Journal of Asian Earth Sciences 35 (2009) 377–390

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Journal of Asian Earth Sciences

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Crustal tomography under the Median Tectonic Line in Southwest Japanusing P and PmP data

Sandeep Gupta a,b,*, Dapeng Zhao b, M. Ikeda c, S. Ueki b, S.S. Rai a

a National Geophysical Research Institute, Hyderabad 500 007, Indiab Department of Geophysics, Tohoku University, Sendai 980-8578, Japanc Department of Civil Engineering, Shikoku Research Institute, Takamatsu 761-0192, Japan

a r t i c l e i n f o a b s t r a c t

Article history:Received 9 June 2008Received in revised form 24 December 2008Accepted 7 January 2009

Keywords:Median tectonic line (MTL)Seismic tomographyReflected wavesLow-velocity anomaliesFluids

1367-9120/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.jseaes.2009.01.004

* Corresponding author. Address: National GeoHyderabad, Andhra Pradesh 500 007, India. Tel.: +923434651, +81 22 2643292.

E-mail addresses: sandeepgupta@ngri.res.in, sandeezhao@aob.geophys.tohoku.ac.jp (D. Zhao).

The Median Tectonic Line (MTL) is the largest active fault system on the Japan Islands and underlies manydensely populated areas. To understand the seismotectonics and to reduce seismic hazard in the MTLregion, we determined a detailed 3-D crustal structure under the region using a large number of arrivaltimes of the first P-waves and Moho-reflected waves (PmP). Results of detailed resolution tests show thatthe addition of PmP data can significantly improve the resolution of the lower crustal structure and theentire crustal structure can be imaged better than that by using P-wave data alone. The obtained tomo-graphic images show that prominent low-velocity (low-V) anomalies are visible in the upper crust alongthe MTL. In addition, some low-V anomalies are visible in the lower crust on either side of the MTL. Theselow-V anomalies can be the weaker sections of the seismogenic crust and can be imprint of the existenceand migration of aqueous fluids released by the dehydration process of the subducting Philippine Seaplate under Southwest Japan.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

The Japanese Islands are broadly divided into Northeastern (NE)and Southwestern (SW) Japan based the geo-tectonic features. InSW Japan, the Pacific plate is descending from the east beneaththe Philippine Sea (PHS) and Eurasian plates, while the PHS plateis subducting beneath the Eurasian plate from the Nankai Troughat a rate of about 4 cm/year in the N50�W direction (Seno et al.,1993). Roughly parallel with the Nankai Trough, the east–westtrending Median Tectonic Line (MTL) is the largest arc-parallel ac-tive fault system in SW Japan and is located above the subductingPHS slab (Shiono, 1980; Zhao et al., 2004; Maruyama and Santosh,2007) (Fig. 1). The MTL fault system was formed probably duringthe Cretaceous and reactivated many times with both right- andleft-lateral strike-slip faultings (Tsutsumi et al., 1991; Tsutsumiand Okada, 1996).

Similar to the San Andreas and North Anatolian fault systems,the long MTL fault system probably does not rupture along its en-tire length during a single earthquake. As implied by its multipleseismic segmented characters, rupturing more likely occurs as suc-cessive, independent events (Tsutsumi and Okada, 1996; Goto

ll rights reserved.

physical Research Institute,1 40 23434700; fax: +91 40

p_ngri@yahoo.co.in (S. Gupta),

et al., 2001; Ikeda et al., 2006). Since A.D. 1500, eight destructiveearthquakes of magnitude (M) 6.5 or greater occurred along theMTL in Shikoku and western Kii Peninsula (Usami, 1996). Manygeological studies have also been made to investigate the pastactivity of the MTL (e.g., Tsutsumi and Okada, 1996; Goto et al.,2001). The earthquake recurrence interval is estimated to be1000–3000 years in Shikoku Island (Tsutsumi and Okada, 1996).But the historical records, geological studies, and the time-predict-able models for earthquake recurrence are not sufficient for fore-casting earthquakes in the MTL for next a few hundred years.Previous studies have shown that the presence of fluids in and be-low the seismogenic zone can affect the long-term structural evo-lution of the fault zone, and can alter the fault-zone strength andlocal stress regime (Sibson, 1992; Hickman et al., 1995). Moreover,Zhao et al. (1996, 2000) have shown the effect of fluids on theshortening of seismic cycles in a tectonically active region, takingthe 1995 Kobe earthquake (M 7.2) as an example in SW Japan.Therefore, studying the crustal heterogeneity (through detailedcrustal velocity structure), in particular, the role of fluids in theMTL fault system which underlies many densely populated areas,becomes very important to understand the seismotectonics andseismic hazard scenario in SW Japan.

Seismic tomography is a very useful tool for clarifying the crus-tal structure, seismotectonics, and volcanism. Using seismictomography, Zhao et al. (2000) have shown that in Japan thepresence and distribution of low-velocity (low-V) anomalies inthe crust and uppermost mantle are closely correlated with the

35

34

33136135134133132131

o

oN

oEo

EoE

oE

oE

oE

oN

oN

130 135 140 145o o o

45

40

35

30

o

o

o

o

o

SEA OF JAPAN

JAPA

NT

RA

NC

H

NANKAI

TROUGH

PACIFICOCEAN

Chugoku

MTL

Kyushu

KiiPeninsula

Shikoku

Longitude

Lat

itude

Fig. 1. Map showing this study area. Solid squares denote the seismic stations used in this study. Solid lines show active faults. MTL: the Median Tectonic Line. Open trianglesdenote the active arc volcanoes. The insert map shows the major plate boundaries in and around Japan.

378 S. Gupta et al. / Journal of Asian Earth Sciences 35 (2009) 377–390

occurrence of large crustal earthquakes as well as the distributionof active arc volcanoes and active faults. In addition, Lees and Nich-olson (1993) suggested that tomographic imaging offers an effec-tive means of investigating active faults at depth. Varioustomographic studies have been carried out in SW Japan to investi-gate 3-D seismic velocity structure beneath the MTL and its rela-tionship to other tectonic features in SW Japan (e.g., Salah andZhao, 2003a; Salah et al., 2005; Ikeda et al., 2006; Wang et al.,2006, and references therein). These studies suggest the existenceof strong lateral heterogeneities in the crust and upper mantle un-der SW Japan. However, the seismic structure of the lower crust isless well determined, as compared with that of the upper crust anduppermost mantle. This is because most earthquakes occur in theupper crust of the continental plate and in the subducting Pacificand PHS slabs, and thus seismic rays do not crisscross well in thelower crust. Addition of later-phase data can improve the lower

33

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35131 132 133 1

0

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20

30131 132 133 1

oE

oE

oE

oE

oE

oE

oN

oN

oN

Longitude

)mk(

htpeD

edutitaL

MTL

)mk(

htpeD

edutitaL

Fig. 2. Three-dimensional hypocenter distribution of the 300 crus

crustal resolution. Recent studies have shown that the use ofMoho-reflected waves (e.g., PmP, SmS, sSmS) can significantly im-prove the imaging of the lower crustal structure (Zhao et al., 2005;Xia et al., 2007; Sun et al., 2008; Lei et al., 2008).

Here we present a detailed 3-D P-wave velocity structure of thecrust beneath the MTL fault system in Shikoku and western KiiPeninsula of SW Japan (Fig. 1) using many high-quality first P-waveand Moho-reflected P-wave (PmP) data.

2. Data and method

We used arrival-time data of first P- and Moho-reflected P-waves (PmP) identified in the three-component seismograms from357 crustal earthquakes which occurred within 45 km on eitherside of the MTL and were recorded by 101 seismic stations

34 135 136

33

34

350 15 30

34 135 136

oN

oE

oE

oE

oE

oE

oN

oN

o

Depth (km)

tal earthquakes used in this study. Vertical exaggeration is 2.

S. Gupta et al. / Journal of Asian Earth Sciences 35 (2009) 377–390 379

(Fig. 1) of the High-Sensitivity Seismic Network (Hi-net), JapanMeteorological Agency (JMA), Japan University seismic networksfor the period from June 2002 to April 2007. Each selected earth-quake was recorded by more than 10 stations. These earthquakeshave local magnitudes from M 2.0 to M 4.0 and have focal depthsbetween 6 and 24 km. During the data selection process, we soughtto make the spatial distribution of the hypocenters as homoge-neous as possible so that we would get a reliable result. Neverthe-less, the hypocenter distribution was heterogeneous, andearthquakes were more concentrated in the central and easternShikoku and western Kii Peninsula (Fig. 2).

Although the crustal reflected waves from the Moho discontinu-ity (PmP) are often visible in seismograms, they have not been usedwidely in seismological research yet. In general, PmP phase is not soeasy to identify because its features (such as arrival time, ampli-tude, and polarity) are controlled by several factors, such as epicen-

bUD

RD

PUD

RD

PmP

UD

TR

UD

TR

TR TR

RD RD

UD (Vertical)

RD (Radial)

TR (Tangential)

P PmP

Ha

Fig. 3. (a) An example of rotated 3-component seismograms of a shallow crustal earthqu(PmP) is visible after the first P-wave arrival. (b) Particle motions of P and PmP phases.

tral distance, focal depth, station elevation, Moho depth, and crustalvelocity structure, and so on. We used the three-component seis-mograms and considered the waveform features of the PmP phasesummarized by the previous studies on the Japan Islands (Nakajimaet al., 2002; Xia et al., 2007; Sun et al., 2008). We identified the PmPphases which have large amplitude, arrive around 0.6–1.2 s afterthe first P arrival, and have particle motion at stations as compres-sional wave. An example of the rotated seismograms and particlemotions of first P and PmP phases for one earthquake-station pairis shown in Fig. 3. After very careful phase identification, we man-ually collected 5488 first P and 572 PmP arrival times from thethree-component, digital seismograms of 300 crustal earthquakesamong 357 selected crustal events. The picking accuracy of the firstP and PmP arrival times is 0.05–0.1 and 0.1–0.2 s, respectively. Raypath distributions of the first P data and the combined (P and PmP)data are shown in Fig. 4a and b, respectively.

132˚ 134˚ 136˚

33˚

34˚

35˚

Longitude

Lat

itud

e

N

E

stationearthquake

c

DP.IKD

YP:2003/01/22/19/01/06.27 134.632 33.970 14.96 M2.7

= 78.40 kmΔ

ake recorded at station DP.IKD (shown in Fig. 3c). A prominent later reflecting phase(c) Locations of the earthquake and station.

380 S. Gupta et al. / Journal of Asian Earth Sciences 35 (2009) 377–390

To determine the 3-D P-wave velocity structure of the crust inthe MTL region, we applied the updated tomography method (Zhaoet al., 2005) that can deal with reflected wave data (PmP, SmS,sSmS, etc.), in addition to the arrivals from direct and convertedphase, which improved the initial tomographic method of Zhaoet al. (1992a). This method uses a 3-D ray-tracing technique to

Fig. 4. Three-dimensional distribution of seismic ray paths: (a) P ra

investigate ray path variations of P and PmP phases due to lateralvelocity variations and can deal with a velocity model that containsseveral velocity discontinuities of complex geometry and with 3-Dvelocity variations everywhere in the model. The 1-D velocitymodel of Yoshii et al. (1974) derived from analysis of explosiondata is used as the initial model for the 3-D tomographic inver-

y paths and (b) P + PmP ray paths. Vertical exaggeration is 5.5.

S. Gupta et al. / Journal of Asian Earth Sciences 35 (2009) 377–390 381

sions. The lateral depth variations of the Conrad and Moho discon-tinuities under the study region (Zhao et al., 1992b; Shiomi et al.,2006) were taken into account in the 3-D ray tracing and the tomo-graphic inversion. The LSQR algorithm (Paige and Saunders, 1982)

Fig. 5. Plane views of P-wave velocity tomography obtained by using the first P-wave daeach map. Red and blue colors denote slow and fast velocities, respectively. The velocity psmall earthquakes during 2003–2007 and large historical earthquakes (1925–2000, M >show the active and Quaternary volcanoes. (For interpretation of the references to colou

with damping and smoothing regulations (Zhao et al., 2005), wasused to solve the observation equations. For the model space, weconsidered 33�–35�N, 131�–136�E and 0–40 km in latitude, longi-tude, and depth ranges, respectively. We set up grid nodes with a

ta (a), both P and PmP data (b). The layer depth is shown on the upper-left corner oferturbation (in %) scale is shown at the bottom. White dots and pink stars show the4.5), respectively, that occurred within 2 km of each depth slice. The red trianglesr in this figure legend, the reader is referred to the web version of this article.)

382 S. Gupta et al. / Journal of Asian Earth Sciences 35 (2009) 377–390

grid spacing of 0.20� and 0.25� in latitude and longitude directions,respectively. Six layers of grid mesh were set up at depths of 1, 5,10, 18, 28, and 40 km.

3. Results and resolution

We determined the 3-D P-wave velocity perturbations at thegrid nodes with hit count >10. The velocity perturbations are rela-tive to the average of the inverted velocity values at each depth.Our preferred final P-wave velocity images are obtained with theoptimal damping parameter determined from detailed analysesof the trade-off between the data variance reduction and the modelnorm. Fig. 5 shows P-wave velocity images in plane view, deter-mined by using only the P data (Fig. 5a) and the combined (Pand PmP) data (Fig. 5b). Large historical earthquakes (Usami,1996) and recent small earthquakes within 2 km of each depthlayer along with the active and Quaternary volcanoes are alsoshown. Fig. 6 exhibits locations of seven vertical cross-sections of

133132131 oE

oE

oE

Longitude

D

D’

G

H

IA

B

Fig. 6. Locations of seven vertical cross-sections, along and across the MTL used for show(Figs. 15–21). Solid squares denote the seismic stations.

0

10

20

30

40

0

10

20

30

40

)mk(

htpeD

132 133

Longitude

5 4 3 2

oE oE

A B

P

P + PmP

a

b

Fig. 7. Vertical cross-section of the P-wave velocity tomography along the MTL (A–B–C li(a), both P and PmP data (b). Red and blue colors denote slow and fast velocities, respectithe small earthquakes during 2003–2007 along the profile. Pink stars show the large histprofile. Vertical exaggeration is 3. (For interpretation of the references to colour in this

the tomographic images, along and across the MTL, as shown inFigs. 7–13. Background seismicity and large historical earthquakes(Usami, 1996) are also shown in the vertical cross-sections.

In order to ascertain the adequacy of ray coverage, spatial reso-lution and the main features of the tomographic images, we con-ducted detailed resolution tests. A direct way of these resolutiontests is to calculate a set of travel-time delays that result from trac-ing the corresponding rays through a synthetic structure asthrough they are real data, and then to compare the inversion re-sult with the initial synthetic structure. In the synthetic tests, thenumbers of seismic stations, events and ray paths are kept thesame as those in the real data set. In this study we conductedtwo kinds of resolution tests (Zhao et al., 1992a, 2005). We firstcarried out the checkerboard resolution tests (CRT) for evaluatingthe adequacy of ray coverage and spatial resolution of the entirestudy area by using only P-wave data (Fig. 14a) and using both Pand PmP data (Fig. 14b). The CRT is just a special form of a syn-thetic test with its diverse input model. To make a checkerboard,

136135134oEo

EoE

35

34

33oN

oN

oN

edutitaL

E F

E’ F’H’

I’

G’

C

ing tomographic images (Figs. 7–13) and their respective restoring resolution tests

134 135 136

1-6 -3 0 3 6 (%)

oE oE oE

C

Moho

Moho

Conrad

Conrad

1927/12/02, M 4.91929/11/20, M 5.8

1987/05/09, M 5.61930/02/11, M 5.3

1934/01/09, M 5.6

ne shown in Fig. 6). Tomographic images determined by using the first P-wave datavely. The velocity perturbation (in %) scale is shown at the bottom. White dots showorical earthquakes (1925–2000, M > 4.5) that occurred within 12 km width from thefigure legend, the reader is referred to the web version of this article.)

0

10

20

30

40

0

10

20

30

40

)mk(

htpeD

33 34 35

Latitude

5 4 3 2 1-6 -3 0 3 6 (%)

oN

Moho

Conrad

P + PmP

Surface trace of MTLE E’

P

Moho

Conrad

oNoN

1955/07/27,M 6.4

a

b

Fig. 9. The same as Fig. 7 but along the profile E–E0 shown in Fig. 6.

0

10

20

30

40

0

10

20

30

40

)mk(

htpeD

33 34 35Latitude

P (a)

oN oN

P + PmP

Moho

Conrad

Moho

Conrad

(b)oN

5 4 3 2 1-6 -3 0 3 6 (%)

F F’

1929/11/20, M 5.81927/12/02, M 4.9

1987/05/09, M 5.6

1930/02/11, M 5.3

1995/01/17, M 7.2(Kobe EQ)

1948/06/15,M 6.7

Surface trace of MTL

a

b

Fig. 10. The same as Fig. 7 but along the profile F–F0 shown in Fig. 6.

0

10

20

30

40

0

10

20

30

40

)mk(

htpeD

33 34 35

Latitude

5 4 3 2 1-6 -3 0 3 6 (%)

oN

MohoP

P + PmPoNoN

1970/09/29, M 4.9

Surface trace of MTLD D’

Conrad

Moho

Conrad

a

b

Fig. 8. The same as Fig. 7 but along the profile D–D0 shown in Fig. 6. The red arrowon the top shows the surface trace of the MTL. (For interpretation of the referencesto colour in this figure legend, the reader is referred to the web version of thisarticle.)

S. Gupta et al. / Journal of Asian Earth Sciences 35 (2009) 377–390 383

positive and negative velocity perturbations (±3%) are assigned tothe alternate 3-D grid nodes in the modeling space, the image ofwhich is straightforward and easy to remember. Therefore, by justexamining the inverted images of the checkerboard, one can easilyunderstand where the resolution is good and where it is poor. Thetest results show that the checkerboard image is recovered downto 18 km depth when only the first P-wave data are used(Fig. 14a), because most of the earthquakes are located in the uppercrust and rays crisscross well from the surface down to about20 km depth (Fig. 4a). When both P and PmP data are used, dueto quite different trajectories of the PmP rays from those of the first

P rays, the ray coverage has improved significantly, especially inthe lower crust (Fig. 4b), and the checkerboard pattern is recoveredin the entire crust (Fig. 14b). Due to the lack of sufficient data in thewestern part of the MTL we could not get good resolution in thatarea for both tests (Fig. 14a and b).

To assess the reliability of the main features appearing in theobtained tomographic images, we carried out another synthetictest, called restoring resolution test (Zhao et al., 1992a). In this test,to make input model for each individual cross-section (shown inFig. 6) we assigned velocity anomalies resembling with the ob-tained velocity images for corresponding cross-section (Figs. 7–13). The synthetic tests were done by using only P-wave data aswell as both P and PmP data. The inverted results of the restoringresolution test (Figs. 15c–21c) show that the velocity anomalies inthe MTL region are well recovered by using the combined data set(P and PmP). Through these synthetic tests, we believe that themain features of the obtained tomographic images (Figs. 5b, 7b–13b) are reliably resolved by our data set and the tomographicmethod. Since we could image the entire crust by using both Pand PmP data than by using only the first P-wave data, in the fol-lowing we discuss the results obtained with the combined data set(P and PmP).

Our tomographic images show that the crustal structure in theMTL region is very heterogeneous. Significant low-V anomalies arevisible down to a depth of nearly 15 km under Shikoku and wes-tern Kii regions (Figs. 5b, 7b, 11b–13b), and they further deepento a depth of nearly 20 km under central and eastern Shikoku re-gions. The recent seismic reflection survey and receiver functionanalysis have also reported the similar trend of low-V anomalies(Kodaira et al., 2002; Yamauchi et al., 2003). The low-V anomaliesare surrounded by several high-velocity (high-V) anomalies (Figs.5b, 7b–13b). In the lower crust (deeper than 20 km), the MTLexhibits high-V, while a few low-V anomalies exist away fromthe MTL (Figs. 5b, 8b–10b, 13b)

Most of the shallow crustal earthquakes (e.g., the destructive1995 Kobe (M 7.2) and 1955 Tokusima (M 6.4) earthquakes), are lo-cated in or around the low-V anomalies. The occurrence of theKobe earthquake in a low-V anomaly is in accordance with the re-sult of Zhao et al. (1996).

0

10

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30

40

0

10

20

30

40

)mk(

htpeD

132 133 134 135 136Longitude

1927/12/02, M 4.91929/11/20, M 5.8

1987/05/09, M 5.61930/02/11, M 5.3

1937/02/27, M 5.9

-6 -3 0 3 6 (%)

oEoE oE oEoE

Conrad

Moho

Moho

Conrad

P

P + PmP

5 4 3 2 1

Surface trace of MTL GG ’

1934/01/09, M 5.6

a

b

Fig. 11. The same as Fig. 7 but along the profile G–G0 shown in Fig. 6.

0

10

20

30

40

0

10

20

30

40

)mk(

htpeD

131 132 133 134 135 136

Longitude

5 4 3 2 1 -6 -3 0 3 6 (%)

Moho

Conrad

P

P + PmPMoho

Conrad

oE oE oEoE oE

oE

HH ’

1936/02/21, M 6.4

1937/02/27, M 5.9

1995/01/17, M 7.2(Kobe EQ)

b

a

Fig. 12. The same as Fig. 7 but along the profile H–H0 shown in Fig. 6.

0

10

20

30

40

0

10

20

30

40

)mk(

htpeD

131 132 133 134 135 136Longitude

5 4 3 2 1-6 -3 0 3 6 (%)

oE

Moho

Conrad

P

P + PmPoE oE

oE oE oE

Moho

Conrad

1955/07/27, M 6.41975/01/23,M 6.1

1975/04/21,M 6.4

II ’Mount TsurumiVolcano

a

b

Fig. 13. The same as Fig. 7 but along the profile I–I0 shown in Fig. 6. The red triangle shows the Mount Tsurumi volcano. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)

384 S. Gupta et al. / Journal of Asian Earth Sciences 35 (2009) 377–390

131˚ 132˚ 133˚ 134˚ 135˚ 136˚

33˚

34˚

35˚5 km

131˚ 132˚ 133˚ 134˚ 135˚ 136˚

33˚

34˚

35˚10 km

33˚

34˚

35˚18 km

131˚ 132˚ 133˚ 134˚ 135˚ 136˚33˚

34˚

35˚28 km

131˚ 132˚ 133˚ 134˚ 135˚ 136˚33˚

34˚

35˚40 km

-3.0-1.501.53.0

Longitude

(%)

Lat

itude

(b) With P + PmP data

131˚ 132˚ 133˚ 134˚ 135˚ 136˚

33˚

34˚

35˚5 km

131˚ 132˚ 133˚ 134˚ 135˚ 136˚

33˚

34˚

35˚10 km

131˚ 132˚ 133˚ 134˚ 135˚ 136˚33˚

34˚

35˚18 km

131˚ 132˚ 133˚ 134˚ 135˚ 136˚33˚

34˚

35˚28 km

(a) With P-wave data

Fig. 14. Results of the checkerboard resolution test (CRT) by using the first P-wave data (a), both P and PmP data (b). The layer depth is shown on the upper-left corner of eachmap. Open and solid circles represent fast and slow velocity perturbations, respectively. The scale for the velocity perturbation (in %) is shown in the right bottom.

S. Gupta et al. / Journal of Asian Earth Sciences 35 (2009) 377–390 385

4. Discussion

In the following we discuss velocity images obtained from thisstudy in cohesion with results of other geophysical and geochem-ical studies carried out in this non-volcanic forearc region of SWJapan.

The cutoff depth of crustal seismicity in the study area is around18 km (Ito, 1999). Mizoue et al. (1983) observed long, continuing

seismic swarm activities in and around Kii Peninsula. Salah andZhao (2003b) studied the 3-D P-wave attenuation (Q) structureof the crust and uppermost mantle beneath SW Japan and identi-fied the zones of low- to average- Q in the upper crust (0–20 kmdepth range) beneath western Kii Peninsula and eastern Shikoku.Low-V anomalies in the epicenter area of the 1995 Kobe earth-quake, eastern Shikoku, western Kii Peninsula and surroundingareas have also been reported by other tomographic studies (Zhao

oE

Moho

Conrad

P

P + PmP

)mk(

htpeD

Longitude-6 -3 0 3 6 (%)

Moho

Conrad

Moho

Conrad

Input Model

A B C

oE oEoE oE

a

b

c

Fig. 15. Results of the restoring resolution test (see text for details) along the MTL (A–B–C line shown in Fig. 6). (a) Input model. Inverted result of the synthetic testsconducted by using the first P-wave data (b), both P and PmP data (c). Red and blue colors denote slow and fast velocities, respectively. The velocity perturbation (in %) scale isshown at the bottom. Vertical exaggeration is 3. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

-6 -3 0 3 6 (%)

Latitude

)mk(

htpeD

Moho

Conrad

Moho

Conrad

Moho

Conrad

P

P + PmP

Input Model

DD ’a

b

c

33 34 35oN

oNoN

0

10

20

30

40

0

10

20

30

40

0

10

20

30

40

Fig. 16. The same as Fig. 15 but along the profile D–D0 shown in Fig. 6.

Latitude

)mk(

htpeD

-6 -3 0 3 6 (%)

Moho

Conrad

Moho

Conrad

Moho

Conrad

Input Model

P

P + PmP

E E’a

b

c0

10

20

30

40

0

10

20

30

40

0

10

20

30

40

33 34 35oN oNoN

Fig. 17. The same as Fig. 15 but along the profile E–E0 shown in Fig. 6.

386 S. Gupta et al. / Journal of Asian Earth Sciences 35 (2009) 377–390

et al., 1996, 2000; Zhao and Mizuno, 1999; Salah et al., 2005; Ikedaet al., 2006).

The magnetotelluric (MT) study in Kii Peninsula (Fuji-ta et al.,1997) have reported two anomalies in the upper crust: a subhori-

zontal conductive layer and a resistive block in the western and thesoutheastern parts of Kii Peninsula, respectively. A conductivelayer has also been revealed in the deep crust. The locations ofthese resistive and conductive bodies agree with the average

S. Gupta et al. / Journal of Asian Earth Sciences 35 (2009) 377–390 387

high-V and low-V anomalies obtained in this study, respectively.3He/4He ratio is high along the MTL in Shikoku Island and particu-larly higher in Kii Peninsula (Dogan et al., 2006; Umeda et al.,

-6 -3 0 3 6 (%)

P

P + PmP

Latitude

Input Model

Moho

Conrad

Moho

Conrad

Moho

Conrad

F F’

)mk(

htpeD

a

b

c0

10

20

30

40

0

10

20

30

40

0

10

20

30

40

33 34 35oN oNoN

Fig. 18. The same as Fig. 15 but along the profile F–F0 shown in Fig. 6.

)mk(

htpeD

G

Long

0

10

20

30

40

0

10

20

30

40

0

10

20

30

40

132 133oE oE

a

b

c

Fig. 19. The same as Fig. 15 but along

2007). Our obtained velocity images in the MTL region are in goodagreement with the results of these previous geophysical and geo-chemical studies conducted for SW Japan.

Zhao et al. (2002) have shown that the large crustal earthquakesoccur in or around the low-V areas in Japan. Furthermore, beneaththe Japan Islands, large crustal earthquakes occur only in the uppercrust (e.g., the 1995 Kobe and the 2000 Tottori earthquakes). Noshallow earthquake occurs in the lower crust and the uppermostmantle except for a few low-frequency microearthquakes in volca-nic areas (Hasegawa and Yamamoto, 1994; Ohmi and Obara, 2002)and in non-volcanic forearc areas (Obara, 2002). However, the low-er crust and the uppermost mantle can play an important role inthe nucleation of large crustal earthquakes because of the exis-tence and migration of arc magma and fluids (usually imaged aslow-V anomalies) released by the dehydration process of the sub-ducting Pacific and PHS slabs (Zhao et al., 2004).

In this non-volcanic forearc region, it is hard to relate low-Vanomalies to arc magma because no volcano exists, and heat flowsand geothermal gradients are low (Yuhara 1973; Okubo, 1993).Salah and Zhao (2003a) found low Vp, low Vs and high Poisson’sratio anomalies beneath the Quaternary volcanoes and westernKii Peninsula and eastern Shikoku. High 3He/4He ratios are relatedwith the presence of mantle helium (possible from the serpenti-nized forearc mantle wedge) transported by aqueous fluids to theEarth’s surface through the fractures within the crust (Doganet al., 2006; Umeda et al., 2007). Therefore, with support fromthese studies the low-V anomalies can be indicative of possiblepresence of fluids. Further, the fracturing in the crust is relatedto the non-volcanic deep tremors caused by fluids liberated fromthe subducting slab. Considering all these observations, we specu-late that the low-V anomalies in the upper crust along the MTL,coinciding with seismicity of the region, represent the weaker sec-tions of the seismogenic crust and are due to possible existence offluids. The presence of fluids in the upper crust can be attributed to

G’

P

P + PmP

Input Model

-6 -3 0 3 6 (%)

itude

Moho

Conrad

Moho

Conrad

Moho

Conrad

134 135 136oE oE oE

the profile G–G0 shown in Fig. 6.

-6 -3 0 3 6 (%)

Moho

Conrad

P

P + PmP

)mk(

htpeD

Longitude

Input Model

II ’

Moho

Conrad

Moho

Conrad

a

b

c

132 133 134 135 136oE131 oE oEoE oE oE

0

10

20

30

40

0

10

20

30

40

0

10

20

30

40

Fig. 21. The same as Fig. 15 but along the profile I–I0 shown in Fig. 6.

-6 -3 0 3 6 (%)

Moho

Conrad

P

P + PmP

Longitude

Input Model

HH ’

)mk(

htpeD

Moho

Conrad

Moho

Conrad

0

10

20

30

40

0

10

20

30

40

0

10

20

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132 133 134 135 136oE131 oE oEoE oE oE

Fig. 20. The same as Fig. 15 but along the profile H–H0 shown in Fig. 6.

388 S. Gupta et al. / Journal of Asian Earth Sciences 35 (2009) 377–390

shallow origins (such as meteoric water, pore fluids and mineraldehydration) and/or to migration of dehydrated fluids of the sub-ducting PHS slab through fracture zones within the crust, whichare imaged as low-V anomalies approaching the lower crust (Figs.8b–10b, 13b).

The earthquake cycle of the MTL fault system in Shikoku is1000–3000 years, and the elapsed time since the most recent his-

torical earthquake in the MTL is about 500 years (Tsutsumi andOkada, 1996). In the case of the 1994 Northridge earthquake (M6.7) in southern California, stress recovery was apparently com-pleted within 2 years after the main shock, as revealed by stress-tensor inversions (Zhao et al., 1997). Similarly, rapid fault healingof the 1995 Kobe earthquake has also been observed by repeatedS-wave splitting analyses of the Nojima fault zone (Tadokoro and

S. Gupta et al. / Journal of Asian Earth Sciences 35 (2009) 377–390 389

Ando, 2002). Thus, the healing time of an active fault system can bevery short. Therefore, fault healing of the MTL fault system hasprobably been completed. In addition, the presence of fluidsaround and below the seismogenic zone can influence the activityof the fault system. As the fluids enter the active faults in the crust(such as the MTL), fault zone friction decrease, and thus fault rup-ture can be triggered to generate large crustal earthquakes.

5. Conclusions

We determined a detailed 3-D P-wave velocity structure of thecrust beneath the MTL fault system in Shikoku and western KiiPeninsula of SW Japan by using high-quality first P-wave andMoho-reflected P-wave (PmP) arrivals from 300 crustal earth-quakes recorded by 101 seismic stations. Our results show thatthe addition of PmP data can significantly improve the resolutionof the lower crust tomography and the entire crust can be betterimaged than by using only the first P-wave data. Results of exten-sive resolution tests show that major velocity anomalies revealedare reliable. Prominent low-V anomalies are visible in the uppercrust along the MTL, and the low-V anomalies are surrounded byseveral high-V anomalies. In the lower crust, the MTL exhibits highvelocity, while a few low-V anomalies exist away from the MTL.Many large crustal earthquakes are located in or around theselow-V anomalies. These low-V anomalies can represent the weakersections of the seismogenic crust and can be imprint of the aque-ous fluids released by the dehydration process of the subductingPHS slab under SW Japan. These seismic images together withother geological, geochemical and geophysical studies can improveour understanding of seismotectonics and can provide useful infor-mation for seismic hazard mitigation in SW Japan.

Acknowledgements

We thank the Hi-net Data Center for providing the waveformdata used in this study. S. Gupta thanks the NGRI director for hisencouragement and support and the Department of Science andTechnology, Government of India for the BOYSCAST fellowship tostudy at Tohoku University, Japan. We thank Dr. Y. Ohta for hishelp at the data processing stage. This work was partially sup-ported by a grant (Kiban-A 17204037) from Japan Society for thePromotion of Science to D. Zhao. Figures were made by using theGMT (Generic Mapping Tools) software by Wessel and Smith(1998). Two anonymous reviewers provided thoughtful reviewcomments which improved the manuscript.

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