Download Deep structure of the northeastern Japan arc

Tectonophysics 403 (2005) 59 – 75
www.elsevier.com/locate/tecto
Deep structure of the northeastern Japan arc and its implications for
crustal deformation and shallow seismic activity
Akira HasegawaT, Junichi Nakajima, Norihito Umino, Satoshi Miura
Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science,
Tohoku University, Sendai 980-8578, Japan
Received 16 September 2004; received in revised form 18 March 2005; accepted 29 March 2005
Available online 10 May 2005
Abstract
Seismic tomography studies in the northeastern Japan arc have revealed the existence of an inclined sheet-like seismic lowvelocity and high-attenuation zone in the mantle wedge at depths shallower than about 150 km. This sheet-like low-velocity,
high-attenuation zone is oriented sub-parallel to the subducted slab, and is considered to correspond to the upwelling flow
portion of the subduction-induced convection. The low-velocity, high-attenuation zone reaches the Moho immediately beneath
the volcanic front (or the Ou Backbone Range) running through the middle of the arc nearly parallel to the trench axis, which
suggests that the volcanic front is formed by this hot upwelling flow. Aqueous fluids supplied by the subducted slab are
probably transported upward through this upwelling flow to reach shallow levels beneath the Backbone Range where they are
expelled from solidified magma and migrate further upward. The existence of aqueous fluids may weaken the surrounding
crustal rocks, resulting in local contractive deformation and uplift along the Backbone Range under the compressional stress
field of the volcanic arc. A strain-rate distribution map generated from GPS data reveals a notable concentration of east–west
contraction along the Backbone Range, consistent with this interpretation. Shallow inland earthquakes are also concentrated in
the upper crust of this locally large contraction deformation zone. Based on these observations, a simple model is proposed to
explain the deformation pattern of the crust and the characteristic shallow seismic activity beneath the northeastern Japan arc.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Arc magmatism; Aqueous fluids; Crustal deformation; Shallow seismicity; Subduction zone; Northeastern Japan arc
1. Introduction
Northeastern Japan is located at a subduction zone,
where the Pacific plate subducts downward into the
T Corresponding author. Tel.: +81 22 225 1950; fax: +81 22 264
3292.
E-mail address: [email protected]
(A. Hasegawa).
0040-1951/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.tecto.2005.03.018
mantle at a convergence rate of 8–9 cm/year and at an
angle of about 308. Many shallow earthquakes occur
beneath the Pacific Ocean mainly along the upper
boundary of the Pacific plate associated with its
subduction. Beneath the land area, shallow earthquakes also occur in the upper crust; many of them are
concentrated in a long, narrow zone extending along
the volcanic front or the central mountainous range
(Ou Backbone Range) which runs through the
60
A. Hasegawa et al. / Tectonophysics 403 (2005) 59–75
middle of the land area nearly parallel to the trench
axis (Fig. 1).
Great progress has been made in the last few
years in understanding the stress concentration
mechanism causing interplate earthquakes beneath
the Pacific Ocean off the northeastern Japan arc.
Asperities are distributed in patches surrounded by
stable sliding areas on the plate boundary. Aseismic
slip in the surrounding stable sliding areas results in
the accumulation of stress at the asperities, and
earthquakes occur when the strength limit of an
asperity is reached leading to sudden slip. It has
gradually become clear that this kind of asperity
model (Lay and Kanamori, 1981) represents an
accurate description of the mechanism of such
earthquakes (Nagai et al., 2001; Yamanaka and
Kikuchi, 2004; Matsuzawa et al., 2002; Okada et
al., 2003, Hasegawa et al., in press).
Understanding the mechanism of stress concentration that leads to shallow inland earthquakes
(intraplate earthquakes) in the arc crust, on the other
had, has advanced more slowly. Why, of the many
active faults, does stress concentrate along just one of
them, leading to slip and an earthquake? It is to be
expected that once slip occurs on an active fault,
producing an earthquake, stress would become concentrated in regions adjacent to extensions of the fault,
but in general, inland earthquakes occur in isolation,
and related earthquakes in adjacent regions are rarely
if ever observed. Why is this so? Our current level of
Fig. 1. Map showing the northeastern Japan arc and its surroundings. Red triangles and thick gray line denote active volcanoes and the volcanic
front, respectively. White arrow indicates the direction of the relative plate motion (Demets et al., 1994). The bathymetry is taken from the Japan
Coast Guard. 1. Iwate volcano, 2. Naruko volcano.
A. Hasegawa et al. / Tectonophysics 403 (2005) 59–75
understanding is not sufficient to explain these facts. It
is clear that this scenario cannot be explained by a
simple model in which an elastic upper crust supports
stress caused by relative plate motion, with slip (and
hence earthquakes) occurring when the stress exceeds
the strength of the fault surface as a plane of weakness
within the crust (Iio, 1996, 1998).
Recent seismic tomography studies in the northeastern Japan arc have provided new information that
shows that water supplied by dehydration of the
subducting slab reaches the upper crust via the mantle
wedge, entrained in an upwelling flow in the mantle
that travels nearly parallel to the slab as a seismic lowvelocity, high-attenuation zone in the mantle wedge.
The sheet-like upwelling flow aligned nearly parallel
to the slab reaches the Moho near the Backbone
Range (or the volcanic front). Consequently, partial
melting is widely distributed along the volcanic front
immediately below the Moho. When the molten
material in such a melting zone approaches the
surface, it cools and partially solidifies, expelling
water contained in the molten material. It is expected
that this water migrates to even shallower levels.
Seismic tomography provides images of the upwelling
paths of water in the upper crust as the low-velocity
zones. The result is the continuous supply of water
expelled from the subducting slab into a region below
the Backbone Range.
Research on surface deformation based on GPS
data has revealed a zone of strain concentration that
extends north–south along the Backbone Range,
representing the local predominance of contractive
deformation in the direction of relative plate motion
along the Backbone Range. This zone of strain
concentration is located above where the upwelling
flow in the mantle wedge reaches the Moho. The
concentrated supply of water originating from the
slab must weaken the crustal material, causing
contractive deformation to occur locally, that is,
anelastic deformation occurs locally even within the
upper crust. It is inferred that since this anelastic
deformation is non-uniform in space, shallow inland
earthquakes serve as a mechanism for making the
overall deformation more uniform. Based on the
present data, we propose this model of stress
concentration mechanism as a model for the occurrence of shallow inland earthquakes in the northeastern Japan arc.
61
2. Mantle wedge structure of the northeastern
Japan arc
Nakajima et al. (2001a,b), using data from the
seismic observation network, the density of which has
recently been increased, calculated the three-dimensional seismic wave velocity structure for the northeastern Japan arc, updating the results of Zhao et al.
(1992). Figs. 2 and 3 show the P-wave velocity (Vp)
and S-wave velocity (Vs) on cross-sections perpendicular to the island arc. In any of the vertical crosssections (a) to (f), the Pacific Plate subducting beneath
the arc is imaged as a strong high-Vp and high-Vs
region. Within the mantle wedge immediately above
the Pacific Plate, low-Vp, low-Vs regions inclined
nearly parallel to the slab and extending from depths
of about 100 to 150 km to the Moho appear clearly.
These regions of low seismic wave speed appear
clearly not only in cross-sections (a), (b), (d) and (f),
which pass through active volcanoes, but also in
cross-sections (c) and (e), which do not include any
volcanoes. This illustrates the existence of a single
sheet-like low-velocity zone inclined nearly parallel to
the slab within the mantle wedge. This low-velocity
zone has high Vp / Vs values. Fig. 4 shows distribution of Vp / Vs ratio at a depth of 40 km. We can see
that a high Vp / Vs (and low Vp, Low Vs) zone is
distributed along the volcanic front immediately
below the Moho. Similar low-velocity zones inclined
nearly parallel to slabs have also been observed in
mantle wedges in other subduction zones (Abers,
1994; Zhao et al., 1995, 1997; Gorbatov et al., 1999),
although none are as clear as those in northeastern
Japan (Figs. 2 and 3).
Seismic attenuation structure provides additional
information on the physical states of the earth’s
interior. Three-dimensional P-wave attenuation structure beneath NE Japan was estimated by a joint
inversion for source parameters, site response and Qp
values (Tsumura et al., 2000). Fig. 5 shows across-arc
vertical cross-sections of Qp values along three lines
in the inserted map. Low Qp (high attenuation) zones
inclined nearly parallel to the slab are clearly seen for
all the cross-sections, although the extent of drop in
Qp-value is not large for cross-sections A and B. The
low-Qp zones are consistent with the inclined low-V
zone in Figs. 2 and 3. Thus there exists an inclined
sheet-like low Vp, low Vs, high Vp / Vs and low Qp
62
A. Hasegawa et al. / Tectonophysics 403 (2005) 59–75
Fig. 2. Across-arc vertical cross-sections of P-wave velocity perturbations along lines in the inserted map of NE Japan (Nakajima et al., 2001a).
The solid line and red triangles at the top represent land area and active volcanoes, respectively. Open and red circles denote earthquakes and
deep, low-frequency microearthquakes, respectively.
zone in the mantle wedge beneath the northeastern
Japan arc.
3. Upwelling flow within the mantle wedge
We infer that the inclined sheet-like low-V and
low-Q zone described above corresponds to the
upwelling flow in the secondary convection (McKenzie, 1969) accompanying slab subduction. Since
temperature increases with depth, the interior of this
upwelling flow is at a higher temperature than the
surrounding region, and as such should have lower
viscosity. In an old plate subduction zone such as
northeastern Japan, water supplied from dehydration
of the subducted slab may form a temporary layer of
serpentine and chlorite in the mantle wedge immediately above (Davies and Stevenson, 1992; Iwamori,
1998), which is then dragged downward to a depth of
150–200 km where dehydration decomposition occurs
(Iwamori, 1998; Schmidt and Poli, 1998). Slightly
low velocity areas are imaged immediately above the
subducted slab in some of vertical cross-sections of
Figs. 2 and 3 (e.g., Fig. 3(b), (d), which might
correspond to this temporary layer of serpentine and
chlorite, although more studies with much higher
resolutions are required to confirm it. The water
released by this dehydration at depth is then transported upward, encountering the upwelling flow at
depths of 100–150 km. The supply of water to the
upwelling flow has the effect of lowering the solidus
temperature. From a comparison of the seismic wave
attenuation structure described in the previous section
(Tsumura et al., 2000) with laboratory experiment
data, the temperature within the low-V, low-Q zone is
estimated to be higher than that of the peridotite wet
A. Hasegawa et al. / Tectonophysics 403 (2005) 59–75
63
Fig. 3. Across-arc vertical cross-sections of S-wave velocity perturbations along lines in the inserted map (Nakajima et al., 2001a). Other
symbols are the same as in Fig. 2.
solidus (Nakajima and Hasegawa, 2003a). Further,
Nakajima et al., in press inferred from the ratio of falloff rates of P-wave and S-wave velocities that melt
inclusions are included in the low-V, low-Q zone,
having aspect ratios of 0.01–0.1 and volume fractions
of 0.1 to several percent.
The existence of such a low-velocity zone inclined
nearly parallel to the slab at depths of less than 150
km, as detected by seismic tomography, has also been
confirmed by numerical simulation of the secondary
convection that accompanies plate subduction. Eberle
et al. (2002) performed a numerical simulation of the
corner flow that accompanies plate subduction using a
temperature-dependent viscosity coefficient, and
found that a low-velocity zone with velocities several
percent slower than in the surrounding region was
generated, which would correspond to the present
upwelling region. The low-velocity zone determined
by Eberle et al. (2002) was aligned nearly parallel to
the slab, was separated from the upper surface of the
slab by about 50 km, and extended to depths of no
more than 125 km, accurately reproducing the lowvelocity zone observed in northeastern Japan (Figs. 2
and 3).
The inferred water transport paths in the northeastern Japan subduction zone are shown schematically in
Fig. 6(a). The upwelling of hot mantle material from
depth and the addition of water may cause partial
melting with a volume fraction on the order of 0.1 to
several percent. Melt is formed both by decompression melting and melting due to water addition. From
the fact that the inclined low-velocity zone is only
clearly observed at depths shallower than about 150
km (Zhao and Hasegawa, 1993), it is inferred that
melting by the addition of water plays an important
role in melt formation. Thus, water that originated
from the slab is eventually incorporated into the melt.
The upwelling flow including this melt eventually
64
A. Hasegawa et al. / Tectonophysics 403 (2005) 59–75
139°
140°
141°
142°
Depth = 40 km
41°
40°
39°
fast directions in the back-arc region are nearly
parallel to the direction of relative plate motion. Most
of stations with such trench-perpendicular directions
are located above the inclined low-velocity zone (i.e.
upwelling flow) in the mantle wedge. The observed
trench-perpendicular fast directions would be
explained by lattice preferred orientation of minerals
caused by flow-induced strain in the mantle wedge
(Ribe, 1992; Tommasi, 1998; Zhang and Karato,
1995). On the contrary, trench-parallel fast directions
are seen in the fore-arc region. Perhaps another
mechanism is working to cause these directions in
the fore-arc mantle wedge.
Seismic tomography research is also providing
important information on the variation of magma
38°
37°
Vp/Vs
1.65
1.70
1.75
1.80
1.85
Fig. 4. Vp / Vs ratio at a depth of 40 km (Nakajima et al., 2001a).
Red triangles denote active volcanoes.
reaches the Moho immediately below the volcanic
front, resulting in the accumulation of large amounts
of melt immediately below the Moho along the
volcanic front. Seismic tomography clearly reveals
this continuous distribution of partially molten material along the volcanic front and immediately below
the Moho as a region of low Vp, low Vs, high Vp / Vs
and low Qp (Figs. 2 through 5). From this point of
view, the volcanic front can be regarded to form
where a sheet-like upwelling flow in the mantle
wedge reaches the Moho.
Seismic anisotropy structure beneath the arc,
shown in Fig. 7 (Nakajima and Hasegawa, 2004),
seems to support the existence of this upwelling flow
in the mantle wedge. Fig. 7 clearly shows a systematic
spatial variation in directions of fast shear-waves. The
Fig. 5. Across-arc vertical cross-sections of P-wave attenuation
structure along lines in the inserted map (Tsumura et al., 2000). Red
and blue colors represent high and low attenuations, respectively,
according to the scale at the bottom. Other symbols are the same as
in Fig. 2.
A. Hasegawa et al. / Tectonophysics 403 (2005) 59–75
65
Fig. 6. (a) Schematic diagram of vertical cross-section of the crust and upper mantle of NE Japan, showing the inferred transportation paths of
aqueous fluids. (b) Schematic 3D structure of the crust and upper mantle of NE Japan showing the upwelling flow with varying thickness in the
mantle wedge.
formation along the island arc. Recently, Tamura et al.
(2002) investigated the distribution of Quaternary
volcanoes in northeastern Japan, and found that the
volcanoes are distributed in long and narrow bands
perpendicular to the island arc, forming 10 clusters of
volcanoes occupying an average width of 50 km.
These cross-arc bands in which Quaternary volcanoes
are concentrated coincide with regions of elevated
topography and low Bouguer gravity anomaly.
Tamura et al. (2002) concluded that volcanoes form
where inclined hot fingers (upwelling regions) distributed across a width of 50 km in the mantle wedge
66
A. Hasegawa et al. / Tectonophysics 403 (2005) 59–75
140°
140.5°
141°
141.5°
142°
40°
39.5°
39°
38.5°
-6
-3
0
3
6
Velocity perturbation (%)
0.21
0.14
0.07
Delay time (sec)
Fig. 7. Direction of fast shear-wave and delay time plotted at each station superposed on shear-wave velocity perturbations in the mantle wedge
(Nakajima and Hasegawa, 2004). Black lines denote the direction of fast shear-wave and length is proportional to the average time delay
between the leading and following shear-waves. Velocity image is the shear-wave velocity perturbations along the inclined low-velocity zone in
the mantle wedge as in Fig. 8(a). Red triangles show active volcanoes. White arrow indicates the direction of the relative plate motion (Demets
et al., 1994).
at a depth of 150 km reach the surface. The repeated
supply of magma from hot fingers in the mantle
wedge to the crust immediately above causes the
bedrock to be uplifted and Quaternary volcanoes to
form. They further concluded that the magma that is
supplied accumulates beneath the Moho, producing
the local low Bouguer gravity anomalies.
To confirm the model of Tamura et al. (2002), we
attempt to image the low-velocity zone in the mantle
wedge with a higher spatial resolution (Hasegawa and
Nakajima, 2004). In this study, the velocity structure
outside of the mantle wedge was fixed to that obtained
earlier by Nakajima et al. (2001a), and the velocity
distribution within the mantle wedge was estimated
using the same data set. The spatial resolution was 10
km or finer in both the horizontal and depth
directions. The distribution of S-wave velocity
obtained is shown in Fig. 8(a). The figure shows the
S-wave velocity perturbations taken along the inclined
low-velocity zone. The value is that along the surface
of minimum S-wave velocity within the mantle
wedge, and thus the figure shows the distribution of
S-wave velocity perturbations along the curved surface joining the core of the low-velocity zone. As
Tamura et al. (2002) predicted, the extent of velocity
drop within the low-velocity zone varies clearly along
the strike of the island arc.
Comparing these results with the topographic map
(Fig. 8(b)), we can see that there is good agreement
between the regions where the velocity drop is locally
particularly strong in the low-velocity zone distributed
from 30 to 150 km depth in the mantle wedge and the
regions where elevations in the topography are high
from the Backbone Range to the back-arc region.
Quaternary volcanoes (red circles) are distributed in
those regions. In addition, low-frequency microearth-
A. Hasegawa et al. / Tectonophysics 403 (2005) 59–75
67
Fig. 8. (a) S-wave velocity perturbations along the inclined low-velocity zone in the mantle wedge of NE Japan. (b) Topography map of NE
Japan. Deep low-frequency microearthquakes were located by the Japan Meteorological Agency and Okada and Hasegawa (2000). Thick lines
denote active faults (Active Fault Research Group, 1991).
quakes (white circles) produced at depths of 25–40
km, believed to be caused by sudden movements of
fluids in the crust (Hasegawa et al., 1991; Hasegawa
and Yamamoto, 1994), are seen to occur immediately
above zones of particularly large velocity drop in the
mantle wedge.
Spatial correlations between the following features
can be clearly seen in Fig. 8: 1) Regional variation of
low-velocity zone distributed from 30 to 150 km
depth in the mantle wedge, 2) The distribution of
low-frequency microearthquakes occurring at 25–40
km depth, 3) The distribution of Quaternary volcanoes at the surface, 4) The distribution of topographical elevations extending from the Backbone
Range toward the back-arc region. The structure of
the crust and upper mantle in northeastern Japan, and
the upwelling flow in the mantle, as inferred based on
these observational facts, are shown schematically in
Fig. 6(b), which shows a three-dimensional expansion of the two-dimensional cross-section in Fig.
6(a). The upwelling flow in the mantle wedge is
sheet-like, with a thickness that varies locally from
place to place, rather than occurring in fingers as
suggested by Tamura et al. (2002). The volcanic front
is formed where this upwelling flow finally contacts
the Moho. As the flow approaches the Moho, it slows
down. The melt contained in the flow accumulates
over a wide area along the volcanic front, immediately below the Moho, resulting in the low-velocity,
high-attenuation zone that is seen to extend over a
wide areas along the volcanic front. Seismic tomography has revealed that in the volcanic zones,
differentiation occurs and magma rises to the middle
crust (see Figs. 12 and 13).
A. Hasegawa et al. / Tectonophysics 403 (2005) 59–75
analysis software. Using this feature, the coordinates
of an isolated observation point can be estimated from
the observation data for that point alone, without
having to form a baseline. For this estimation,
parameters estimated in advance to high precision
by JPL including orbital histories of GPS satellites,
clock errors and the Earth’s rotation are used. Data
were analyzed using this Precise Point Positioning
(PPP) technique (Zumberge et al., 1997).
East–west components of horizontal strain rates
estimated from observational data from January 1997
to December 2001 are shown in Fig. 9. Constraints
have been applied so as to ensure that the strain rate is
continuous in space (Miura et al., 2002; Sato et al.,
2002). The east–west components are shown as the
direction in which the deformation that accompanies
the plate convergence predominates; north–south
41°
-1
Within the sheet-like upwelling flow, in regions of
the back-arc side where the sheet is locally thick and
there is a large amount of melt, part of the melt
sometimes separates from the upwelling flow before it
reaches the Moho along the volcanic front. According
to the estimate by Nakajima et al., in press obtained
using the rates of decrease of Vp and Vs, the volume
fractions of melt within these regions of the back-arc
side in the upwelling flow are on the order of 0.1% to
several percent. The separated melt rises straight
upward in the plumes, and accumulates beneath the
Moho. Part of it continues to rise and penetrates into
the crust, forming volcanoes and uplifting the bedrock. We infer that this is how the concentrations of
Quaternary volcanoes and elevations of topography
extending from the volcanic front toward the back-arc
region formed, and probably this formation process
continues today. The alternation of the regions where
Quaternary volcanoes are concentrated and regions
without volcanoes in the direction along the island arc
is presumed to be due to the variation of partial
melting in the upwelling flow in the mantle wedge at
depths from 30 to 150 km along the island arc.
-5
0
5
0
40°
-1
4. Zones of concentrated deformation along the
Backbone Range
0
10-7/yr.
-2
0
68
-2
-1
-1
39°
-2
-2
-1
-1
-1
38°
-1
Observational data on the surface deformation field
obtained from the nationwide GPS continuous observation network (GEONET) of the Geographical
Survey Institute of Japan have provided a great deal
of information that was previously impossible to
obtain, such as that related to the temporal and spatial
variations of interplate coupling at plate boundaries.
Suwa et al. (2003) and Sato et al. (2002) have
analyzed data from the GEONET and the observational network of Tohoku University from 1997 to
2001 seeking to clarify surface deformation in the
Tohoku region. GIPSY-OASIS II (GPS Inferred
Positioning System-Orbit Analysis and Simulation
Software II), developed by the Jet Propulsion Laboratory (JPL) of the American National Atmospheric
and Space Administration (NASA) was used for GPS
data analysis. This analysis software estimates parameters such as clock drift in satellites and receivers as
probability variables without the need to take double
phase differences. This is a big advantage over other
-2
-2
-2
-1
37°
139°
140°
141°
142°
Fig. 9. Distribution of horizontal east–west strain rate estimated
from GPS observations for the period from 1997 to 2001 (Sato et
al., 2003). Contour interval is 100 ppb/year. Red triangles denote
active volcanoes.
A. Hasegawa et al. / Tectonophysics 403 (2005) 59–75
components are much smaller than east–west components. It can be seen from Fig. 9 that there is a beltlike zone in which contractive deformation is concentrated along the Backbone Range (or the volcanic
front). The concentrated zone in which contractive
deformation predominates in the direction of relative
plate motion is therefore distributed in a long and
narrow band that runs throughout Tohoku along the
Backbone Range.
69
41°
-5
0
5
10-7/yr.
40°
5. Deformation of the arc crust and shallow inland
earthquakes—their relationship with fluids
39°
As shown in Section 3, the inclined sheet-like
upwelling flow in the mantle wedge reaches the Moho
along the volcanic front, that is, the Backbone Range.
The distribution of the Vp / Vs ratio immediately
below the Moho is shown in Fig. 4. The upwelling
flow, imaged as low-Vp, low-Vs, high-Vp / Vs and
low-Qp regions, is distributed nearly continuously
along the volcanic front, immediately below the
Moho. The melt incorporated into the upwelling flow
either butts up against the bottom of the crust or
penetrates into the crust. When it cools in the crust
and partially solidifies, water is expelled from it and
moves upward. Thus, water of slab origin is supplied
continuously to the shallow part of the crust along the
Backbone Range. The presence of water is consistent
with the concentration of low-frequency microearthquakes (Hasegawa et al., 1991; Hasegawa and
Yamamoto, 1994) at depths near the Moho, and with
S-wave reflectors at intermediate crustal depths (Hori
et al., 2004) along the Backbone Range. The presence
of water can be expected to weaken the crustal
material and to produce local contractive deformation
under a compressive stress field. We infer that this
happens in the concentrated deformation zone along
the Backbone Range as shown in Fig. 9. This
concentrated deformation zone is also the location of
considerable present microearthquake activity, as
shown in Fig. 10.
The deformation pattern of the arc crust in northeastern Japan inferred from these observed facts is
schematically shown in Fig. 11(a). As melt cools and
solidifies, water that have separated from the melt
sometimes moves suddenly in the lower crust, and is
observed as deep low-frequency microearthquakes in
38°
37°
139°
140°
141°
142°
Fig. 10. Distribution of horizontal east–west strain rate for the
period from 1997 to 2001 (Sato et al., 2003), and shallow
earthquakes located by the seismic network of Tohoku University
for the same period.
the lowermost crust (Hasegawa et al., 1991; Hasegawa and Yamamoto, 1994). The water forms a sill at
intermediate crustal depths and accumulates, perhaps
corresponding to the bright S-wave reflectors that
have been detected across a wide area along the
Backbone Range (Matsumoto and Hasegawa, 1996;
Hori et al., 2004). In the Backbone Range, the
temperature is locally increased by the infiltration of
high-temperature material from the upper mantle, and
the bottom of the seismogenic layer (the boundary
between brittle and ductile layers) is locally elevated
(Hasegawa and Yamamoto, 1994; Hasegawa et al.,
2000). Corresponding to this, the observed crustal
heat flow has locally high values in the Backbone
Range (Furukawa, 1993; Tanaka and Ishikawa, 2002).
The water continues to rise and reaches the upper
crust, causing plastic deformation in some part of the
brittle upper crust.
70
A. Hasegawa et al. / Tectonophysics 403 (2005) 59–75
Backbone Range
(a)
* *
* *
**
*
*
*
*
WEST
*
contraction & uplift
(partly anelastic deformation)
small earthquakes EAST
large earthquake
seismogenic zone
brittle to
ductile transition
low-V
S-wave reflectors
lower crust
Moho
*
upper mantle
*
low-V
low-Q
*
*
low-F events
upwelling flow
(b)
Backbone Range
elastic deformation
partly anelastic
deformation elastic deformation
large contraction
reverse
fault
small contraction
reverse
fault
large contraction
Fig. 11. (a) Schematic illustration of across-arc vertical cross-section of the crust and uppermost mantle, showing the deformation pattern of the
crust and the characteristic shallow seismic activity beneath NE Japan. (b) Map view schematically showing the deformation pattern of the
upper crust.
In the Backbone Range, where the seismogenic
layer is locally thin and melt and water are
distributed in the lower crust, the entire crust will
be locally weak in comparison with the surrounding
region. For this reason, the arc crust, which is being
compressed in the direction of relative plate motion,
deforms elastically outside of the Backbone Range,
but anelastically in part within the upper crust along
the Backbone Range, which can be expected to
cause local contraction and uplift. We infer that the
concentrated deformation region shown in Fig. 9 was
formed in this way, although some extension is
observed locally around Iwate volcano, which is
probably related to the volcanic activity of Mt.
Iwate, which started in 1998. Numerical simulation
studies are essential to obtain a quantitative model
having spatial perturbations of elastic and viscous
rheological constants which can explain the observed
amount of the deformation, however, they are left for
future studies.
Research on surface deformation based on analysis
of GPS data (Sato et al., 2003) is steadily revealing
A. Hasegawa et al. / Tectonophysics 403 (2005) 59–75
evidence of uplifting zones along the Backbone
Range as predicted by the present model. Local
contractive deformation along the Backbone Range
is perhaps caused by asseismic slip on the deep
extension of faults and/or by plastic volume deformation in the lower crust, leading to stress concentration
in the upper crust immediately above. Anelastic
deformation may also occur partially, in the upper
crust. This eventually leads to the rupture of the whole
upper crust, producing a shallow inland earthquake
that makes the deformation uniform in space (Iio et
al., 2000, 2002). Anelastic contractive deformation
along the Backbone Range including the upper crust
causes numerous shallow microearthquakes as it
advances, as seen in Fig. 10.
Fig. 9 shows that, there is one more long, narrow
region on the fore-arc side where contractive deformation predominates, in addition to the Backbone
Range where the upwelling flow in the mantle wedge
reaches the Moho. This region, in northern Miyagi
Naruko volcano
140.25 E
0
71
Prefecture and southern Iwate Prefecture, also has a
concentration of shallow microearthquakes (Fig. 10).
This region includes the hypocenters of the 1900
Northern Miyagi earthquake (M7.0) and the 1962
Northern Miyagi earthquake (M6.5). Vp, Vs and Vp /
Vs ratios in east–west vertical cross-section along a
line across this region are shown in Fig. 12. In this
region, a large amount of data is available from
densely spaced temporary observation stations, making imaging possible at higher spatial resolution
(Nakajima and Hasegawa, 2003b). The cause of the
concentrated deformation zone on the fore-arc side,
which could not be understood from the image
immediately below the Moho (Fig. 4), can perhaps
be understood from Fig. 12. In addition to the lowvelocity zone extending from below the Moho
beneath the Backbone Range to immediately below
the Naruko volcano, there is another low-velocity
zone that branches off and extends to the eastern side
(the fore-arc side).
Naruko volcano
141.25 E
dv(%)
10
Depth (km)
10
5
20
0
30
-5
40
(a)
dVp
50
0
Depth (km)
20
30
-10
dVs
(d) 0
10
Depth (km)
(b)
C1
10
C2
R1
R2
+
+
100
+
C3
+
10
+
20
40
Vp/Vs
(c)
50
Vp/Vs
1.61 1.68 1.75 1.82
1.89
bright S-wave reflectors
low-F microearthquakes
1000
1
Distance (km)
1
Ma g .
5
Electrical Resistivity
Fig. 12. EW vertical cross-sections of (a) P-wave and (b) S-wave velocity perturbations, (c) Vp / Vs (Nakajima and Hasegawa, 2003b), and (d)
electrical resistivity (Mitsuhata et al., 2002) along line a in Fig. 10. Rectangles in (a), (b) and (c) show the range of cross-section in (d) in both
horizontal and vertical directions. Red circles and dots denote low-frequency microearthquakes and shallow earthquakes, respectively. Red lines
indicate S-wave reflectors (bright spots) (Hori et al., 2004), and red triangles on the top denote active volcanoes. Open circles in (d) indicate
shallow earthquakes.
72
A. Hasegawa et al. / Tectonophysics 403 (2005) 59–75
Nakajima and Hasegawa (2003b) inferred from the
rates of decrease of Vp and Vs that about 1% melting
occurs in the upper mantle, while several percent
melting occurs in the lower crust, with about 0.3–5%
water in the upper crust in this low-velocity zone. An
MT survey conducted in the hypocentral region of the
1962 Northern Miyagi earthquake detected a clear
low-resistivity zone in almost exactly the same
location as the low seismic velocity zone as shown
in Fig. 12(d) (Mitsuhata et al., 2002). Immediately
above this zone there is a sheet-like distribution of
microearthquakes, dipping to the west, representing
aftershocks of the 1962 Northern Miyagi earthquake
(Kono et al., 1993).
From these observations, we infer that water of
slab origin is supplied not only to the Backbone
Range but also to the hypocentral region of the 1962
Northern Miyagi earthquake on the fore-arc side. It is
conceivable that this causes local contractive deformation in this region as well as in the Backbone
Range.
In the location where contractive deformation
occurs locally, not only are microearthquakes concentrated (Fig. 10), but large earthquakes that rupture
the entire seismogenic layer also occur. We infer that
contractive deformation occurs principally as anelastic
deformation where the entire crust including the upper
crust has been locally weakened. The observations
given below suggest that since this kind of anelastic
deformation does not proceed uniformly in space,
large earthquakes that cause the overall contractive
deformation to become uniform occur at locations of
smaller contractive deformation.
Depth (km)
0
A
Fig. 13 shows the Vp / Vs ratio on a vertical crosssection along the Backbone Range. Regions of high
Vp / Vs ratio, believed to be regions of partial melting,
are distributed immediately beneath two volcanic
areas, in the north and south, reaching intermediate
crustal depths. In these two areas, the amount of melt
supplied from the mantle wedge, and consequently the
amount of water, must be greater than the area
between the two areas. Accordingly, within these
two areas, it can be expected that weakening of the
crust will be considerable and that local contractive
deformation will proceed rapidly. If this is the case,
then stress will be concentrated in the area between
these two areas, perhaps causing a reverse fault
earthquake to occur at the edge of the Backbone
Range (or inside it), as shown schematically in Fig.
11(b). In fact, the fault plane of some large earthquakes such as the 1896 Rikuu earthquake (M7.2)
was not within these two volcanic areas, but at the
western and eastern edges of the area (or inside it)
between them (Active Fault Research Group, 1991).
Even within the volcanic area, it appears that the
same kind of phenomenon is taking place, although
on a smaller scale. Fig. 14 shows the S-wave velocity
distribution at a depth of 4.5 km in the Onikobe area
of northern Miyagi Prefecture (Onodera et al., 1998),
which is part of the above-mentioned volcanic area. In
this area, the lower boundary of the seismogenic layer
(the brittle to ductile transition zone) is relatively
shallow, on the order of 7 km (Hasegawa et al., 2000).
The estimated velocity distribution shows that the
velocity within the caldera is low while that outside
the caldera is high. It is expected that more water is
Mt. Naruko Mt. Kurikoma
Mt. Akitakoma
B
10
20
30
B
40
50
0
50
100
150
A
Distance (km)
Vp/Vs
1.61
1.68
1.75
1.82
1.89
bright S-wave reflectors
low-F microearthquakes
Fig. 13. NS vertical cross-section of Vp / Vs structure in NE Japan along the line in the inserted map (Nakajima et al., 2001b). Other symbols are
the same as in Fig. 12.
A. Hasegawa et al. / Tectonophysics 403 (2005) 59–75
73
Fig. 14. S-wave velocity perturbations at 4.5 km depth (Onodera et al., 1998) and fault planes of earthquakes (Umino et al., 1998) in the
Onikobe area shown in the inserted map. Fault planes of earthquakes with magnitudes greater than ~5 are indicated by rectangles. Arrows in
each fault plane show slip vectors. Small circles denote aftershocks of the M5.9 Onikobe earthquake sequence in 1996. Caldera rims are
indicated by bold lines (Yoshida, 2001), and red triangles denote active volcanoes.
supplied within the caldera than outside the caldera,
and consequently there will be considerable anelastic
contractive deformation within the caldera. In 1996
there was considerable seismic activity in this region,
with the largest earthquake a M5.9 event. In this
region, where the seismogenic layer is locally thin,
and at depths of around 7 km, the M5.9 earthquake
was sufficient to rupture the entire seismogenic layer
(Umino and Hasegawa, 2002). From Fig. 14, relatively large earthquakes for this region (M5 class)
occur not inside the calderas, but around them. In
particular, the M5.9 earthquake occurred between the
Sanzugawa caldera and the Onikobe caldera. Thus,
the M5.9 earthquake occurred in the region between
the calderas to compensate for the delay in the
progress of anelastic contractive deformation. This
phenomenon is similar to that shown schematically in
Fig. 11(b), but on a smaller scale.
6. Concluding remarks
In the northeastern Japan arc, shallow earthquakes
are concentrated in a region of large contractive
deformation in the direction of relative plate motion.
Research based on a comparison of crustal horizontal
deformation rates over the last 100 years has
previously confirmed that such a region also corresponds to a region of low seismic velocity (Hasegawa
et al., 2000). Based on these observations, Hasegawa
et al. (2000) inferred the upwelling of water from
depth to weaken the crust and increase local crustal
74
A. Hasegawa et al. / Tectonophysics 403 (2005) 59–75
contraction rates, resulting in shallow crustal earthquakes in such areas.
In the present paper, this concept was extended,
and a simple model was proposed based on the highresolution three-dimensional velocity structure determined by seismic tomography and detailed crustal
deformation determined by GPS. The model facilitates our understanding of the processes of deformation in the island arc and the occurrence of shallow
inland earthquakes in northeastern Japan. Although
the validity of this model must await future verification, at least water plays an important role in crustal
deformation and in the occurrence of shallow inland
earthquakes.
Acknowledgments
We wish to express our thanks for comments by H.
Iwamori, Y. Iio, T. Matsuzawa, and two anonymous
reviewers which have contributed importantly to
improve this paper. This work was partially supported
by a grant from the Ministry of Education, Culture,
Sports, Science and Technology of Japan.
References
Abers, G.A., 1994. Three-dimensional inversion of regional P and S
arrival times in the East Aleutians and source of subduction
zone gravity highs. J. Geophys. Res. 99, 4395 – 4412.
Active Fault Research Group, 1991. Active Faults in Japan, Rev.
Ed. University of Tokyo Press, Tokyo. 437 pp. (in Japanese).
Davies, J.H., Stevenson, D.J., 1992. Physical model of source
region of subduction zone volcanics. J. Geophys. Res. 97,
2037 – 2070.
Demets, C., Gorden, R.G., Argus, D.F., Stein, S., 1994. Effect of
recent revisions to the geomagnetic reversal time scale on
estimates of current plate motions. Geophys. Res. Lett. 21,
2191 – 2194.
Eberle, M.A., Grasset, O., Sotin, C., 2002. A numerical study of the
interaction between the mantle wedge, subducting slab, and
overriding plate. Phys. Earth Planet. Inter. 134, 191 – 202.
Gorbatov, A., Domingues, J., Suarez, G., Kostoglodov, V., Zhao, D.,
Gordeev, E., 1999. Tomographic imaging of the P-wave velocity
structure beneath the Kamchatka peninsula. Geophys. J. Int.
137, 269 – 279.
Furukawa, Y., 1993. Depth of the decoupling plate interface
and thermal structure under arcs. J. Geophys. Res. 98,
20005 – 20013.
Hasegawa, A., Yamamoto, A., 1994. Deep, low-frequency microearthquakes in or around seismic low-velocity zones beneath
active volcanoes in northeastern Japan. Tectonophysics 233,
233 – 252.
Hasegawa, A., Nakajima, J., 2004. Geophysical constraints on slab
subduction and arc magmatism, AGU Geophys. Monograph,
150, IUGG volume 19, 81–94,2005.
Hasegawa, A., Zhao, D., Hori, S., Yamamoto, A., Horiuchi, S.,
1991. Deep structure of the northeastern Japan arc and
its relationship to seismic and volcanic activity. Nature 352,
683 – 689.
Hasegawa, A., Yamamoto, A., Umino, N., Miura, S., Horiuchi, S.,
Zhao, D., Sato, H., 2000. Seismic activity and deformation
process of the crust within the overriding plate in the northeastern Japan subduction zone. Tectonophysics 319, 225 – 239.
Hasegawa, A., Uchida, N., Igarashi, T., Matsuzawa, T., Okada, T.,
Miura, S., Suwa, Y., in press. Asperities and quasi-static slips on
the subducting plate boundary east off Tohoku, NE Japan,
SEIZE volume, Columbia University Press.
Hori, S., Umino, N., Kono, T., Hasegawa, A., 2004. Distinct S-wave
reflectors (bright spots) extensively distributed in the crust and
upper mantle beneath the northeastern Japan arc. J. Seismol.
Soc. Jpn., Ser. 2 (56), 435 – 446 (in Japanese).
Iio, Y., 1996. A possible generating process of the 1995 southern
Hyogo prefecture earthquake—stick of fault and slip on
detachment. J. Seismol. Soc. Jpn., Ser. 2 (49), 103 – 112 (in
Japanese).
Iio, Y., 1998. jhmin—a role on the generation of earthquake. J.
Seismol. Soc. Jpn., Ser. 2 (50), 273 – 281 (in Japanese).
Iio, Y., Kobayashi, Y., Sagiya, T., Shiozaki, I., 2000. Water in the
lower crust controls the deformation of the arc crust. Earth Mon.
247, 37 – 44 (in Japanese).
Iio, Y., Sagiya, T., Kobayashi, Y., Shiozaki, I., 2002. Waterweakened lower crust and its role in the concentrated
deformation in the Japanese Islands. Earth Planet. Sci. Lett.
203, 245 – 253.
Iwamori, H., 1998. Transportation of H2O and melting in
subduction zones. Earth Planet. Sci. Lett. 160, 65 – 80.
Kono, T., Nida, K., Matsumoto, S., Horiuchi, S., Okada, T.,
Kaihara, K., Hasegawa, A., Hori, S., Umino, N., Suzuki, M.,
1993. Microearthquake activity in the focal area of the 1962
Northern Miyagi earthquake (M6.5). J. Seismol. Soc. Jpn., Ser.
2 (46), 85 – 94 (in Japanese).
Lay, T., Kanamori, H., 1981. In: Simpson, D.W., Richards, P.G.
(Eds.), Earthquake Prediction, An International Review, Maurice Ewing Ser., vol. 4. A.G.U, pp. 579 – 592.
Matsumoto, S., Hasegawa, A., 1996. Distinct S-wave reflector in
the mid-crust beneath Nikko-Shirane volcano in the northeastern
Japan arc. J. Geophys. Res. 101, 3067 – 3083.
Matsuzawa, T., Igarashi, T., Hasegawa, A., 2002. Characteristic
small-earthquake sequence off Sanriku, northeastern
Honshu, Japan. Geophys. Res. Lett. 29 (11), doi:10.1029/
2001GL014632.
McKenzie, D.P., 1969. Speculations on the consequences and
causes of plate motions. Geophys. J. R. Astron. Soc. 18, 1 – 32.
Mitsuhata, Y., Ogawa, Y., Mishina, M., Kono, T., Yokokura, T.,
Uchida, T., 2002. Electromagnetic heterogeneity of the seismigenic region of 1962 Northern Miyagi Earthquake, northeastern
Japan. Geophys. Res. Lett. 28, 4371 – 4374.
A. Hasegawa et al. / Tectonophysics 403 (2005) 59–75
Miura, S., Sato, T., Tachibana, K., Satake, Y., Hasegawa, A., 2002.
Strain accumulation in and around Ou Backbone Range,
Northeastern Japan as observed by a Dense GPS Network.
Earth Planets Space 54, 1071 – 1076.
Nagai, R., Kikuchi, M., Yamanaka, Y., 2001. Comparative study
on the source processes of recurrent large earthquakes in
Sanriku-oki region: the 1968 Tokachi-oki earthquake and the
1994 Sanriku-oki earthquake. J. Seismol. Soc. Jpn., Ser. 2 (54),
267 – 280 (in Japanese).
Nakajima, J., Hasegawa, A., 2003a. Estimation of thermal structure
in the mantle wedge of northeastern Japan from seismic
attenuation data. Geophys. Res. Lett. 30 (14), 1760,
doi:10.1029/2003GL017185.
Nakajima, J., Hasegawa, A., 2003b. Tomographic imaging of
seismic velocity structure in and around the Onikobe volcanic
area, northeastern Japan: implications for fluid distribution.
J. Volcanol. Geotherm. Res. 127, 1 – 18.
Nakajima, J., Hasegawa, A., 2004. Shear-wave polarization
anisotropy and subduction-induced flow in the mantle wedge
of northeastern Japan. Earth Planet. Sci. Lett. 225, 365 – 377.
Nakajima, J., Matsuzawa, T., Hasegawa, A., Zhao, D., 2001a.
Three-dimensional structure of Vp, Vs, and Vp/Vs beneath
northeastern Japan: implications for arc magmatism and fluids.
J. Geophys. Res. 106, 843 – 857.
Nakajima, J., Matsuzawa, T., Hasegawa, A., Zhao, D., 2001b.
Seismic imaging of arc magma and fluids under the central part
of northeastern Japan. Tectonophysics 341, 1 – 17.
Nakajima, J., Takei, Y., Hasegawa, A., in press. Quantitative
analysis of the inclined low-velocity zone in the mantle wedge
of northeastern Japan: a systematic change of melt-filled pore
shapes with depth and its implications for melt migration, Earth
Planet. Sci. Lett.
Okada, T., Hasegawa, A., 2000. Activity of deep low-frequency
microearthquakes and their moment tensors in northeastern
Japan. J. Volcanol. Soc. Jpn. 45, 47 – 63 (in Japanese).
Okada, T., Matsuzawa, T., Hasegawa, A., 2003. Comparison of
source areas of M4.8+-0.1 earthquakes off Kamaishi, NE
Japan—are asperities persistent feature? Earth Planet. Sci. Lett.
213, 361 – 374.
Onodera, M., Horiuchi, S., Hasegawa, A., 1998. Three-dimensional
seismic velocity structure in and around the focal area of the
1996 Onikobe earthquake based on Vp/Vs inversion. J. Seismol.
Soc. Jpn., Ser. 2 (51), 265 – 279 (in Japanese).
Ribe, N.M., 1992. On the relation between seismic anisotropy and
finite strain. J. Geophys. Res. 97, 8737 – 8747.
Sato, T., Miura, S., Tachibana, K., Satake, Y., Hasegawa, A., 2002.
Crustal deformation around Ou Backbone range, northeastern
Japan observed by a dense GPS network. J. Seismol. Soc. Jpn.,
Ser. 2 (55), 181 – 191 (in Japanese).
Sato, T., Miura, S., Suwa, Y., Hasegawa, A., Tachibana, K., 2003.
Crustal deformation of the NE Japan arc as derived by
continuous GPS observations. IUGG 2003 Scientific Program
and Abstracts, p. B11. JSG01/08P/D-006.
75
Schmidt, M., Poli, S., 1998. Experimentally based water budgets for
dehydrating slabs and consequences for arc magma generation.
Earth Planet. Sci. Lett. 163, 361 – 379.
Suwa, Y., Miura, S., Hasegawa, A., Sato, T., Tachibana, K., 2003.
Inter-plate coupling beneath the NE Japan arc Inferred from 3
dimensional crustal deformation. IUGG 2003 Scientific Program and Abstracts, p. B10. JSG01/08P/D-002.
Tamura, Y., Tatsumi, Y., Zhao, D., Kido, Y., Shukuno, H., 2002. Hot
fingers in the mantle wedge: new insights into magma genesis in
subduction zones. Earth Planet. Sci. Lett. 197, 105 – 116.
Tanaka, A., Ishikawa, Y., 2002. Temperature distribution and focal
depth in the crust of the northeastern Japan. Earth Planets Space
54, 1109 – 1113.
Tommasi, A., 1998. Forward modeling of the development of
seismic anisotropy in the upper mantle. Earth Planet. Sci. Lett.
160, 1 – 13.
Tsumura, N., Matsumoto, S., Horiuchi, S., Hasegawa, A., 2000.
Three-dimensional attenuation structure beneath the northeastern Japan arc estimated from spectra of small earthquakes.
Tectonophysics 319, 241 – 260.
Umino, N., Hasegawa, A., 2002. Inhomogeneous structure of the
crust and its relationship to earthquake occurrence. In: Fujinawa,
Y., Yoshida, A. (Eds.), Seismotectonics in Convergent Plate
Boundary. Terrapub, Tokyo, pp. 225 – 235.
Umino, N., Matsuzawa, T., Hori, S., Nakamura, A., Yamamoto, A.,
Hasegawa, A., Yoshida, T., 1998. 1996 Onkobe earthquakes and
their relation to crustal structure. J. Seismol. Soc. Jpn., Ser. 2
(51), 253 – 264 (in Japanese).
Yamanaka, Y., Kikuchi, M., 2004. Asperity map along the
subduction zones in northeastern Japan inferred from regional
seismic data. J. Geophys. Res. 109, B07307, doi:10.1029/
2003JB002683.
Yoshida, T., 2001. The evolution of arc magmatism in the NE
Honshu arc Japan. Sci. Rep. Tohoku Univ. 36, 131 – 149.
Zhang, S., Karato, S., 1995. Lattice preferred orientation of olivine
aggregates deformation in simple shear. Nature 375, 744 – 777.
Zhao, D., Hasegawa, A., 1993. P-wave tomographic imaging of the
crust and upper mantle beneath the Japan Islands. J. Geophys.
Res. 98, 4333 – 4353.
Zhao, D., Hasegawa, A., Horiuchi, S., 1992. Tomographic imaging
of P and S wave velocity structure beneath northeastern Japan.
J. Geophys. Res. 97, 19909 – 19928.
Zhao, D., Christensen, D., Pulpan, H., 1995. Tomographic
imaging of the Alaska subduction zone. J. Geophys. Res. 100,
6487 – 6504.
Zhao, D., Xu, Y., Wiens, D.A., Dorman, L., Hildebrand, J., Webb,
S., 1997. Depth extent of the Lau back-arc spreading center and
its relation to subduction processes. Science 278, 254 – 257.
Zumberge, J.F., Heflin, M.B., Jefferson, D.C., Watkins, M.M.,
Webb, F.H., 1997. Precise point positioning for the efficient and
robust analysis of GPS data from large networks. J. Geophys.
Res. 102, 5005 – 5017.