Japan Trench. Tsunamis inundated wide
areas along the coasts of the Tohoku and Kanto districts facing the Pacific Ocean.
In this study, the influence of tsunamis caused by the Tohoku Earthquake on landforms
such as coastal ridges in three coastal lowlands; the Sendai, Ishinomaki andKujukurihama, will be discussed.

II. Sendai Lowland
1. Geomorphological development of coastal ridges
The Sendai lowland faces the Pacific Ocean; it is around 50 km in length from north tosouth and 10–20 km wide from east to west. The Nanakita, Natori and Abukuma Rivers
flow through the lowland into Sendai Bay. Most parts of the lowland have an altitude ofless than 5 m, and the landforms in this area consist of natural levees along rivers andthree coastal ridges, which are numbered I to III from landward to seaward (Fig. 2).
The present-day landforms and deposits in the lowland were analysed by Matsumoto―

3―

R. NanakitaN▲uplands
coastal ridges
natural levees
backmarshes

R. Hiroseabandoned
channels
embankments

R. Natori

Sendai Bay

Inundation Limit of Tsunami
from Tohoku Earthquake(Haraguchi and Iwamatsu, 2011)
0

1

2

km

Fig. 2 Inundation area of tsunami from Tohoku Earthquake in the northern part of Sendai Lowland
Landform classification is based on GSI Map (GSI HP)(1981; 1984), Tamura and Masuda (2005) and Tamura et al. (2006). Matsumoto (1981;1984) determined the geomorphological development of the coastal ridges during the
Holocene using 14C dating.
According to the results of those studies, coastal ridges I, II and III in the northern partof the lowland were constructed around 5,000 cal BP, 2,500 cal BP and 700 cal BP,
respectively. In the central part of the lowland, coastal ridges I, II and III developed during
4,500 to 4,000 cal BP, 3,300 to 1,000 cal BP and around 1,000 cal BP, respectively.
Formation of coastal ridge I was completed at around 2,900 cal BP in the southern part ofthe lowland.

―4―Influence of Tsunamis on Coastal Lowlands

2. Influence of tsunami on the lowland
At Sendai Bay, the maximum height of tsunami waves was recorded as more than 8.6 m
at the Ishinomaki coast, which is located in the northern part of the bay (Japan
Meteorological Agency: JMA, 2012). Shishikura et al. (2012) examined the distribution oftsunami deposits in the Sendai lowland. According to that study, in the northern part ofthe lowland, sandy deposits reached between 2.71 and 3.40 km inland, and muddy
deposits reached 2.88 to 3.93 km inland. This is in contrast to the limit of inundation,
which was 3.80 to 5.14 km inland (Geographical Survey Institute: GSI, 2014). In thecentral part of the lowland, the inland limits of sandy and muddy deposits were 3.14 km
and 4.54 km, respectively. The limit of inundation was 5.05 km in this area. In the southernpart of the lowland, sandy and muddy deposits were present up to 2.89 km and 3.05 km
inland, respectively, whereas, the limit of inundation was 3.72 km. These findings suggestthat the distribution of tsunami deposits was smaller than the inundated area.
Coastal ridges I to III were all influenced by the tsunami (Haraguchi and Iwamatsu,
2011; GSI, 2014). However, the tsunami reached neither the natural levees along theNatori, Hirose and Nanakita Rivers nor the inner embankments in the backmarshes (Fig.
2). Abe et al. (1990) and Sawai et al. (2007) reconstructed the inundated areas of ancienttsunamis accompanying massive earthquakes in the Sendai lowland, on the basis ofanalyses of historic documents, archaeological excavations and deposits in the lowland.
According to these results, the tsunami from the Jogan Earthquake, which occurred in
869 AD, reached around 4 km inland, causing inundation of the backmarshes behind thecoastal ridges.

III. Ishinomaki Lowland
1. Geomorphological development of coastal ridges
The Ishinomaki lowland, which is a beach ridge plain, is located northeast to the Sendai
lowland and in the lower reaches of the Old Kitakami River, facing the northern part of
Sendai Bay (Fig. 3). The lowland is around 10–20 km in length from east to west and 50
km wide from north to south. Most parts of the Ishinomaki lowland have an elevation―

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above sea level of less than 3 m. Five coastal ridges are distributed about 9 km inlandfrom the present-day coastline (Matsumoto, 1984; Ito, 1999; 2003). The innermost coastal
ridge began to develop during the Holocene transgression (Ito, 2003).

2. Influence of tsunami on the lowland
In the western part of the Ishinomaki lowland, the inland limits of the distribution of
sandy and muddy deposits were 1.87 km and 2.20 km, respectively (Shishikura et al.,2012); however, the limit of inundation was 2.55 km inland (GSI, 2014). Although theouter three coastal ridges were influenced by the tsunami from the Tohoku Earthquake,the tsunami did not reach the areas around the inner two coastal ridges (Haraguchi and
Iwamatsu, 2011; GSI, 2014)(Fig. 3).

IV. Kujukurihama Lowland
1. Geomorphological development of coastal ridges
The Kujukurihama lowland is a typical beach ridge plain of Japan; it is oriented
northeast-southwest, and it is around 60 km in length facing the Pacific Ocean. As thelowland is located on the margin of the Kanto Basin, crustal uplift occurred during theLate Quaternary. All rivers in the lowland flow into the Pacific Ocean and intersect thecoastal ridges. The coastal ridges can be classified into three zones, numbered I to III
from landward to seaward, and are generally better developed in the southwestern than in
the northeastern part of the lowland (Matsubara, 2014)(Fig. 4, 5).
Moriwaki (1979; 1982) examined the geomorphological development in the Kujukurihama
lowland after the culmination of the Holocene transgression around 7,000 cal BP. The
results of that study showed that the coastal ridges had already developed between 6,500
and 6,000 cal BP. Completion of the formation of coastal ridges I, II and III occurred
during the periods 5,500 to 4,500 cal BP, 4,500 to 2,500 cal BP and around 1,500 cal BP,
respectively.
Sampling of the Holocene deposits was conducted along the Magame River in thecentral part of the lowland by Masuda et al. (2001a; b). From geological cross-sections, a―6―

Influence of Tsunamis on Coastal Lowlands
N

mountains and hills

R. Old Kitakami▲

coastal ridges

backmarshes

natural levees

abandoned
channels
embankments
reclaimed lands

Ishinomaki Bay

0

Inundation Limit of Tsunami
from Tohoku Earthquake(Haraguchi and Iwamatsu, 2011)

1 km

Fig. 3 Inundation area of tsunami from Tohoku Earthquake in Ishinomaki Lowland
Landform classification is based on GSI Map (GSI HP)

buried abrasion platform occurs at around −10 m beneath coastal ridge I around 7 km
inland from the present-day coastline. This suggests that at least coastal ridge I developed
on top of an abrasion platform during the Holocene.

2. Influence of tsunami on the lowland
The tsunami from the Tohoku Earthquake also influenced the Kujukurihama lowland.
The maximum tsunami height in this region was recorded as 2.5 m at the Choshi coast,
which is situated at the northeastern end of the Kujukurihama lowland (JMA, 2012).
Fujiwara et al. (2011; 2012) examined the distribution of the deposits carried by the
―

7―

▲

N

Paciﬁc Ocean

hills and uplands

coastal ridges
backmarshes
Inundation Area of Tsunami
from Tohoku Earthquake(Haraguchi and Iwamatsu, 2011)

0

1 km

Fig. 4 ‌Inundation area of tsunami from Tohoku Earthquake in southwestern part of Kujukurihama
Lowland―8―

Influence of Tsunamis on Coastal Lowlands

N▲

Paciﬁc Ocean

hills and uplands
coastal ridges

backmarshes
Inundation Area of Tsunami
from Tohoku Earthquake(Haraguchi and Iwamatsu, 2011)
01 km

Fig. 5 ‌Inundation area of tsunami from Tohoku Earthquake in northeastern part of Kujukurihama
Lowland―9―

tsunami from the Tohoku Earthquake, while Haraguchi and Iwamatsu (2011) and GSI(2014) showed the inundation areas (Fig. 4, 5). Based on these studies, it is suggested
that the tsunami reached the outermost ridge III in the Kujukurihama lowland. Tsunami
deposits were observed at locations behind the outermost coastal ridge, around 1 km
inland from the present-day coastline in the central part of the lowland.

V. Influence of Tsunamis on Coastal Ridges
As the Sendai and Ishinomaki lowlands are located along Sendai Bay, it is consideredthat the scale of the tsunami was large in both areas. However, the tsunami influenced thecoastal ridges differently in these areas. Although all of coastal ridges I to III were
influenced by the tsunami in the Sendai lowland, the tsunami did not reach the areas
around the inner two coastal ridges in the Ishinomaki lowland. The difference was
because the Sendai and Ishinomaki lowlands belong to different types of coastal lowlands.
The landforms of the Sendai lowland are coastal ridge-backmarsh systems. However, in
the Ishinomaki lowland, which is a beach ridge plain, coastal ridges are more strongly
developed than in the Sendai lowland. Thus, it is inferred that the difference in thelandforms of the coastal lowlands caused the differences in the inundation areas.
According to Matsubara (2000; 2003; 2015), the coastal ridges in the lowlands may havebeen important landforms for human settlement during the Holocene. Human activity has
been influenced by the development of coastal ridges and devastating volcanic events.
Time-lags are recognized between the final stages of coastal ridge formation and whenhumans began to advance and settle on the ridges. The time-lags represent the fact that it
took thousands of years for the coastal ridges to become stabilized and free from theinfluence of sea water, such as high waves, high tides and tsunamis. This is supported by
the fact that coastal ridges, particularly the outermost ridges, were influenced by thetsunami following the Tohoku Earthquake.

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Influence of Tsunamis on Coastal Lowlands

VI. Conclusions
The influence of the tsunami from the Tohoku Earthquake on the landforms in three
coastal lowlands; the Sendai, Ishinomaki and Kujukurihama, was examined. The results
can be summarized as follows.
The coastal ridges in each area, particularly the most seaward ridges, were influenced
by the tsunami following the Tohoku Earthquake. All the coastal ridges in the Sendai
lowland, which contains coastal ridge-backmarsh systems, were influenced by thetsunami. However, the tsunami did not reach the areas around the inner two coastal
ridges in the Ishinomaki lowland, which is a beach ridge plain environment. In addition,
the tsunami did not reach natural levees along the rivers and inner embankments in the
backmarshes of the Sendai lowland.
These results suggest that the inundation area of tsunamis is influenced by small-scale
landforms in coastal lowlands.

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