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Tectonophysics 364 (2003) 191–213

3-D seismic structure of Kii Peninsula in southwest Japan:

evidence for slab dehydration in the forearc

Mohamed K. Salah*, Dapeng Zhao

Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan

Received 15 May 2002; accepted 12 February 2003

Abstract

Kii Peninsula is located in the forearc region of southwest Japan and has distinct structural and tectonic features

characterized by high seismicity in the crust and the subducting Philippine Sea slab, high surface heat flow, high 3He/4He

isotopic ratio, and a local change in the geometry of the subducting slab. We have tried to determine detailed 3-D P and S

wave velocity structures of this region using a large number of high-quality arrival time data recorded by dense seismic

networks on the Japan Islands. From the obtained seismic velocities, we further estimated 3-D distributions of Poisson ratio,

crack density, saturation rate and porosity parameter in the study area. Our results show significant heterogeneities in the

crust and upper mantle wedge characterized by low seismic velocities, high Poisson ratio, high values of crack density,

saturation rate and porosity. These results suggest the existence of fluids in the crust and mantle wedge resulting from the

dehydration of the subducting Philippine Sea slab, which can explain the observed geophysical and geochemical features in

Kii Peninsula.

D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Kii Peninsula; Seismic tomography; Crack density; Saturation rate; Poisson ratio; Porosity parameter

1. Introduction and Nagasaki, 1996). The difference in arc orientation

The Kii Peninsula is located in the forearc region

of the Southwestern Japan Arc, where the Philippine

Sea plate has been subducting northwestward beneath

the Eurasian plate since the Middle Miocene (Fig. 1).

The Philippine Sea plate resumed subduction at ca. 6

Ma after a halt or slowdown of subduction during the

period 10–6 Ma (Kamata and Kodama, 1994; Itoh

0040-1951/03/$ - see front matter D 2003 Elsevier Science B.V. All right

doi:10.1016/S0040-1951(03)00059-3

* Corresponding author. Tel.: +81-89-927-9652; fax: +81-89-

927-9640.

E-mail address: salah@sci.ehime-u.ac.jp (M.K. Salah).

led to oblique subduction of the Philippine Sea plate

beneath the Southwest Japan Arc through the Nankai

Trough and normal subduction beneath the Ryukyu

Arc through the Ryukyu Trench (Fig. 1b). One of the

major geologic events in the Southwest Japan Arc is

the acceleration of dextral-fault movement along the

Median Tectonic Line (MTL) which extends 900 km

in an east–west direction (Kamata and Kodama,

1999).

Geophysical and geochemical studies of the region

yield basic clues to the understanding of the tectonic

framework and suggested that the Kii Peninsula is a

special region in southwest Japan. Seismological

s reserved.

Fig. 1. (a) Distribution of active and Quaternary volcanoes on the Japan Islands. Curved lines show the trenches, which represent the major plate

boundaries around the Japanese region. Black square shows the present study area. (b) Index map showing the tectonic framework around the

Japanese Islands (after Kamata and Kodama, 1999). HVZ, Hohi Volcanic Zone; MTL, Median Tectonic Line.

M.K. Salah, D. Zhao / Tectonophysics 364 (2003) 191–213192

M.K. Salah, D. Zhao / Tectonophysics 364 (2003) 191–213 193

monitoring has been carried out in the region (Mizoue

et al., 1971, 1983; Nakamura, 1992). Mizoue et al.

(1971) detected seismic reflectors in the crust. Mizoue

et al. (1983) mapped seismicity in detail and found

that earthquake foci cluster at three different depths.

Magnetotelluric measurements by Fuji-ta et al. (1997)

revealed two upper crustal anomalies: one is a sub-

horizontal conductive layer dipping northwest, at

depths of 5–10 km; the other is a resistive block at

depths of 2–15 km. The former is interpreted as a

flysh unit, whereas the latter is consistent with the

high-velocity structure detected by refraction experi-

ments and may be a lost continental fragment of the

Kuroshio paleoland. Sano and Wakita (1985) and

Wakita et al. (1987) studied the geographical distri-

bution of the 3He/4He ratio in the Kinki district which

is located in the southwestern corner of Kii. They

found that the ratio in Kinki is strikingly different

from those found in the rest of the Japanese Islands,

including Shikoku which lies next to Kinki. They

suggested the presence of a shallow magma body

beneath the area from observations of high emanation

of 3He of magmatic origin, high-temperature hot

springs, high terrestrial heat flow, hypocentral distri-

bution of microearthquakes, a local irregularity in the

geometry of the subducting Philippine Sea plate, and

the presence of molten materials inferred from the

characteristic phases of reflected waves in seismo-

grams. Seno et al. (2001) studied the seismicity in the

subducting Philippine Sea plate beneath southwest

Japan and suggested that, in the Kii Peninsula area,

aqueous fluids released from the serpentinized mantle

may have initiated partial melting in the mantle

wedge, as indicated by the presence of high 3He/4He

ratio in the natural gases and the shallow seismic

swarms in this region (Wakita et al., 1987).

The availability of dense seismic networks in Japan

has allowed us to collect a large number of high-

quality data from local microearthquakes. We applied

a tomography method to body-wave arrival times to

estimate P and S wave velocities and from these we

tried to deduce variations in lithology and physical

properties of rocks. Because rocks with differing

physical states can have similar seismic velocities,

seismic velocity alone is not a sensitive indicator of

variable rock property. For this reason, it is often useful

to consider ratios and products of seismic parameters

to differentiate three-dimensional variations in the sub-

surface. A brief review of how rock properties relate to

seismic velocity and attenuation can be found in

Sanders et al. (1995) and Lees and Wu (2000).

Poisson ratio is a useful indicator of lithology and

geofluids. Its average value is 0.25 for Earth’s crust

and upper mantle (Holbrook et al., 1988). Further-

more, Poisson ratio is sensitive to cracks and their

aspect ratios in rock matrix (Koch, 1992). The product

of compressional and shear wave velocities, VpVs, on

the other hand, has been used to delineate porosity in

sedimentary rocks (e.g. Iverson et al., 1989; Serrano et

al., 2002). It has been observed that lower VpVs

indicates an increase of porosity. O’Connell and

Budiansky (1974) (hereafter we call it OB74) devel-

oped a self-consistent theory that relates changes in

seismic velocity and Poisson ratio to crack density and

saturation rate of rocks. Although many previous

researchers have studied the effects of microcracks

and fluids on elastic properties of rocks and their role

in earthquake dynamics, little work is done to inves-

tigate the in situ status and distribution of cracks and

fluids in the Earth’s crust. The determination of

distributions of cracks and fluids can improve our

understanding of the generating process of large

crustal earthquakes (e.g. Zhao and Mizuno, 1999).

Although tomographic studies for the entire Japan

Islands have been conducted by Zhao et al. (1994,

2000), there has been no detailed tomography study

for the Kii area. Previous studies have shown that the

Kii Peninsula region has distinct structural and tec-

tonic features compared to the rest forearc areas in

southwest Japan. The aim of this work is to determine

detailed three-dimensional structure of seismic veloc-

ity and a few other geophysical parameters and

correlate them with geophysical and geochemical

measurements carried out so far in southwest Japan

to better understand the structure and seismotectonics

of the Japan subduction zone.

2. Data selection

Fig. 2 shows the 3-D distribution of 3648 local

earthquakes that we used in the present study. We

have used 488 aftershocks of the January 17, 1995

Kobe earthquake (M 7.2) recorded by 37 permanent

and 30 portable stations (Fig. 3), 2858 events selected

from the Japan University Network Earthquake Cata-

Fig. 2. 3-D distribution of the 3648 events used in this study.

M.K. Salah, D. Zhao / Tectonophysics 364 (2003) 191–213194

log (JUNEC) (July 1985–December 1994), and 302

events recorded by the High Sensitivity Seismograph

Network (Hi-net) which is installed and maintained by

the National Research Institute for Earth Science and

Disaster Prevention (NIED). These events are located

between latitudes 33–36j5VN and longitudes 134–

137jE, with focal depths shallower than 100 km.

Each event is recorded by at least 10 stations. The

error in the hypocentral location does not exceed 3 km

for all events. We tried to select events that have a

uniform distribution in the study area; however, we

can see that earthquakes tend to cluster in the south-

western corner of Kii Peninsula and along the after-

shock area of the 1995 Kobe earthquake. The majority

of the crustal events have focal depths between 0 and

20 km. Most of the intermediate-depth events are

confined to the southwestern part of the study area

and occurred in the subducting Philippine Sea

slab. These 3648 events generated 92,119 P and

48,600 S arrivals recorded by the seismic stations

shown in Fig. 3. The accuracy of arrival times is better

than 0.1 s for most of the data. The 30 portable

seismic stations are distributed mainly in the after-

shock area of the 1995 Kobe earthquake and were

deployed following the Kobe mainshock (Hirata et al.,

1996). The JUNEC stations are operated by eight

national universities in Japan and are equipped with

short-period and broadband seismographs (Tsuboi et

al., 1989). The NIED Hi-net is a newly established

seismic network (Obara, 2002). Each station consists

Fig. 3. Distribution of Hi-net (solid circles), JUNEC (solid squares) and portable (solid triangles) seismic stations used in the present study.

M.K. Salah, D. Zhao / Tectonophysics 364 (2003) 191–213 195

of a three-component velocity seismometer with a

natural frequency of 1 Hz installed at the bottom of

a borehole with a depth of 100–200 m. The data are

digitized at each station with a sampling frequency of

100 Hz, and then the data packets attached with the

absolute time information from a Global Positioning

System clock are transmitted to the data center

(Obara, 2002). The 3648 events were recorded by a

total of 146 seismic stations in the study area (59

JUNEC stations, 30 Portable stations and 57 Hi-net

stations). We can see that these stations cover the

study area densely and uniformly (Fig. 3).

3. Methods and analysis

In this study, we have used the tomography

method of Zhao et al. (1992a) to determine the 3-

D P and S wave velocity (Vp, Vs) structures in the

study area. Although the conceptual approach of this

method is derived from Aki and Lee (1976), it has

some additional features. The technique can deal

with the complex geometry of seismic velocity

discontinuities existing in the study area such as

the Moho, Conrad or the subducting slab boundary,

and it uses an efficient 3-D ray tracing scheme to

compute travel times and ray paths. We set up a 3-D

grid in the study area with a grid spacing of 15 km

in the horizontal direction, 6–10 and 10–18 km in

depth in the crust and upper mantle, respectively

(Fig. 4). Hypocenter locations and velocities at the

grid nodes are taken as unknown parameters. The

velocity at any point in the model is calculated by

linearly interpolating the velocities at the eight grid

nodes surrounding that point. For details of the

method, see Zhao et al. (1992a, 1994).

The interpretation of tomographic images is not an

easy job. For example, a given velocity anomaly can

Fig. 4. 3-D configuration of the grid net adopted in the present study. Grid spacing is 15 km in the horizontal direction and 6–18 km in depth.

M.K. Salah, D. Zhao / Tectonophysics 364 (2003) 191–213196

be attributed to a thermal or chemical variation.

Therefore the interpretation is usually nonunique.

Compared to seismic velocity itself, Vp/Vs ratio (or

Poisson ratio) is a better indicator for the content of

fluids and/or magma. After the Vp and Vs images are

determined from travel time inversions, we can use

the relation:

ðVp=VsÞ2 ¼ 2ð1� tÞ=ð1� 2tÞ ð1Þ

to determine Poisson ratio (t) distribution (Zhao and

Negishi, 1998).

The velocity product VpVs has been related to

porosities in sedimentary rocks (Pickett, 1963;

Tatham, 1982, Serrano et al., 2002). If other physical

parameters are held constant, VpVs decreases with in-

creasing porosity in sedimentary rocks (Iverson et al.,

1989). In our analysis, we calculated the perturbation

of VpVs values obtained by tomographic inversion to

the same product of initial velocities and draw it as a

percentage.

The self-consistent crack theory developed by

OB74 was used to determine crack density and

saturation rate in the study area. Assume that flat

circular cracks are aligned randomly in the crust and

are filled with fluids or air. According to OB74, the

crack density parameter e can be defined as:

e ¼ Nha3i; ð2Þ

where N is the number of microcracks per unit volume

and hai is the average radius of circular cracks.

Suppose that N1 cracks per unit volume are dry and

M.K. Salah, D. Zhao / Tectonophysics 364 (2003) 191–213 197

that N2 =N�N1 cracks are saturated, the saturation

rate f is defined as:

f ¼ N2

N: ð3Þ

With such definitions of crack density and saturation

rate, OB74 showed that the shape and aspect ratio of

spheroidal cracks would affect little on the relation

between e, f and seismic velocity. The OB74 theory

predicts the following relations:

K=K ¼ 1� 16

9

1� t2

1� 2t

� �ð1� fÞe; ð4Þ

E=E ¼ 1� 16

45

�1� t2

�3ð1� fÞ þ 4

2� t

� �e; ð5Þ

G=G ¼ 1� 32

45

�1� t

�1� f þ 3

2� t

� �e; ð6Þ

e ¼ 45

16

t � t1� t2

� �

� ð2� tÞ½ð1� fÞð1þ 3tÞð2� tÞ � 2ð1� 2tÞ ; ð7Þ

where K, E, G and t are bulk modulus, Young’s

modulus, shear modulus and Poisson ratio for the

cracked volume, respectively. K, E, G and t are the

same quantities for the uncracked volume. The fol-

lowing general relations hold between seismic veloc-

ity and elastic moduli:

Vp=Vp ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffið1� tÞð1þ tÞKð1þ tÞð1� tÞK

s; ð8Þ

Vs=Vs ¼ffiffiffiffiffiffiffiffiffiffiG=G

q; ð9Þ

Vp=Vs

Vp=Vs¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffið1� tÞð1� 2tÞð1� 2tÞð1� tÞ

s; ð10Þ

where Vp, Vs are P and S wave velocities for cracked

volume. Vp and Vs are velocities for uncracked rocks.

Thus crack density (e) and saturation rate (f) can be

estimated from observed seismic velocity and Poisson

ratio using Eqs. (4)–(10). Zhao and Mizuno (1999)

applied this theory to estimate the 3-D distribution of

crack density and saturation rate in the source area of

the 1995 Kobe earthquake. Since temperature and

lithologic changes would also affect seismic veloc-

ities, one should be careful when applying the OB74

crack theory to tomographic results. Information on

surface heat flow, geotherms, surface geology and

lithology in a study area should be referred to clarify

what are the main reasons for the changes of seismic

velocity. One should be particularly careful for vol-

canic areas where velocity variations may be mainly

caused by high temperature (Zhao et al., 1996).

The crack theory has been further developed since

the work of OB74 by, e.g. Bruner (1976), Davis and

Knopoff (1995) and Takei (2002). They pointed out

that the OB74 theory becomes less accurate for cases

of large crack density. However, our tomography

shows that the changes of seismic velocities (Vp, Vs)

are less than 5–6%, which leads to e < 0.25. Hence weconsider the OB74 model is viable for our study area,

at least to the first order. One distinctive advantage of

theOB74model is its conciseness. Therefore we prefer

to use the OB74 model in this work.

4. Results and resolution

The crust and uppermost mantle under our study

area consist of three layers separated by the Conrad

and Moho discontinuities. In the three layers, the

initial values of Vp are 6.0, 6.7 and 7.75 km/s,

respectively. The corresponding values of Vs are

3.5, 3.8 and 4.35 km/s, respectively. The Conrad

depth is 15–18 km, and the Moho depth is 34–35

km in the study area (Zhao et al., 1992b). The lateral

depth variations of the Conrad and Moho discontinu-

ities (Zhao et al., 1992b) were taken into account in

the 3-D ray tracing and the tomographic inversion.

However, we found that the velocity changes due to

the Conrad and Moho depth variations are less than

0.5% in the final tomographic images.

We conducted several checkerboard resolution tests

(Zhao et al., 1992a, 1994) at different grid spacings to

examine the resolution scale of the present data set. To

make a checkerboard, we assigned positive and neg-

ative velocity anomalies of 3% to all the 3-D grid

Fig. 5. Result of a checkerboard resolution test of P wave velocity (see text) for four representative depth layers. The grid separation is 15 km.

Solid and open circles denote high and low velocities, respectively. The depth of each layer is shown below the map. The perturbation scale is

shown on the right.

M.K. Salah, D. Zhao / Tectonophysics 364 (2003) 191–213198

Fig. 6. The same as Fig. 5 but for S wave velocity.

M.K. Salah, D. Zhao / Tectonophysics 364 (2003) 191–213 199

Fig. 7. Distribution of P wave velocity images (in %) at four depth slices. The perturbation scale is shown on the right. Circles and crosses

denote low and high velocities, respectively. Star represents the epicenter of the 1995 Kobe mainshock. Thin lines show active faults. MTL is

Median Tectonic Line. The depth to each layer is shown below the map.

M.K. Salah, D. Zhao / Tectonophysics 364 (2003) 191–213200

Fig. 8. The same as Fig. 7 but for S wave velocity.

M.K. Salah, D. Zhao / Tectonophysics 364 (2003) 191–213 201

Fig. 9. The same as Fig. 7 but for Poisson ratio. Circles and crosses denote high and low values of Poisson ratio, respectively. The perturbation

scale (from + 10% to � 10%) is shown on the right.

M.K. Salah, D. Zhao / Tectonophysics 364 (2003) 191–213202

Fig. 10. Percent perturbations of VpVs at four crustal layers. Circles and crosses denote low and high values of VpVs, respectively. Star

represents the location of the 1995 Kobe mainshock. Thin lines show active faults. MTL is Median Tectonic Line. The perturbation scale is

shown on the right.

M.K. Salah, D. Zhao / Tectonophysics 364 (2003) 191–213 203

Fig. 11. The same as Fig. 10 but for the crack density parameter. Circles and crosses denote high and low crack densities, respectively.

M.K. Salah, D. Zhao / Tectonophysics 364 (2003) 191–213204

Fig. 12. The same as Fig. 10 but for saturation rate. Circles and crosses denote high and low saturation rate, respectively.

M.K. Salah, D. Zhao / Tectonophysics 364 (2003) 191–213 205

M.K. Salah, D. Zhao / Tectonophysics 364 (2003) 191–213206

nodes. Synthetic arrival times are calculated for the

checkerboard model. Numbers of stations, events and

ray paths in the synthetic data are the same as those in

the real data. The inverted image of the checkerboard

suggests where the resolution is good and where it is

poor. Figs. 5 and 6 show the results of the tests at four

representative layers for both Vp and Vs structures

with a grid spacing of 15 km in the horizontal

direction. The resolution is generally good for the

crust and the upper mantle layers where most of the

events are located. However, some edge portions have

relatively poor resolution, which was expected. On the

other hand, in the checkerboard resolution test with a

grid spacing of 25 km in horizontal direction (not

shown), all parts have very good resolution and the

synthetic anomalies are generally well recovered.

Figs. 7–12 show the Vp, Vs, Poisson ratio (t),porosity parameter (VpVs), crack density (e) and

saturation rate (f) distributions at four representative

depth layers. We have also conducted a number of

inversions by adopting different initial models and

different grid spacing, and using slightly different data

sets. We found that the overall pattern of the velocity

structure as shown here is stable and the change in the

amplitude of the velocity anomalies is generally small

( < 0.5%). In the following, we discuss the main

features of Vp, Vs and Poisson ratio in the crust and

upper mantle. For the other geophysical parameters,

we will restrict our discussion on only crustal layers

because values of crack density, saturation rate and

porosity thus obtained are valid only in the crust

(OB74, Iverson et al., 1989).

Large velocity variations are imaged in the study

area. For the crustal layers, Vp and Vs are generally

low along the Median Tectonic Line and the western

part of Kii Peninsula. Low Vs is also seen along the

aftershock area of the 1995 Kobe earthquake. On the

other hand, higher than average seismic velocities are

observed in the southern and eastern parts of the

peninsula. In the crust, Poisson ratio is generally high

along the aftershock area of the 1995 Kobe earth-

quake and most of the southern and western parts of

Kii Peninsula (Fig. 9a,b).

At shallow depth layers (depths 2 and 8 km), higher

values of porosity, crack density and saturation rate are

revealed along most parts of the Median Tectonic Line

and the aftershock area of the 1995 Kobe earthquake

(Figs. 10–12). However, some areas in the lower crust

show high values for both porosity and crack density,

which is in good agreement with the lithological

(mainly acidic) composition of that area (Fuji-ta et

al., 1997). In this region, there is no volcano, and

hence changes in Vp, Vs, Poisson ratio (t) and

porosity parameter (VpVs) could be attributed mainly

to cracks and fluids rather than high temperature (Zhao

and Negishi, 1998), though a fraction of the velocity

changes may be caused by lithological variations. We

can clearly notice that the crack density and saturation

rate show considerable heterogeneity and their overall

pattern is similar to that of Vs and Poisson ratio

distributions, respectively. High crack density (e) areasare generally consistent with high saturation rate (f)areas (Figs. 11 and 12).

In the upper mantle layers (40 and 55 km depths),

low Vp and low Vs anomalies are clearly seen along

or south of the Median Tectonic Line (Figs. 7 and 8).

High Vp, high Vs and low Poisson ratio anomalies at

55 km depth under Kii Peninsula may show the image

of the subducting Philippine Sea slab.

5. Discussion

The origin of lateral heterogeneity in seismic veloc-

ities in the crust andmantle is one of the most important

issues in the geodynamical application of seismic

tomography. In particular, distinguishing thermal and

chemical origins of heterogeneity is critical because the

dynamic significance of heterogeneity is quite different

for the two cases (Karato and Karki, 2001). For

example, when lateral variation in seismic velocity is

due to lateral variation in temperature, then low- (high-)

velocity regions represent higher (lower) than average

temperatures and should have lower (higher) density.

When velocity heterogeneity is due to chemical heter-

ogeneity, however, such a conclusion may not be valid.

According to Karato and Karki (2001), the velocity and

density heterogeneity in the upper mantle is affected

significantly by the heterogeneity in chemical compo-

sition. On the other hand, seismic velocity variations

are generally attributed to temperature changes in

volcanic areas, and to fluids and cracks in the forearc

region in southwest Japan (Zhao et al., 2000).

In the present study area, many researchers have

conducted various geophysical and geochemical stud-

ies. Sano and Wakita (1985) and Wakita et al. (1987)

M.K. Salah, D. Zhao / Tectonophysics 364 (2003) 191–213 207

found that the geographical distribution of 3He/4He

ratio reflects well the geotectonic structure of the

Japanese Islands. In northeastern (NE) Japan, a clear

geographical difference in the 3He/4He ratio exists

between the forearc and back arc regions. The ratio is

lower in the forearc region and higher along the

volcanic front (Fig. 1a) and in the back arc side. These

results suggest that the mantle-derived helium in the

volcanic areas is associated with the diapiric uprise of

magma. On the contrary, in southwestern (SW) Japan

there is no clear geographical contrast in the 3He/4He

ratio. Some samples in western Kii Peninsula show

high 3He/4He ratios, which is consistent with the

distribution of terrestrial heat flow data and reflects

that the geotectonic structure in Kii is different from

those of other regions of Japan. They interpreted the

high 3He/4He ratios observed in Kinki and Chugoku

district as indicative of renewed or incipient magma-

tism due to a descending young and warm slab. An area

with a diameter of 100 km in southwest Kii, which

exhibits anomalously high 3He/4He ratio is calledKinki

spot (Wakita et al., 1987). Since 3He is the primordial

component derived from the mantle, any difference in

the concentration in a sample can be regarded as a dif-

ference in the contribution from the mantle, irrespec-

tive of changes in sample type (Wakita et al., 1987).

In an analogous study in the southern Apennine

axial zone, Italy, Italiano et al. (2000) found that there

is a significant contribution of mantle-helium to gas

manifestations, which strongly suggests that the

studied emissions are related to melts intruded into

the crust. However, these melts are present in the form

of dike-shaped intrusions rather than a proper magma

chamber (Italiano et al., 2000). Therefore, the observed

systematic distribution of higher content of 3He in the

back arc region and lower in the forearc region has

been interpreted as generated by magmatic activities.

Although high emanation of 3He is indicative of

the presence of a magma source beneath Kii, no

Quaternary volcanism is observed in the area. The

past volcanisms in the Middle Miocene and the Upper

Cretaceous are not sufficient to explain the observed

high 3He/4He ratios. For this reason, it was suggested

that diapiric rise of magma is expected to occur in the

region (Wakita et al., 1987).

Seno et al. (2001) studied seismicity in different

segments of the Philippine Sea plate in southwest

Japan and found that slab seismicity beneath Kii

Peninsula is exceptional where slab events occur

down to a depth of 80 km, deeper than in other areas.

Moreover, in this area, Hori et al. (1985) detected

events that do not accompany later crustal phases and

suggested that these events might occur within the

mantle portion of the Philippine Sea slab. This implies

in turn that, beneath Kii Peninsula, dehydration of the

serpentinized mantle of the Philippine Sea slab is

occurring, and that the mantle portion of the Philip-

pine Sea slab subducting beneath this area was

hydrated before subduction (Seno et al., 2001).

Finally, aqueous fluids released from the serpentinized

mantle may have initiated partial melting in the

mantle wedge. Similarly, a tomographic study of the

Juan de Fuca plate that is subducting beneath the

North American plate revealed an extensive low

velocity zone above the subducted slab at about 45

km depth (Zhao et al., 2001). This low velocity zone

is interpreted as a result of the presence of hydrated

(serpentinized) mantle, with a supply of water from

the metamorphic dehydration of the subducted slab.

In this work we have detected low-velocity, high-

Poisson ratio, low-VpVs, high crack density and high

saturation rate anomalies in the crust and upper mantle

under Kii Peninsula (Figs. 7–12 and 13–15 along

vertical cross sections). All these results suggest the

existence of fluids resulting from the dehydration of

the subducting Philippine Sea slab, which is also

supported by former geophysical and geochemical

observations. Low velocity anomalies are visible in

the top 15 km of the crust (Figs. 13–15). A clear Y-

shaped low Vp anomaly is visible that extends from

the upper mantle to the upper crust, while Poisson ratio

only shows minor or moderately high values (Fig. 14).

Nakajima et al. (2001) investigated the 3-D structure of

Vp, Vs and Vp/Vs beneath NE Japan and found

differences in Vp/Vs ratio with depth. The average

value of Vp/Vs ratio is f 1.69 in the upper crust,

f 1.75 in the lower crust, and f 1.77 in the upper-

most mantle. They stated that these differences in Vp/

Vs ratios mainly reflect the lithological variations with

depth and interpreted the low-V and low-Vp/Vs areas

beneath active volcanoes in the upper crust by the

presence of H2O and low-V and high-Vp/Vs areas in

the lower crust and the mantle wedge are caused by

inclusions of partial melts within rocks. Watanabe

(1993) proposed that Vp/Vs ratio can be used to

distinguish partially molten rocks and rocks containing

Fig. 13. Vertical cross sections of Vp, Vs and Poisson ratio along line A–AV(insert map). Slow velocities and high Poisson ratio are shown in

red, high velocities and low Poisson ratio are shown in blue. White circles show seismicity in 10-km-wide zone along the profile. The

perturbation scale is shown at the bottom.

M.K. Salah, D. Zhao / Tectonophysics 364 (2003) 191–213208

Fig. 14. The same as Fig. 13 but along cross section B–BV.

M.K. Salah, D. Zhao / Tectonophysics 364 (2003) 191–213 209

Fig. 15. The same as Fig. 13 but along cross section C–CV. Star shows the hypocenter of the January 17, 1995, Kobe earthquake (M 7.2).

M.K. Salah, D. Zhao / Tectonophysics 364 (2003) 191–213210

M.K. Salah, D. Zhao / Tectonophysics 364 (2003) 191–213 211

water. Vp/Vs increases with increasing fluid fraction

for a rhyolite melt. On the other hand, for water, Vp/Vs

decreases as the fluid fraction increases to 10 vol.%,

then it increases. Taking into account these studies, we

consider that the low-V and average to slightly high

Poisson ratio under Kii Peninsula at crustal depths are

related to the presence of fluids. The low velocity and

high Poisson ratio in the upper mantle wedge above

the slab may contain a small amount of melts. It is still

difficult to say conclusively whether these fluids are

water or partial melts.

Heat flow values are generally low in the forearc

region and higher in the back arc region but Yamano et

al. (1984) observed a high value of terrestrial heat flow

Fig. 16. Shallow (0–20 km depths) (a, b) and deep (20–50 km depths) seis

to December 1989 (a, c) and from January 1990 to December 1994 (b, d

in the forearc region of southwest Japan. The anom-

alously high terrestrial heat flow is detected in the

western part of the Kii Peninsula (e.g. Okubo, 1993).

The value is approximately two times higher than that

found in the forearc region and is almost identical to

that in the back arc region.

Long-continuing swarm activities of shallow earth-

quakes have been observed in and around Kii Pen-

insula (Mizoue et al., 1983). It has been suggested that

fluids are related to earthquake occurrence and swarm

activities (e.g. Sano et al., 1986; Wakita et al., 1987;

Scholz, 1990). Fig. 16 shows the crustal and slab

seismicity in two time intervals. It is clear that the

Kinki spot area has intense and continuous seismicity,

micity (c, d) in southwest Japan. Seismicity occurred from July 1985

), respectively.

M.K. Salah, D. Zhao / Tectonophysics 364 (2003) 191–213212

which may be related to the abundant fluid compo-

nents in this region.

Fuji-ta et al. (1997) studied the crustal structure of

Kii Peninsula using the magnetotelluric (MT) method

and found two upper crustal anomalies. One is a

subhorizontal conductive layer dipping northwest at

5–10 km depth while the other is a resistive block at 2–

15 km depth. The location of the resistor and conductor

bodies agrees well with high and low velocity anoma-

lies we found in this study (see Figs. 2 and 4 in Fuji-ta et

al., 1997 and compare themwith the velocity anomalies

in the cross sections in our Figs. 13–15).

6. Conclusions

1. A large number of local earthquake arrival times are

used to determine detailed 3-D seismic velocity and

Poisson ratio structures in Kii Peninsula in south-

west Japan. From the obtained Vp and Vs, we

further estimated 3-D distribution of Poisson ratio,

crack density, saturation rate and porosity in this

region. The availability of the six physical param-

eters enables us to better understand the structure

and tectonics of the southwest Japan forearc region.

2. Low velocity and high Poisson ratio anomalies are

revealed under Kii Peninsula in the crust and the

mantle wedge above the subducting Philippine Sea

slab, suggesting the existence of fluids resulting

from the slab dehydration. This may explain the

geophysical and geochemical observations in Kii,

such as the microearthquake swarm activities, high

terrestrial heat flow and high 3He/4He isotopic ratio.

3. A low velocity and high Poisson ratio anomaly is

detected at the source area of the 1995 Kobe earth-

quake (M 7.2), which confirmed the previous results

by Zhao et al. (1996). We found that this anomaly

further extends to the upper mantle wedge just above

the subducting Philippine Sea slab. This result indi-

cates that the fluids that contributed to the initiation

of the Kobe earthquake (Zhao et al., 1996) come

from the dehydration of the Philippine Sea slab.

Acknowledgements

We thank H. Tani for his help at the data pro-

cessing stage. Two anonymous reviewers provided

helpful comments, which improved the manuscript.

Some figures in this paper were made using Generic

Mapping Tools (GMT) software written by Wessel

and Smith (1998). This work was partially supported

by a grant (Kiban-B No. 11440134) from the Japan

Society for the Promotion of Science.

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