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Crustal structure of the Bonin Trough
Tectonophysics,
345
181 (1990) 345-350
Elsevier Science Publishers B.V., Amsterdam
Crustal structure of the Bonin Trough H. Katao ‘, R. Hino 2, H. Kinoshita ’ Earthquake Research instifute, ’ Observation
3 and S. Nagumo
‘,*
University of Tokyo, Tokyo 113 (Japan)
Center for Prediction of Earthqaakes and Volcanic Eruptions, Faculty of Science, Tohoku University, Sendai 980 (Japan) ’ Deparimeni of Earth Sciences, Chiba University, Chiba 260 (Japan)
(Received May 6,1989; revision accepted November 3, 1989)
ABSTRACT Katao, H., Hino, R., Kinoshita, H. and Nagumo, S., 1990. Crustal structure of the Bonin Trough In: M. Kono and B.C. Burchfiel (Editors), Tectonics of Eastern Asia and Western Pacific Continental Margin. Tectonopbysics, 181: 345-350 A seismic refraction study using OBSH and explosives was carried out in the Benin Trough. The P-wave velocity model along the trough axis consists of 4 layers; the velocities, VP, of these layers are from top to bottom 2.0, 3.6, 6.3 and 7.8 km s-’ respectively. The thickness of the crust is 13 km at the northern part of the trough, and becomes thicker toward the south. The negative free-air anomaly pattern in the trough can be explained by thickening of the crust.
Introduction The Bonin area has many interesting features. The seismicity pattern and the dip of deep seismic plane of the Izu-Bonin Arc changes at the north end of the Bonin area. The Benin Plateau and a line of seamounts collide with the Izu-Benin Trench along the southern part of the Bonin Trough. One of the largest free-air gravity anomalies in the world is observed over the Benin Ridge. The Bonin Trough is developed between the (young volcanic) Shichito-Iwoto Ridge and the (old fore-arc) Bonin Ridge. The depth of the trough is mostly more than 4000 m, and the bottom has smooth topography. The Bonin Trough is interpreted to have formed in Tertiary time, but is presently inactive (Honza and Tamaki, 1985). The structure and origin of the trough is not well understood. The determination of the fine seismic structure of the trough is
* Present address: Hawaii Institute of Geophysics, University of Hawaii, Honolulu, HI 96822, USA. ~~*1951/~/$03.50
Q 1990 - Elsevier Science Publishers B.V.
needed to decipher the tectonic history of this area. Few refraction studies have been carried out in the Izu-Bonin Arc region. A crustal section across the northern part of the Izu-Benin Arc and Trench along latitude 32”N was presented by Hotta (1970). The crustal structure along latitude 23.5 o N (the junction area of the Mariana Arc and the Bonin Arc) was obtained by Murauchi et al. (1968). Tanahashi et al. (1981) carried out a sonobuoy refraction survey to resolve the shallow structure in the Bonin Trough. In the late fall of 1987, as a part of the Japanese Lithosphere Research Program (DELP: Dynamics and Evolution of the Lithosphere Project), geophysical and geological research cruises were made in the Bonin area (Japanese DELP Research Group on Back-arc Basins, 1989). During the DELP 1987 cruises, seismic refraction experiments were carried out in the Bonin Trough along two perpendicular profiles (Hino et al., 1989; Katao et al.a, 1989). They were named Line A (trending north-south) and Line B (trending east-west). In the following, we report the preliminary results for the N-S trending line (Line A; Fig. 1).
H. KATAO
346
Experiment and processing
calculated time
The objective derive
of the survey
the seismic
structure
on Line A was to
along
the trough
from the sea floor to the uppermost ocean
bottom
(OBSHs) along
seismometers
(BT-1 to BT-7, Table
the 335 km of Line
free-fall
pop-up-type
(BT-6D)
was equipped
system
(Kasahara
were equipped
mantle.
with
Eight
A. All of them One
a digital
et al., 1985),
and
with direct analogue
were
fired
along
about
3.5 km. Explosives
from
this
a running
line.
the others sys-
size was 5-20 kg) were
Spacing
research
of the shots
All were
tems. (Nagumo et al., 1982) Large explosive charges were used as controlled seismic sources. Four large explosions (charge size 300-400 kg) and 91 small shots (charge
by LORAN-C system.
was
prior stored
observed
by the
vessel. Lo-
were determined
combined
ac-
with the NNSS
All the OBSHs data
were
on a series
processed
by means
supported
were success-
converted
by a graphic
to digital
The digital
of magnetic
tapes,
of a mini-computer display
data and
system
system.
An example of the seismograms is presented. All show the vertical component of the geophone without
an amplitude
Figure
correction.
2a is a record
BT-7 which is located A. The reduced
vessel.
fracted
were
the arrival
the research
to data processing.
were fused and dropped Shot times
wave
and
after the operation.
analogue
records
recording
speed
of shots and OBSHs
fully retrieved
OBSH
recording
cations
water
towed behind
navigation
1) were deployed
the ship’s
of the direct
hydrophone curately
hydrophones
instruments. with
axis
from
ET AL.
phase
section
velocity from
obtained
at the northern lower
at station end of Line
is 8.0 km sP ‘. The recrust
appears
as first
30”N
27’N
14.1”E Fig. 1. Location
of DELP 1987 refraction
experiments.
Solid circles represent
positions
143”E of the OBSHs
used in this study.
CRUSTAL
STRUCTURE
TABLE
OF THE BONlN
347
TROUGH
1
Location
of OBSHs
and periods
of observation.
Latitude
Longitude
Depth
Period (JST)
N
E
(m)
deployment
BT-1
26O 01.95’
141”41.08’
3455
Nov. 12
3:30
Nov. 17
19:19
BT-2
26O37.95’
141°41.08’
3650
Nov. 11
23:20
Nov. 17
23:49
BT-3
27 a 01.99’
141O33.43’
4005
Nov. 11
19:20
Nov. 18
4:38
BT-4
27O 37.99’
141 o 29.70’
4140
Nov. 11
15:00
Nov. 18
9:16
BT-5
28’= 07.98’
141O25.96’
4160
Nov. 11
11:30
Nov. 18
14:30
28O40.64’
141O22.02
4055
Nov. 11
3124
Nov. 18
19:20
28 O40.74’
141O22.12’
4055
Nov. 11
3:18
Nov. 18
19:15
29°08.00’
141O18.43’
4010
Nov. 10
18:15
Nov. 18
23:34
Station
BT-6 BT-6D
*
BT-7 * Digital
breaks traced
retrieval
OBSH.
at distances beyond 20 km, and can be up to a distance of about 60 km. The
slightly less than 8.0 km SC*, and the intercept time of P, is about 8 s. The record section obtained at BT-4 which is
apparent velocity of this phase is about 6.0 km s-r, and increases continuously with distance. The P,, phases are observed at distances beyond 60 km
located at the center of the line is shown in Fig. 2b. The reduced velocity is 8.0 km s-‘. The P,
from the station. Although amplitude of the P,, phase is very small, it can be detected clearly by the large explosions. The apparent velocity of P, is
phase is observed beyond 60 km from the OBSH, and has an intercept time of about 8 s similar to that of BT-7.
a
BT-7
b
BT-4
BT-1
C
0
Fig. 2. Examples explosions
of record
sections.
along Line A. for station
The reduced
velocity
is 8.0 km SC’ with no amplitude
BT-7 with only the vertical Record
component
section of station
correction.
of the geophone. BT-1.
(a) Record
(b) Record
section
section of dynamite for station
BT-4. (c)
H. KATAO
348
The record section at BT-1 which is located at southern end of the line is shown in Fig. 2c. Although the records are noisy due to bottom currents, the P,, phase can be seen beyond about 80 km from the OBSH. The intercept time of P,, increases to about 9 s, and suggests that the Moho discontinuity is deeper below the southern part of the trough.
w
Cross polrlt
N
Distance
too
(kmf
0
too
200
lb)
E ET14 --1
\1
5 H - 10 II z 2 15
(6.5)
(6.5)
1 20 km
Fig. 4. Crustal
Modeling of the velocity structure was performed using two-dimensions ray tracing. Models were refined by forward modeling, until the calculated travel times coincided with the observed travel times within 0.1 s. An example of a ray tracing is shown in Fig. 3. High-density airgun shooting was not carried out along Line A. multi-ch~el reflection surveying also were not carried out along the northern half of Line A. Therefore, it was impossible to
BV3
O BTl2
20’
Structure modeling
ET AL.
structure
along
Line B obtained
by Hino et al.
(1989).
derive the fine structure of the shallow crust along Line A. A refraction survey using five OBSHs and high-density airgun shooting was carried out along Line B during the same cruise (Hino et al., 1989). The model of the crustal structure along Line B (Fig. 4) was assumed to be similar to the shallower structure of Line A. It is believed that the sediment structure along Line A is more complicated than along Line B. The multi-channel profile shows contamination of transparent layers and opaque material near the trough axis (Abe et al., 1989). The preens model for P-wave velocity (VP) structure along Line A is shown in Fig. 5. This model consists of four layers below the water layer (about 4 km thick, VP= 1.5 km s-r): (1) the sediment layer (about 2 km thick), (2) an intermediate layer (about 2 km thick), (3) a lower crustal layer (lo-14 km thick), and (4) the mantle, P-wave velocities of these layers from top to bottom are 2.0, 3.6, 6.3 and 7.8 km s-r respectively. The Moho depth increases from 17.5 km below Distance
N
0 #-J&l_L_l
j
-
(km)
100 / I ~_._LL.L,Y_II-j-1
1.5 I__
200
300 / / I 1
-
._.
.-.----
m
BT-5 BT-5. (a) Observed ponent adopted.
record
of the geophone Dots
t-_
of two-dimensional
represent
section record.
ray
travel
6.1
com-
correction time.
(b)
.____ i
---%---
for station
with only the vertical No amplitude
theoretical diagram.
tracing
--I
3.6 6.3
3. Example
!
I.5 __^_ __._..____.---~
-----------__
Fig.
S '.I-._
is Bay
Fig. 5. Preliminary Line A. Numerals
model
of P-wave
represent
velocity
structure
P-wave velocity in km s-‘.
along
CRUSTAL
STRUCTURE
OF THE
BONIN
TROUGH
the northern half of the trough to 21.0 km below the southern end of the line. The total crustal thickness ranges from 14 to 18 km. Although the water depth is much deeper in the Bonin Trough, its Moho depth is similar that of other regions of the Izu-Benin Arc (Murauchi et al., 1968; Hotta 1970). The negative free-air gravity anomalies in the Bonin Trough become larger toward the south (Ishihara et al., 1981) although the water depth is shallower in the southern part of the trough. It is difficult to explain this paradox by only a variation of sediment thickness. The gravity anomalies suggest that the Moho is deeper in the southern part of the trough as shown in our seismic model. (Recently, the model along Line B has been refined (Hino, 1989) confirming the main results on Line A.) Discussion
In Fig. 6, the velocity structure of the Benin Trough is compared with that of other basins. The Bonin Trough has an intermediate crustal thickness between continental and oceanic crust. The crustal thickness of the Yamato Basin in the Japan Sea, is 13-15 km, similar to that of the Bonin
Vp (km/s) 2.0
4.0
6.0
6.0
Fig. 6. AverageP-wave velocity structure of the Bonin Trough compared with other ocean basins. References: Yamato Basin, Hirata et al. (1987); North Sea, Barton and Wood (1984).
349
Trough, although the velocity structure of the Yamato Basin shows typical oceanic features (Hirata et al., 1987; Katao, 1988). The crust of the Bonin Trough is dominated by the 6.3 km s-’ layer. The second layer of our model has a P-wave velocity of 3.6 km s-l which is low when compared to typical oceanic basement, and the velocity gradient of the layer is not large when compared to that of typical oceanic layer 2. Even if our second layer consists of consolidated sediments (pyroclastics?), the velocity of the third layer (6.3 km s-r) is higher than the topmost part of typical oceanic layer 2. Thus, the crustal structure of the Bonin Trough is not identical to that of typical oceanic crust. Rather, this model seems to be similar to the structure of stretched continental crust, such as in the Central Graben of the North Sea (Barton and Wood, 1984). In this study, the diving wave through the lower part of the crust continuously increases its apparent velocity, and we cannot identify a velocity boundary in the lower part of the crust. The variation in the velocity gradient of the lower crust must be examined by further studies of amplitude and waveform analyses. The detailed reflection study shows that the basement of the trough dips eastward in a series of steps. This feature can be interpreted to be the result of normal faults formed by tension during the rifting of the trough in Tertiary time (Abe et al., 1989). The spreading in the Bonin Trough probably ceased during rifting of the island arc crust. The deepening of the basement and Moho discontinuity toward south in the trough revealed by the present analyses is in good agreement with the distribution of the negative free-air gravity anomaly (Ishihara et al., 1981). There are two negative depressions in the free-air gravity anomaly, a weak low of - 100 mgal in the northem part and a -200 mgal low in the southern part of the trough. This distribution is interpreted to be due partly to a deeper heavier basement or mantle layer dipping toward the south. A qualitative judgement shows that the seismic structure of the Moho boundary can explain the distribution of the free-air gravity anomaly fairly well. This deepening of the Moho boundary can be partly
n. KATAO
350
explained by some drag force of the subducting Pacific lithosphere underneath this region or by a simple fragmentation of the lithosphere of Philippine Sea plate due to collision with the seamount chain (Bonin Plateau). Conclusion
A seismic structure model along the axis of the Bonin Trough was obtained by refraction experiments using explosives and OBSHs. Beneath the water layer, the top layer of the crust is about 2 km thick with a I$ of 2.0 km s-l. The second layer is about 2 km thick with a VPof 3.6 km s-r. The lower crustal layer is about lo-14 km thick with a VP of 6.3 km s-t. The depth of Moho discontinuity is about 18-21 km below the sea surface. The I$ of the uppermost mantle is 7.8 km s-‘. The sedimentary basement as well as the Moho boundary is deeper in the southern part of the trough. The negative free-air anomary pattern in the trough is explained by the deepening of layer boundaries. Acknowledgements
Authors are greatly indebted to Drs. K. Suyehiro, T. Ouchi, H. Tokuyama, A. Nishizawa, Mrs. S. Koresawa, S. Abe, A. Kubo and all other participants of DELP for their help and valuable suggestions. Thanks are also due to Captain Miki and crew members of R/V “Wakashio-maru”, Nippon Salvage Co. Ltd. for their help and corporation during the surveys. Dr. B.C. Burchfiel kindly reviewed this manuscript. References Abe, S., Tokuyama, H., Kuramoto, S., Suyehiro, K., Nisbizawa, A. and Kinoshita, H., 1989. Report on DELP 1987 cruises in the Ogasawara area, II. Seismic reflection studies in the Ogasawara Trough. Bull. Ear&q. Res. Inst. Univ. Tokyo 64: 133-147. Barton, P. and Wood, R., 1984. Tectonic evolution of the North Sea basin: crustal streching and subsidence. Geophys. J.R. Astron. Sot., 79: 987-1022. Hino, R., 1989. A study on the crustal structure beneath the Benin Trough. MS. Thesis, Tohoku University, Tohoku.
ET AL.
Hino, R., Nishizawa, A., Suyehiro, K., Kinoshita, H.. Abe, S., Ouchi, T., K&o, A., Koresawa, S. and Nagumo, S., 1989. Report on DELP 198’7cruises in the Ogasawara area, Part III. Seismic refraction experiment using airgun-OBSH for fine crustal structure across the Ogasawara Trough along an east-west track line. Bull. Earthq. Res. inst. Univ. Tokyo, 64: 149-162. Hirata, N., Kinoshita, H., Suyehiro, K., Suyemasu, M., Mtsuda, N., Ouchi, T., Katao, H., Koresawa, S. and Nagumo, S., 1987. Report on DELP 1985 cruises on the Japan Sea, Part II. Seismic refraction experiment conducted in the Yamato Basin, Southeast Japan Sea. Bull. Earthq. Inst. Univ. Tokyo, 62: 341-365. Honza, E. and Tamaki, K., 1985. The Benin Arc. In: A.E.M. Nairn, F.G. Stehli and S. Uyeda (Editors), The Ocean Basin and Margins, Vol. 7A. pp. 459-502. Hotta, H., 1970. A seismic section across the Izu-Ogasawara Arc and Trench. J. Phys. Earth, 18: 125-141. Ishihara, T.. Murakami, F., Miyazaki, T. and Nishimura, K., 1981. Gravity survey. In: The Geological Investigation of the Ogasawara (Bonin) and Northern Mariana Arcs, Cruise GH79-2, 3 and 4. Geological Survey of Japan, Tokyo, pp. 45-78. Japanese DELP Research Group on Back-arc Basins, 1989. Report on DELP 1987 cruises in the Ogasawara Area, I. General outline. Bull. Earthq. Res. Inst. Univ. Tokyo, 64: 119-131. Kasahara, J.. Takahashi, M., Matsubara. T. and Komiya, M., 1985. Mass storage digital ocean bottom seismometer and hydrophone (DOBSH) controlled by microprocessors using ADPCM voice synthesizing. Bull. Earthq. Res. Inst. Univ. Tokyo, 60: 23-37. Katao, H., 1988. Seismic structure and formation of the Yamato Basin. Bull. Earthq. Res. Inst. Univ. Tokyo, 63: 51-86. Katao, H., Nagumo, S., Koresawa, S., Hino, R., Nishizawa, A., Suyehiro, K., Ouchi, T., Kubo, A., Ishibashi, M., Ono, Y., Baba. H. and Kinoshita, H., 1989. Report on DELP 1987 cruises, IV. Explosion seismic refraction studies in the Ogasawara Trough. Bull. Earthq. Res. Inst. Univ. Tokyo, 64: 163-177. Murauchi, S., Den, N., Asano, S., Hotta, H., Yoshii, T., Asanuma, T., Hagiwara, K., Sato, T., Ludwig, W.J., Ewing, J.I., Edger, N.T. and Houtz, R.E., 1968Crustal structure of the Philippine Sea. J. Geophys. Res., 73: 3143-3171. Nagumo, S.. Kasahara, J., Koresawa S. and Murakami, H., 1982. Acoustic release ocean-bottom seismometer (ERIAR81). Bull. Earthq. Res. Inst. Univ. Tokyo, 57: 125-132 (in Japanese). Tanahashi, M., Tamaki, K. and Okuda, Y., 1981. Sonobouy refraction study. In: The Geological Investi~tion of the Ogasawara (Boninf and Northern Mariana Arcs, Cmise GH79-2, 92-94.
3 and 4. Geological Survey of Japan, Tokyo, pp.
345
181 (1990) 345-350
Elsevier Science Publishers B.V., Amsterdam
Crustal structure of the Bonin Trough H. Katao ‘, R. Hino 2, H. Kinoshita ’ Earthquake Research instifute, ’ Observation
3 and S. Nagumo
‘,*
University of Tokyo, Tokyo 113 (Japan)
Center for Prediction of Earthqaakes and Volcanic Eruptions, Faculty of Science, Tohoku University, Sendai 980 (Japan) ’ Deparimeni of Earth Sciences, Chiba University, Chiba 260 (Japan)
(Received May 6,1989; revision accepted November 3, 1989)
ABSTRACT Katao, H., Hino, R., Kinoshita, H. and Nagumo, S., 1990. Crustal structure of the Bonin Trough In: M. Kono and B.C. Burchfiel (Editors), Tectonics of Eastern Asia and Western Pacific Continental Margin. Tectonopbysics, 181: 345-350 A seismic refraction study using OBSH and explosives was carried out in the Benin Trough. The P-wave velocity model along the trough axis consists of 4 layers; the velocities, VP, of these layers are from top to bottom 2.0, 3.6, 6.3 and 7.8 km s-’ respectively. The thickness of the crust is 13 km at the northern part of the trough, and becomes thicker toward the south. The negative free-air anomaly pattern in the trough can be explained by thickening of the crust.
Introduction The Bonin area has many interesting features. The seismicity pattern and the dip of deep seismic plane of the Izu-Bonin Arc changes at the north end of the Bonin area. The Benin Plateau and a line of seamounts collide with the Izu-Benin Trench along the southern part of the Bonin Trough. One of the largest free-air gravity anomalies in the world is observed over the Benin Ridge. The Bonin Trough is developed between the (young volcanic) Shichito-Iwoto Ridge and the (old fore-arc) Bonin Ridge. The depth of the trough is mostly more than 4000 m, and the bottom has smooth topography. The Bonin Trough is interpreted to have formed in Tertiary time, but is presently inactive (Honza and Tamaki, 1985). The structure and origin of the trough is not well understood. The determination of the fine seismic structure of the trough is
* Present address: Hawaii Institute of Geophysics, University of Hawaii, Honolulu, HI 96822, USA. ~~*1951/~/$03.50
Q 1990 - Elsevier Science Publishers B.V.
needed to decipher the tectonic history of this area. Few refraction studies have been carried out in the Izu-Bonin Arc region. A crustal section across the northern part of the Izu-Benin Arc and Trench along latitude 32”N was presented by Hotta (1970). The crustal structure along latitude 23.5 o N (the junction area of the Mariana Arc and the Bonin Arc) was obtained by Murauchi et al. (1968). Tanahashi et al. (1981) carried out a sonobuoy refraction survey to resolve the shallow structure in the Bonin Trough. In the late fall of 1987, as a part of the Japanese Lithosphere Research Program (DELP: Dynamics and Evolution of the Lithosphere Project), geophysical and geological research cruises were made in the Bonin area (Japanese DELP Research Group on Back-arc Basins, 1989). During the DELP 1987 cruises, seismic refraction experiments were carried out in the Bonin Trough along two perpendicular profiles (Hino et al., 1989; Katao et al.a, 1989). They were named Line A (trending north-south) and Line B (trending east-west). In the following, we report the preliminary results for the N-S trending line (Line A; Fig. 1).
H. KATAO
346
Experiment and processing
calculated time
The objective derive
of the survey
the seismic
structure
on Line A was to
along
the trough
from the sea floor to the uppermost ocean
bottom
(OBSHs) along
seismometers
(BT-1 to BT-7, Table
the 335 km of Line
free-fall
pop-up-type
(BT-6D)
was equipped
system
(Kasahara
were equipped
mantle.
with
Eight
A. All of them One
a digital
et al., 1985),
and
with direct analogue
were
fired
along
about
3.5 km. Explosives
from
this
a running
line.
the others sys-
size was 5-20 kg) were
Spacing
research
of the shots
All were
tems. (Nagumo et al., 1982) Large explosive charges were used as controlled seismic sources. Four large explosions (charge size 300-400 kg) and 91 small shots (charge
by LORAN-C system.
was
prior stored
observed
by the
vessel. Lo-
were determined
combined
ac-
with the NNSS
All the OBSHs data
were
on a series
processed
by means
supported
were success-
converted
by a graphic
to digital
The digital
of magnetic
tapes,
of a mini-computer display
data and
system
system.
An example of the seismograms is presented. All show the vertical component of the geophone without
an amplitude
Figure
correction.
2a is a record
BT-7 which is located A. The reduced
vessel.
fracted
were
the arrival
the research
to data processing.
were fused and dropped Shot times
wave
and
after the operation.
analogue
records
recording
speed
of shots and OBSHs
fully retrieved
OBSH
recording
cations
water
towed behind
navigation
1) were deployed
the ship’s
of the direct
hydrophone curately
hydrophones
instruments. with
axis
from
ET AL.
phase
section
velocity from
obtained
at the northern lower
at station end of Line
is 8.0 km sP ‘. The recrust
appears
as first
30”N
27’N
14.1”E Fig. 1. Location
of DELP 1987 refraction
experiments.
Solid circles represent
positions
143”E of the OBSHs
used in this study.
CRUSTAL
STRUCTURE
TABLE
OF THE BONlN
347
TROUGH
1
Location
of OBSHs
and periods
of observation.
Latitude
Longitude
Depth
Period (JST)
N
E
(m)
deployment
BT-1
26O 01.95’
141”41.08’
3455
Nov. 12
3:30
Nov. 17
19:19
BT-2
26O37.95’
141°41.08’
3650
Nov. 11
23:20
Nov. 17
23:49
BT-3
27 a 01.99’
141O33.43’
4005
Nov. 11
19:20
Nov. 18
4:38
BT-4
27O 37.99’
141 o 29.70’
4140
Nov. 11
15:00
Nov. 18
9:16
BT-5
28’= 07.98’
141O25.96’
4160
Nov. 11
11:30
Nov. 18
14:30
28O40.64’
141O22.02
4055
Nov. 11
3124
Nov. 18
19:20
28 O40.74’
141O22.12’
4055
Nov. 11
3:18
Nov. 18
19:15
29°08.00’
141O18.43’
4010
Nov. 10
18:15
Nov. 18
23:34
Station
BT-6 BT-6D
*
BT-7 * Digital
breaks traced
retrieval
OBSH.
at distances beyond 20 km, and can be up to a distance of about 60 km. The
slightly less than 8.0 km SC*, and the intercept time of P, is about 8 s. The record section obtained at BT-4 which is
apparent velocity of this phase is about 6.0 km s-r, and increases continuously with distance. The P,, phases are observed at distances beyond 60 km
located at the center of the line is shown in Fig. 2b. The reduced velocity is 8.0 km s-‘. The P,
from the station. Although amplitude of the P,, phase is very small, it can be detected clearly by the large explosions. The apparent velocity of P, is
phase is observed beyond 60 km from the OBSH, and has an intercept time of about 8 s similar to that of BT-7.
a
BT-7
b
BT-4
BT-1
C
0
Fig. 2. Examples explosions
of record
sections.
along Line A. for station
The reduced
velocity
is 8.0 km SC’ with no amplitude
BT-7 with only the vertical Record
component
section of station
correction.
of the geophone. BT-1.
(a) Record
(b) Record
section
section of dynamite for station
BT-4. (c)
H. KATAO
348
The record section at BT-1 which is located at southern end of the line is shown in Fig. 2c. Although the records are noisy due to bottom currents, the P,, phase can be seen beyond about 80 km from the OBSH. The intercept time of P,, increases to about 9 s, and suggests that the Moho discontinuity is deeper below the southern part of the trough.
w
Cross polrlt
N
Distance
too
(kmf
0
too
200
lb)
E ET14 --1
\1
5 H - 10 II z 2 15
(6.5)
(6.5)
1 20 km
Fig. 4. Crustal
Modeling of the velocity structure was performed using two-dimensions ray tracing. Models were refined by forward modeling, until the calculated travel times coincided with the observed travel times within 0.1 s. An example of a ray tracing is shown in Fig. 3. High-density airgun shooting was not carried out along Line A. multi-ch~el reflection surveying also were not carried out along the northern half of Line A. Therefore, it was impossible to
BV3
O BTl2
20’
Structure modeling
ET AL.
structure
along
Line B obtained
by Hino et al.
(1989).
derive the fine structure of the shallow crust along Line A. A refraction survey using five OBSHs and high-density airgun shooting was carried out along Line B during the same cruise (Hino et al., 1989). The model of the crustal structure along Line B (Fig. 4) was assumed to be similar to the shallower structure of Line A. It is believed that the sediment structure along Line A is more complicated than along Line B. The multi-channel profile shows contamination of transparent layers and opaque material near the trough axis (Abe et al., 1989). The preens model for P-wave velocity (VP) structure along Line A is shown in Fig. 5. This model consists of four layers below the water layer (about 4 km thick, VP= 1.5 km s-r): (1) the sediment layer (about 2 km thick), (2) an intermediate layer (about 2 km thick), (3) a lower crustal layer (lo-14 km thick), and (4) the mantle, P-wave velocities of these layers from top to bottom are 2.0, 3.6, 6.3 and 7.8 km s-r respectively. The Moho depth increases from 17.5 km below Distance
N
0 #-J&l_L_l
j
-
(km)
100 / I ~_._LL.L,Y_II-j-1
1.5 I__
200
300 / / I 1
-
._.
.-.----
m
BT-5 BT-5. (a) Observed ponent adopted.
record
of the geophone Dots
t-_
of two-dimensional
represent
section record.
ray
travel
6.1
com-
correction time.
(b)
.____ i
---%---
for station
with only the vertical No amplitude
theoretical diagram.
tracing
--I
3.6 6.3
3. Example
!
I.5 __^_ __._..____.---~
-----------__
Fig.
S '.I-._
is Bay
Fig. 5. Preliminary Line A. Numerals
model
of P-wave
represent
velocity
structure
P-wave velocity in km s-‘.
along
CRUSTAL
STRUCTURE
OF THE
BONIN
TROUGH
the northern half of the trough to 21.0 km below the southern end of the line. The total crustal thickness ranges from 14 to 18 km. Although the water depth is much deeper in the Bonin Trough, its Moho depth is similar that of other regions of the Izu-Benin Arc (Murauchi et al., 1968; Hotta 1970). The negative free-air gravity anomalies in the Bonin Trough become larger toward the south (Ishihara et al., 1981) although the water depth is shallower in the southern part of the trough. It is difficult to explain this paradox by only a variation of sediment thickness. The gravity anomalies suggest that the Moho is deeper in the southern part of the trough as shown in our seismic model. (Recently, the model along Line B has been refined (Hino, 1989) confirming the main results on Line A.) Discussion
In Fig. 6, the velocity structure of the Benin Trough is compared with that of other basins. The Bonin Trough has an intermediate crustal thickness between continental and oceanic crust. The crustal thickness of the Yamato Basin in the Japan Sea, is 13-15 km, similar to that of the Bonin
Vp (km/s) 2.0
4.0
6.0
6.0
Fig. 6. AverageP-wave velocity structure of the Bonin Trough compared with other ocean basins. References: Yamato Basin, Hirata et al. (1987); North Sea, Barton and Wood (1984).
349
Trough, although the velocity structure of the Yamato Basin shows typical oceanic features (Hirata et al., 1987; Katao, 1988). The crust of the Bonin Trough is dominated by the 6.3 km s-’ layer. The second layer of our model has a P-wave velocity of 3.6 km s-l which is low when compared to typical oceanic basement, and the velocity gradient of the layer is not large when compared to that of typical oceanic layer 2. Even if our second layer consists of consolidated sediments (pyroclastics?), the velocity of the third layer (6.3 km s-r) is higher than the topmost part of typical oceanic layer 2. Thus, the crustal structure of the Bonin Trough is not identical to that of typical oceanic crust. Rather, this model seems to be similar to the structure of stretched continental crust, such as in the Central Graben of the North Sea (Barton and Wood, 1984). In this study, the diving wave through the lower part of the crust continuously increases its apparent velocity, and we cannot identify a velocity boundary in the lower part of the crust. The variation in the velocity gradient of the lower crust must be examined by further studies of amplitude and waveform analyses. The detailed reflection study shows that the basement of the trough dips eastward in a series of steps. This feature can be interpreted to be the result of normal faults formed by tension during the rifting of the trough in Tertiary time (Abe et al., 1989). The spreading in the Bonin Trough probably ceased during rifting of the island arc crust. The deepening of the basement and Moho discontinuity toward south in the trough revealed by the present analyses is in good agreement with the distribution of the negative free-air gravity anomaly (Ishihara et al., 1981). There are two negative depressions in the free-air gravity anomaly, a weak low of - 100 mgal in the northem part and a -200 mgal low in the southern part of the trough. This distribution is interpreted to be due partly to a deeper heavier basement or mantle layer dipping toward the south. A qualitative judgement shows that the seismic structure of the Moho boundary can explain the distribution of the free-air gravity anomaly fairly well. This deepening of the Moho boundary can be partly
n. KATAO
350
explained by some drag force of the subducting Pacific lithosphere underneath this region or by a simple fragmentation of the lithosphere of Philippine Sea plate due to collision with the seamount chain (Bonin Plateau). Conclusion
A seismic structure model along the axis of the Bonin Trough was obtained by refraction experiments using explosives and OBSHs. Beneath the water layer, the top layer of the crust is about 2 km thick with a I$ of 2.0 km s-l. The second layer is about 2 km thick with a VPof 3.6 km s-r. The lower crustal layer is about lo-14 km thick with a VP of 6.3 km s-t. The depth of Moho discontinuity is about 18-21 km below the sea surface. The I$ of the uppermost mantle is 7.8 km s-‘. The sedimentary basement as well as the Moho boundary is deeper in the southern part of the trough. The negative free-air anomary pattern in the trough is explained by the deepening of layer boundaries. Acknowledgements
Authors are greatly indebted to Drs. K. Suyehiro, T. Ouchi, H. Tokuyama, A. Nishizawa, Mrs. S. Koresawa, S. Abe, A. Kubo and all other participants of DELP for their help and valuable suggestions. Thanks are also due to Captain Miki and crew members of R/V “Wakashio-maru”, Nippon Salvage Co. Ltd. for their help and corporation during the surveys. Dr. B.C. Burchfiel kindly reviewed this manuscript. References Abe, S., Tokuyama, H., Kuramoto, S., Suyehiro, K., Nisbizawa, A. and Kinoshita, H., 1989. Report on DELP 1987 cruises in the Ogasawara area, II. Seismic reflection studies in the Ogasawara Trough. Bull. Ear&q. Res. Inst. Univ. Tokyo 64: 133-147. Barton, P. and Wood, R., 1984. Tectonic evolution of the North Sea basin: crustal streching and subsidence. Geophys. J.R. Astron. Sot., 79: 987-1022. Hino, R., 1989. A study on the crustal structure beneath the Benin Trough. MS. Thesis, Tohoku University, Tohoku.
ET AL.
Hino, R., Nishizawa, A., Suyehiro, K., Kinoshita, H.. Abe, S., Ouchi, T., K&o, A., Koresawa, S. and Nagumo, S., 1989. Report on DELP 198’7cruises in the Ogasawara area, Part III. Seismic refraction experiment using airgun-OBSH for fine crustal structure across the Ogasawara Trough along an east-west track line. Bull. Earthq. Res. inst. Univ. Tokyo, 64: 149-162. Hirata, N., Kinoshita, H., Suyehiro, K., Suyemasu, M., Mtsuda, N., Ouchi, T., Katao, H., Koresawa, S. and Nagumo, S., 1987. Report on DELP 1985 cruises on the Japan Sea, Part II. Seismic refraction experiment conducted in the Yamato Basin, Southeast Japan Sea. Bull. Earthq. Inst. Univ. Tokyo, 62: 341-365. Honza, E. and Tamaki, K., 1985. The Benin Arc. In: A.E.M. Nairn, F.G. Stehli and S. Uyeda (Editors), The Ocean Basin and Margins, Vol. 7A. pp. 459-502. Hotta, H., 1970. A seismic section across the Izu-Ogasawara Arc and Trench. J. Phys. Earth, 18: 125-141. Ishihara, T.. Murakami, F., Miyazaki, T. and Nishimura, K., 1981. Gravity survey. In: The Geological Investigation of the Ogasawara (Bonin) and Northern Mariana Arcs, Cruise GH79-2, 3 and 4. Geological Survey of Japan, Tokyo, pp. 45-78. Japanese DELP Research Group on Back-arc Basins, 1989. Report on DELP 1987 cruises in the Ogasawara Area, I. General outline. Bull. Earthq. Res. Inst. Univ. Tokyo, 64: 119-131. Kasahara, J.. Takahashi, M., Matsubara. T. and Komiya, M., 1985. Mass storage digital ocean bottom seismometer and hydrophone (DOBSH) controlled by microprocessors using ADPCM voice synthesizing. Bull. Earthq. Res. Inst. Univ. Tokyo, 60: 23-37. Katao, H., 1988. Seismic structure and formation of the Yamato Basin. Bull. Earthq. Res. Inst. Univ. Tokyo, 63: 51-86. Katao, H., Nagumo, S., Koresawa, S., Hino, R., Nishizawa, A., Suyehiro, K., Ouchi, T., Kubo, A., Ishibashi, M., Ono, Y., Baba. H. and Kinoshita, H., 1989. Report on DELP 1987 cruises, IV. Explosion seismic refraction studies in the Ogasawara Trough. Bull. Earthq. Res. Inst. Univ. Tokyo, 64: 163-177. Murauchi, S., Den, N., Asano, S., Hotta, H., Yoshii, T., Asanuma, T., Hagiwara, K., Sato, T., Ludwig, W.J., Ewing, J.I., Edger, N.T. and Houtz, R.E., 1968Crustal structure of the Philippine Sea. J. Geophys. Res., 73: 3143-3171. Nagumo, S.. Kasahara, J., Koresawa S. and Murakami, H., 1982. Acoustic release ocean-bottom seismometer (ERIAR81). Bull. Earthq. Res. Inst. Univ. Tokyo, 57: 125-132 (in Japanese). Tanahashi, M., Tamaki, K. and Okuda, Y., 1981. Sonobouy refraction study. In: The Geological Investi~tion of the Ogasawara (Boninf and Northern Mariana Arcs, Cmise GH79-2, 92-94.
3 and 4. Geological Survey of Japan, Tokyo, pp.