Download Morphology of the distorted subducted Pacific slab beneath the

Physics of the Earth and Planetary Interiors 156 (2006) 1–11
Morphology of the distorted subducted Pacific slab
beneath the Hokkaido corner, Japan
M.S. Miller a,∗ , B.L.N. Kennett a,1 , A. Gorbatov b,2
a
Research School of Earth Sciences, The Australian National University, Mills Road, Building 61, Canberra, ACT 0200, Australia
b Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia
Received 10 May 2005; received in revised form 1 January 2006; accepted 17 January 2006
Abstract
The intersection of the Japan and Kurile arcs is expressed as a cuspate feature at the trench, a bend in the Japanese islands,
and a complex lithospheric structure and is known as the Hokkaido corner. The Pacific plate is subducting beneath the two arcs
in the northwest Pacific at different velocities, which has resulted in an arc–arc collision and distortion of the subducting oceanic
lithosphere. Using P and S wave tomographic images and seismicity the distorted shape of the subducted Pacific plate beneath
the Hokkaido corner can be interpreted in three dimensions. The Pacific plate is imaged as continuous along the Japan–Kurile arc
with dip angle increasing from south to north, but at the arc–arc junction the geometry is complex and appears crumpled. Beneath
the Hokkaido corner there are two features in the tomography that are clearly imaged: a region of low velocity at approximately
50–150 km depth between 142.5–145◦ E and 41–43◦ N and a distorted, buckled lower slab boundary. These two unusual characteristics
in the slab morphology are likely to be related to deformation of the subducted Pacific plate at the arc–arc junction.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Subducted slab; Japan and Kurile arcs; Upper mantle; Tomography; 3D imaging; Arc–arc junction
1. Introduction
Along the northwest Pacific margin the Pacific plate
is converging at a rate of approximately 8.2–9.2 cm/year
to the northwest beneath the Kurile and Japan arcs (Seno
et al., 1993; DeMets et al., 1994). At the Japan arc the
Pacific plate is subducting approximately perpendicular
to the arc, but is subducting obliquely at the Kurile arc,
∗
Corresponding author. Tel.: +61 2 6125 3803 (O);
fax: +61 2 6125 8253.
E-mail addresses: [email protected] (M.S. Miller),
[email protected] (B.L.N. Kennett),
[email protected] (A. Gorbatov).
1 Tel. +61 2 6125 4621 (O); fax: +61 2 6257 2737.
2 Tel. +61 2 6249 9376 (O); fax: +61 2 6249 9999.
0031-9201/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.pepi.2006.01.007
which results in an arc–arc collision. The junction of the
two island arcs is expressed as a sharp cuspate feature
and bend in the northern Japanese islands known as the
Hokkaido corner (Fig. 1). The slab geometry beneath
the Hokkaido corner is very complex and distorted as
a result of the collision of the two arcs. The deformation of subducting plate at the arc–arc junction may be
associated with interplate and lithospheric earthquakes
that occur in the region (Suzuki and Kasahara, 1996;
Katsumata et al., 2003) and expressed in the uplift of
the Hidaka Mountains (Okada, 1983; Miyamachi and
Moriya, 1984, 1987; Miyamachi et al., 1994; Kimura,
1996; Moriya et al., 1998). The unique setting and geological characteristics of the region results in a complicated model, but we present a new interpretation of
the subducting slab morphology beneath the Hokkaido
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M.S. Miller et al. / Physics of the Earth and Planetary Interiors 156 (2006) 1–11
Fig. 1. Map of the major features in northwest Pacific including ETOPO5 bathymetry (1998) contoured in 1 km increments, volcano locations
from the Smithsonian Institute volcano index convergence rates of the Pacific plate (Seno et al., 1993 and Zang et al., 2002), and location of the
cross-sections in Figs. 3 and 4.
corner using 3D ray tracing tomographic inversions and
seismicity.
2. Previous studies
Initial studies of subduction zone structures along
areas of sharp bends, such as the junction of the Japan
and Kurile arcs, suggested the slab would be contorted
or discontinuous (Isacks and Molnar, 1971). Later investigation of the subducting slab morphology beneath the
northwestern Pacific integrated seismicity and seismic
tomography data to define the geometry (Burbach and
Frohlich, 1986; Zhou et al., 1990; Chiu et al., 1991; van
der Hilst et al., 1991, 1993; Fukao et al., 1992; Zhao et
al., 1992; Zhao and Hasegawa, 1993; Miyamachi et al.,
1994; Gudmundsson and Sambridge, 1998; Moriya et
al., 1998). Many of these studies suggested the Pacific
plate changes dip near the Hokkaido corner from 40◦
to 50◦ at the Kurile arc to approximately 30◦ beneath
the Japan arc and that the slab varies in thickness along
the arc. Previous research has investigated the geometry of the slab beneath the northwest Pacific margin, but
few studies have discussed the slab morphology at the
Hokkaido corner in detail.
Chiu et al. (1991) were pioneers in three-dimensional
imaging of subducting slabs in the Western Pacific. They
defined the top of the subducting Pacific plate as an
envelope of selected hypocenters and used this data to
illustrate the slab surface in three dimensions. Their
models for the Japan and Kurile arcs did not illustrate
any distortion in the slab, but detailed the contrast in
subduction angle of the gently dipping central Japan
zone and the steeply dipping Kurile zone in the north.
The transition was featured as a smooth surface with
M.S. Miller et al. / Physics of the Earth and Planetary Interiors 156 (2006) 1–11
curved, fan shaped contours in their models. They found
that shallow to intermediate depth seismicity was not
disrupted at the Hokkaido corner and noted that seismic waves propagated efficiently from events in the
Kurile and Hokkaido areas to the stations on the eastern coast of Hokkaido, therefore suggested a continuous
slab.
An inverse method was applied to P and S wave
data from local events by Miyamachi et al. (1994) to
model the three-dimensional velocity structure beneath
northern Japan. This inversion proposed the top of the
Pacific plate ranged in depth from 50 to 170 km and
the subduction angle was greater beneath Hokkaido
than along northeastern Japan. The upper slab boundary in their model exhibited a gentle curved shape
that approximately mirrored the trench shape, but without a sharp bend often shown at the Hokkaido corner. The estimated thickness of the subducting slab
ranged from 90 km along the Kurile arc and 110 km
along the northern Japan arc from the local earthquake
data.
An alternative approach was used to create a threedimensional model of subducting slabs along the
Western Pacific by contouring slab-related seismicity
to approximate the top of the slab and the bottom
defined as a constant 200 km thick oceanic lithosphere
(Gudmundsson and Sambridge, 1998). The morphology
was simplified since the slab extent was based on earthquakes and a projection of constant thickness, yet slab
complexity and general characteristics of the geometry
along the arc were well defined. The model depicted
the bend at the Japan–Kurile arc junction and the associated change in dip along the plate boundary as a
smooth curved shape in their regionalized upper mantle
model and provided another method to model the slab
morphology.
Early investigations were limited by lower resolution
images making smaller scale structures more difficult to
resolve and made assumptions to simplify the modeling
process. As technology and data quality has improved
more recent studies have been able to define the geometry of the Japan–Kurile arc–arc junction in more detail
using enhanced methodology. Katsumata et al. (2003)
recently investigated the geometry of the deep seismic zone associated with the subducting Pacific plate
beneath the Hokkaido corner using hypocenters from
events between July 1999 and July 2001. Their work
confirmed the change in dip of the subducting plate on
each side of the arc–arc junction, but the lateral transition
of the subduction angle was more significant than in previous models by Miyamachi et al. (1994) and Hasegawa
et al. (1994).
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3. Tomographic controls
The models presented in this study are based on
regional body wave joint tomographic inversion of
Gorbatov and Kennett (2003) together with a new
P-wave tomographic inversion were produced from
the same arrival-time data. The detailed joint inversion datasets used an inversion algorithm introduced
by Kennett et al. (1998) that was then adapted by
Gorbatov and Kennett (2003) to include 3D ray
tracing. In this method the trajectory of seismic-ray
propagation between source and receiver through the
three-dimensional structure of the earth is included,
which improves the resolution of gradients and strong
variations in wave speeds. The joint inversion used
P and S arrival-time data with the same source and
receiver from the global catalog of Engdahl et al. (1998),
which are plotted in Fig. 2. In order to optimize data
coverage and ensure the final images can be directly
compared, single rays with both P and S readings were
picked for events and stations within the study area.
The surrounding Western Pacific mantle structure was
parameterized into a non-overlapping grid of 5◦ × 5◦
with 16 layers ranging from 35 to 200 km down to a
total depth of 1600 km. The study area consisted of a
grid of 19 layers with cells 0.5◦ × 0.5◦ in the uppermost
mantle, 1◦ × 1◦ in the transition zone and lower mantle,
and 2◦ × 2◦ beneath continents to a depth of 1500 km.
The final dataset contained 900,000 pairs of P and S ray
paths and used the ak135 model as reference (Kennett
et al., 1995). This nested iterative approach resulted
in P and S models that were then used in a non-linear
joint tomographic inversion for bulk-sound and shear
wave speed inversions (Gorbatov and Kennett, 2003).
Then a separate P-wave tomographic inversion was
obtained with a similar non-linear scheme using the
same dataset as in the joint tomography and with the
same inversion parameters, but using a standard tomographic formulation. Although the inversion schemes
used for joint tomography and the P-wave tomography
are different, the three inversions can be compared
with careful consideration of the smoothing (damping)
parameters. We present a subset of the previously
published shear wave-speed model and the new P-wave
model to illustrate the structure of the subducting Pacific
plate beneath the junction of the Japan and Kurile arcs.
4. Tomographic images
The tomographic images presented have a maximum depth of 900 km and over the region between
136◦ –149◦ E and 37◦ –48◦ N. The large number of seis-
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Fig. 2. World maps illustrating (A) the distribution of seismic stations from which arrival times were recorded as triangles and (B) epicenter
distribution (as dots) of earthquakes selected from Engdahl et al. (1998) catalog and that were used in the inversions.
mic stations and earthquake events in the area, recorded
teleseismic events, and small cell size allow for the
structure of the subducting slab to be well resolved.
A convenient way to measure the data coverage is the
sum of the lengths of the ray segments traversing each
cell. This is illustrated in Fig. 3 with ray density plots,
which show very consistent high ray density across the
regions where subduction zone features are expected
beneath the Japan and Kurile arcs. Detailed resolution tests using a synthetic model, in Gorbatov and
Kennett (2003), concluded the image of the subducted
slab could be recovered along its complete extension
using the same inversion parameters as in the observed
data.
Fast wavespeed anomalies used to define the subducting slab in the P-wave inversion and shear wave-speed
images are clearly imaged down to at depth of least
700 km (Figs. 4–6). The dip of the Pacific plate decreases
from north to south along the Japan–Kurile arc, consistent with previous studies (Chiu et al., 1991; van der
Hilst et al., 1993; Miyamachi et al., 1994; Katsumata et
al., 2003), and is illustrated in the cross sections through
the P-wave and shear wave-speed tomographic models
(Figs. 4 and 5). Beneath the Kurile arc the subducting
slab is dipping at approximately 50◦ and the seismicity
falls within the high velocity zone (Figs. 4 (K–K ) and 5
(B–B and C–C )). Beneath the Japan arc the Pacific slab
is well defined as dipping at approximately 30◦ and then
gradually transitions into a horizontal structure above the
660 km discontinuity (Fig. 4 (J–J )).
An interesting low velocity feature is present in the
tomographic images, which suggests a complex shape
M.S. Miller et al. / Physics of the Earth and Planetary Interiors 156 (2006) 1–11
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Fig. 3. Maps and cross-sections, respectively, representing density of ray-length segments in log 10 km. (A) P-wave and (B) S-wave seismic rays
illustrate the ray density in plan view sliced along 275 km depth and (C) P-wave and (D) S-wave are cross-sections along 42◦ N latitude.
in the neighborhood of 41–43◦ N, 145◦ –147◦ E, and
50–150 km depth. This feature lies approximately below
the Hokkaido corner, where the Japan and Kurile arcs
meet at a point (Fig. 1). A series of slices through the
P-wave and shear wave-speed models at the Hokkaido
corner show the anomalous feature in the upper mantle
(Figs. 5 and 6). The P-wave images have lower amplitudes in comparison to the shear wave-speed images,
but both illustrate the prominent low velocity feature
in the upper 150 km to the west of the slab. The shear
wave speed images have stronger fast velocity perturbations that illustrate an approximately linear top of
the subducting plate, but a discontinuous lower boundary. The bottom of the subducting slab appears crumpled in the upper mantle in both P and S inversions,
unlike the top slab surface, which is a more smooth
and coherent feature (Fig. 6). It appears that the subducting Pacific plate at the Hokkaido corner becomes
very distorted and does not bend smoothly at the arc–arc
junction.
5. Three-dimensional morphology
The complex morphology of the subducting Pacific
plate beneath northern Japan and the Kurile islands has
been interpreted from a combination of seismicity and
seismic tomography. A three-dimensional visualization
of the slab geometry beneath the Hokkaido corner was
created by systematically picking the top and bottom of
the slab from the P-wave inversion in Figs. 4–6. The top
and bottom of the slab was defined by picking the maximum gradient of velocity in the P-wave model, which
compensates for the lower resolution in the northern portion of the model where there are fewer seismic stations.
The high velocity anomalies that were used to define the
slab geometry are subjective, but with care in picking
the maximum gradient rather than a constant value contour the slab structure can be extracted irrespective of
amplitude of the velocity perturbations.
The interpreted points were then used to create
a schematic model of the subducting Pacific plate
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Fig. 4. Cross-sections of the P-wave velocity perturbation from locations shown in Fig. 1 with hypocenters of events during 1967–2002
with magnitude greater than 4.5 from the NEIC/USGS catalog as yellow dots. Cyan colors represent fast velocity perturbations and Brown
colors represent slow velocity perturbations relative to ak135 (Kennett
et al., 1995). Cross-section K–K illustrates the relatively steep dipping
Pacific plate beneath the Kurile arc and J–J shows the more shallow
dipping slab beneath the Japan arc. The Z units are in km depth. (For
interpretation of the references to colour in this figure caption, the
reader is referred to the web version of the article.)
(Fig. 7(A–C)). The interpreted top of the slab picks were
used to form a surface that illustrates the top of the subducting Pacific plate and the bottom of the slab picks
were used to model the bottom of the slab. Using the two
sets of interpreted points surfaces representing the top
and bottom of the subducting slab were created. The two
surfaces were used to define the solid three-dimensional
shape and extent of the slab. The three-dimensional interpreted models illustrate the change in dip of the slab
beneath the Japan and Kurile arcs in Fig. 7(A and B)
and intra-plate seismicity (Fig. 7(C)) when the slab is
viewed transparently. This new model agrees with previous models in which the subducting Pacific plate dip
angle is between 40–50◦ beneath the Kurile arc and
approximately 30◦ beneath the Japan arc. The subducting slab appears to be continuous and have a coherent
transition between the two subduction angles near the
arc–arc junction at the Hokkaido corner.
Fig. 5. Three-dimensional (A) P-wave tomographic model and (B)
shear wave-speed model sliced along the cross-sections shown in Fig. 1
(A–A to F–F ) to illustrate the Pacific plate subducting beneath the
Japan and Kurile arcs. Fast perturbations are in cyan and slow perturbations are in brown relative to ak135 (Kennett et al., 1995). The
Asian coastline is in purple, plate boundary is a broken black line, and
Quaternary volcanoes from the Smithsonian volcano index are plotted as yellow triangles. (For interpretation of the references to colour
in this figure caption, the reader is referred to the web version of the
article.)
The detailed morphology of the slab at the Hokkaido
corner is illustrated in Fig. 8(A and B) using contour maps of the top and bottom of the interpreted
slab surfaces. The depth of the top and bottom of
the slab is contoured in 10 km increments down to
a maximum depth of 260 km. The slab appears to
have a generally smooth surface except along the bottom of the slab near the Hokkaido corner. The steepening of the subduction angle from south to north
along the arc and the complex shape is apparent in
M.S. Miller et al. / Physics of the Earth and Planetary Interiors 156 (2006) 1–11
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Fig. 6. Three-dimensional P-wave and shear wave speed tomographic images sliced approximately perpendicular to the trench near the Hokkaido
corner to illustrate the anomalous features in the subducting Pacific plate and the mantle wedge. The locations of the cross-sections are shown in
Fig. 1 as dotted lines. The volcano locations are projected on to the profiles. The Z units are in km with a maximum depth of 450 km.
the rapid change in dip in the slab surface (Fig. 6B).
The bottom of the slab has a more contorted morphology, which illustrates the complex nature of the junction of the Japan and Kurile arcs. The contour maps
present a more complex and detailed image of the
slab morphology than those contour maps produced by
Miyamachi et al. (1994) and Katsumata et al. (2003). The
most distorted region is at the Hokkaido corner, which
is located at approximately 42.5◦ N and 145–146◦ E
(Figs. 5–8B).
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M.S. Miller et al. / Physics of the Earth and Planetary Interiors 156 (2006) 1–11
Fig. 7. (A–D) Interpreted three-dimensional model of the morphology and geometry of the subducting Pacific plate beneath the Hokkaido corner.
The Japanese coastline is in purple, current plate boundaries are represented by black dashed lines. (A) Represents the method of picking the top
of the slab (green dots) and the bottom of the slab (yellow dots) from the P-wave images. (B and C) Illustrate the interpreted model and (D) shows
the earthquake hypocenters for events greater than 4.5 during 1967–2002 from the USGS/NEIC catalog and are color coded with depth. (Note: the
slab does not converge at a point in the NW. It is an effect of the perspective.) (For interpretation of the references to color in this figure caption, the
reader is referred to the web version of the article.)
6. Discussion
Hokkaido is a unique region because it lies at the junction of two subduction zones that are both seismically
and volcanically active. The Hidaka Mountains, which
strike approximately NE–SW on the island of Hokkaido
are an expression of the collision of the northern Japan
and southern Kurile arcs. Many of the studies done in
this region have focused on the crustal structure and its
relationship to the arc–arc junction, but few have investigated the slab morphology beneath this region.
Moriya (1986), Moriya et al. (1998) and Miyamachi
and Moriya (1984, 1987) investigated the structure
beneath the Hidaka Mountains using seismic observations and their models suggest the complex velocity
structure of the area is due to the oblique collision of
the northern Japan (Honshu) and southern Kurile arcs.
It was proposed that the variation of rate and angle of
convergence along the arc (Fig. 1) resulted in a complex
overlap structure in the crust and in the transition zone.
Miyamachi and Moriya (1984) imaged an inclined low
velocity zone at a depth to 70 km beneath the Hidakda
Mountains. Miyamachi et al. (1994) suggested that this
depth coincided with the location of the top of the slab
and could be interacting; they speculated on the thickness
of the Pacific plate based on seismic velocity distribution
(range of 90–110 km), but the lower plate boundary was
an unknown parameter in their models.
Katsumata et al. (2003) also studied the seismicity in
the region to image the shape of deep seismic zone at
the Japan–Kurile arc junction. In the process of modeling the seismic zone they found an unusual cluster of
M.S. Miller et al. / Physics of the Earth and Planetary Interiors 156 (2006) 1–11
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Fig. 8. (A–B) Contour maps of the top and bottom of the interpreted Pacific slab surfaces with contours in 10 km increments with Hokkaido coastline
in blue, the Pacific plate boundary in red, and earthquake events with magnitude greater than 4.5 between 1967 and 2002 from the USGS/NEIC
catalog. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of the article.)
intra-plate earthquakes where the dip of the Pacific plate
changes at the Hokkaido corner. Focal mechanisms for
this near vertical linear cluster were interpreted to have
predominately extensional characteristics. From these
results they suggest this cluster is a slab-cracking zone
that could develop into a slab tear due to the rapid lateral change in subduction angle at the arc–arc junction
(Katsumata et al., 2003). This near vertical cluster is
located at a depth range of 50–110 km near 42.5◦ N and
143.5◦ E. Although the new model presented here does
not image a tear, a low velocity anomaly in the slab as
seen in the tomographic images in Figs. 5 and 6 is located
in the same region.
A different explanation for the shallow low velocity zone and earthquake cluster found by Katsumata
(2003) near the Hokkaido corner can be explained by
its position beneath a cluster of Quaternary volcanoes
in central Hokkaido (Figs. 1, 6, and 9). The low velocity zone in the upper mantle extends from approximately 125 km to near the surface beneath the Tokachi,
Daisetsu, Nipesotsu-Upepesanke, and Shikaribetsu volcanic groups along B–B . These volcanoes in central
Hokkaido (Figs. 1, 5, 6, and 9) are composed of a series
of overlapping andesitic to dacitic lava domes and stratovolcanoes trending perpendicular to the plate boundary.
The correlation between this series of volcanoes that
trend along a line running NW–SE, which is unlike the
other volcano groups in Hokkaido, and the presence of
a low velocity anomaly (LVZ) in the mantle wedge is
evidence for magma generation and conduits to the volcanoes (Fig. 9).
Similar low velocity zones interpreted as the source
and path of magma for volcanoes have been imaged in
other regions in Japan and South America (Nakajima et
al., 2001; Wyss et al., 2001; Tamura et al., 2002; Husen
et al., 2003), but not in this specific region. The low
velocity in both P-wave and shear wave speed images do
not appear continuously along the Japan arc, as noted in
Fig. 9. Map of the Hokkaido corner illustrating the relationship of the
low velocity zone (LVZ) and the active volcanic front.
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M.S. Miller et al. / Physics of the Earth and Planetary Interiors 156 (2006) 1–11
previous research (Tamura et al., 2002), but there appears
to be a basic correlation between volcanic groups and
low velocity zones in the mantle wedge. Cross-section
B–B in Fig. 6 images the localized low velocity zone
extending from about 125 km depth up to the near surface
where the volcanic group is located, which contrasts the
more typical mantle wedge geometry features such as
those observed in E–E . This low-velocity feature is also
present in the recent work of Wang and Zhao (2005) (in
their Fig. 9 sections E–E and F–F ) using a different data
set, but is neither commented upon nor interpreted.
The position of the cluster of volcanoes in central
Hokkaido lie above the most crumpled and distorted portion of the subducting Pacific plate. The contour maps
in Fig. 6 illustrate the position of the volcanoes relative to the top of the slab, which are at approximately
the same depth as the other active volcanoes in the area
(∼150 km depth). The position of these volcanoes relative to the main volcanic front and is similar to location of
the Klyuchevskoy and Sheveluch volcanic groups north
of ∼55◦ N on the Kamchatka peninsula (Gorbatov et al.,
1997). The Kamchatka volcanoes are shifted west of the
main axis of the volcanic front and lie above a portion
of the slab that has changed dip and become shallower
(Figs. 8 and 9). West of this uplifted area beneath central
Hokkaido, where the cluster of volcanoes are located,
the depth of the top of the slab is deeper and thicker due
to the change in dip and thickness of the slab. Above
this thickened oceanic lithosphere there appears to be a
gap in the volcanic front. Beyond the gap there is a cluster of stratovolcanoes in southwest corner of Hokkaido,
including Usu, Shikotsu, Komaga-take, among others,
which are at least 200 km away from the closest volcano in central Hokkaido, Tokachi (Figs. 1, 6, and 9).
It is possible that the gap in the volcanic front may
be due to the thickened slab beneath this region,
which is due to the collision of the Japan and Kurile
arcs.
The subducting slab morphology beneath the
Hokkaido corner is very complex, but through the combination of seismicity, tomography, and plate velocities the complexity can be understood in terms of the
collision of the Japan and Kurile arcs. The subducted
plate beneath the northwestern Pacific margin is continuous, but changes geometry as the slab subduction angle
decreases from north to south as the slab thins. Directly
beneath the Hokkaido corner a low velocity zone in the
upper mantle (70–125 km depth) is imaged, which may
be related to volcanism in the area or another result of the
collision of the two arcs. The present complex state of the
Kurile and northern Japan arcs with apparent crumpling
may be a result of the evolution of the arc–arc junction,
which appears to have retreated to its present position
over the last 15 million years.
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