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Tectonophysics 403

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 low-

velocity 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

0040-1951/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.tecto.2005.03.018

T Corresponding author. Tel.: +81 22 225 1950; fax: +81 22 264

3292.

E-mail address: hasegawa@aob.geophys.tohoku.ac.jp

(A. Hasegawa).

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 earth-

quakes 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

(2005) 59–75

A. Hasegawa et al. / Tectonophysics 403 (2005) 59–7560

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

Fig. 1. Map showing the northeastern Japan arc and its surroundings. Red t

front, respectively. White arrow indicates the direction of the relative plate m

Coast Guard. 1. Iwate volcano, 2. Naruko volcano.

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 concen-

tration 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 con-

centrated 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

riangles and thick gray line denote active volcanoes and the volcanic

otion (Demets et al., 1994). The bathymetry is taken from the Japan

A. Hasegawa et al. / Tectonophysics 403 (2005) 59–75 61

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 north-

eastern 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 low-

velocity, 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 occur-

rence of shallow inland earthquakes in the north-

eastern Japan arc.

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-dimen-

sional seismic wave velocity structure for the north-

eastern 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 perpen-

dicular to the island arc. In any of the vertical cross-

sections (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 distribu-

tion 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 struc-

ture 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

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.

A. Hasegawa et al. / Tectonophysics 403 (2005) 59–7562

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 (McKen-

zie, 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 immedi-

ately 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 trans-

ported 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

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.

A. Hasegawa et al. / Tectonophysics 403 (2005) 59–75 63

solidus (Nakajima and Hasegawa, 2003a). Further,

Nakajima et al., in press inferred from the ratio of fall-

off 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 low-

velocity zone observed in northeastern Japan (Figs. 2

and 3).

The inferred water transport paths in the northeast-

ern 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 decompres-

sion 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

Depth = 40 km

1.65 1.70 1.75 1.80 1.85

Vp/Vs

139° 140° 141° 142°

37°

38°

39°

40°

41°

Fig. 4. Vp /Vs ratio at a depth of 40 km (Nakajima et al., 2001a).

Red triangles denote active volcanoes.

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) 59–7564

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 mate-

rial 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

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

,

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.

A. Hasegawa et al. / Tectonophysics 403 (2005) 59–75 65

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) dis-

tributed across a width of 50 km in the mantle wedge

140° 140.5° 141° 141.5° 142°

38.5°

39°

39.5°

40°

-6 -3 0 3 6Velocity perturbation (%)

0.21 0.14 0.07Delay 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).

A. Hasegawa et al. / Tectonophysics 403 (2005) 59–7566

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 sur-

face 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-

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).

A. Hasegawa et al. / Tectonophysics 403 (2005) 59–75 67

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 volca-

noes at the surface, 4) The distribution of topo-

graphical 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 expan-

sion 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, immedi-

ately 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 tomog-

raphy has revealed that in the volcanic zones,

differentiation occurs and magma rises to the middle

crust (see Figs. 12 and 13).

-1

-5 0 5

41°

A. Hasegawa et al. / Tectonophysics 403 (2005) 59–7568

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 bed-

rock. 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.

-2

-2

-2

-2

-2-2

-2

-1

-1

-1

-1

-1

-1

-1

-1

00

0

10-7/yr.

37°

38°

39°

40°

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 e

al., 2003). Contour interval is 100 ppb/year. Red triangles denote

active volcanoes.

4. Zones of concentrated deformation along the

Backbone Range

Observational data on the surface deformation field

obtained from the nationwide GPS continuous obser-

vation 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 observa-

tional 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 Labo-

ratory (JPL) of the American National Atmospheric

and Space Administration (NASA) was used for GPS

data analysis. This analysis software estimates param-

eters 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

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

t

40°

41°

-5 0 5

10-7/yr.

A. Hasegawa et al. / Tectonophysics 403 (2005) 59–75 69

components are much smaller than east–west compo-

nents. It can be seen from Fig. 9 that there is a belt-

like zone in which contractive deformation is con-

centrated 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.

139° 140° 141° 142°37°

38°

39°

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.

5. Deformation of the arc crust and shallow inland

earthquakes—their relationship with fluids

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 microearth-

quakes (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 north-

eastern 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

the lowermost crust (Hasegawa et al., 1991; Hase-

gawa 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.

** *

**

***

**

**

WEST EAST

seismogenic zone

brittle toductile transition

lower crust

Moho

upper mantle

large earthquake

small earthquakes

S-wave reflectorslow-V

low-F events

upwelling flow

Backbone Range

(partly anelastic deformation)

* * *

contraction & uplift

low-Qlow-V

Backbone Range

partly anelastic deformation

large contraction

small contraction

large contraction

elastic deformation elastic deformation

reverse fault

reverse fault

(a)

(b)

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.

A. Hasegawa et al. / Tectonophysics 403 (2005) 59–7570

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 71

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 deforma-

tion 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 defor-

mation predominates, in addition to the Backbone

Range where the upwelling flow in the mantle wedge

reaches the Moho. This region, in northern Miyagi

140.25 E 141.25 E

Dep

th (

km)

Dep

th (

km)

Naruko volcano

(

bright S-walow-F micro

0

10

20

30

40

50

0

10

20

30

40

50

1.61 1.68 1.75 1.82 1.89Vp/Vs

dVp(a)

Vp/Vs(c)

(

Fig. 12. EW vertical cross-sections of (a) P-wave and (b) S-wave velocity

electrical resistivity (Mitsuhata et al., 2002) along line a in Fig. 10. Rectan

horizontal and vertical directions. Red circles and dots denote low-frequenc

indicate S-wave reflectors (bright spots) (Hori et al., 2004), and red triang

shallow earthquakes.

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, mak-

ing 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 low-

velocity 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).

dv(%)Naruko volcano

Ma g .1 5

++

+ +

+

C1C2

C3

R1R2

d)

Distance (km)

Dep

th (

km)

0

10

20

1000

100

10

1

Electrical Resistivityve reflectorsearthquakes

-10

-5

0

5

10

dVsb)

perturbations, (c) Vp/Vs (Nakajima and Hasegawa, 2003b), and (d)

gles in (a), (b) and (c) show the range of cross-section in (d) in both

y microearthquakes and shallow earthquakes, respectively. Red lines

les on the top denote active volcanoes. Open circles in (d) indicate

A. Hasegawa et al. / Tectonophysics 403 (2005) 59–7572

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 defor-

mation in this region as well as in the Backbone

Range.

In the location where contractive deformation

occurs locally, not only are microearthquakes con-

centrated (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.

Dep

th (

km)

0 50 1

Distance (km

A0

10

20

30

40

50

1.61 1.68 1.75 1.82 1.8

Vp/Vs

Mt. Naruko Mt. Kurikoma

Fig. 13. NS vertical cross-section of Vp/Vs structure in NE Japan along the

the same as in Fig. 12.

Fig. 13 shows the Vp/Vs ratio on a vertical cross-

section 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 earth-

quakes 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

00 150

)

B

bright S-wave reflectorslow-F microearthquakes

9

A

B

Mt. Akitakoma

line in the inserted map (Nakajima et al., 2001b). Other symbols are

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.

A. Hasegawa et al. / Tectonophysics 403 (2005) 59–75 73

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, rela-

tively 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 corre-

sponds 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

A. Hasegawa et al. / Tectonophysics 403 (2005) 59–7574

contraction rates, resulting in shallow crustal earth-

quakes in such areas.

In the present paper, this concept was extended,

and a simple model was proposed based on the high-

resolution three-dimensional velocity structure deter-

mined by seismic tomography and detailed crustal

deformation determined by GPS. The model facili-

tates our understanding of the processes of deforma-

tion in the island arc and the occurrence of shallow

inland earthquakes in northeastern Japan. Although

the validity of this model must await future verifica-

tion, 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.

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