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Seismic hazard in Takamatsu Japan from fault trenching
and paleo-liquefaction studies
S. Hasegawaa,*, W.D.L. Finnb
aSafety Systems Construction Engineering, Kagawa University, 2217-20 Hayashi-cho, Takamatsu City, Kagawa 761-0396, JapanbAnabuki Chair of Foundation Geodynamics, Kagawa University, 2217-20 Hayashi-cho, Takamatsu City, Kagawa 761-0396, Japan
Abstract
Takamatsu is a harbor city of 330,000 people on the northern shore of Shikoku Island, southwest Japan. Earthquakes in the Nankai
Trough, typically of magnitude M 8 þ , have been considered the primary source of seismic hazard for the island and Takamatsu. A
major active fault system, the Median Tectonic Line, runs across the Shikoku from east to west near the north shore. There is no
documented historical seismicity associated with this major fault system in Shikoku or with associated faults such as the 20 km Nagao
Fault near Takamatsu. Therefore the trenches were cut across some of the more important faults to expose the record of past seismicity.
Additional data on past seismicity is derived from paleo-liquefaction studies in the Takamatsu plain. This paper describes the trenching
and paleo-seismic studies and discusses the implications of the findings for seismic hazard in Takamatsu.
q 2002 Published by Elsevier Science Ltd.
Keywords: Seismic hazard assessment; Historical earthquake; Active fault; Paleo-liquefaction; Nankai Trough
1. Introduction
Takamatsu, the capital city of Kagawa Prefecture, is a
harbor city of 330,000 people on the northern shore of
Shikoku Island, southwest Japan (Fig. 1). Shikoku Island is
located on the Eurasian Plate. The Philippine Sea Plate is
subducting beneath the Eurasian Plate at the Nankai Trough,
120–180 km from the south coast of Shikoku. Earthquakes
in the Nankai Trough, typically of magnitude M 8 þ , have
been considered the primary source of seismic hazard for
the island and Takamatsu.
Takamatsu is located about 100 km southwest of Kobe
and had strong tremor of 4 on JMA seismic intensity scale
(6–7 on modified Mercalli scale) from the Kobe earthquake.
The Kobe earthquake alerted everyone in Japan to the huge
damage potential of smaller local earthquakes.
A major fault system, the Median Tectonic Line (MTL),
runs across the Shikoku from east to west near the north
shore. There is no historical seismicity associated with this
major fault system in Shikoku or with associated faults such
as the 20 km Nagao Fault near Takamatsu [1]. Therefore the
trenches were cut across some of the more important faults
to expose the record of past seismicity. Additional data on
past seismicity is derived from palaeo-liquefaction studies
in the Takamatsu plain. This paper describes these trenching
and liquefaction studies and discusses the implications of
the findings for seismic hazard in Takamatsu.
2. Geological setting
Takamatsu City is situated in the Takamatsu plain. The
plain is mainly composed of the alluvial fan of the Koto
River and the subordinate flood plains of the Shin, Kasuga
and Honzu rivers, deltas of these rivers, and landfills along
the shore (Fig. 2). Downtown Takamatsu is located mainly
on deltaic deposits and urban areas are developing on the
Koto Fan and the flood plains of other rivers.
The Koto Fan, formed since the glacial age, consists
mainly of gravels and sand. The surface of the fan is covered
by thin sandy and silty sediments of Holocene age, except
for the present riverbeds. Several paleo-chanells of Yayoi
period (300 B.C. –300 A.D.) are recognized by microtopo-
graphy and have been confirmed at archaeological sites.
The flood plains of minor rivers consist of soft sandy and
muddy sediments. The delta is composed of mainly soft
sandy and silty sediments of Holocene age. The delta
sediments are less than 10 m in thickness [5] and cover the
Pleistocene Fan deposits of the Koto River (Fig. 3).
0267-7261/02/$ - see front matter q 2002 Published by Elsevier Science Ltd.
PII: S0 26 7 -7 26 1 (0 2) 00 1 13 -6
Soil Dynamics and Earthquake Engineering 22 (2002) 901–909
www.elsevier.com/locate/soildyn
* Corresponding author.
E-mail address: [email protected] (S. Hasegawa).
Fig. 1. Active faults in and near Shikoku Island.
Fig. 2. Outline of topography and geology of Takamatsu area. Outline of liquefaction site is shown in Table 2.
S. Hasegawa, W.D.L. Finn / Soil Dynamics and Earthquake Engineering 22 (2002) 901–909902
Reclaimed lands have been formed for paddy fields and
salt pans since the 17th century and the salt pans were filled
up in 1970s. They are composed of soft sandy sediments,
dredged soils and waste materials.
The surrounding hills are composed of late Cretaceous
granitic rocks and middle Miocene volcanic rocks.
3. Historical destructive earthquakes
The earthquakes along the Nankai Trough (Nankai
earthquakes) have been documented since 684 A.D. [13].
The mean recurrence time of the Nankai earthquakes is
about 200 years before 1605 A.D. and about 100 years after
1605. However, recent paleo-seismological studies using
pale-liquefaction data have revealed that there is a
possibility of undocumented Nankai earthquakes before
1605 [10]. The historical record of earthquakes on this plate
boundary is particularly well described since 1596 in
Takamatsu (Table 1). Unfortunately, most of the historical
documents are said to have been burned during the Warring
States period of late 16th century.
The 1596 earthquake in Takamatsu is the only confirmed
earthquake relating to the activity of an active inland fault. It
is correlative with the Keicho–Kinki earthquake caused by
the Arima–Takatsuki Fault running from Kobe to Kyoto
[10,13]. Trench excavation surveys across the MTL in the
Tokushima area, 20 km south of Takamatsu, suggests that
the latest faulting on the MTL in Tokushima was during or
after 16th century A.D. and may also be correlated to the
1596 Keicho–Kinki earthquake [8].
The 1707 Hoei earthquake (M 8.4), one of the greatest
earthquakes in Japan, caused collapses of many houses,
dikes, and slopes and a Tsunami of 1.8 m in height in
Takamatsu area. The description in historical documents
suggests that liquefaction must have occurred, but the
Fig. 3. Geologic profile of Takamatsu area.
Table 1
Historical destructive earthquakes in Takamatsu area
Date Name of earthquake Magnitude Epicenter JMA seismic
intensity
Description
1596.9.5 Keicho–Kinki 7.5 ^ 1/4 Kyoto Arima–Takatuki Fault 5 Collapse of shines and temples
1707.10.28 Hoei 8.4 Nankai Trough 33820N 135890E 6 Collapse of Mt Gokenzan
Liquefaction
Tsunami 1.8 m
1854.12.24 Ansei–Nankai 8.4 Nankai Trough 33800N 1358E 6 Liquefaction?
Tsunami 0.3 m
1946.12.21 Nankai 8.0 Nankai Trough 338020N 1358370E 5 Liquefaction?
Settlement
Complied from Usami [13] and JSCE Shikoku (1998).
S. Hasegawa, W.D.L. Finn / Soil Dynamics and Earthquake Engineering 22 (2002) 901–909 903
localities were not described. A noteworthy event during the
earthquake was the collapse of one peak of Mt Gokenzan.
The 1854 Ansei–Nankai earthquake (M 8.4) caused
collapses of many houses, dikes, and slopes and a tsunami of
0.3 m in height in the Takamatsu area. The fill-type dam of
Manouike, the largest irrigation pond in Japan, collapsed
during the earthquake. Liquefaction may have occurred, but
we have not found a documentation on it yet.
The seismic intensity of 1946 Nankai earthquake is
smaller than that of the Hoei and Ansei–Nankai earth-
quakes. It caused collapses of many houses, dikes, bridges
in Takamatsu area [2]. Severe damages were reported in
landfills and in reclaimed salt pans. Ground subsidence
occurred during the earthquakes. One cause of settlement is
regional crustal movement but liquefaction may have also
caused the settlement.
4. Active fault trenching
4.1. Nagao Fault
The Nagao Fault is a 20 km length active fault near
Takamatsu. It runs along the boundary between the
Takamatsu plain and southern hills.
Four trench excavation surveys across the Nagao Fault in
1990, 1996, 1997, and 2000 have revealed the recent
activities of the fault [3,11]. The fault trenching at Tanaka in
Miki town in 1997 indicates that the latest faulting occurred
after 7000 years B.P. and the former faulting occurred in
29,000–32,000 years B.P. [2].
The trenching at Hikami–Miyashita in Miki town in
2000 have confirmed that the latest faulting occurred
after the Kofun period (300–600 A.D.), and it remains a
possibility of after the Heian period (794–1192 A.D.)
(Fig. 4.). This historical faulting of the Nagao Fault has
not been confirmed by historical documents yet.
The recurrence time of the faulting is estimated to be more
than 30,000 years. The Nagao Fault is estimated to produce
an earthquake of M 7 þ , as judged by fault length (20 km)
and its net slip (1.5–2.0 m). However, the probability of
future activity of the Nagao Fault is extremely low.
4.2. Median tectonic line
The MTL is one of the most predominant and active
faults in Japan. The fault system has predominantly right-
lateral displacement in the Quaternary period in Shikoku
and western Kii. The rate of right-lateral slip of the fault
during the late Quaternary is estimated to be several meters
per 1000 years in Shikoku. The active faults of the MTL
are inferred to cause a magnitude of 7–8 class earthquakes
on the basis of the fault lengths and the amount of
displacement [7].
Many trench excavation surveys across the MTL have
been conducted since 1980. The trench excavation survey at
Ichiba town in Tokushima prefecture 20 km south of
Takamatsu has revealed that the Chichio Fault slipped
around 2000 years B.P. (Yayoi period). The lastest event of
the Chichio Fault as determined by trenching is dated as 16th
century, but has not yet been documented historically [8].
The recurrence time of the faulting is estimated to be
around 1500 years. The right-lateral displacement by the
Chichio Fault is estimated to be approximately 7 m. This
indicates that the MTL was an earthquake source fault of
M 8 class. However, the probability of future activity of the
MTL fault is relatively low.
5. Paleo-liquefaction
Ten paleo-liquefaction sites have been reported by
archaeological survey in Takamatsu plain [6,9]. Eight sites
are located on the Koto Fan and two sites are located on the
delta (Fig. 2, Table 2). The reason why there are few
reported paleo-liquefaction sites on deltaic deposits and
reclaimed land is lack of archaeological sites.
The interesting fact is that the paleo-liquefaction sites
are concentrated on the eastern part of the Koto Fan.
Detailed microtopographical and archaeological surveys
have revealed that most of the paleo-liquefaction sites are
located at the buried channel or back swamp on the fan
(Fig. 5). The log of a excavation at Kagawa University on
eastern part of the fan have confirmed that loose medium-
sized well-sorted sand layer is covered by soft silty
sediments and that the water table is near the surface
(Fig. 6). This suggests that liquefaction could occur on the
eastern part of the Koto Fan.
Eight sites show small sand dikes and two sites show
thick gravel dikes (Fig. 7). The age of sand dikes roughly
correspond to Nankai earthquakes, but other possibilities
also remain. The age of liquefied gravels at two sites is
middle Yayoi period (about 200 B.C. –50 A.D.). This strong
liquefaction is believed to have been caused by an
earthquake on the MTL, because the Nagao Fault did not
fault during Yayoi period and the Chichio Fault of the MTL
20 km south of the sites must have caused a M 8 class
earthquake in Yayoi period (about 2000 years B.P.).
6. Preliminary seismic hazard assessment
6.1. Previous seismic hazard assessment in Takamatsu
Kagawa Prefecture [4] and Takamatsu City [12] have
conducted the seismic hazard assessments described below.
(1) Nankai Trough (M 8.4). The seismic intensity on the
Takamatsu plain is mostly 6-lower on the JMA scale. The
risk of liquefaction is very high on reclamed lands, and
high on the delta areas and flood plains of the Shin and
Kasuga rivers. The risk of liquefaction is very low on the
Koto Fan.
S. Hasegawa, W.D.L. Finn / Soil Dynamics and Earthquake Engineering 22 (2002) 901–909904
Fig. 4. Profile of a trench across the Nagao Fault at Hikami–Miyashita in 2000. Sediments younger than 30,000 years B.P. are displaced about 1.5 m vertically by latest faulting of the Nagao Fault [11].
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(2) Median tectonic line (M 7.7). The seismic intensity on
Takamatsu plain is mostly 6-upper on JMA scale. The risk
of liquefaction is very high on fill-up lands, the delta area
and flood plains of the Shin and Kasuga rivers. The risk of
liquefaction is very low on the Koto Fan.
(3) Nagao Fault (M 7.1). The seismic intensity on the
Takamatsu plain is mainly 6-upper on the JMA scale, but
areas of 7 are widely distributed on the flood plains of the
Shin and Kasuga rivers. The risk of liquefaction is very high
on reclaimed lands, the delta areas and flood plains of the
Shin and Kasuga rivers. The risk of liquefaction is very low
on the Koto Fan.
6.2. Seismic risk in Takamatsu
Paleo-earthquakes in Takamatsu from fault trenching
and paleo-liquefaction studies are shown in Fig. 8 Paleo-
seismic studies suggest that Nankai earthquake is the most
probable scenario earthquake in Takamatsu. They also
suggest that an earthquake is not likely to occur on the MTL
Table 2
Paleo-liquefaction sites in Takamatsu area
No. Site (name of ruins) Type Age Topography (microtopography) Possible earthquake
1 Kawaminami-nishi Sand dike 1450–1800 A.D. Delta (ancient shore) 1707 Hoei?
2 Kawaminami-higashi Sand dike ca. 1700–1800 A.D. Delta (ancient shore) 1707 Hoei?
3 Rokujo-Josho Sand dike Before 1600 A.D. Fan (margin) Nankai?
4 Gufukujiryo Sand dike ca. 400 B.C. –300 A.D. Fan (buried channel) Nankai?
5 Kukoatochi Sand dike 1600–1850 A.D. Fan (buried channel) 1707 Hoei? 1854 Ansei?
6 Hikkonbara Sand dike 300–1500 A.D. Fan (buried channel) Nankai?
7 Matsubayashi Gravel dike 200 B.C. –50 A.D. Fan (back swamp) MTL?
8 Nishihaze-doi Sand dike 200 B.C. –300 A.D. Fan (back swamp) ?
9 Tsuzukijo Sand dike Before 1800 A.D. Fan (margin) ?
10 Ikkaku Gravel dike 50–300 A.D. Fan (buried channel) MTL?
Complied from Oshima [9] and Kinoshita [6].
Fig. 5. Microtopography and paleo-liquefaction sites on the eastern part of the Koto Fan, central Takamatsu [9].
S. Hasegawa, W.D.L. Finn / Soil Dynamics and Earthquake Engineering 22 (2002) 901–909906
in the next 50 years. Although a scenario earthquake by
the Nagao Fault is the most destructive for Takamatsu,
paleo-seismic studies suggest that this is not likely to occur.
Therefore, next Nankai earthquake is the most realistic
scenario earthquake in Takamatsu.
The recent 2000 Tottori-ken Seibu and 2001 Geiyo
earthquakes suggest that other scenario earthquakes are
necessary for better seismic hazard assessment. Tottori-ken
Seibu earthquake indicates the possibility of shallow crustal
earthquakes that are not recognized from active faults.
Another is a big intra-slab earthquake like Geiyo
earthquake.
7. Conclusions
Conclusions of this paper are as follows:
1. The Nagao Fault near Takmatsu has the potential to
produce M 7 class earthquake and is the most destructive
earthquake in Takamatsu. However, the probability of
future activity of the Nagao Fault is extremely low, based
on trenching surveys across the fault.
2. The MTL located 20 km south of Takamatsu has
potential to produce M 8 class earthquakes and is
the second most destructive earthquake source for
Fig. 6. Trench excavation log at Kagawa University of eastern part of the Koto Fan, central Takamatsu.
Fig. 7. Liquefaction of gravel at Matsubayashi ruin Kagawa University of eastern part of the Koto Fan, central Takamatsu.
S. Hasegawa, W.D.L. Finn / Soil Dynamics and Earthquake Engineering 22 (2002) 901–909 907
Fig. 8. Paleo-earthquakes in Takamatsu from fault trenching and paleo-liquefaction studies.
S. Hasegawa, W.D.L. Finn / Soil Dynamics and Earthquake Engineering 22 (2002) 901–909908
Takamatsu. However, the probability of future activity of
the MTL is relatively low based on trenching surveys
across the fault.
3. The next earthquake in the Nankai Trough, typically
of magnitude M 8 þ , is considered the most realistic
scenario earthquake for Takamatsu based on studies
of paleo-liquefaction sites and historical documents.
The seismic intensity on the Takamatsu plain is
estimated to be mostly 6-lower on the JMA scale.
The risk of liquefaction is very high on reclaimed
lands, and high on the deltas and flood plains of the
small rivers.
4. The recent 2000 Tottori-ken Seibu and 2001 Geiyo
earthquakes suggest that other scenario earthquakes
are necessary for better seismic hazard assessment.
Acknowledgements
The authors thank Mr A. Saito of Shikoku Research
Institute for furnishing the size distribution data and Mr
K. Oshima of Takamatsu City Hall for offering the
archaeological data.
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