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1.1.1

1980 COCORP Brown, 1991 Lithoprobe Cook et al., 1992

laminate

Improved Mapping Method for Deep Crustal Imaging - Application to Wide-angle Refl ectionData in the Southwest Japan Arc and its Interpretation -

Tetsuya TAKEDA

Solid Earth Research Group,National Research Institute for Earth Science and Disaster Prevention, Japan

ttakeda@bosai.go.jp

Abstract

Wide-angle refl ection data often show remarkable refl ections from the deep crust. In order to obtain new images of crustal inhomogeneity beneath the southwest Japan arc, I propose an improved mapping method designed for sparsewide-angle refl ection data. The method, an improved form of the Common Scatter-Point (CSP) stacking method, has theadvantages of both the Common Mid-Point (CMP) and CSP stacking methods, which means that it can add a migrationeffect to the CMP method, and has less of the ghost curves that results from the CSP method. When applied to twosets of wide-angle reflection data (the 1988 Kawachinagano-Kiwa profile and the 1989 Fujihashi-Kamigori profile),the method provided clear images of the subducting Philippine Sea plate and the island-arc’s lower crust. The mainresults are as follows: (1) Intra-plate seismic activity is concentrated within the oceanic mantle, which suggests that theoceanic mantle may be subject to dehydration embrittlement; and (2) The lower crust beneath the southwest Japan archas strong inhomogeneity, with a wavelength ranging from several to a dozen kilometers, which may result from crustalreconstruction due to igneous activity that has occurred since the Cretaceous.

Key words : Common Scatter-Point (CSP) stacking method, Wide-angle refl ection, The 1988 Kawachinagano-Kiwa profi le, The 1989 Fujihashi-Kamigori profi le, The Philippine Sea plate

Allmendinger et al., 1986Moho

MohoMoho 3-5km

Hale and Thompson, 1982

1.2

1985,

68 2005 9

176

1999km

Ikami et al., 1986, Takeda et al., 2004, 1997

PnMoho

32-35km27km

Iwasaki et al., 2001P S

Moho

TASP Sato et al.,1998Iwasaki et al., 1999

Vibroseis®

Lithoprobe

pop-up1999

Vibroseis®

Moho

1.3

Chang et al. 1989Zelt et al. 1998 Chang et al.

1989

Zelt et al. 1998 syntheticOBH Ocean-Bottom Hydrophone

OBH 2km4%

1991 Matsu’ura et al. 19911991

CMP

CMP

100m

1999

S/N

S/N1 100m

1.4

1.1Moho

1.11989 S-1

6km/sec reduceFig. 1.1 Example of the record section of the refraction survey

the 1989 Fujihashi-Kamigori profi le conducted by the Research Group for Explosion Seismology RGES . The reduction velocity is 6km/sec. Wide-angle refl ections from the crustal lower part are identifi ed in the area enclosed by the trapezoid.

177

2

Moho

2.2.1

LithoprobeLithoprobe

Cook et al., 1992

1995 2.1

1/30 1/500

1.5km

150-200km

CMP

2

2.2 CMP2.2.1 CMP

2.1 a2

CMP Common MidPoint

2.1 b

NMO NMO

CMP

CMPP S

multiple NMO

n S/N n1/2

CMP

2.1 Lithoprobe Southern Canadian CordelleraCook et al., 1992

1995Table 2.1 Specification comparison between a continental deep

seismic survey of the Southern Canadian Cordellera Cook et al., 1992 from the Lithoprobe project, and

a wide-angle reflection/refraction survey from the 1989 Fujihashi-Kamigori profile Research Group for Explosion Seismology, 1995 .

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178

2.2.2 CMPCMP 1 2

CMP

CMP

CMP S/NCMP

2.32.3.1

2.2

t=0t=t0

t=t0

3t=t0

2.4.3

2.2

Fig. 2.2 Scheme of the Common Scatter-Point CSP stacking method. Amplitudes are allocated on the isochrone of the scatter traveltime left . The scatter point is emphasized after the allocated amplitudes are stacked right .

2.1 CMP a 2 b NMOc NMO d CMP

Fig. 2.1 Scheme of the Common Mid-Point CMP stacking method. a Simple model with a singlehorizontal refl ector triangles and squares indicate shot points and receivers, respectively b BeforeNMO correction c After NMO correction d CMP stacking after NMO correction.

179

2.3.2

0.5-2km

2.42.4.1

2 2.2

CMP

CMP

2.4.2

2.3t=t0

1

0A t=t0 θ

G L t=t0

dl0 F control factor

θcontrol factor θ

θ

| θ F | > π/2 0

control factorθ 0 | θ |θ π/2 control

factor θcontrol factor

| θ |θ F| θ |θ

control factor F2.4 F→FF θ

θ =0θF→0FF 0 | θ |θ

π/2control factor F→FFCMP CMP

F→0FF

control factor CMP

control factor

2.2 CMPTable 2.2 Comparison between Common Mid-Point CMP and

Common Scatter-Point CSP stacking methods.

2.3 I

Fig. 2.3 Scheme of the improved mapping method. Amplitudes areallocated on the isochrone of the scatter traveltime using aweighting function.

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180

CMP

θ θ2

2.5 AB

SPA RPB 2

φ

θ φφ 3

F θ φ

φφ

control factor CMP

control factor

2.4.3

2.6TSPRTT

TSPRTT TSPTT + TPRTT 4

2.5 P 2

θ P ABφ

Fig. 2.5 Geometry of the angles θ andθ φ . θ is an angle between θthe normal line and the bisector of the scatter angle at the scatter point P. φ is an angle between the horizontal line φand the tangent at P.

2.62

Fig. 2.6 Scheme of making a scatter traveltime map. Maps of traveltime from a shot point upper left and from areceiver point lower left . A scatter traveltime map ismade from a summation of both the traveltime maps

right .

2.4 II control factorcontrol factor

CMP

Fig. 2.4 Schematic diagrams of the control factor’s effect. The improved mapping method comes close to the CMP stacking method as the control factor increases, and it comes close to the CSP stacking method as the control factor decreases.

181

TSP

TPR

2

1 1

Hole and Zelt 1995

TRPTT

TRTT P TPRTT 5

TSPRTT

TSPRTT TSPTT + TRPTT P 6

2.4.4

2λ

rvr

7

Rayleigh 1/4

1 23 control factor

1 rz

8

2s

in

9

3 control factorcontrol factor

control factor θ θ max

z d

10

3

2.4.5control factor

CMPcontrol

factor

3.3.1

1 23 control factor 4

control factor 41

NMO

CMP

2

3 control factor

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182

control factorCMP

4

250m 2

Hole and Zelt 1995synthetic Zelt and Smith 1992

convolutionsynthetic 100Hz

8.3Hz 1

3.2NMO

3.1a CMP

NMO 4NMO

b

NMONMO

100% c d NMO150% 200%

b

150%

200% 150%

4

3.270km

3.1 NMO aNMO b

c150% d 200%

Yilmaz 2001Fig. 3.1 Stretching effect caused by NMO correction Yilmaz,

2001 . a A synthetic CMP gather before NMO correction, b after NMO correction without stretch limits, c after muting using a stretch upper limit of 150 percent, and d after muting using a stretch upper limit of 200 percent.

3.270km

140km140km

Fig. 3.2 Schematic diagrams of the stretching effect caused by theimproved mapping method. Scatter traveltime maps showthe cases in which the source-receiver offsets were 70km

left and 140km right . The case of 140km has anisochrone contour with a wider interval.

3.3Fig. 3.3 Synthetic horizontal multi-layered model.

183

140km

1secNMO

NMO1

2 3

180km×50km 3.3

15km 25km 35km 45km6.1→6.4km/sec 6.4

→6.9km/sec 6.9→7.6km/sec 7.6→8.0km/sec1 1km 179

1990

3.44

3.5150% 170%

180% 190% 200% 210% 250%

3.4 synthetic 6km/sec reduce

Fig. 3.4 Synthetic seismograms reduced by a velocity of 6km/sec.

3.5 150%, 170%, 180%, 190%, 200%, 210%, 250%

Fig. 3.5 Effects of stretch upper limits in the improved mapping method. The stretch upper limits are 150, 170, 180, 190, 200, 210, and 250% as well as no limit.

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8

15km30km

45km 60km

150%

35km 40km 250%150%

35km70km

190%190%

190%3.3

3.62

3.6

43.7

21 I

1 23

1 2 II

III IVII 1 2

III 2

3.6

Fig. 3.6 Traveltime of head waves calculated using a finite difference method. In case of a large offset, head waves propagating in the lower layer at a high velocity are faster than direct waves.

3.7 4

Fig. 3.7 Calculation of traveltime map without head waves in a four-horizontal-layered model.

185

I IV I 1II 2

V

3.2

70 km3.8

1.5sec3.9

2sec140km 3.10

70km

10km 3.11

control factor F=1FF3.12

080km

80km

2.1L

3.8 70km

Fig. 3.8 Scatter traveltime map for a case with a source-receiver offset of 70km with left , and without right head waves.

3.9 70km

Fig. 3.9 Difference between scatter traveltime maps with and without head waves for an offset of 70km.

3.10 140km

Fig. 3.10 Scatter traveltime map for a case with a source-receiver offset of 140km with left , and without head waves

right .

3.11 140km

Fig. 3.11 Difference between scatter traveltime maps with and without head waves for an offset of 140km.

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190% F=10FF3.13

190%3.14 70km

190%20km

190

140km

40km 70km

3.15 2θ θ

control factor | θ | controlfactor | θ |

control factor

3.4 control factorcontrol

factor controlfactor CMP

2

10° 20° 3.1620km 40km

3.13

190% control factor 10

Fig. 3.13 Seismic sections applied with the improved mapping method using a stretch upper limit of 190% and a control factor of 10. With left and without right head waves.

3.14 190%70km

140kmFig. 3.14 Mapped area using a stretch upper limit of 190% for

offsets of 70km left , and 140km right .

3.15 70km 140km

θFig. 3.15 Distributions of θ for offsets of 70kmθ left and 140km

right .

3.12

control factor 1Fig. 3.12 Seismic sections applied with the improved mapping

method using no stretch upper limit and a control factor of 1. With left and without right head waves.

187

60km 3 2km 4590km× 50km synthetic

3.17 29 control factorF

1, 2.5, 5, 10, 20, 40, 80, 160, 320 93.18 2

F=1FFF=320FF CMP

F=1FF2 10° 20°

F=320FF CMP

120° 18°

10°control factor

F=20FF2

F=80FFcontrol factor

20°

F=2.5FF 20°F=5FF 20°

F=10FF 18°control factor

18° 10°10°

control factor

3 control factor

F=1FFF=320FF

3F=80FF

F

3.5 control factor

3.4control factor

9synthetic

0.15sec3.19

synthetic 3.20

3.16 210° 20°

Fig. 3.16 Synthetic velocity model. The two left dipping refl ectors have dip angles of 10° and 20°, respectively.

3.17 synthetic 20km6km/sec reduce

Fig. 3.17 Example of synthetic wave forms without noise. The shot point is located at an offset of 20km. The reduction velocity is 6km/sec.

68 2005 9

188

control factorF=1FF

F=5FF F=10FFF=20FF

40 S/NF

F=160FFF=320FFF=40FF

CMP

control factor

control factorcontrol factor

3.19 synthetic20km 5-10Hz

6km/sec reduceFig. 3.19 Example of synthetic wave forms with noise. The shot

point is located at an offset of 20km. The reduction velocity is 6km/sec.

3.18 synthetic control factor control factor 1, 2.5, 5, 10, 20, 40, 80, 160, 320

Fig. 3.18 Effect of the control factor of the improved mapping method, which is applied to synthetic data without noise. The control factor range is from 1 to 320. A white dashed line indicates a refl ector of the synthetic velocity model.

189

F=10FF 80control factor

3.6

control factor

control factorCMP

control factor

control factor

4.4.1

19881992 1989

1995 21988

1991CMP

1989 -

200kmPn Moho

3.20 synthetic control factor control factor 1, 2.5, 5, 10, 20, 40, 80, 160, 320

Fig. 3.20 Effect of the control factor of the improved mapping method, which is applied to synthetic data with noise. The control factor range is from 1 to 320. A white dashed line indicates a refl ector of the velocity model used.

68 2005 9

190

4.24.1 fl ow chart

2

8HzBandpass

S

S

P SVS P

S PBandpass

F-K

aliasingVp/Vs 1.73 S

S4.2 S

AGC Automatic Gain Control

S

DeconvolutionDeconvolution

DeconvolutionWiener Robinson and

Treitel, 1980 Wiener

4.3

4.1 fl ow chartFig. 4.1 Flow chart to process wide-angle reflection data in this

study.

4.2 SS

Fig. 4.2 Example of observed wave forms before left and after right the S-wave removal. First arrivals with large

amplitudes are also removed.

4.3 Deconvolution

0.6sec 0.07secFig. 4.3 Application of deconvolution. Before left and after

right deconvolution using an operator length of 0.6sec and a prediction lag of 0.07 sec.

191

Zelt and Smith 1992

8km8km 2

31

0.1sec

3

0.1sec2

4.3 19884.3.1

5 1988 11

19881992

4.4

5Ichikawa,

1980 km

30-40Ito et al., 1996

4cm/yearSeno et al., 1993

1944 M7.91946 M8.0

684 176Ando, 1975 Ando 1975 19441946

65km 8615km

S-1 S-6 64.5

100Hz AD

4.4 1988 1989 - 1995Fig. 4.4 Geological map of the Kinki district including the 1988 Kawachinagano-Kiwa and the 1989 Fujihashi-Kamigori profi les, after

the Geological Survey of Japan 1995 .

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192

4.5 19881995 5 2001

41997

Fig. 4.5 Location map of the 1988 Kawachinagano-Kiwa profi le. Stars and open circles indicate shot and receiver points, respectively. Hypocenters of micro-earthquakes from May 1995 to April 2001 Nakamura et al., 1997 are superposed.

4.6 1988 S-1 5-8Hz6km/sec reduce

Fig. 4.6 Record section S-1 for the 1988 Kawachinagano-Kiwaprofile using a bandpass filter of 5-8Hz. The reductionvelocity is 6km/sec.

4.7 1988 S-6 5-8Hz6km/sec reduce

Fig. 4.7 Record section S-6 for the 1988 Kawachinagano-Kiwa profile using a bandpass filter of 5-8Hz. The reduction velocity is 6km/sec.

4.8 19888km

Fig. 4.8 Upper crustal velocity model for the 1988 Kawachinagano-Kiwa profi le after traveltime inversion.

4.9 1988

Fig. 4.9 Model parameters used for the traveltime inversion of the upper crustal velocity model for the 1988 Kawachinagano-Kiwa profile. The boundary and velocity nodes are indicated by squares and circles, respectively.

193

, 1992 S-14.6 15km 30km

24.6 a 35km

4.6 b S-6 4.74.7 c

5-7sec 50km-20-0km 24.7 d

P SS

4.3.28km

4304.8

4.94.1

4.10

95 4.10.2km

0.05km/sec 30.02km/sec

65km Pn

Sasaki et al., 197022km 6.4km/sec

35km 6.8km/sec 35km 7.6km/sec4.11

4.83.1-4.5km/sec

4.1 1988

Table 4.1 Resolutions and standard deviations of the model parameters used to invert the upper crustal velocity model for the 1988 Kawachinagano-Kiwa profi le.

4.10 1988

Fig. 4.10 Ray path diagram used for the inversion of upper the crustal velocity model for the 1988 Kawachinagano-Kiwa profi le.

4.11 1988

Fig. 4.11 Lower crustal velocity model for the 1988 Kawachinagano-Kiwa profi le. The velocity of the lower crust is assumed to refer to the 1989 Fujihashi-Kamigori this study and Kurayoshi-Hanafusa Yoshii et al., 1974 profi les.

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194

500m

1km 5.6km/sec

4.3.3control factor 1, 2.5, 5, 10, 20,

40, 80, 160, 320 94.12 3.5

3.19F=1FF

FF=20FF 40

F=80FFF

F=20FF 80

F=40FF 4.1330km

S-1 2

10° 15° 25kmS-6

33km 235km

20° 30°50km

Moho

4.12 1988 control factor 1, 2.5, 5, 10, 20, 40, 80, 160, 320

Fig. 4.12 Seismic sections of the 1988 Kawachinagano-Kiwa profi le. The range of the control factor FF is from 1 upper left to 320 lower right .

195

35km5

4.3.41991 CMP

4.14

1991

20-30kmS-6

4.7 21991 1

4.14 a1991

40km 14.14 b CMP

125km

35km

19914.14 c CMP

4.13 1988F=40FF

Fig. 4.13 Seismic section of the 1988 Kawachinagano-Kiwa profi le and its interpretation F=40FF .

4.14 1991 a2 b c

Fig. 4.14 Comparison between this study and the previous study. Cross-sections for this study left and for Yoshii 1991 right . a Undetectable double refl ectors, b undetectable multi refl ectors, c undetectable

change of the dip angle.

4.15 19891985 1 1994 12

1997Fig. 4.15 Location map of the 1989 Fujihashi-Kamigori profile. Stars and open

circles indicate shot and receiver points, respectively. Hypocenters of micro-earthquakes from May 1995 to April 2001 Nakamura et al., 1997are superposed.

68 2005 9

196

4.4 19894.4.1

19891995 4.15

4.4S-3

1987

Yoshii et al., 1974 1

2 30-40km 3 Pn7.8km/sec

4cm/yearSeno et al., 1993

Watanabe et al. 1990 1997

50km

198860-70km

210km

4.16 1989 S-1 3-8Hz6km/sec reduce

Fig. 4.16 Record section S-1 of the 1989 Fujihashi-Kamigori profile using a bandpass filter of 3-8Hz. The reduction velocity is 6km/sec.

4.17 1989 S-4 3-8Hz6km/sec reduce

Fig. 4.17 Record section S-4 of the 1989 Fujihashi-Kamigori profile using a bandpass filter of 3-8Hz. The reduction velocity is 6km/sec.

4.18 19898km

Fig. 4.18 Upper crustal velocity model for the 1989 Fujihashi-Kamigori profi le after traveltime inversion.

4.19 1989

Fig. 4.19 Model parameters used for traveltime inversion of theupper crustal velocity model for the 1989 Fujihashi-Kamigori profi le. The boundary and velocity nodes areindicated by squares and circles, respectively.

197

1371.5km

S-1 S-44 500-800kg

70km

S-1 S-4180km 7km/sec

Pn 4.16 4.17S-1

reverberation2sec

S30km

S-1S/N S-4

4.4.28km

3104.18

4.194.2 4

70km4.20

90%3 99

4.290% 0.2km

0.1km/sec

Pn

40km

8km148

Pn 234.21

4.224.3

4.23 530% 4.3

690% 60%

0.05km/sec 0.6km8km 4.18

1km

S-1 S-31km

S-2 S-3 3.4km/secS-1 S-2 2.9km/sec

S-3S-4 5.3km/sec

Moho34km

7.6km/sec 25km 6.3km/sec 6.6km/sec

4.4.3F 1, 2.5,

5, 10, 20, 40, 80, 160, 320 94.24

70km 4 S/N

control factorcontrol factor

F=1FF CMP F=320FF

F 1 2.515-35km

F

F

F=20FF 80 S/N

CMP

F=40FF4.25

km25km 35km

10km refl ective zone

S-4refl ective zone

Moho33km

68 2005 9

198

4.2 1989

Table 4.2 Resolutions and standard deviations of the modelparameters used to invert the upper crustal velocitymodel for the 1989 Fujihashi-Kamigori profi le.

4.20 1989

Fig. 4.20 Ray path diagram used for the inversion of the upper crustal velocity model for the 1989 Fujihashi-Kamigoriprofi le.

4.21 1989 -

Fig. 4.21 Lower crustal velocity model for the 1989 Fujihashi-Kamigori profi le after traveltime inversion.

4.22 1989

Fig. 4.22 Model parameters used for the traveltime inversion of the lower crustal velocity model for the 1989 Fujihashi-Kamigori profi le. The boundary and velocity nodes areindicated by squares and circles, respectively.

4.3 1989

Table 4.3 Resolutions and standard deviations of the modelparameters used to invert the lower crustal velocitymodel for the 1989 Fujihashi-Kamigori profi le.

199

4.24 1989 control factor 1, 2.5, 5, 10, 20, 40, 80, 160, 320

Fig. 4.24 Seismic sections of the 1989 Fujihashi-Kamigori profi le. The range of the control factor FF is from 1 upper left to 320 lower right .

4.23 1989

Fig. 4.23 Ray path diagram used for the inversion of the lower crustal velocity model for the 1989 Fujihashi-Kamigori profi le.

4.25 1989F=40FF

Fig. 4.25 Seismic section of the 1989 Fujihashi-Kamigori profi le and its interpretation F=40FF .

68 2005 9

200

50km140-160km

refl ective zoneMoho

5

5.5.1 19885.1.1

4.13 25km50km

35km 20° 30°33km

2

1999

5.1Kodaira et al. 2001 2002

165km185km

150km2002

5.2 6°

12°2

2002 4km /sec1km

225km 2002

24km/sec

1.5km2002

1994160km

1997 5.3

19973°→5°→10°

5.4 1997

19971997

10°20° 2

10° 20°15° 3 10°

20km 15°24km 20° 38km

5.1 1999

2002Fig. 5.1 Location map of the seismic survey line conducted in

1999 solid line Kurashimo et al., 2002 . The lineintersects with the Nankai trough, in the eastern part of Shikoku Island and the Chugoku District.

5.2

2002Fig. 5.2 Geometry of the subducting Philippine Sea plate and the

crustal velocity structure Kurashimo et al., 2002 . Thehypocenters are also superimposed.

201

25km15°

1997

19972 4.2-4.6km/sec 5.3-5.9km/

sec 21.8km

2 2

5km/sec1.9km 1997 2

1.8km 223

1997

3°→5°→10° 20°

35km 30°2

33km2002

12°

19884.13 5.5

1995 7 2001 6

20°45km

30°

5.3 19941997

Fig. 5.3 Location map of the seismic line off of the Kii Peninsula in 1994 the easternmost north-south line in the figure

Nishisaka, 1997 .

5.41997

Fig.5.4 Geometry of the subducting Philippine Sea plate and the crustal velocity structure off of the Kii Peninsula

Nishisaka, 1997 .

5.5 1988

Fig. 5.5 Seismic section for the 1988 Kawachinagano-Kiwa profi le superimposed with hypocenters, which were provided by the Earthquake Observation Center of the Earthquake Research Institute at the University of Tokyo.

68 2005 9

202

10km

5.61.5km

5.7

5.85.5 1km

210±2.5km

7km1997

19975.2

5.6 5.5 1988 -

Fig. 5.6 Errors of the hypocenters shown in Fig. 5.5.

5.71997

Fig. 5.7 Velocity structure used for hypocentral determination leftmost Nakamura et al., 1997 .

5.8 1988

Fig. 5.8 Seismic section for the 1988 Kawachinagano-Kiwa profi le based on the same velocity structure used for the hypocentral determination.

203

10km

Seno et al. 2001

2000

40-60km 640-690Ulmer and Trommsdorff, 1995 Peacock and Wang

19995.9

40-60km14km 5.8

5.1.24.13 20km 25km

10 15°

underplating2 Matsuda and Isozaki 1991

5.10

underplating

25km

19972

33km

5.9 Peacock and Wang 1999

Peacock and Wang 1999Fig. 5.9 Thermal structure of the subducting Philippine Sea plate

from off of Shikoku Island Peacock and Wang, 1999 .

5.10 Matsuda and Isozaki 1991

Fig. 5.10 Schematic section of accretionary prism Matsuda and Isozaki, 1991 .

68 2005 9

204

Moho

Moho

Moho 1Moho

2 Moho

3 Moho

MohoMoho

5.2 19895.2.1

4.2525-35km refl ective

zonekm 10km

4.13

refl ectivesill

Mooney and Meissner, 1992

ductile fl owlaminate

sill

MohoMoho 34km

10km

Moho

Mooney and Meissner 1992Moho

3-5km

Braile and Chang 1986Moho

5.11

Moho

5.11 Braile and Chang 1986 Moho

Fig. 5.11 Moho transition zone model Braile and Chang, 1986 . The transition zone consists of random, thick-variable, high- and low-velocity lamellae.

205

MohoMoho

Moho

sill

Moho

5.2.2 50km

50km4.25

PS S-1

P S6km/sec reduce 5.12 a

3.46km/sec reduce 5.12 b5.12 b

1/1.73 2P

S50km S P

50kmP S

SP 1.2-3.0

Vp/VsS

5.12 1989 S-1 a 3-8Hz6km/sec

reduce b S2-5Hz 3.46km/sec reduce

Fig. 5.12 Record sections S-1 of the 1989 Fujihashi-Kamigoriprofile using a a bandpass filter of 3-8Hz and areduction velocity of 6km/sec, and b a bandpass fi lter of 2-5Hz and a reduction velocity of 3.46km/sec toclarify the S-wave refl ections.

5.13

Furukawa et al. 1998

Fig. 5.13 upper Distribution of heat fl ow data. The contour lines show estimated heat flow. lower Heat flow profile in the cross-arc direction using heat fl ow data from the rectangle in the upper fi gure Furukawa et al., 1998 .

68 2005 9

206

Furukawaet al., 1998 60-70mW/m2

5.13

50km

4cm/year Seno et al., 1993

Watanabe et al., 1990 ,1997 1997

1985-1994 10 13

5.14

50km

Seno

et al. 1993

5.1470km 5.1.1

14km

55km

50km

Seno et al. 2001

50km

6.Moho

control factorcontrol factor

CMP

synthetic

190%

control factorsynthetic

control factorcontrol factor 1 320

CMP

CMP

control factor

syntheticcontrol factor

5.14 1984 19941997

Seno et al. 1993

Fig. 5.14 Depth contours of the subcrustal earthquakes1984~1994 in southwestern Japan Nakamura et al.,

1997 . The solid triangles indicate the leading edgewhere the seismicity disappears. The solid line indicatesthe depths of the subcrustal earthquakes extrapolated along the plate motion as shown by Seno et al. 1993 .

207

2

CMP

30km 20° 30°

25-35kmkm- 10km refl ective zone

Moho10km

MohoBraile and Chang 1986

Moho50km

S P

Moho 19881989

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Accepted : April 20, 2005

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CMPCMP migration

2 1988 1989

1988 1989