Active Tectonics

cindIntraplate Paleoseismic Activities,
Southwest Japanby

Eikichi Tsukuda
Geological Survey of Japan, 1-1-3 Higashi , Tsukuba, 305 JapanTo be published in Bulletin of the Geological Survey of Japan, Vol. 47 No. 8(1996)

CONTENTS
Abstract
1. Introduction
2. Active tectonics of southwest Japan
2.1

zonal arrangement of southwest Japan arc

2.2

Activity of the Median Tectonic Line

2.3

Forearc basins of southwest Japan

2.4

Deformation of the Shikoku zone

2.5

Seto'uchi shear zone

2.6

Right-lateral shear zone of central Kyushu

2.7

Development of tectonic landform of southwest Japan arc

2.8

Zonal arrangement of Ryukyu arc and Okinawa trough

2.9

Junction between the southwest Japan arc and the Ryukyu arc

3. Paleoseismicity of southwest Japan
3.1

Migration of large earthquakes in central Japan

3.2

Intraplate paleoseismic activities of southwest Japan arc

4. Faults of the 1995 Hyogo-ken nanbu (Kobe) earthquake
5. Discussions
6. Conclusions
References

Active Tectonics and Intraplate Paleoseismic Activities,
Southwt:!st Japan
Eikichi '-SUKUDA
Geological Survey of Japan
1-1-3 Higashi, Tsukuba, 305 Japan

Abstract

In the frrst part of this paper, the author examined active structures ofsouthwest Japan and pointed out that a westward movement of the forearc slivercaused by the oblique subduction of the Philippine Sea plate plays a very importantrole in the active tectonics of the Southwest Japan and Ryukyu arcs.
The Southwest Japan arc is divided into six major arc-parallel structural zones
from characteristics of active structures.

The Median Tectonic Line (MIL), one of

the most active faults on land in Japan, is situated as the main boundary fault which
separates the arc into two, namely forearc and backarc.

The Shikoku zone (forearc

ridge zone) on the south of MIL is charructerized by broad mega-folds whose axes arealmost perpendicular to MIL.

These arc-parallel compressional structures are

explained by that a westward translation rate changes along the forearc. This tectonic
situation is caused by that obliquity of two plates decreases to the west because of theconcave shape of the plate boundary.

On the north of MTL exists the depressionalzone which is named Seto' uchi or Seto Inland Sea.

This 80 to 100 km wide

depression is composed of en-echelon basins and ridges whose axes are oriented to
NE-SW, about 30 to 40 degrees to MTL.

The author named this zone as "Seto' uchi

shear zone" (SSZ) because these structures are explained as a drag motion caused by aright-lateral strike slip faulting of MTL.

1SSZ extends to the volcanic region of central Kyushu where E-W trending enechelon grabens and normal faults are intensively developed.

The U nzen grabenshows a typical pattern of normal faults around volcano in E-W compressional stressfield.

These en-echelon grabens are also understood as elemental structures in a

dextral shear zone.
The tectonic framework of Southwest Japan arc is also applicable to the
Ryukyu arc system.

The southwestward migration of forearc sliver in the southern

Ryukyu arc is apparently coupled with the development of en-echelon grabens in the
backarc, Okinawa trough.

The forearc is characterized by the development of

normal faults perpendicular to arc.

This forearc extension is explained by that

migration rate increases in the south.

In the northern Okinawa Trough active faults

are very rare because of small migration rate of forearc sliver.

This suggests that the

major active tectonic framework of the Okinawa trough, that is, extensional rightlateral shear zone is not directly connected to the central Kyushu which belongs to thebackarc of Southwest Japan arc.
Secondly, the author discussed historical earthquakes and their source faults
in southwest Japan, and he pointed out that clustering and migration of earthquakes
since the early 16th century are significant in the Kinki and Chubu districts.

It wassuggested that the 1596 Fushimi earthquake caused by the movement of the ArimaTakatsuki Tectonic Line and Rokko fault system from distribution of damaged areaand some faulting evidences at the archeological excavation sites.

The faults of the

1995 Hyogo-ken Nanbu earthquake, which probably did not move during the 1596
earthquake, is located in the southwestelm segment of the above cited fault system.
This estimation is proved by the recent intensive trench excavation surveys along thefault system. The 1995 faulting area is located in MIL seismic zone, one of four
potential seismic zones designated by Tsukuda (1978).
term seismic cycle of 1300 to 1600 years.

2

It is found that there is a long

During this long-term seismic cycle

seismic area shifts significantly in active tectonic provmces.

Seismic period ofwestern SSZ comes little after the beginnig of that of Kinki district (eastern SSZ).
According to the seismotectonic framework of dextral shear movement in southwest
Japan and seismotectonic cycle, somewhere along MTL could be a potential seismic
fault in the near future.

Key words: active fault, forearc sliver, oblique subduction, graben, strike-slip fault,
arc-arc junction, Seto'uchi shear zone, Southwest Japan arc, Ryukyu arc, Philippine
Sea plate,Eurasian plate,paleoseismology,

central Kyushu,

Hyogo-kennanbu

earthquake,

Median Tectonic Line, Arima-Takatsuki Tectonic Line, Rokko

fault system,,

1 . Introduction
For the purpose to understand a seismotectonic environment, it is essentially
important to describe clearly an active tectonic framework which would be illustrated
by accumulation of long-tenn defonnation.

An individual large seismic activity is a

key factor to construct active structures as a major incremental strain step.

The author

discussed the development of active structures of southwest Japan (Fig. 1) in some
papers (Tsukuda, 1992; 1993) and conclusively pointed out that a westward
movement of forearc sliver driven by oblique subduction of the Philippine sea is a key•

feature for the active tectonics of southwest Japan (Fig. 2) .
Oblique subduction is a very popular feature in convergent two plates.

Inthe early period of plate tectonics, Fitch (1972) introduced an importance of obliquesubduction in island arc tectonics and proposed a decoupling hypothesis of continental
margins.

He sited the Median Tectonic Line as an example of transcurrent fault aswell as the Great Sumatran fault of Indonesia, the Philippine fault of Philippines andthe Alpine fault of New Zealand.

Matsuda and Veda (1971) , and Kaizuka (1972;,3

1975) also found a very significant rOile of arc-parallel strike-slip fault
arc into two, forearc and backarc.

•

which divides

Recently, tectonics of Japanese islands is being

discussed with Eurasian plate movernent (e.g. eastward movement of Amurian plate;
Ishibashi, 1986; Kimura et al, 1986). It is however, concluded that effects of obliquesubduction of Philippine Sea plate is much more substantial in southwest Japan(Tsukuda, 1992; 1993).

Migration of forearc sliver caused by oblique subduction isa very common feature in modern subduction zones (Beck, 1983; Jarrard, 1986).
In this paper, again characteristics of active tectonics of southwest Japan will
be discussed with newly obtained data with special reference to deformation of forearc
sliver together with backarc shear.

In addition, paleoseismic activity related to theactive tectonics will be discussed based upon recent excavation surveys of active
faults .
Active faults are well described as for their distribution in Japan except forsubmarine areas or some metropolium areas covered with thick Quaternary deposits(Research Group for Active Faults of Japan, 1991).

In the case of active structuresof southwest Japan,. it seems, however, to be difficult to understand theirdevelopment process because orientations and natures of active faults are not so simple.
This paper will solve this question.
The author himself participated in several mapping projects in southwest
Japan, for example, as 1 to 500,0100 scale Neotectonic map series (Tsukuda et al,
1982; 1985; Yamazaki et al, 1984) and strip map series of active fault system(Tsukuda et al, 1993 and Mizuno et al, 1994) in southwest Japan.

He also

contributed to excavation surveys of active faults (e. g. Tsukuda and Yamazaki, 1984;
Tsukuda, 1985a; 1989c; 1995a) and paleoseismological studies (Tsukuda, 1987;
Sangawa et al, 1986a, 1986b).

Discussions of this paper are based upon very

original studies mentioned above and with his colleagues of Geological Survey ofJapan.

4

2. Active tectonics of southwest Japan
2.1 Zonal arrangement of southwest Japan arc
The Southwest Japan arc can be divided into six arc-parallel zones from aview point of tectonic landforms and active faults distribution (Fig. 3).

They arenamed as 1) Inner trench slope zone, 2) Forearc basin zone, 3) Forearc ridge zone
(Shikoku zone), 4) Seto' uchi (shear) zone, 5) Chugoku zone, 6) San'in zone, fromthe Nankai trough on the south, respectively (Tsukuda, 1992).
Among them, active tectonic features of the forearc basin zone, Shikoku zone
and Seto' uchi shear zone will be discussed as well as right-lateral shear motion of theMedian Tectonic Line (MIL).

2.2 Activity of the Median Tectonic LineThe most significant activce structure of Southwest Japan arc is the rightlateral strike-slip faulting of the Median Tectonic Line (MfL).

Researches on MfL

has progressed intensively and extensively during 1970's.

With respect to thedisplacement history, Ichikawa (1980) recognized five stages since early Creatceous.
Roughly speaking, left-lateral displacement is predominant during Creataceous to
Tertiary ages (Hara et al, 1980). To the contraty, right-lateral displacement is very

It can be said that dextral movement is asignificant during recent 1 million years.

very recent feature in its long geologic history.
recognized by Kaneko (1966).

Recent activity of MfL is flfSt

He suggested some geomorphological features

showing right-lateral strike slip displacement along the fault.

After his works,

Okada (1968, 1970, 1971, 1973a, 1973b) continued to investigate extensively on the
Quaternary faulting

ofMfL and summerized in Okada (1980).

5

He concluded that

1) active segment of M1L is restricted from western Kii peninsula in the east to
western Shikoku in the west for about 300 km long, 2) average dextral slip-rate of
M1L is estimated to be about S to Srrun/year in central to eastern Shikoku, 3) total
amount of right-lateral displacement in the Quaternary is estimated to be several
kilometers at eastern Shikoku, 4) amount and direction of vertical displacement are
different from place to place along the fault, and the maximum vertical displacement of

1. S km is obtained at the Ishizuchi fault scarp.

The beginning age of dextral slip of

M1L is estimated about O. S to 1 Ma by Sangawa (1977, 1978) Okada and Sangawa(1978), and Hujita (1980) .

Although there are many discussions on its activity andslip sense since their original works, very active dextral motion of MTL is now widely
accepted.

•

Shimamoto (1989) proposed a thin-skinned island arc lithosphere from cruststrength. Fig. 4 illustrates using lower limits of microearthquake distribution (Regional
Observation Center of Earthquake Prediction of Kyoto University, 1988; Okano et al,
1983; Okano and Kimura, 1988; Shiraki Microearthquake Observatory, 1990) basedon Shimamoto's idea

It is reasonable that MrL is located at a thinnest part oflithosphere which is weaker than other parts.
Shiono (1980; 1992) modified Fitch (1972), s decoupling hypothesis andpointed out the importance of leading edge position of subducting plate to explain that
active portion of M1L at present is limited .

•2 .3 Forearc basins of southwest Japan
South of MTL, the forearc sliver is not a rigid plate and has characteristic
deformed structures.

It can be divided into two zones, namely the zone of forearc

rise (Shikoku zone) and that of forearc basins. Both of them are characterized by folds
and reverse faults oriented at high angle to MTL, although the making processes for
those structures are significantly different from each other.

6

The forearc basin zone is composed of five major basins divided by reverse
faults and folds (Awata and Sugiy~ 1989 ; Okamura et al, 1986; 1987) as shown in
These surface structures characterized by inverted L-shaped structural highs

Fig. 5.

(Sugiy~1992; 1994) are explained to has been made by low angle oblique thrustfaulting (Ando, 1975), as a coseismic rebound to the subducting plate.
(structural unit) is

fundamentally fonned

defonnations (Awata andSugiy~

Each basinby the accumulation of coseismic

1989; Sugiy~ 1992; 1994). These units

indicate the segmentation of interplate seismogenic mega-thrust (A-B-C of Fig. 4).
The Tonankai earthquake of 1944 was caused by the movement of C and D segments
in Fig. 5.

The N ankai earthquake of 1946 is A B and Z segments.

Maemoku(1988a,b) and Maemoku and Tsubono (1990) found that arc-parallel movement withintervals of 300 to 1500 years contributed the elevation of structural highs.

This

event considered to be made by faulting of N -S trending reverse faults along structural
highs by Awata (1991) and Sugiyama (1992).

They call this faulting event Genroku-

type earthquake (Matsuda, 1978).

The northern end of this zone is located on the margin of forearc basins.

N-

S structural high axis like Muroto peninsula does not continue to the North (Fig. 6).

2 .4 Deformation of the Shikoku z:one
In the Shikoku zone, NNW-SSE trending structures are predominant like
synclines of Kii-suido and Bungo-s:uido, Mt. Tsurugi anticline and Hijikawa
monocline, which indicate an arc-parallel shortening (Fig. 6).

This evidence,

together with the right-lateral strike-slip motion of MTL, supports the hypothesis ofwestward movement of forearc sliver.

The Shikoku zone can be recognized as aforearc ridge zone which is significantly compressed to the island arc direction(Fig. 8).

It is a very rare case in forearc defonnation, but this arc-parallel compression of
forearc is a very rare example which is theoretically shown in MaCaffery (1992).

7

Hijikawa area in the west of Shikoku shows a monoclinal structure whose
axis is drawn from the uplift-rate contours and studies of the late Quaternary terrace
defonnation along the Hijikawa river (Fig. 7; Tsukuda, 1992).
Satoh et al(1994) studied microearthquakes around the borders between

Ehime, Kagawa and Tokushima prefectures along M1L and revealed very shallow
hypocenter distribution less than 8 kIn.

This earthquake swann (S of Fig. 6) is in thewest of Mt. Tsurugi anticline located in the flexure of westerly inclined fold (Fig. 8).
This is the same situation as the Wakayama microearthquake swann (Wakayama
Earthquakes of magnitude more than 4.0

Microearthquake Observatory, 1992).

clearly show thrust faulting with E-W P-axis (ditto).
The maximum compressional axis obtained from earthquake mechanism
analyses is oriented to ENE-WSW (Kimura and Okano, 1992), almost parallel orslightly oblique to the wne.

This maximum stress orientation is concordant to thefolds development mentioned above.

The wne continues to the east in the area ofthe Mikawa earthquake, of which maximum compression axis is oriented to ENEWSW (Fig. 3; Ando, 1974)

2.5 Seto'uchi shear zone
The Seto' uchi longitudinal basin (Setonaikai or Seto inland sea) on the northof MTL has characteristic dextral shear structures.

The Seto' uchi basin is divided

into four major en-echelon basins, narnely Osaka bay, Harima-nada, Hi'uchi-nada,
Iyo-nada The long axis of these basins (syncline axis) is oriented to NE-SW, at theangle of 30 to 40 degrees to MTL.

The northwestern margin of basin is bounded by

dextral reverse faults dipping to the northwest. These synclines and anticline withfaults can be explained as structures in a right-lateral shear wne.
summarizes the active structures of SSZ.

The Fig. 9

These faults and folds are recognized as

thrust shears (P-shear) and drag folds, respectively, in a shear zone (Wilcox et8al, 1973; Skempton, 1966).

This shear zone about 80- 100 km wide is named as"Seto'uchi shear zone" (SSZ; Tsukuda, 1992).
We can see a typical structures of shear zone in Osaka bay and Harima-nada
area which is located in the eastern end of SSZ (Fig. 10).

Fig. 11 shows a strain

ellipsoid together with faults of different orientations in right-lateral shear zone.
Most of the northwestern marginal faults of Osaka bay belong to D-shear, P-shear andreverse fault, increasing the angle to shear zone, getting larger vertical component
The Osaka group surrounding ()Saka bay is well surveyed and one of type
Quaternary sediments in Japan (Huzita and Kasama, 1982; Huzita and Maeda, 1985;
Mizuno et al, 1990; Mizuno, 1992).

It is suggested that topography or landscape of

this area has been made during the last 1 or 0.7 Ma by Sangawa (1986b).

In otherwords, Osaka basin is made by the movement of northwestern marginal faults of
Arima- Takatsuki Tectonic Line and Rokko fault system which caused 1596 Keicho-

Fushimi earthquake and 1995 Hyogo-ken nanbu earthquake.

The thickness of ()Saka

group reaches as much as 2000 m in front of the higher Rokko mountains .

This

Osaka bay is strongly affected by E-W compression, which may be a effect of the
Pacific plate (Huzita, 1980; Kuwahara, Jl985) which can be seen by the developmentof thrust faults of Uemachi fault and Ikonla fault in the east.
Other basins in the west of SSZ have very limited structural information.
There are, however, some hints for tectonics, that is, right-lateral strike slip ofIwakuni fault system (Higashimoto, 1983; 1986, Tsukuda, 1985) of Yamaguchi
prefecture, Koi fault in the west of Hiroshima city and Chojagahara fault to the northof Onomichi city.
SSZ extends to central Kyushu of volcanic region (Matsumoto, 1979) where
E- W trending en echelon grabens and normal faults are intensively developed asshown in next section.

Fig. 12 shows stress trajectories of maximum compression

in and around SSZ, mainly obtained from seismological data (Shiono, 1980; Kimura

9and Okano, 1992) and active faults data (Research Group for Active Faults of Japan,
1991; Tsukuda et al, 1982; 1985).

This shear motion is not able to see in geodetic

surveys (Hashimoto, 1990).

2.6 Right-lateral shear zone of central Kyushu
The central Kyushu is the western extension of Seto' uchi shear zone (Fig. 1
and Fig. 9; Tsukuda, 1992). There is no tectonic boundary between Setonaikai in theeast and central Kyushu except for the volcanic front (Fig. 13).

In this volcanic

region, four major volcanogenic grabens are developed (Fig. 14).

They are called,

Unzen, Yufu-Tsurumi, Kuju-Haneyama and Aso grabens.

A dextral shear

deformation is very clear in central Kyushu although Tada (1984; 1985) proposed
north-south rifting model (Hatanaka and Shimazaki, 1988; Fig. 15).
trajectories of maximum compression support this shear motion (Fig. 16).

Stress

It is easily

understood that hydrothermal activity and high pore pressure in a volcanic region arecontributing to the development of normal faults .

The U nzen graben shows a typical

distribution of normal faults around volcano in E-W compressional stress field(Kaneko, 1973). Fig. 17a illustrates active faults of terrestrial (Chida, 1979, Tsutsumi,1978) and submarine (Nakai et al, 1982; Geographical Institute of Japan, 1988) area s.
Leaving from volcanic centers, normal faults tend to settle their orientation to E-W ofregional maximum horizontal stress .

This is very-concordant with stress field fromseismological analysis (Yamashina and Mikami, 1977).
this graben is 42 km and 12 km, respectively.

The length and width ofThis characteristic fault distribution

coincide with the dike pattern of Spanish peaks (Ode, 1957).
different from rifting of Iceland and C>kinawa trough.

This pattern is quite

Fig. 17b shows a stresstrajectories obtained from active faults. «)hta (1972;1987) recognized Chijiwa caldera,
but there is no evidence of active faults.

The U nzen graben typically shows a fault

distribution pattern of E-W compressional stress field, developed and modified by the10volcanic activity (uplifting of magma) of Unzen volcanoes.
The general direction of grabens is oriented at the angle of 40-50 degrees tothe trend of the shear zone.

This means that there is no positive or negative zone-

nonnal dilation in the shear zone (Ramsay cmd Huber, 1983).
Fig. 18a shows active fault distribution around Yufu and Tsurumi volcanoes .

Thisgraben area has been called as follows, Beppu-nishi (west) and Beppu-higashi(east)
grabens by Chida (1979),

Hayami grabens by Ikeda (1979)Beppu-bay

grabens by Chida (1992).

Tsukuda (1993) named this graben as Yufu-Tsurumi

graben taken from names of volcanoes .
compared with Unzen graben.

and YufuinThis graben pattern is a little twisted

This is the reason why eruption centers of Yufu-

dake and Tsurumi-dake is a little shifted.

Recently, Shimazaki et al (1986) and

Nakata et al (1989) surveyed distribution and activity of submarine active faults innorthern Beppu bay.

In the southern margin of Beppu bay no active faults are found ,but the author expects active faults there according to significant topographic relief (Fig.
19).

The ratio of width to length of graben is 1:3 or 1:4.

This is almost the sameratio as Unzen graben.
Kuju-Haneyama graben (Fig. 20) and Aso graben (Fig. 21) are also
developed by the same manner, mentioned above (Tsukuda, 1993).

Volcanic

activities looks to be working as hydro fracturing for stress measurement.
The rifting model of the Central Kyushu proposed by Tada (1984, 1985) is
not supported by the study of active faults.

Hashimoto and Jackson (1993) reviewed

his analysis and did not support his results either.

This orientation of grabens as well

as fault pattern is completely different frorn that of the "extensional" Okinawa trough.

2.7 Development of tectonic landform of southwest Japan arc
In the east of Southwest Japan arc, the oblique subduction of the Philippine
Sea plate is quite obvious (Seno, 1977; Seno et al, 1993), which gives the driving

11

~

.....

_ ..

force of westward translation of forearc sliver (Fig. 13).

In the west, it becomes notclear because the trench axis rotates counterclockwise.

This means that there is no

driving force for the westward translation.

The gradient of translation rate along theforearc sliver to west can make the arc parallel compressional structures of Shikoku
zone.
Fig. 22 shows generalized distribution of displacement across the Southwest
Japan arc.

This explains zonal arrangement of active structures. It is very interestingthat a right-lateral shear is expected along the outer ridge between the inner trench slop
and the forearc basin zone.

Some kinds of extensional structure are expected

between Shikoku zone and forearc sliver zone.
It is very

Fig. 23 illustrates the basin development in southwest Japan.

interesting that basins of different orientation, origin and structure are formed during
thesame period.

2.8 Zonal arrangement of Ryukyu arc and Okinawa troughIn the middle and south of Okinawa trough, the development of en-echelon
grabens is significant.

Orientation and density of active normal faults in Okinawa

trough are shown in rose diagram (Kato et al, 1988; Fig. 24). In the southern Okinawa
trough, faults are oriented at low angle (15 degrees) to the zone of Okinawa trough.
Sibuet et al (1987) and Ota and Kaizuka (1991) suggested that this en-echelon grabens
are developed by the dextral shear in the Okinawa trough.
Fig. 25 shows several examples on orientation of normal faults in a shear
zone. Data are obtained from Taupo-Rotorua depression (Cole, 1984: New ZealandGeological Survey, 1973),

Fizroy trough in Australia (Smith, 1968) and Guatemara-

Montagua fault zone (Plafker, 1976).

According to the text book of Ramsay and

Huber (1983), It is very clear that Okinawa trough belongs to extensional (positive
dilational) shear zone.

12

~-

~

~

-

-

~

-=

.

-_. -

_.
---

The most remarkable active structure of forearc sliver of the Ryukyu arc is a
development of transverse normal faults (Fig. 26) which is typically shown in Miyako
island (Kawana and Pirazzoli, 1989).
extension.

This structure indicates an arc-parallel

Many examples are reported concerning on forearc extension (Kimura,

1985; Kuramoto and Konishi, 1989; MaCaffery, 1992).
This interesting structural phenomena of forearc extension and backarc
dextral shear can be explained by that the: migration rate of southern part of the Ryukyu
forearc is much higher than that of northern part because of a convex shape of Ryukyu
arc-trench system.

The structural contrast between forearc compression ofSouthwest Japan arc and forearc extension of Ryukyu arc depends on only shape ofcurvature of arcs .

2.9 Junction between the southwest Japan arc and the Ryukyu arc

It is very significant that the movement of forearc sliver is very
the tectonics of oblique subduction.

important in

Fig. 27 shows the speed of forearc slivertranslation is coupled with backarc shear deformation.

The oblique subduction of thePhilippine Sea plate to the Eurasia plate plays a very important role in development ofbackarc shear zone.

The arc-parallel translation rate changes as a function of theobliquity between the two plates.

The higher migration rate of the forearc sliver inthe southern Ryukyu arc is apparently coupled with the development of en-echelon

In the northern Okinawa trough active

grabens in the backarc, Okinawa trough.

faults are quite rare (Kato et al, 1988) because of low migration of forearc sliver.
This suggests that the major active tectonic framework of the Okinawa trough, that is,extensional right-lateral shear zone is not directly connected to the central Kyushu
which belongs to the Southwest Japan arc.

13

---

--

3. Paleoseismicity of southwest Japa n
In the latter half of this paper, paleoseismic activities in southw est
will be discussed together with active tectonics shown in the previo

Japan
It

us chapter.

is very important to develop some works in order to identify a source
historical seismic event, because we may fmd precautional faults (Matsthe basis of "characteristic earthquake tnodel" (Schwartz and Coppe

fault for auda, 1980) on

rsmith , 1984).

the Philip pine
Nakat a et al (1990) showe d us a V(~ry successful example ofearthquake of 1990.

Each seismic event works as an incremental displacement forthe on-going tectonics. Therefore, a long term (more than 1500 years)

seismic activity

which is especially possib le in southw est Japan with long historical

records, should

show us a deformation proces s and time-space relationship of faultin

g in the area.

This may also provid e a clue to predict a future ocurrence of earthq

uakes in theactive tectonic system in southw est Japan.

3.1 Migra tion of large earthq uakes in central Japan
Historically in Japan, there were many large earthquakes which
severe damages .

causedSome of them are well recorded and documented (Table 1).

the Chubu areas
Tsukuda (1987) examined historical ecuthquakes in the Kinki andfrom the earthquake catalog by Usami (1975, 1987), and then discus

sed their source

faults from some geologic evidences, and pointed out that clusteringand eastwardinmigration of earthquakes since the early 16th century are significant

Kinki area

the central

It was concluded that the 1596 Fushim i earthquake caused bythesystem frommovement of the Arima-Takatsuki tectonic line and Rokko fault
14distribution of damaged area and sonle faulting evidences at the archeologicalexcavation sites.

The faults of the 1995 Hyogo-ken nanbu earthquake, which

probably did not move during the 1596 earthquake, is located in the southwestern
margin of the above cited fault system.

This estimation is being examined by the

many trench excavation surveys after the earthquake to fmd faults with high possibilityof movement.

Tsukuda (1987) paper also forecasted that the future seismic activity

would be concentrated outside of the past faulting area and this seems to be correct.
The trench excavation surveys have been conductedwidely in Japan to

The author himself participated many

identify paleoseismic events on faults .

excavation surveys in last 15 years and found recurrence intervals are long, more than
1500 years in southwest Japan.

This is quite different from faults to the east of

the Itoigawa-Shizuoka Tectonic Line, for example the Nagano active fault system(Tsulruda et aI, 1995a), Itoigawa-Shizuoka Tectonic Line active fault system(Okumura et aI, 1994), the Northern Tzu fault system.

Kinld district
Sangawa (1986) investigated the detailed topography of the Konda-yama
tumulus (burial mound, 430m long) in tbe eastern end of Osaka p lain. This tumulus issaid to have been built for the Emperor Ohjin in the forth or fifth century. He foundthat the tumulus deformed by the Konda fault of 4 km long, Vertical displacement ismeasured 1.8 m, which may represent an historic destructive event.

He concluded

that the event was the earthquake of 1510 because there was no other destructive
earthquake reported in the eastern Osaka plain in our history. Magnitude

15

for

the

earthquake is calculated to be 7.1 fornn the equation M - (log D+4.0) /0.6 (M atsuda,
1975; 1977). D (in meter) is the surface displacement accompanying an earthquake.
The long-term vertical slip rate (s) of 0.25-0.4 m / 1000 years for the Konda fault isobtained from terrace deformation analysis.

The recurrence interval (R) is estimatedat 5000 to 7500 years from the equation log R =0.6 M-(log S+4.0) (Matsuda, 1977).
From the emp irical basis of the relation between magnitude and surface fault length,surface faults of the earthquake of magnitude 7.1 extends at least 20 km. Therefore, it
is very possible that the north-south trending eastern marginal fault system of the
Osaka plain composed of the Konda fault and Ikoma fault,

generated theearthquake of 1510.
The following large earthquake after the 1510 event in the Kinki area is 1596
(Keicho-Fushimi) earthquake.
the 1596 earthquake.

There were many discussions on the source fault forOn the basis of hazard area illustrated by Usami (1987), it issuggested that the Arima-Takatsuki Tectonic Line fault system and Rokko fault
system of the northwestern margin of Osaka basin is the source faults of theearthquake.

Recently, in some archeological excavation sites found geologic

evidences of faulting between 15 and 17 century layers.

Trench excavation surveys

on one segment of the Arima-Takatsuki fault performed in 1995 strongly su~sts
that the 1596 event was causled by

the fault(Sangawa,

1995; personalcommunication).
The epicenter of 1662 earthquake (M=7.8) is located in Biwa lake (Usami,1987).

The western coast region of the lake suffered severe damage and it is16

reported that a significant area of the coast had been submerged into the lake,
Sangawa and Tsukuda (1986b) investigated ancient picturesque maps for comparison
with present topography.

The detailed bathymetric map shows the ancient mainroad is now below the water.
estimated at about 2-3 m.

Th(~ tectonic depression from the earthquake was

Fault scarp lets near the coast that cut the terrace dated

younger than 5700 years B.P. show 2.0-2.7 m of vertical displacement, the amountof which is comparable the depression.

The submerged zone was recognized along

the entire western coast of Biwa lake. The total length of surface rup ture is exp ected
to be more than 50 Ian for the earthquake magnitude of 7.8.

Consequently, it isconcluded that the 1662 event was caused by the movement of the western marginal
fault system of Biwa lake, which is composed of en echelon faults and is closely
related to the develop ment of Biwa lake.

Recent archeological excavations in Biwa

lake indicated that the previous events were occurred in some time middle of Yayoi
period (BC 0+-50) and in about 3000 years B.P. (Sangawa and Tsukuda, 1986).
After the 1662 earthquake, 158 years later, the Hikone earthquake of 1819
(M =7.4) occurred in central Kinki.

The seismic intensity map for the earthquake(U sami, 1987) suggests that the Jmarginal faults of the Suzuka mountains wereactivated although no geological evidences has not been found yet.
earthquake (M=7.6) occurred in 1854.

The 19a-Ueno

Yokota et al (1976) found some surface

evidences of surface rup tures of the earthquake along a segment of the Kizugawa
fault system.

Seismic intensity map of Usami(1987) suggests the Kuwana and

Yokkaichi faults may be activated during the earthquake.

17

Between 1819 and 1854

-

-

~

-

-

~

--

-

-

-

-

--

earthquakes the so-called Kyoto earthquake (M =6.5 +/- 0.2) occurred locally in the
Kyoto city area.

After this earthquake, 37 years later, the great Nobi earthquakeof magnitude 8.0 occurred in the Chubu district, an adjoining seismic area.

Chubu district
The Atera fault system in the Chubu district is known as one of the mostactive fault in Japan.

It dissects central Japan with a northwest-southeast strike

and a length of about 70 km. This fault shows left-lateral slip with a verticalcomponent of northeast side up .

Lateral slip rate is estimated to be about/ 1000 years (Sugiyama and Matsuda, 1965).

3-5 m

Many excavation survey had been

performed since 1981 (Tsukuda and Yamazaki (1984).
shows an average recurrence interval of 1700 years .

The latest four events

The most recent event for the

Atera fault is estimated to be the great earthquake of 1586 (so-called Tensho
earthquake, M=7.9-8.1 by Usami, 1987), from following data,(1) Usami(1987) located the epicenter in the northwest border of Gifu
prefecture from a damage area distribution.

(2) Dai-itokuji, a abandoned temple on

the fault is reported to have been collapsed completely during the 1586 earthquake.
En-echelon fractures still remain covered by thick forest near the temple.

(3)

There is a legend that the 1586 earthquake made a pond, which is a wet muddy rice
field now which is explained to be a sag pond from a geomorphological view point.
(4) Large scale land slides during the earthquake are found to the southeast of thefault system, recently (Matsushitna, 1994 personal communications).

18(5) The1586 event was supported by recent excavation surveys along the fault by Toda et al(1995).
Therefore, the recognition of the Atera fault as one of precaution faults by
Matsuda (1981) is denied (Tsukuda et aI, 1993).

Sugiyama et al (1994) revealed

that the M iboro fault system which is the northwest extension of the Atera fault
system also disp laced during the 1586 earthquake.

This evidence suggests thatfaults of 1586 Tensho earthquake extends as long as 85 km in total.
After the T ensho earthquake, the Kaga earthquake of 1799 was caused by
the movement of the trust fault (M orimoto fault) at Kanazawa city (Sangawa, 1986)
in the northward extension of the Tl~msho rupture zone.

The earthquake of 1858

(M=6.9) on the Atotsugawa fault (Matsuda, 1966; Research Group for Excavation of
the Atotsugawa Fault, 1983) in the east of the northwest fault termination point of
the Tent earthquake.
The Nobi earthquake of 1891 (M=8.0, Matsuda, 1974) occurred after theseseismic area in the east.

It seems that the occurrence of the Nobi earthquake isclosely related to the earthquake migration in the Kinki and Chubu districts.

Close

to the both ends of the Nobi earthquake faults ruptured in 1945 (Fukozu fault,
Mikawa earthquake) and 1948 (Fukui earthquake).
The 1925 Tajima earthquake, M=6.8 and the 1927 Kita-tango earthquake,
M=7.3 occurred after more than 6000 years interval (Tsukuda et al, 1989a; 1989b;
1993) in the tectonic province of San'in zone (Fig. 3) northwest of central Kinki area,
which had been active during 16 to 19 centuries.

The 1872 Hamada earthquake(M=7.1) and the 1943 Tottori earthquake (M=7.2) occurred in this zone.
19

The time-space distribution of earthquakes and their faults since the 16
century is illustrated in Fig. 28 .

It is interpreted that most of faults in the core areaof central Japan had been activated during the last 1300 years as discussed above.
It is recognized by geological observations that an active fault causes " characteristic

earthquakes" (Schwartz and Coppersmith, 1984) with a long recurrence interval.
A seismic chain reaction or clustering can be seen in seismotectonic zones inintraplate Japan. With this view, Tsukuda(1987) designated four "potential seismic
zones", which has almost same meaning as precaution fault of criterion 1 in Matsuda(1981).

Potential seismic zones are characterized by low seismicity at present, no

historic major earthquakes and tectonic significance.

This will be discussed in thenext section.

3 .2 Intraplate paleoseismic actilvities of southwest Japan arc
According to recent progresses of paleoseismology, the author describes along-term seismic behavior in southwest Japan for more than 2000 years .

Some ofthe active faults in this area are at present being investigated and much more detailed
studies are required.

It is, however, hard to avoid pointing out the following

features .
(1) Shifting of seismic active are41 or partitioning of activities is recognized insouthwest Japan.

Fig.29 shows the time-space distribution of paleoseismic

activities obtained from the data set of Usami (1987; Table 1) and excavation
surveys.

The activity of central Kinki district has more than 1500 years interval.

Shifting phenomena for seismic .activity from central Kinki to both western Chubu

20and Tango-San'in districts are recognized after c.a. 6th century, as happened
similarly during 19th century.
Seismic area of Kinki from 16th to 19th century is located in the eastern end
and extension of SSZ, where active faults are densely populated and show highly
compressive structures, namely north-south trending reverse faults of Uemachi
faul~ Ikoma fault and western marginal faults of Biwa lake.

The Kinki district

may be seen as a "compressive bend" to the dextral shear zone of SSZ (Fig. 9).
In the west of

Seto'uchi district, Beppu bay to Aki-nada area, two ofseismic activity is recognized since the end of 16th century.

The epicenter ofNovember 4, 1596 (M=7.0) earthquake is located in Beppu bay.

There is alegendary story that the pre-existing Driu island was subsided under the sea during
the earthquake, although no geological evidences are reported.

This ispresumably caused by the southern marginal fault of the Yufu-Tsurumi graben(Fig. 18 and 19).

This earthquake was followed by the earthquakes of March

17, 1649 (M=7.0) and January 4, 1686 in the Iyo-nada area.
activity seems to be related to the geological structure.

The trend of
After 179 years

quiescence, the earthquake of December 26, 1854 (M=7. 3-7.5) occurred atsouthern end of Iyo-nada.

This earthquake was followed by the earthquakes ofOctober 12, 1857 (M=7.3) and June 2, 1905 (M=6.7) at Aki-nada area.

It isvery interesting that these two series of active period and period of intermission
coincides with the seismic pattern of Kinki district mentioned in the previous
section.
(2) From historical earthquakes in central Japan, Tsukuda (1987) proposed four major
potential seismic zones (Fig. 30) outside of previously active seismic zone (Fig.
28) as follows,

I. Median Tectonic Line (MTL) seismic zone:
The right-lateral slip rate and recurrence interval of MTL are estimated 5-10m

2111000 years (Okada, 1980) and 1500-2000 years, respectively.

The most

recent event of MIL is obtained as 5 to 8 century in the central to westernsegments in Shikoku (Yamazaki et al, 1993; Okada, 1993).

Tsukuda(1996a)

revealed by trench excavation surveys that no historic earthquakes since 16th
century occurred along the N egoro fault of the Kinki segment of MIL.

The

most possible earthquake appeared in the catalog of U sami (1987) is theearthquake of 734, by that the Koyasan temple was reportedly damaged.
Recently, in the eastern segment of Shikoku, the latest event was reported to be
younger than 16 century (Okada, 1993).

II. Itoigawa-Shizuoka Tectonic Line seismic zone:
ISTL is a major tectonic boundary together with MfL in Japan.

In general,

this tectonic line composed of left-lateral strike slip faults with some amount ofvertical component.

The most recent event along IS TL is estimated at 841

AD At Gofukuji fault (Okumura et al, 1994), 1700 to 2500 years B.P. at
Okaya site (Research Group for the Okaya fault, 1984), and 1000 to 1400
years B. P. at Fujimi site (rv1iyoshi and Research group for the ItoigawaShizuoka tectonic Line, 1984) by excavation surveys in central segments ofthis zone. The recurrence interval, slip-rate and average slip for single event isestimated to be shorter than 1000 years, 9.4+-4.5 mm (since c.a. 6000 years
B.C. and 7.5 +-1.5 m, respectively (Okumura et ai, 1994).
III. Central Japan Alps (CJA) seismic zone:
CJA is characterized by thrust faults on both flanks of central Japan Alpsmountains.

Inadani fault system belongs to this seismic zone.

There is no

reliable information on paleoseismic activity in the zone.
IV. Noto peninsula (NP) seismic zone:
NP seismic zone is characterized by very short dip-slip faults with low
seismicity. There is no reliable information on paleoseismic activity in the zone

22

yet.
This was partly proved by the hazardous Hyogoken-nanbu earthquake of1995.

The 1596 Fushimi earthquake was determined to have been produced

from the Arima- Takatsuki Tectonic Line and Rokko fault system from distributionof damaged area and some faulting evidences at the archeological excavation sites.
Their own characteristic earthquake is very different from the faults of the 1995
earthquake.

The faults of the 1995 Kobe earthquake, which probably did not

moved during the 1596 earthquake,is located in the western margin of the above

cited fault system.

The surface faults area is clearly located in the seismic zone of

MIL cited above.

Faults of Hyogoken-nanbu earthquake and their rupturing

process are discussed in the next section.
Two phenomena mentioned above indicate a long-tenn seismotectonic cycle(Tsukuda, 1989b) of c.a. 1300 to 1600 years, and are stongly suggesting the
movement of MIL in the near future.

4. Faults of the 1995 Hyogo-ken nanbu (Kobe) earthquake
The 1995 Hyogo-ken nanbu (Kobe) earthquake (M=7. 2) caused severe
damages in Kobe area. Death toll reached more than 6000.

After the earthquake,there are many arguments on existence of faults in the Kobe area.

The author

maintained his opinion that no faults moved in the urban area from geological andseismological view points. Terrible hazards downtown Kobe area may be caused byfaulting directivity and/or focusing (Uld amplification of ground motion caused bysubsurface structures.

On the faults and their rupturing process, Tsukuda (1995)

concluded as follows,
The right-lateral strike slip fault of the earthquake is composed of three major
segments, namely NE-SW trending Nojima fault (10 km) at northern Awaji island, E23

W trending Takaiso fault (6 kIn) at the: Akashi strait and NE-SW trending Suma fault(9 km) at western Kobe, which extends about 25 Ian long in total.

The Nojima fault

displaced as much as 2. 1m right laterally at the central part (Awata et al, 1995).

Fig.

shows active faults distribution

Theobtained from lzaki and Kaneko (1960).

Takaiso fault close to which the epicenter is located, played a major role in faulting
process from the view point of active tectonics of the Seto' uchi inland sea which is aright-lateral shear zone (SSZ; Tsukuda, 1992). But, the details are still unknown
because of heavy traffic and high flow rate of sea water in the strait.

Fig. 32 showsthat Takaiso fault has a same orientation as the Arirna- Takatsuki Tectonic Line which isa major fault in SSZ as well as MIL.

The right-lateral displacement of the Takaiso

fault is estimated about 1. 3m from IGPS measurements between two piers of the
Akashi strait bridge under construction (Yamagata, 1995 personal communications).
High activity of the Takaiso fault is suggested by that the submarine channel of the
Akashi strait is significantly displaced in the right lateral motion across the Takaiso
fault.
No reliable observations of surface breaks were reported along the Suma fault,
although geodetic measurements by lIashimoto et al (1995) suggest a movement of
deeper part of the fault.

No surface rupture were not reported either along the footof Rokko mountains to the east along the Suwayama fault and Uzugamori fault, where
damages are limited.
The sigmoidal geometric pattern of three fault segments mentioned above
makes two major compressional bends (barriers) which seems to be the locations ofearthquake nucleation.

Early phase: of aftershocks, three to four hours right after themain shock show the rupture was terminated at somewhere central Kobe where aextensional bend and jog is recognized.

Regional GPS measurements (Yoshida et al,

1995; Hashimoto et al, 1995) also indicate abrupt decrease of displacements aroundthe same area.

It is concluded that fault ruptured at the compressional bend between

24the Takaiso and the N ojima faults at the initial moment and propagated to anothercompressional barrier in the east between the Takaiso and Suma faults, which is thesecond break point. Then the rupture was terminated at extensional bends between
Suma, Nagata and Suwayama faults. In the south of the Nojima fault recognized is areleasing bend where is the place of fault termination. Fault ruptures were terminatedreleasing bends in both ends.

This faulting pattern is shown in Fig. 33 and is very

similar to recent earthquakes generalized as "A rupture starts from a compressional
barrier and stops at an extensional barrier" by Tsukuda (1991).
This fault rupturing process is consistent with the result of Kikuchi (1995)
obtained from distant seismographs. Dlirection of his initial bilateral fault break may be
the average direction of two faults, Takaiso and Nojima faults.
This 1995 event, as terrible as it was, was not "the big one" for the area. It
did not contribute the mountain building of Rokko mountains and depression of Osaka
bay described as "Rokko movement" by Huzita, 1968; Huzita and Kasama, 1982).
From paleoseismologica1 studies mentioned in the previous chapter, we may say thatthe 1596 Keicho-Fushimi earthquake was the main event which caused muchhazardous damages around the bay area (Usami, 1987; Sangawa, 1993).

Recenton-going trenching studies after the earthquake are proving this conclusions.

v. Cliscussions.
Fig. 34 shows a timing of great earthquakes of Nankai trough andearthquakes inland.

From the figure, we can read without hesitation that inland

earthquakes are crowded on and in front of the lines of N ankai trough earthquake as
pointed out several seismologists.

The Hyogo-ken nanbu earthquake may be one of

the examples before the great earthquake of Nankai trough.

It is quite possible toexpect that some other large earthquakes inland may occur in the near future.

MTL isone of the candidates for future earthquake generator as discussed in previous section.
It is very important to recognize the 100 years seismic cycle together with a long-tennseismotectonic cycle of 1300 to 1600 years for earthquake prediction Reseaches.
Recently two volcanoes in central Kyushu, namely Unzen and Kuju
volcanoes which belong to the Seto' uchi shear zone become very active.
be related to the strain release of right-lateral shear.

This might

It is reported that the most recent

eruption of the Kuju volcano has occurred after the 1700 years quiescence and its
recurrence intervals are estimated to be about 1000 to 2000 years.
The author strongly suggests that it is very important to analyze quantitatively
the defonnation process of SSZ for long-term earthquake prediction in southwest
Japan

VI. Conclusions
The author discussed active tectonic features of the Southwest Japan and
Ryukyu arcs, and also paleoseismic activities of the area, and made following
conclusions:
1. The Oblique subduction of 1the Philippine Sea plate plays a very importantrole in the active tectonics of Southwest Japan arc.
2. Tectonic contrast between thle forearc rise zone named as the Shikoku zone
and the backarc depression named as the Seto' uchi shear zone, separated by
the right-lateral strike slip fault, the Median Tectonic Line is totally explainedby the westward movement of the forearc sliver of the Southwest Japan arc
caused by oblique subduction of the Philippine Sea plate.
3. It is recognized that a significant arc-parallel shortening in the Shikoku zone26

which is made by an existence of gradient in an arc-parallel translation ratefrom east to west as a function of the obliquity between the two plates.
4. The Seto' uchi shear zone of Seto Inland sea is composed of four majorbasins which are bounded by NE-SW trending and NW dipping reverse fault
with right-lateral shear component.
5. The Seto' uchi shear zone continues to the central Kyushu volcanic region.
where en-echelon volcanotec:tonic grabens are developed. Characteristic fault
pattern are typically shown in the Un zen graben.
6. Development of active faults of the Okinawa islands and the Okinawa troughis also controlled by the southwestward extensional translation of forearc
sliver.
7. Arc-parallel compression and extension of forearc slivers are recognized in
the Southwest Japan arc and the Okinawa arc, and they mainly depend on
concave or convex shapes of island arcs or plate boundaries .
8. A shifting pattern of seismic area in southwest Japan is found during the last2000 years. This is thought to be closely related to the right-lateral shear
tectonics which has some long-term seismic cycle for about 1300 to 1600
years.
9. Ruptures of the 1995 Hyogo-ken nanbu earthquake started fromcompressionalbarrier and stopped at extensional barriers, as seen commonly
in Tsukuda (1991).
10. Faults of the 1995 Hyogo-ken nanbu earthquake belongs to MTL seismic
zone, one of four potential seismic zones. It is suggested that MTL might be apotential fault for the future JTIovement.

Acknowledgment
I would like to express sincere thanks to Professor Hara of the Hiroshima
University for his continuous support and encouragement since my graduate school27

term.

I also thank to my colleagues of Geological Survey of Japan, Dr. H .,Kinugasa, Dr. A. Sangawa, Mr. Awata, Mr. Sugiyama and Dr. Okumura, and Dr. Y.
yamazaki of Tokyo Metropolitan University

for their continuous and fruitful

discussions .

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•

,

40

Figures and tables
Figure. 1 Active structures in the Southwest Japan arc.
Solid arrow indicates the direction of the forearc translation. A shear zone named as
Seto'uchi shear zone is shaded on the north of the Median Tectonic Line (MTL).
Figure 2 Oblique subduction of the Philippine sea plate and westward forearc migration of theSouthwest Japan arc.
Active tectonic features of the Seto' uchi shear zone and the forearc sliver are very
different each other.
Figure 3 Arc-parallel zonal arrangement of Southwest Japan arc.
Each zone has its own characteristic features of active structures .
Figure 4 Generalized profile of Southwest Japan arc.
Micro-earthquakes are observed in the area of gray color which is recognized as an island
arc lithosphere.
Figure 5 Separated forearc basins of Southwest Japan arc (from Awata & Sugiyama, 1989)
Z: Hyuga basin, A: Tosa b., B: Muroto b., Cl: Shima b., C2: Kumano b,. D: Enshu b., a:
Ashizuri promontory, m: Muroto p., s: Shionomisaki p., d: Daiozaki p., 0: Omaezaki p ..
Figure 6 Tectonic structures of the Shikoku zone.
Contours indicate the average uplift rate during last IMa (Omori, 1990).
K: Kii-suido syncline, T: Mt. Tsurugi anticline, M: Muroto anticline, H: Hijikawa
monocline, A: Ashizuri anticline, B: Bungo-suido syncline, S: the microearthquake
swarm in shallow crust west of Mt. Tsurugi.
N-S or NNW-SSE structures suggest arc-parallel compression.
Figure 7 Westward tilting along the Hijikawa river, western Shikoku.
a: Alluvium along the Hijikawa river, b: tilting area c: direction of tilting.
Figure 8 Generalized profile of the Shikoku zone
Microseismic activity is significant in shaded areas.
Figure 9

East-west compression area of Kinki and Chubu in the east of the Seto'uchi shear zone.

Figure 10 Active tectonic structures in the east of the Seto'uchi shear zone.
I: Ikoma faults, U: Uemachi fault, R: Rokko faults, 0: Osaka-wan fault.
Figure 11

Orientation of elemental structures expected in a right-lateral shear zone.

Figure 12 crl stress trajectory derived from earthquakes and active faults.
indicates the Seto' uchi shear zone .

Dotted area

Figure 13 Development of en-echelon folds in non-volcanic region and grabens in volcanic
region, in the right-lateral shear zone, Southwest Japan.
Figure 14 En-echelon grabens in the central Kyushu.
Dotted area indicates the right-lateral shear zone (Seto'uchi shear zone).
YT:Yuhu-Tsurumi graben, KH:Kuju graben, As:Aso graben, Uz:Unzen graben.
Figure 15 The displacement field related to the baseline modified from Hatanaka & Shimazaki(1988).
Direction of the displacement in the northern Kyushu is almost parallel to theshear zone.
M.T.L.: Median Tectonic Line.

1

Figure 16 crl stress trajectory derived fronl earthquakes and active faults .
indicates the Seto'uchi shear zone of central Kyushu .

Dotted area

Figure 17 a: Active nonnal faults of the Uozen graben.
See Fig. 14 for the location of this figure . Notice that nonnal faults are not arranged inparallel. It is inferred to be made by E-W oriented compressional stress condition.
b: Orientation of maximum horizontal stress in the Unzen area.
Diameter of the circle is about 25 km.
Figure 18 a: Active nonnal faults in the Yufu-Tsurumi graben.
Y: Mt. Yufu, T: Mt.Tsurumi, A-A': profile line of Fig. 14
b: Orientation of maximum horizontal stress in the area.
Diameter of the circle is about 25 kIn .
Figure 19

Profile along the line A-A' ofFilg. 18 a. showing nonnal faults in the Beppu bay.

Figure 20 Active nonnal faults in the Kuju-Haneyama grabens
Am: Mt. Amagoi, Ni: Mt. Noine, Ke: Mt. Kuenohira, Hn: Mt. Haneyama, Wi: Mt. Waita,
Km: Mt. Kameishi.
Figure 21

Active faults of the Aso graben

Figure 22

Displacement distribution across zones of Southwest Japan arc.

Figure 23 Development active tectonic basins of southwest Japan.
These basins are fonned the same tectonic regime, that is, oblique subduction of thePhilippine sea plate.
Figure 24 Orientation of active faults in the Okinawa trough, modified from Kato et al (1988).
a: angle between faults and the trough direction.
Figure 25 Orientation of normal faults to a shear zone.
a: angle between the trend of shear zone and nonnal faults.
range of a in each shear zone.

Solid bars indicate the

Figure 26 Active faults characteristics of the forearc and backarc from the Southwest Japan arc
to the Ryukyu arc.
The explanation of the figure is in the text.
Figure 27 Southwestward translation of the forearc sliver accompanied with the right -lateralThe
shear of backarc caused by the oblique subduction of the Philippine Sea plate.
development of en-echelon grabens in the backarc is closely related to the migration of
forearc sliver. Double open triangles indicate a relatively higher translation rate thansingle ones.
Solid arrows indicate the direction of relative convergence between thePhilippine sea plate and the Eurasian plate.
Figure 28 Faults of historical intraplate large earthquakes (M>6.5) since the 16th century.
Starting from the 1510 Settsu earthquake, earthquakes migrated to the central Kinki
district. After the 1854 Iga-Ueno earthquake, seismicity become very high outside of thecentral Kinki. (Modified from Tsukuda, 1987)
Figure 29 Time-space diagram of paleoseismic activities in southwest Japan showing apartitioning of activity.
Shaded areas indicate periods of high seismicity estimated from trench excavation
surveys and archeological surveys. B: Biwako area, K: Kobe area. Details ofearthquakes are shown in Table 1 .

2

Figure 30 Major potential seismic zones surrounding the core of central Japan which had been
active during the past 500 yrs . (from Tsukuda, 1987).
MTL:Median Tectonic Line seismic zone; CJA: Central Japan S.Z.; ISTL: ItoigawaShizuoka Tectonic Line S.Z.; NP: :Noto peninsula S .Z ..
Figure 31 Fault distribution and submarine topography in the Akashi strait.
1A,2P, 3P, 4A: Anchorage and pier of the Akashi strait bridge under construction
Figure 32 Active faults of the Arima-Takatsuki Tectonic Line and the Rokko fault system.
Faults of the 1995 Hyogo-ken Nanbu earthquake is located in the western margin of thefault system which is the source of 1596 Fushimi earthquake.
Figure 33 Estimated fault rupturing proce:ss of the Hyogo-ken Nanbu earthquake from theview point of the fault geometry.
The initial break started from the compressional bend(l) between the Nojima fault and the
Takaiso fault. Ruptures were propagated bilaterally, then the second break started fromthe compressional barrier(2) in the east. Finally ruptures were tenninated in theextensional bends(3) or jogs in the (~t and south.
Figure 34
Relation between inland historic earthquakes of southwest Japan and greatearthquakes along the Nankai trough since the 16 century.
Vertical dashed lines indicate the tinring of great plate boundary earthquakes along the
Nankai trough.
Table 1 Historic earthquakes greater than or equal to M6.5 during the past 1300 years.
original source is mainly obtained from Usami (1987) .

3

The

Active faults~ Strike-slip fault~ Thrust fault, - Normal fault
Active folds
A- Anticline
A-Syncline

Chugoku~Monocline

Figure. 1 Active structures in the Southwest Japan arc .
Solid arrow indicates the direction of the forearc translation. A shear zone named as
Seto'uchi shear zone is shaded on the north of the Median Tectonic Line (MTL).

'Eurasia prate30°~ ~. ~ ~~ Pfiifippine :ea prate

~.

~

~

~------~-------------~----~~----------+-----~
~

...

. .

.

~

,

'V

~

~~

Figure 2 Oblique subduction of the Philippine sea plate and westward forearc migration of theSouthwest Japan arc.
Active tectonic features of the Seto' uchi shear zone and the forearc sliver are very
different each other.

300

400 (km)

100
I

200

0
I
..c

0>
::J

Q)

0>
"'C

a:
Japan Sea(k~) I"-.....J

San'in~.

..

ChugokuQ)..-

rSeto'uchi~..

Shikoku..

Forearc basin

::J

0

-c

0

"-

Inner
trenchslope

Philippine Sea Plate
50

Figure 4 Generalized profile of Southwest Japan arc.
Micro-earthquakes are observed in the area of gray color which is recognized as an island
arc lithosphere.

r-

.@
~

c

co

z

• Osaka_-.--------~-~~~;;~i-~~---- (~)~
r-, ~ :/}:<\>\rrrj/\\\x.~\\\\\>#.{~:~:.:: ~::.:::d; .~.:7)"~::~":~
it;--A··,-··,-···\_- ~--,

\7.{>n··:/Y::::::k·:\.».:.' -, - - - - - ..
I
foteat . .-.:.:- Z .. .-.:-.:; j ..: ..............:}U.:-.. . ::: . . :. .-.:-:.-. s <:/j:j{::::::.£ :::::: q..'.:.:./.:..'~..'.. } C#;
~aS\"

.

.. o"e:\.\~:\{}':':' ·... .~·.< oJ·'::2r:z:;...:&·:··::::::·:.-:{ J' ./.j;;;[.A/ ::'::'::"::':}.".'~.; /' ~~ __ -------------\- - - - - - ----, pet.----Inner trench 5 0--- - \..-A./.C2:::.:.::

~ . ~
~

~~;..

---------- --------

--

.

..'.'

:: . ' ~::;' ~ANt<A~\i~~ . ;.. L ..:::: . ); ··,,,r11
o!

100
!

200km
I

Figure 5 Separated forearc basins of Southwest Japan arc (from Awata & Sugiyama, 1989)
Z: Hyuga basin, A: Tosa b., B: Muroto b., C1: Shima b., C2: Kumano b,. D: Enshu b., a:
Ashizuri promontory, m: Muroto p., s: Shionomisaki p., d: Daiozaki p., 0: Omaezaki p ..

••••

•••

o

50km

Figure 6 Tectonic structures of the Shikoku zone.
Contours indicate the average uplift rate during last IMa (Omori, 1990).
K: Kii-suido syncline, T: Mt. Tsurugi anticline, M: Muroto anticline, H: Hijikawa
monocline, A: Ashizuri anticline, B: Bungo-suido syncline, S: the microearthquake
swarm in shallow crust west of Mt. Tsurugi.
N-S or NNW -SSE structures suggest arc-parallel compression.

100kmo

Yawatahama

B

Figure 7 Westward tilting along the Hijikawa river, western Shikoku.
a: Alluvium along the Hijikavv'a river, b: tilting area c: direction of tilting.

Arc-parallel compression of forearc
<l.J
C
:J

U

<l.J

c

C

0.

0

..c

"en
:J

<l.J
C

<l.J
C

U

u

0

c

V')

>.
V')

......

Ero'......
V')

ro

ro

0

~

~

0)

ro
......
ro

"~

ro
~

<l.J
C

u
C

U

:J

:J

I-

>.
V')

..-i

......
ro......

Vl

N

..c

-

Vl

2

V')

~

V')

I

c

:J

co

+-

-+
Shikoku•

Earthquake swarm

Figure 8 Generalized profile of the Shikoku zone
Microseisrnic activity is significant in shaded areas"Kii

: E-W compressional area
:Kinki and Chubu districts.....

ChugokUc.....

"YYjj)
Figure 9

East-west compression area of Kinki and Chubu in the east of the Seto'uchi shear zone.

~

w
(J1
o

MTL

9.
---~~

~.

.

~
I

2,Okm

0,,-, _'---.....J

Figure 10 Active tectonic structures in the east of the Seto'uchi shear zone.
I: Ikoma faults, U: Uemachi fault, R: Rokko faults, 0: Osaka-wan fault.

•

Figure 11

Orientation of elemental structures expected in a right-lateral shear zone . .

~' /

•

'x;:/
.

.

/ . /l/' I .// / I" / .
'.
/.
i/ / fYC(\)2 /\.1 /

.I /

./

/'

~.

/

./

,

~

I

/

/

"/,/'
/

<0

/

./

I
/

..

• '"

En-echelon grabens

Volcanic region••~

En-echelon folds

Non-vocanic region

Figure 13 Development of en-echelon folds in non-volcanic region and grabens in volcanic
region, in the right-lateral shear zone, Southwest Japan.

N34r-------+--------------__~.f)3

32l--~~~~..

- - - _...

lOOk

3l~-----4------------~~----~-----------------+--~

13

Figure 14 En-echelon grabens in the central Kyushu.
Dotted area indicates the right-lateral shear zone (Seto'uchi shear zone).
YT:Yuhu-Tsurumi graben, KH:Kuju graben, As:Aso graben, Uz:Unzen graben.

o

100 kmI

I

1982-1891
1m--+
Displacement
Figure 15 The displacement field related to the baseline modified from Hatanaka & Shimazaki(1988).
Direction of the displacement in the northern Kyushu is almost parallel to theshear zone.
M.T.L.: Median Tectonic Line.

N
34r--------+----------·------~

·0

3

•

32 t------'--;y;.;..~~..t....

..

- - -- -31~-------4----------·--~~----~--------------------~--~

13

Figure 16 al stress trajectory derived from earthquakes and active faults.
indicates the Seto'uchi shear zone of central Kyushu.

Dotted area

a H
Q

t'l

Isahaya•32'50'

Shimabara

Shimabara
Bay~

Tachibana
Bay ...

~,~11,.

o

5

10km

b

~-"-"~"_'_'_'_I_,-·_I.

" ,,'" , ''.... ,-._._'_1_ 1_.

-,_, ___ 1---_-1---'-'-'----_1_'_----1-

o

5

10km

I

Figure 17 a: Active normal faults of the {Jnzen graben.
See Fig. 14 for the location of this figure. Notice that normal faults are not arranged inparallel. It is inferred to be made by E-W oriented compressional stress condition.
b: Orientation of maximum horizontal stress in the Unzen area.
Diameter of the circle is about 25 km.

ao5c====--__

10km
33' 20'

b
o

5

10km

_1_._._._._._,_ ..... '_' ....'-,Beppu Bay- ____ '_'_1_1-__
~,

....

....... ,_ ..

-'...:.~, ,....,....

'-'-'~::':::"....

' ,.... ,....,..-,_1 ___ ._'-'''''' ,....

Figure 18 a: Active nOImal faults in the Yufu-Tsurumi graben.
Y: Mt. Yufu, T: lV1t.Tsurumi, A-A': profile line of Fig. 14
b: Orientation of maximum horizontal stress in the area.
Diameter of the circle is about 25 km.

1\

A

300

.zoo
100

-100

Figure 19

Profile along the line A-A' of Fig. 18 a. showing normal faults in the Beppu bay .

.,...,-,- 'to ~.,..

V).......

~

~

~~
~__

N'

-~

-""''''~p. " .......

_.... ""~.,.f-~~Wi.u1I 'I't~
~

Am

I

- ....-..... ---~~ ... ~" '-..
~~.-"'~~~~~~~ ~-4""''\~~""'"'~~~_~-f'~.~~ . . . ~J-..

.....". -Km~'" .... ~~.:.

~

~.

If"AI

I

~

1, .... /","A:";;:;:,.,....'~~.#"~....

3rlO'

-

~

..... "' ...... _.....

/' ~

-

,~~

.... ""-4 -' :44404-'"""-'-

o

5

~

Active nonnal faults in the Kuju-Haneyama grabens
Am: Mt. Amagoi, Ni: Mt. Noine, Ke: Mt. Kuenohira, Hn: Mt. Haneyama, Wi: Mt. Waita,
Km: Mt. Kameishi.

Figure 20

10 km

'-<' "T" • ...,.... -ro' "'.

33·0'
".,.

irl
~

b

M

, ~Io:.....b f,'<~~ ~~~~....

/~
t:.~'~j,..[.t
~J

~

~

.~~

,<'" T·,..

f..

~

~

~

-(

A

Kura-dake "'-..

"y.",,""""

»",

-',~

,

~

y

~

--(y.

..'7:...•.••.... ;' •. .. .

);

y '

i
.

"'(~

..t

~.

>-.

-\

""

-)

(

,--- 'r-.~
.4
~

."
,('

--roY

>-

32·50'

~
.

~

~

~

.;'_

/

,

.

y~

r~

.

~ .J- .~

/

--i

•
.

~

~
/
. / , < ,",-,'",,,,'"

.

~
~.~

/r//
'

.

Cal def a ,f..f.~,<'y '( ;.y"--"
\

Y
y '

"<.. . .L . ...l--"".J,.J

./

f

o

1
Figure 21

Active faults of the Aso graben

5

10km

WL-/_ ---~--"-'E

San'in Zone

Chugoku Zone

Seto'uchi Shear Zone'~-;::;~~~. ~·Median Tectonic Line

Shikoku Zone

Forearc Basin Zone

Inner Trench Slope

Nankai Troughb

Figure 22

Displacement distribution across zones of Southwest Japan arc .

Development of tectonic basinsin the Southwest Japan arc, ' i [i'}l't{~;; ~}i.......

----..

,,

Tr----..

Backarc
SetouchiShearzone
~

...

MTL
Forearc rise zone

Forearc...

Forearc basin zone

Plate boundary~ Oblique subduction of~~ Philippine Sea Plate
Figure 23 Development active tectonic basins of southwest Japan.
These basins are formed the same tectonic regime, that is, oblique subductionof the Philippine sea plate.

Northern
Okinawa -rroughu=2t-3r

I
I

I

.

I

I
I

I

MiddleOkinawa Troughu::; 20

I
I

N

I

0

/
.f

/

"

Southern
Okinawa Trough

~
/6-..

/

~;

~

• 6-

-$..
••

"

/

/

/

/

u~15°

/

•

•

..

•

N

124

121S

128

Figure 24 Orientation of active faults in the Okinawa trough, modified from Kato et al (1988) .
a: angle between faults and the trough direction.

a

a
10

20

I.Central Kyushu
2.0kinawa trough3.Taupo-Rotorua depression
4.Guatemala-Motagua fault zone
5.Fitzroy Trough

30

-

40

-

50

60

70

80

-

.........

Normal fault(graben)~~ Right-lateral shear zone"

Figure 25 Orientation of nonnal faults to a shear zone.
a: angle between the trend of shear zone and nonnal faults.
range of a in each shear zone.

Solid bars indicate the90

.....

oFigure 26 Active faults characteristics of the forearc and backarc from the Southwest Japan arc
to the Ryukyu arc.
The explanation of the figure is in the text.

--•-I.II..'........
,.-.-.
--.
u:::
•
•

I•

-

e::~

.g::
c:::

-

&::
-.;;;

$=

e

E

.~

Figure 27 Southwestward translation of the forearc sliver accompaniedwith theshear of backarc caused by the oblique subduction of the Philippine Sea right-lateralplate.
The
development of en-echelon grabens in the backarc is closely related tothe migration of
forearc sliver. Double open triangles indicate a relatively higher translation rate thansingle ones.
Solid arrows indicate the direction of relative convergence between thePhilippine sea plate and the Eurasian plate.

o.

100 Ikm,,

1925 1927\",. 1718~. :-.~."'"~

Figure 28 Faults of historical intraplate large earthquakes (M>6.5) since the 16th century.
Starting from the 1510 Settsu earthquake, earthquakes migrated to the central Kinki
district. After the 1854 Iga-Ueno earthquake, seismicity become very high outside of thecentral Kinki. (Modified from Tsukuda, 1987)

•

4-Time-space diagram of Paleoseismicity in southwest Japan
M >7
1948.

762

•

Western Chubu andHokuriku districts

1891 •
1909-745...........•..1}25.
•• 715·······.............
·· ··1··········•........................1945.. ... .....
.1662
96 1819.

1

Central Kinki district

Tanba-Harima andTango-San'in districts

•

1854.
1510
863 ······r····· · ········1.!~~............ .r······ ················ ·1·963····

701

•

1927.
192bj

"'

1943

880

......................... .......•........

...

........

.1872~.~~ ... ~~~.1649

Western Seto'uchi district

.1596

Be

I Eastern Shikoku

1596

?

Mid.-western ShikoAD

L . . . - '- - - - - - ' - - - - - - - - - " -

-500

.1854

........ ....•..........................
734?

Kii peninsula

Median Tectonic Line

1857

0

500

1000

1500

Figure 29 Time-space diagram of paleoseismic activities in southwest Japan showing a partitioning of activity.
Shaded areas indicate periods of high seismicity estimated from trench excavation surveys andarcheological surveys. B: Biwako area, K: Kobe area. Details of earthquakes are shown in Table 1.

2000

•

Figure 30 Major potential seismic zones surrounding the core of central Japan which had been
active during the past 500 yrs. (from Tsukuda, 1987).
M1L:Median Tectonic Line seismic zone; CJA: Central Japan S.Z.; ISTL: ItoigawaShizuoka Tectonic Line S.Z.; NP: Noto peninsula S.Z ..

•

..

...

•

\

\"

o.
/Fautt~'\::;J

Eplcenter of 1995 Hyogoken-nanbu
earthquake by Japan Meteoiogical Agency~\..]QJ

Depth of sea floor (m)"-.\-. ~', .(.'2- -3 km...~~:::.f'~-=:':~~X~~,.

Sea floor channel of Akashi strait

Figure 31 Fault distribution and submarine topography in the Akashi strait.
lA, 2P, 3P, 4A: Anchorage and pier of the Akashi strait bridge under construction

.Kyot•..........

----

Active faults of NW of
Osaka bay.....
".
.......

Major faults of the Hyogoken Nanbu earthquake

N\

Seismic Intensity 7(JMA scale)

~

K\

..
ecton\c Une...........

1" aKatsu.
...:.......
... .. :...... .. ::r:•••~ ........::::i."?)":.::::::.:~::::::...:::: .......

. .: :. . . f . /~. . .......
th_e_M_a_in_s-,h_o_C_k.---_-- - :K:-=-o~be:~~~::~:~::::·: :;:; ;:: .....~
0 ,?>~

Epicenter of

L - -_ _ _Nara ••.....

Osaka bay
Kishiwada

P__.......2C?\<.m0....
, _ _ _1...

Figure 32 Active faults of the Arima-Takatsuki Tectonic Line and the Rokko fault system.
Faults of the 1995 Hyogo-ken Nanbu earthquake is located in the western margin of the fault
system which is the source of 1596 Fushimi earthquake.

••Compressional bend
Initial breakCompressional bend
Second break

T Regional tectonic stress~ Extentional bend

rupture termination••

Figure 33 Estimated fault rupturing process of the Hyogo-ken Nanbu earthquake from theview point of the fault geometry.
The initial break started from the compressional bend(1) between the Nojima fault and the
Takaiso fault. Ruptures were propagated bilaterally, then the second break (2)started fromthe compressional bamer in the east. Finally ruptures were tenninated in the extensionalbends (3)or jogs in the east and south .

•

'.

•

•

Great earthquakes along the Nankai trough and inland earthquakes in southwest Japan
Tanba-Harima,=and Tango-San'in'=-----:=

Western Chubu and

,--_.

?

Hokuriku

1586 .

-

==
--_.

,=
- .,

-

~
-

.."..

-

a

1596

1510

Western Seto'uchi andCentral Kyushu

1=

I'

1662

1819

=

1-

:«'('t.."':

1-

a =
-

!=

a

18.S

-

=

Q-'

a 1945

a a

[=-=
---

a

1119481

1, 1909

~

1

.ls;
~

1963

1943'

1891

""'"
1~»--',1=-=-.....,

--

e:a

i,=
--

""'"

,..,'«

Central Kinki

"'-

-

-,

1925a

1872

11' =
......

I~

~a

a

'i:

i~
iE
I-~

1927a

=-

!

I

11

1686

I

1649

1596

1857

-

I
i

,'"

1

I1-"

Median Tectonic Line

1900

,ds?

-

16bo

1700

1800

1900

2000 AD

Figure 33
Relation between inland historic earthquakes of southwest Japan and greatearthquakes along the Nankai trough since the 16 century .
Vertical dashed lines indicate the timing of great plate boundary earthquakes along the
Nankai trough.

Table 1
Historic earthquakes greater than or equal to M6.5 during the past 1300 years
The original source is mainly obtained fromUsami( 1987).

•

•
•

•

Oate(y/M/O)

Magnitude

715/07/04

6.5-7.5

Tenryu rive r

137.8

35.1

715/07/05

6.5-7.0

Mikawa

137.4

34.8

734/05/18

7.0

Kii

136.1

34.3

745/06/05

7.9

Mino

136.5

35.4

Location(Earthquake name)

Long.(E)

Lat.(N)

762/06/09>=7.0

Mino,Hida, Shinano

137.5

36.0

827/08/11

6.5-7.0

Kyoto

135.7535.0

8411?1?

>=6.5

Matsumoto

138.0

36.2

863/07/10>=7.0

Ecchu-Echigo

138.1

37.1

868/08/03>=7.0

Harima, Yamash iro

134.8

34.8

887/08/26

7.4

N. Shinano

138.1

36.6

938/05/22

7.0

Kyoto , Kii

135.8

35.0

976/07/22>=6.7

Yamashiro,Omi

135.8

34.9

1070/12101

6.0-6.5

Yamashiro , Yarnato

135.8

34 .8

1185/08/13

7.4

Omi, Yamashiro, Yamato

135.8

35.0

1317/02124

6.5-7.0

Kyoto

135.8

35.1

1325/12105

6.25-6.75

Northern Omi

136.1

35.6

1449/05/13

5.75-6.5

Yamashiro , Yarnato

135.7535.01502101/28

6.5-7.0

SW Echigo

138.2

37.2

1510109/21

6.517.0

Settsu, Kawachi

135.6

34.6

1586/01/18

7.7-7.9

Hida, Mino (Tensho)

136.9

36.0

1596/09/05

7.25

Kyoto, Settsu (Fushimi)

135.6

34.65

1640/11/23

6.25-6.75

Kaga-Taiseiji

136.2

36.3

1662/06/16

7.3-7.5

Yamashiro, Omi

135.9535.2

1666/02101

6.75

W. Echigo

138.2

37.1

1718/08/22

7.25

Ina, Mikawa

137.9

35.3

1751/05/20

7.0-7.4

Echigo, Ecchu

138.1

37.1

Kaga

136.6

36.6

Ise, Mino, Omi (Hikone)

136.3

35.2

Kyoto

135.6

35 .1

Nagano (Zenkoji)

138.2

36.7

1799/06/29

6.5

1819/08/02

7.0-7.5

1830108/19

6.5 +1- 0.2

1847/05/08

7.4

1854/07/09

7.0-7.5

Iga-Ueno, Yokkaich i

136.0

34.75

1858/04/09

7.0-7.1

Kaga, Echizen (Hietsu)

137.2

36.4

1891/10/28

8.0

Mino, Owari (Nobi)

136.6

35.6

1909/08/14

6.8(Anegawa)

136.3

35.4

1918/11/11

6.5(Omachi)

137.8836.45

1925/05/24

6.8(N.Tajima)

134.8

35.6

1927/03/07

7.3(N.Tango)

135.1535.53

1930/11/26

7.3(N. Izu)

139.0

35.1

1943/09/10

7.2(Tottori)

134.0835.52

1945/01/13

6.8(Mikawa)

137.0

34.7

1948/06/28

7.1(Fukui)136.2036.17

1952103107

6.5

Ishikawa pref. (Off Taiseiji)

136.2036.48

1961/08/19

7.0

E. Fukui pref. (Kita-Mino)

136.4636.01

1963/03/27

6.9

Wakasa bay (Off Echizenmisaki)

135.77

35.78(Matsushiro)

138.2

36.5

1966/08/03-

6.4

1969/09/09

6.6(C. Gifu pref)

137.0435.47

1974/05/09

6.9(Off. Izu Pen. )138.4834.34

1978/01/14

7.0(Off. Izu-Oshirna)139.15

34.46

1984/09/14
1995/01/17

6.8
7.2

(W. Nagano pref)(S . Hyogo pref.)137.56135.03

35.82
34.36

•

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