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駿河湾奥部大陸棚の地質構造」
梶 琢 ・根元謙次 ・山崎晴雄 ・生野静香 ・松田健也
“Geological Structure of the Continental Shelf in the Northern Part of Suruga Bay”
Taku KAJI,Kenji NEMOTO,Haruo YAMAZAKI,Shizuka SHONO and Takeya MATSUDA
Abstract
Suruga bay is a structural bay with a considerable number of geographical features.The Suruga Trough runs
from north to south,and the Fujikawa Fan Delta is in the northern part of Suruga Bay.From seismic data on the
continental shelf in the northern part of Suruga Bay,we clarified the formation processes of the geographical features
from detailed sea-beam data and determined the displacement of the basement in the Wurm ice age due to the
subduction of the Philippine Sea Plate. As a result, unique geographical features were found to exist on the
continental shelf in the northern part of Suruga Bay.We distinguished some of the geographical features and assumed
a wavy surface and two gradual slopes.Moreover,we found that the base sank gradually from the region off Ueda
to that off Fuji River from seismic data.However,large displacement of the base was confirmed to the west of Fuji
River.This large displacement of base might show geographical features change from Wurm.
1. Introduction
The northern margin of the Philippine Sea Plate
subducts in Suruga Bay on the west side of the Izu
Peninsula,forming Suruga Trough toward north-north-
east,a deep,long,and slender valley more than 2000m
from Nankai Trough, off the southwestern coast of
Japan(Fig.1).This area has been subject to numerous
investigations, including an investigation using a Shin-
kai 2000 submersible.
The base of the Izu Peninsula is exposed on the
eastern slope, but has sedimentary rocks that are not
highly agglomerated distributed on its western slope in
the Suruga Trough;and its geography comprises repeat-
ing steep slopes and flat plains.(Kato et al.,1983;Kato
and Yamazaki, 1985;Yamazaki and Kato, 1986;Kit-
azato,1988;Koyama et al.,1992;Niizuma et al.,1992;
etc.). Moreover, the topographic high of Senoumi-tai,
which is a part of the fold belt that continues from Udo
Hill to the south,enters along the west side of the trough
(Shiba et al., 1990). Senoumi-tai is thought to be an
outer ridge,while the Senoumi Basin is in the deep-sea
terrace to the west. Thus, the tectonic relief that
develops on the hanging side of the plate subduction
zone is admitted to the west side of the Suruga Trough.
However,the Fujikawa Fan and Ukishimagahara subsi-
dence regions are located on the land side of the Suruga
Trough. Moreover, the active fault zone (Fujikawa
fault belt),where the maximum speed of displacement
occurs,is on the west side of the Fujikawa Fan.These
two active faults that run south-north are responsible
for the remarkable geographical features at the bound-
aries between the mountains, hills, and plains. These
mountains are composed of Fujikawa Group and Kan-
bara Conglomerate,which are thick sediments from the
第6巻第2号(2008)
東海大学紀要海洋学部「海―自然と文化」 第6巻第2号 1-14頁(2008)Journal of The School of Marine Science and Technology,Tokai University,Vol.6,No.2,pp.1-14,2008
2008年6月9日受理
*1 首都大学東京都市環境科学研究科地理環境科学研究科専攻(Tokyo Metropolitan University/urban environmental science/Geography)
*2 東海大学海洋学部海洋資源学科(Tokai University/Marine Mineral Resources)
*3 首都大学東京都市環境学部地理環境コース地形地質学教室(Tokyo Metropolitan University/urban environmental science/Geography)
*4 ジオ・サーチ株式会社(Geo Search CO.,LTD.)
*5 国際航業株式会社(Kokusai Kogyo CO.,LTD.)
Pleistocene to Lower Pleistocene. Yamazaki (1992)
thought that the terrigenous sediments that had filled
the trough were added to the west side of the subduction
zone.Moreover,the active fault was an imbricate thrust
that diverged from the plate boundary fault to the
hanging side.An accretionary complex appears on the
land surface in the northern part of Suruga Bay.This is
a peculiar region,with a geology that is not commonly
seen worldwide.However,the geographical features of
the active structure are indistinct due to the large
amount of deposit supplied by the Fuji River that forms
the abyssal fan (Sho et al., 1995) on the continental
shelf. The depth of the shelf edge is an index of the
diastrophism,because the continental shelf is a coastal
plain created by the invasion or sedimentation of land
during the period of the maximum decrease in the sea
level during a past ice age.It is known that the eastern
(Izu)side of Suruga Bay is 70±50m deeper than the
western (Omaezaki) side (Nakamura et al., 1989).
However,the depth of the shelf edge,which is subject to
local sedimentation and the process of erosion,does not
show the same period of the formation.The flat reflec-
tion surface distributed from the neritic region to the
shelf edge across the entire area of the continental shelf,
which is seen in the seismic data off Udo Hill, was
defined as reflection L (Nemoto et al., 1988). We
defined same surface as reflection L to planation surface
of Wurm maximum,the distribution was pursued,and
the amount of diastrophism was clarified quantitatively
in this research. The purpose of this research was to
clarify the distribution of the planation surface from
detailed bathymetry and seismic data on the continental
shelf in the northern part of Suruga Bay,and to clarify
the amount of diastrophism.
2. Research method
Fig.1 shows the study area in the northern part of
Suruga Bay.The continental shelf is not well developed,
and numerous small submarine canyons exist on the
narrow continental shelf from the coast off Fujikawa to
Tagonoura (Misawa, 1993, 1994). Misawa (1993)
東海大学紀要海洋学部「海―自然と文化」
Taku KAJI,Kenji NEMOTO,Haruo YAMAZAKI,Shizuka SHONO and Takeya MATSUDA
Fig.1 The location map of study area.
defined a gulley as a valley where the toe of the slope
disappears at the middle of the continental slope(-400
~-600m depth),and a channel as where several valleys
join and continue to the Suruga Trough.These gullies
and channels developed as roads that supply sediment
from Fujikawa to the ocean.Moreover, the upper flat
surface(0~-30m)and lower flat surface(-50~-120m)
are identified on the continental shelf in the eastern part,
off the coast of Ueda (Fig.2a).The edge of the lower
flat surface corresponds to the continental margin.
Misawa (1994)divided the deposit on the continental
shelf in this region into four layers by using seismic
reflection data;these corresponded to the upper part of
the alluvium,the central part of the alluvium,and the
region under the alluvium.The distribution of the ero-
sion surface is also known in the northern part of
Suruga Bay: it exists at 130m off Sagara (Ishii and
Nemoto,1995),130±10m off Ookuzure(Yamamoto et
al.,1998),110±10m off Udo Hill(Nemoto et al.,1998;
Ishii and Nemoto, 1998), 105±5m in Uchiura Bay
(Nemoto et al.,1989).We compared reflection L with
the unconformity surface formed during the Wurm
glacial maximum, because this surface is a smooth
erosion surface distributed at about-100m overall.This
erosion surface was formed in the vicinity of the coast-
line during the period of relative stability in the sea level
at the Wurm glacial maximum (20000 BP);it is found
worldwide(Motegi,1970).Therefore,it is thought that
the edge depth of the erosion surface is the same as that
when the erosion surface was formed. In other words,
the amount of diastrophism that has taken place since
the last ice age can be presumed by clarifying the depth
distribution of the erosion surface.
In this research,we used bathymetric data collected
by Chubu Regional Bureau, Ministry of Land, Infras-
tructure,Transport and Tourism,from November 2001
to February 2002. This data was obtained with an
EM3000 multibeam sounding device,which can collect
very detailed data at a resolution of a few meters;the
traverse line interval is 100~500m.We also used seismic
reflection data collected by the Japan Coast Guard in
Suruga Bay in 1977;the traverse line interval is about
1km.
3. Result
3-1. Geographical features
We divided the area of study into two segments
based on the differences in their geographical features:
the eastern and western parts of Tagonoura (Fig.1).
・Continental shelves off Hara-Tagonoura
We created a grid with a lattice interval of 4m from
the multi fan-beam data,and also plotted a bathymetric
map with a contour interval of 10m (Fig.2a). Cross-
Geological Structure of the Continental Shelf in the Northern Part of Suruga Bay
Fig.2 (a):The bathymetric map off Hara-Tagonoura.(b):The topographical cross section off Hara-Tagonoura.
第6巻第2号(2008)
sections of the geographical features with a fineness
ratio of 1:3 (VE×3)were also created from the grid
(Fig.2b).The cross-sections were labeled BP-1(Bath-
ymetric Profile-1)through to BP-14 from east to west
(Fig.2a). In addition, the relief map off Hara-
Tagonoura viewed from the southwest is shown in Fig.3.
The four conversion points are clearly seen in the
cross-sections (Fig.2a):their depths are -10,-30, -50,
and-120m.Thus,two flat surfaces exist in this area:one
is the slope from-30 to-50m with an average inclination
of 100/1000,and the other is from-50 to-120m with an
average inclination of 60/1000. The slopes from the
shore line to-10m and from-30 to-50m have an inclina-
tion of 270/1000 and 830/1000, respectively. The shelf
edge is at a depth of-120m,beyond which is the slope of
the continental shelf.The two flat surfaces(-10~-30m
and -50~-120m)are remarkable off Hara (Fig.2b;
BP1-6),but the lower flat surface does not develop off
Ueda in the western part of Hara(Fig.2b;BP7-14).The
upper flat surface(-10~-30m)and lower flat surface
(-50~-120m)correspond to the upper layer and lower
layer, respectively, in Misawa’s (1993) classification.
The upper flat surface is also seen in the relief map as
a gradual surface (Fig.3);it has a width of 200m off
Hara, 200m off Ueda, and 150m off Tagonoura. The
edge of the continental shelf can be seen to have a wavy
surface at the depth of-120m.The edges of the upper
flat surface off Hara, Ueda, and Tagonoura are at
depths of-20,-30,and-20m,respectively.The widths of
the lower flat surface off Hara and Ueda are 530 and
225m, respectively. In the western part off Ueda, the
lower surface disappears,and instead,a wavy surface,
seen as a repetition of the valleys and ridges in the
bathrymetric map,develops (Fig.2a).The valleys and
ridges of the wavy surface have a strike that is aligned
north-south. The ridges have a width (EW)of 200~
300m,amplitude of 250~300m,and height of 10~30m.
They develop from depths of-30m off Ueda(BP-6),and
increase in scale and amplitude toward the west (Figs.
2a and 3). These wavy surfaces are seen as ridges
(BP-7 and 10)and valleys (BP-9, 11, and 13) in the
cross-sections. However, these geographical features
are not seen off Tagonoura,where the continental shelf
does not develop at all.
・Continental shelves off R.Fuji
The bathymetric map with a contour interval of
10m(Fig.4),the cross-section of the north-south strike
(Fig.5),and relief map (Fig.6)are shown.The relief
map (Fig.6)shows the different characteristic to the
east and west of the boundary off Samejima and the Fuji
River. The wavy surface is continuous off Samejima
from Tagonoura,and its ridges have a maximum length
of 1.3km,width (EW)of 400m,and height of 30~50m
(Fig.4). The valleys between the ridges develop from
depths of -30m off Samejima (Fig.5). However, the
forms of the ridges are greatly different in the western
part off Samejima. An uneven surface that has three
large valleys and ridges exists on the continental shelves
and slopes off Fuji River.The ridge forms the triangular
pyramid type that the top part is sharpened(Fig.5;BP
-19),and the valley on both sides of it is gentle(Fig.5;
BP-21).This uneven surface is distributed from -70 to
-250m,and has a larger ridge with a maximum width
(WE) of 600m and a maximum height of 120m. A
Fig.3 The relief map off Tagonoura-Hara.
東海大学紀要海洋学部「海―自然と文化」
Taku KAJI,Kenji NEMOTO,Haruo YAMAZAKI,Shizuka SHONO and Takeya MATSUDA
swollen surface larger than the wavy surface is seen
between the gentle valleys on the continental shelves off
Fukiagenohama.This is distributed from -30 to -250m,
and its ridge has a maximum width of 1.2km and a
maximum height 120m.The uneven surface forms the
triangular pyramid type that has sharpened top(Fig.5;
BP-19)off Fuji river,but swollen surface forms the top
which is smoothness off Fukiagenohama in the relief
map (Fig.6).
The two flat surfaces (upper and lower flat sur-
faces)exist in the eastern part off Ueda;however, a
wavy surface develops instead of the lower flat surface
in the western part off Ueda.The lower flat surface is
seen in all the regions;and the wavy surface also exists
in all regions except the regions off Tagonoura and the
eastern part off Samejima.The continental shelves are
Fig.4 The bathymetric map off Fuji River.
Fig.5 The topographical cross section of off Fuji.
第6巻第2号(2008)
Geological Structure of the Continental Shelf in the Northern Part of Suruga Bay
not developed at all off Tagonoura.The uneven surface
consists of the triangular pyramid type ridge with a
sharpened top and a gentle slope off Fuji River. The
swollen surface consists of the swollen ridge and gentle
slope off Fukiagenohama.
3-2. Geological structure of the continental shelves
A strong reflection is observed in the seismic reflec-
tion data for the layers of the continental shelves.This
reflection continues smoothly over the whole area of
Suruga Bay.It is thought that this strong reflection is an
erosion surface because it forms a smooth surface in the
vicinity of the shelf edge marked by the sea level during
the Wurm ice age(20000 BP),and it is compared with
reflection L (Nemoto, 1988). In this research, we
assumed this erosion surface to be an acoustic basement
and especially examined the changing depth of the
basement edge.The sediment on this acoustic basement
is defined as the continental shelf sediment in this study.
The positions of the recordings are shown by the solid
lines in Figs.3 and 6,and by SP①~SP on the solid
line corresponding to the number of seismic reflection
recordings.
・Continental shelves off Hara-Tagonoura
The six records(SP-1~SP-6)of the seismic reflec-
tion and its interpretation charts are shown in Fig.7.A
distinct reflection is seen in the vicinity of-110~-120m
in all sections, and this is assumed to be the acoustic
basement. The seismic reflection records are shown
from the vicinity of-30~-40m,and the structure of the
upper flat surface seen in the geographical cross-section
is not shown.The arrow in the figure indicates the edge
of the acoustic basement (Fig.7). The depths of the
basement edges off Hara (SP1,2)and Ueda(SP3)are
-115m and -120~-125m,respectively.Continental shelf
sediment reaching a maximum thickness of 90m is
distributed on the acoustic basement.A strong reflection
that follows the acoustic basement is seen at-80m in the
continental shelf sediment;this is divided on the bound-
ary of -80m into Layer 2 (lower layer) and Layer 1
(upper layer).Layer 2,with a thickness of 40m,forms
the lower flat surface in SP-1 and SP-2 from the coast
off Hara to Ueda;Layer 1 is 30m in thickness and is
piled above Layer 2. However, as Layer 1 reaches 40
~50m above Layer 2, its sediments continue to the
continental slope(Fig.7;SP4-6).
・Continental shelves off Fuji River
Seven seismic reflection records and the interpreta-
tion charts for SP-13(off Fukiagenohama)to SP-7(off
Samejima) are shown in Figs.8 and 9. The spatial
Fig.6 The relief map off Fuji River-Fukiagenohama.
東海大学紀要海洋学部「海―自然と文化」
Taku KAJI,Kenji NEMOTO,Haruo YAMAZAKI,Shizuka SHONO and Takeya MATSUDA
Fig.7 Seismic profiles and interpretations off Hara-Tagonoura (SP1-6).
第6巻第2号(2008)
Geological Structure of the Continental Shelf in the Northern Part of Suruga Bay
Fig.8 Seismic profiles and interpretations off Samejima-Fuji(SP7-10).
東海大学紀要海洋学部「海―自然と文化」
Taku KAJI,Kenji NEMOTO,Haruo YAMAZAKI,Shizuka SHONO and Takeya MATSUDA
distribution pattern of the sediments is quite different
from that in the eastern part of SP-6,and the continen-
tal shelves are wider in the western part of SP7 than in
the eastern part of Tagonoura. Layer 2 (20-40m in
thickness)and Layer 1(20m in thickness)pile up to the
continental margin on the flat basement that is distribut-
ed widely off Samejima. Layer 1 is very thick, and
reaches the continental margin with the maximum layer
in the seismic profile having a thickness of 60m (SP-9,
10)off Fuji River.The wavy surface and uneven surface
are seen on the continental slope in the seismic profile
(SP-9~13).However,the reflection from the inside of
Layer 1 is hardly seen (SP-9~11),because the reflec-
tion from the surface strengthens off Fuji River.
The wavy surface off Samejima (SP-7,8)and the
uneven surface off Fuji River(SP-9,10)have different
geographical features and scales in the seismic profiles
of the coast off Fuji River from Samejima(Fig.8;SP-
7~10).The uneven surface off Fuji River(SP-9,10)is
largest than the wavy surface off Samejima (SP-7,8).
The maximum thickness (60m)of the sediment layer
off Fuji River on the basement is more developed than
off Samejima(20~40m in thickness;Fig.8).The depth
of the basement margin is -105~-125m off Samejima
(SP-7,8)and -105m off Fuji River (SP-9, 10;Fig.8).
However, the depth of the basement margin off
Fukiagenohama(SP-12,13)is-90m,and its distribution
is shallower than that in the eastern part. In addition,
the fault that displaced the basement and a part of
continental shelf sediment are seen off Fukiagenohama
in SP-13(cross-section of ridge).
Fig.9 Seismic profiles and interpretations off Fukiagenohama (SP11-13).
第6巻第2号(2008)
Geological Structure of the Continental Shelf in the Northern Part of Suruga Bay
4. Discussion
・The topography of the continental shelf and the
sediment structure
The topographical features are obviously different
to the east and west of the boundary of Tagonoura in
this research.The features in each region are collective-
ly shown in Table 1: the edge depth of the acoustic
basement and the distance(that is assumed to be width
of the continental shelf)from the edge of the acoustic
basement to the present shore lines are shown in Fig.10.
The width of the continental shelf is different to the east
and west of the boundary of Tagonoura (Fig.10 and
Table 1).The width of the continental shelf is narrow
(500~800m)to the east of Tagonoura,while,the west
of Tagonoura has a continental shelf that is consider-
ably wider(700~1050m).The upper flat surface and the
lower flat surface of the bathymetric map and cross
section off Hara (Figs.2a and b)can respectively be
compared with Layer1 and Layer 2 in the seismic reflec-
tion record(Fig.7;SP1 and 2).Moreover,the lower flat
surface disappears, and instead, the wavy surface
develops from the coast off Ueda to Tagonoura. The
sediments (Layer1) develop to the shelf edge in that
region (Fig.7; SP3-6). In addition, the width of the
continental shelf is very narrow from the region off
Ueda to Tagonoura (Fig.10). In short, the lower flat
surface (Layer 2, -50~-120m) is not seen, the upper
surface of the sediment is contiguous with the continen-
tal slope when the width of the continental shelf is
narrow(SP4-SP6),and the continental shelf sediment
(Layer1) on the basement develops toward the shelf
edge.On the other hand,the lower flat surface develops
when the width of the continental shelf is wide, and
continental shelf sediment (Layer 1)does not develop
toward the edge of the continental shelf (Fig.7, SP-1,
and SP-2).This sediment (Layer 1)that is contiguous
with the continental shelf slope is shown in Fig.7(SP5
and 6), and the sediment with the irregular reflection
can be seen on the continental shelf slope.The sediments
in SP-5 and 6 are thought to be the slump sediments that
slipped down to the continental shelf slope by landslides.
The topography of wavy surface with repeated valleys
and ridges is possibly formed by landslides.Therefore,a
lower flat surface remains in -50~-120m because it is
Fig.10 Distribution of the acoustic basement (Wurm maximum)margin and width of continental shelves.
▲ :Acoustic basement margin ■ :Width of continental shelves
東海大学紀要海洋学部「海―自然と文化」
Taku KAJI,Kenji NEMOTO,Haruo YAMAZAKI,Shizuka SHONO and Takeya MATSUDA
not buried so much by the continental shelf sediment on
the basement off Hara.However,off Ueda to Tagonour-
a,the continental shelf is narrow and sediment (Layer
1)on the basement continues to the continental slope,
while a part has slipped down:this forms wavy surface.
This wavy surface was not formed by present processes,
but by the sea level rise during the postglacial age,
because it is thought that the present sedimentation is
the uppermost layer from the coastline to -10m that is
shallower than wave base (Fig.2b). The sediments
(Layer 1)on the continental shelf reach from the conti-
nental shelf edge to the continental slope in the seismic
reflection profiles of the off Samejima and off R.Fuji
(Fig.8).It is thought that the sediments on the continen-
tal shelf reach the continental slope because R.Fuji that
is the supply source of the sediment is near though the
width of the continental shelf is overall wide compared
with eastern part of Tagonoura. The continental sedi-
ments reach the continental slope and piles up thick
(Fig.10,Table.1),because the width of the continental
shelves decreases to off R.Fuji from Samejima(Fig.8;
SP-9,10).The ridge of the triangular pyramid type on
the slope of the continental shelf where this is shown in
SP9 and 10 as a result is formed.In addition,the gentle
valley between the ridge and the ridge is thought that is
the route to transport the deposit from Fujikawa to the
trough bottom,and the cause of unclearness an internal
structure by diffused reflection of the seismic reflection
profile (SP9-11)is a large amount of gravel supplied
from Fuji River.The swelled surface was admitted off
Fukiagenohama(Fig.4).The existence of the fault that
cuts continental sediments(Layer 1)on the continental
shelves and the acoustic basement like the block can be
admitted by using the seismic reflection profile that cut
from east to west (Fig.9; SP-13). In a word, it is
thought that the acoustic basement was displaced by
some diastrophism.Table1 (T-①~④)shows the rela-
tionship between the continental shelf sediments
(Layers 1and 2) and the width of the continental
shelves.
・Relation between width of continental shelf and
displacement of acoustic basement
The tendency of distribution of basement depth and
the width of the continental shelves turns on the bound-
ary of Tagonoura.The width of the continental shelves
has narrowed from Hara to Tagonoura. The distribu-
tion of basement depth gradually becomes deep from
Hara to Tagonoura,and it is the deepest at Tagonoura.
Table 1 Conclusion of bathymetric feature and distribution.
第6巻第2号(2008)
Geological Structure of the Continental Shelf in the Northern Part of Suruga Bay
In a word,Tagonoura where the continental shelves don’
t develop at all is a region that subsides most. The
basement depth becomes gradually shallow in the west-
ern part of Tagonoura except a part of Samejima. In
addition,the width of the continental shelves is wider in
western part of Tagonoura than eastern part of
Tagonoura.The relation between the width of continen-
tal shelves and the displacement magnitude is admitted,
because the width of continental shelves is wide at
upheaved region,on the other hand the width of conti-
nental shelves is narrow at subsided region.The cause is
not clear though it is thought that the difference of a
regional environment is a cause in a part of exception
region.
・The distribution depth of the basement and displace-
ment magnitude
The depth of the edge of the acoustic basement
become gradually deeper from Hara (-115m)to Same-
jima (-125m). However, the west side of R.Fuji and
Fukiagenohama become gradually shallower from -105
to -90m.The displacement is 35m,if depth of the base-
ment edge compared Fukiagenohama (-90m) with
Samejima(-125m).It is thought that this value obvious-
ly contains the amount of diastrophism. The displace-
ment is a possibility of the sea side extension fault of
Fuji River Fault Group including the Iriyamase Fault,
because Iriyamase Fault that elevates the west side on
land of the shallowest basement off Fukiagenohama.
However,displacement magnitude (1.75m/ky)of 35m
every 20ky might be too smaller than displacement
magnitude(7m/ky)of Iriyamase fault, if it is thought
the continuation of Iriyamase fault. Therefore, two
possibilities are thought as a cause of those differences.
First, the displacement magnitude on the sea side is
smaller than the land side.Secondarily,there is a possi-
bility that great fluctuations have occurred also in the
western part of this research region. These upheaval
tendency of the basement is same as a thrust structure
like stairs by sinking of Philippine Sea Plate on the west
side of the trough, and it is displayed as the displace-
ment of the superficial geographical features.
5. Summary
1. The geographical features of the continental shelves
in the northern part of Suruga Bay can be divided
into four kinds (upper flat surface and lower flat
surface off Hara,upper flat surface and wavy sur-
face of the section from Ueda to Samejima, upper
flat surface and wavy surface off Fuji River,upper
flat surface and swollen surface off Fukiageno-
hama).These geographical features on the continen-
tal shelves (upper flat surface, lower flat surface,
wavy surface,swelled surface)were formed by the
relation between the change in the thickness of the
continental sediment (Layers 1 and 2)on the base-
ment and the width of the continental shelves.
2. A relation between the width of continental shelves
and the displacement magnitude is observed. In
short,the width of the continental shelf is greater at
the upheaved regions,while it is narrow at the subsid-
ed regions.
3. The erosion surface(20000 BP)of the Wurm maxi-
mum inclines gradually from east (off Hara)to off
Samejima(-115~-120m);it upheaves up to-90m in
the maximum on the west side of Samejima.These
features are the same as the reverse fault structure
on the land side and the stair structure on west side
of the trough, which are caused by plate sinking.
Thus,the fault structures of Fuji and the stair struc-
tures on the west side of the trough are also seen on
the continental shelf in the northern part of Suruga
Bay.This displacement is a result of the east-west
compression due to the subsidence of the Philippine
Sea Plate.
6. Acknowledgments
We appreciate the generosity of the Japan Coast
Guard in making the seismic data available to us.We
also thank our colleagues at the Chubu Regional Bureau,
Ministry of Land, Infrastructure,Transport and Tour-
ism for their support with the multibeam data.We wish
to express our deep gratitude to the abovementioned
organization and individuals.
References
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第6巻第2号(2008)
Geological Structure of the Continental Shelf in the Northern Part of Suruga Bay
要 旨
駿河湾は,フィリピン海プレート北縁のプレート沈み込み境界である駿河トラフが存在する構造性の湾である.駿河湾
の海底地形については潜水船をはじめ多くの研究がなされてきた.その結果,駿河トラフの東側斜面には伊豆半島の基盤
が露出するのに対し,西側斜面には固結度の高くない堆積岩が分布し,急斜面と平坦面が繰り返すスラスト構造が認めら
れる.また,駿河トラフ西側には石花海堆,石花海海盆といった外縁隆起体や前弧海盆がみられる.このように駿河トラ
フを境に東西の地質構造が異なることが知られている.同様にして,駿河トラフ延長の陸側の構造も調査されており,富
士川周辺地域には南北走向の断層帯が認められている.しかし,富士川前面の大陸棚から大陸棚斜面にかけては,富士川
からの大量の堆積物供給による富士川扇状地が発達しており,駿河トラフと陸側との接続部の地質構造については良く知
られていない.本研究では,シービームによって得られた詳細な海底地形データと音波探査データをもとに,駿河湾奥部
の大陸棚から大陸棚斜面にかけての地質構造を明らかにした.特に音波探査断面よりWurm最終氷期の浸食平坦面の分
布を明らかにし,Wurm以降の構造運動を明らかにした.その結果,富士川沖以西で音響基盤が隆起していることが認
められた.これは駿河トラフ西側の階段状構造や富士川河口断層帯の構造と調和的である.また,海底地形データも用い
ることにより,堆積構造と地形の関係についても考えた.
東海大学紀要海洋学部「海―自然と文化」
Taku KAJI,Kenji NEMOTO,Haruo YAMAZAKI,Shizuka SHONO and Takeya MATSUDA