地球環境研究,Vol.12(2010)

Activity Evaluation of Fukaya Fault Zone Basedon a Study of a Subordinate Fault
NAKAMURA Yosuke＊, TAMURA Toshikazu＊,
KIKUCHI Takao＊＊ and FURUTA Tomohiro＊＊＊Keywords: Fukaya fault, subordinate fault, fluvial terrace, recurrence interval
unit in the past.

1. Introduction

During construction work of Rissho University En-

According to data published by the Headquarters

ergy Center a new cross section of sediments corre-

for Earthquake Research Promotion (2005), about 25

sponding to the Konan II terrace (and covering soil

km long Fukaya fault belongs to the middle part of alayer) (Arai et al., 2002) was created (Fig. 2, Fig. 3).

fault zone extending NW-SE direction and bounding

At the beginning we anticipated to see new sediments

NW Kanto Plain. Its estimated average displacement

belonging to the ones later than Konan I (and cover-rate is 0.2-0.4 mm/1000 years and its latest activity

ing soil layer) as we believed that gently sloping cliff

was about 2,500 yBP within the last 6,200 years (Fig.

between Konan I and Konan II terrace was formed by1). The amount of displacement per single earthquake

river erosion (Fig. 4). The lithology of the sediments,

event is 5 to 6 mm with average time interval ofhowever, closely resembled to the one belonging toevents 13,000 to 30,000 years. Also it is estimated that

Konan I terrace (On-Pm1: 100ka; Machida and Arai,

Kanto Plain NW marginal fault was active as a single

2003). Especially, yellow-green pumiceous bed visible

Fig. 1
Distribution of active faults around Kumagaya [modified from The Headquarters for Earthquake Research Promotion(2005)]. Fukaya F : Fukaya fault, Hirai F : Hirai fault, Konan F : Konan fault, Ayasegawa F : Ayasegawa fault.
＊

Faculty of Geo-environmental Science, Rissho University
502, 2-2-15, Sekimae, Musashino City, Tokyo, 180-0014, Japan＊＊＊
JR Kyushu consultants company＊＊

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Activity Evaluation of Fukaya Fault Zone Based on a Study of a Subordinate Fault (Nakamura, Tamura, Kikuchi, Furuta)

Fig. 2
Geomorphological map around the Kumagaya Campus of Rissho University [modified from Arai et al. (2002)]. Ko1 Te :Konan 1 Terrace, Ko2 Te : Konan 2 Terrace, Ko3 Te : Konan 3 Terrace, W-Y RB Te : Wada-Yoshino river right bank Terrace.

Fig. 3
Campus map of the Kumagaya Campus of Rissho University [modified from Arai et al. (2002)].

to unaided eyes (first discovered by Kikuchi, one ofanalysis including refractivity measurements of pum-the authors) intercalated with grayish white silt inice, air photo analysis and topographic survey of thethe middle of the sediments was identified to becliff have been carried out.

Ontake I Ash based on lithology and stratigraphic position. If so, the cliff between Konan I and Konan II
terrace is not created by erosion but fault movement.
To prove that this cliff is a fault scarp, mineralogical70

2. Summary of topography and geology
Konan Plateau (Fig. 2) is a plateau on the right地球環境研究,Vol.12(2010)

Fig. 4
A new cross section of sediments at the Rissho University Energy Centerbank (south side) of Arakawa River and belongs tophoto of the core, Fig. 3a). The lower part of thethe southern part of an old river fan created bybrown loam between 230 and 240 cm depths shows

Arakawa River. Konan Plateau is underlain by overdull yellowish brown color probably suggesting tran-

15 m thick gravel layer. The gravel layer is overlain

sition from eolian to water-laid deposition. From theby about 3 m thick gray clay and in turn overlain bydepths 245 cm and below consists of water-laid clay

less than 2 m Kanto Loam. The gray clay is interca-

containing rounded pebbles 5 mm in size (Nakamura

lated with yellow-green and white pumice beds.

et al., 2005). Nakamura et al. (2005) also found andAmong the pumice beds, yellow-green one contains

described several volcanic ash beds including those of

biotite and zircon, and because of this it is interpreted

AT and K-Tz in the soil covering Konan I surface nearto belong to Ontake I Ash (Yanagida et al., 1982).

Rissho University Unidence, and estimated the timing of emergence above the sea of Konan I surface to

3. Descriptions of drill core and results of refractive index measurements

be after K-Tz fall and before Aso-4, that is, about

The sequence of drill core consists of four layers inOn the other hand, lithofacies of sediments ob-

descending order: cultivation soil, Kuroboku soil,

served on the outcrop is: from the top to be: artificial

brown loam (eolian), and clay (water-laid) (Fig. 5a).

piled-up soil, brown eolian loam, and water-laid clay

The soft and light-weight black-brown to dark brown(Fig. 5b). From the ground surface to the depth of 70

cultivation soil is down to 50 cm. At 50 to 80 cm

cm is artificially piled-up soil for laying water pipe.

depths is a transition layer from Kuroboku soil toKuroboku soil and upper part of brown loam beds

brown loam containing pale brown humus. From 85

seen in the drill core are not recognized. Instead,to 225 cm is a massive eolian loam showing no layer-

about 50 cm thick chocolate-colored loam is recog-

ing mixed with brown to dark brown materials andnized. It is underlain by silt bed about 2 m thick.

considered to belong to the so-called Kanto Loam. In

Within this silt bed at depths 170 cm is yellow-green

addition, at 120 cm depth, a white layer probably vol-pumice bed and at 300 cm depth is medium-grained

canic ash is observed (this is not clearly visible in thesand layer about 15 cm thick. As described before, the90,000 yBP:.

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Activity Evaluation of Fukaya Fault Zone Based on a Study of a Subordinate Fault (Nakamura, Tamura, Kikuchi, Furuta)

Fig. 5Columnar sections of the drilling core (near the east of Unidence : Fig. 5a) and Outcrop ((Energy Center : Fig. 5b). Locations are shown in Fig. 3.

pumice bed occurring at 170 cm depth is correlated tooccurs within lower silt bed of chocolate-colored loam

4. Description of topography due to fault activities around Kumagaya campus of Rissho University(identified to be Musashino Loam bed referring onAs lithofacies and ages of constituting sediments ofprevious studies), its color is yellow-green, and in-Konan I and Konan II surfaces are found to be identi-cludes biotite.

cal, cliff between Konan I and Konan II surfaces

Ontake I Ash bed (On-Pm1; 100,000 yBP) because itIn this study, minerals contained in the pumice bed

seems to be created not by erosion but displacement

are identified with polarizing microscope and refrac-with fault movement. As a means to prove this possi-

tive indices of volcanic glass and hornblende are meas-

bility, topographic details with stereographic viewing

ured to help identify if the bed is correlated to Ontakeof air photos are carried out. At the place shown inI. Under the microscope, volcanic glass (pumice), feld-Fig. 3, topographic flexure with uplifted south side isspar, hornblende, orthopyroxene, biotite, and zircon

confirmed (Fig. 6). The flexure seems to have formed

are identified. The assemblage closely resembles tothrough several earthquake events since about hun-that of On-Pm1 reported by Machida and Arai (2003).

dred thousand years in the past. Topographic crossRefractive index of glass is n=1.5011-1.5027 (meansection obtained with total station method revealed1.5021) while that of hornblende n2 =1.6845-1.6864

about 3.7 m vertical displacement at ground surface(mean 1.6854). Those are close to the values reported

level while about 3.2 m at On-Pm1 horizon (Fig. 7).

by Machida and Arai (2003) for On-Pm1. Therefore it

Here the reason why the displacement is larger foris safe to conclude that the pumice bed observed on

the ground surface than that for On-Pm1 seems to be

the outcrop is correlated to On-Pm1.

on the fact that the surface soil on the footwall was
somewhat removed when building and playgroundwere constructed.
Distribution of active faults around Kumagaya is72

地球環境研究,Vol.12(2010)

Fig. 6
Campus map and active faults around the Kumagaya Campus of Rissho University.

Fig. 7
The Profiles of deformed terraces across the Kumagaya campus area of Rissho University (A-Aline). Location of the profiles are shown in Fig. 6.

shown in Fig. 1. In the map, the position of newly

earthquake event is estimated to be 70 cm and totalfound fault scarp of this study is shown by the letter

displacement of loam bed (about 30 thousand years orR. This fault scarp is related to subordinate fault ofolder) to be 2.5 m. The displacement found this time

Konan fault which itself is a subordinate fault ofin Rissho University campus is about 3.2 m for theFukaya fault (Fig. 8: modified from Yamaguchi et al.,last 100 thousand years. Assuming the displacement

2000). Referring to Mizuno et al. (2002) and The

per single earthquake event related to Konan fault toHeadquarters for Earthquake Research Promotion

be 70 cm or less, the displacement topography corre-(2005), displacement of Konan fault per single

sponds to repeated movements of once for 2073

Activity Evaluation of Fukaya Fault Zone Based on a Study of a Subordinate Fault (Nakamura, Tamura, Kikuchi, Furuta)

Fig. 8Underground structure in the Konan Plateau (modified from Yamaguchi et al., 2000). In the map, the position of newly
found fault scarp of this study is shown by the letter R. This fault scarp is related to subordinate fault of Konan fault whichitself is a subordinate fault of Fukaya fault

thousand years (the earthquake itself is related to theactivity of Fukaya fault).

1. A study of newly created outcrop with construction work of Rissho University Energy Center re-The latest activity of Konan fault, however, seems

vealed existence of On-Pm1 pumice in sediments

to have occurred after 6,200 yBP and before 2,500 yBP

constituting Konan II surface (Arai et al., 2002).

(The Headquarters for Earthquake Research Promo-

This fact indicates that the cliff between Konan Ition, 2005), suggesting that the possibility of recur-and Konan II is not an erosinal one as has been as-rence movement of this fault seems to be low. On thesumed in the past, but the one created by fault

other hand, for Kushibiki fault, another subordinate

movement.

fault of Fukaya fault, recurrence interval of the2. Stereographic examination of air photos of Risshoearthquakes was found to be once for 10 thousand

University campus area confirmed a topographic

years based on a trenching study (Sugiyama et al.,flexure (about3.2m) with south side uplifted.

2009). As recurrence intervals of the earthquakes in

3. The flexure is interpreted to have formed by sev-

several subordinate faults are about 10 thousand

eral earthquakes since about 100 thousand years.

years, and that of Fukaya fault is suggested to be less

Assuming displacement of 70 cm or less at Konan

than 10 thousand years.

fault per single earthquake, the displacement to-Lastly this study suggests that a cliff created bypography found in Rissho University Kumagaya

fault movement misidentified to be erosion cliff (orcampus suggests displacements repeatedly occur-

vice versa) may possibly be common in the world.

ring at a rate of once for every 20 thousand years

Misidentification of the origin of surface cliff could

or so.

lead to erroneous interpretation of topo- and geohistory of the area. We d like to draw attention toReferences

this fact.

Arai, T., Shimazu, H., and Yuichi Suzuki., 2002, Data in thephysical environment in the Kumagaya Campus of Rissho
University, Regional Studies, 43, 25-32 (in Japanese).

5. Conclusions
In this study the following conclusions are reached.

74

Machida, H., and Arai, F., 2003. Atlas of tephra in andaround Japan. Univ. Tokyo Press, 337 pp (in Japanese).

地球環境研究,Vol.12(2010)

Mizuno, K.., Fusejima, Y., Sugai, T., Sugiyama, Y., Hosoya,

pp.79-111 (in Japanese with English abstract).

T., and Yamazaki, H., 2002, Paleoseismological study ofThe Headquarters for Earthquake Research Promotion,

the Fukaya fault system in Okabe and Konan Towns,

2005, On the long term evaluation for northwestern part

Saitama Prefecture, central Japan, Annual Report on Ac-of Kanto Plain fault zone, Report from the Headquarters

tive Fault and Paleoearthquake Researches, 2, 25-40, Geo-for Earthquake Research Promotion (in Japanese withlogical Survey of Japan, National Institute of Advanced

English abstract). http://www.jishin.go.jp/main/chousa/

Industrial Science and Technology (in Japanese with Eng-05mar̲kanto/index.htm

lish abstract).

Yamaguchi, K., Kano, N., Yokokura T., Kiguchi, T.,

Nakamura. Y., Kikuchi, T., Kurishita, K., Lee, Jae Sub., andYokota, T., Tanaka, A., and Sato, H., 2000, Seismic Reflec-Takamura, H., 2005, Tephrochronology of the Kantotion Survey around the Fukaya Fault - Subsurface Struc-

Loam Formation on the Konan 1 Terrace (part 1). (inture between the Konan Terrace and the Menuma

Japanese)

Lowland -, Journal of the Seismological Society of Japan,Sugiyama, Y., Shintani, K., Miyawaki, R., and Miyawaki,A., 2009. Peexamination of fault distribution and activityof the Hirai-Kushibiki fault zone in the Kanto-heiyahokuseienfaultzone,

centralJapan,

based

53, 2, 115-125, Seismological Society of Japan (in Japanesewith English abstract).
Yanagida, M.Momose,M. and Okuma,Y. 1982, Geomorphiconchronology of river terraces along the Arakawa, central

geomorphological and drilling surveys. Annual Report onJapan, Geographical research of Graduate Students,

Active Fault and Paleoearthquake Researches, Vol.9,

Komazawa University, 12, 3-14 (in Japanese).

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Activity Evaluation of Fukaya Fault Zone Based on a Study of a Subordinate Fault (Nakamura, Tamura, Kikuchi, Furuta)

Activity Evaluation of Fukaya Fault Zone Basedon a Study of a Subordinate Fault
NAKAMURA Yosuke＊, TAMURA Toshikazu＊, KIKUCHI Takao＊＊ and FURUTA Tomohiro＊＊＊＊Faculty of Geo-environmental Science, Rissho University
502, 2-2-15, Sekimae, Musashino City, Tokyo, 180-0014, Japan＊＊＊
JR Kyushu consultants company＊＊

Abstract:
An active fault, which displaced Konan terrace surface about 3.2 m about 100 thousand years
ago, was newly discovered within the Kumagaya campus area of Rissho University. This fault isinterpreted to be a subordinate fault of Konan fault, which itself is a subordinate fault of Fukaya
fault, and is estimated to have repeated its displacement movements once in about 20 thousandyears considering evidence on earthquake activities of Konan fault. The cliff formed by displacement by the fault has been interpreted to be an erosion cliff. This suggests that such misidentification of movement cliff as erosion cliff or vice versa may exist in other cases in the world including
Japan.

Keywords: Fukaya fault, subordinate fault, fluvial terrace, recurrence interval

副次断層の調査に基づく深谷断層の活動性評価中村＊

立正大学地球環境科学部

要洋介＊・田村俊和＊・菊地隆男＊＊・古田＊＊〒180-0014 武蔵野市関前2-2-15-502

智弘＊＊＊

＊＊＊

JR 九州コンサルタンツ株式会社旨：
立正大学熊谷キャンパス内にて､ 約100,000年前に形成された江南面を約3.2m 変位させる活断層を新たに確認した｡ この活断層は深谷断層の副次断層である江南断層のさらに副次的な断層であると解釈され､ 江南断層におけるこれまでの地震活動を考慮すると約20,000年に１回程度の割合で断層変位を繰り返してきた計算となる｡ 今回新たに発見した江南面の変動崖は従来の研究においては浸食崖であると解釈されており､ 日本はもとより世界中においても変動崖が浸食崖と誤認されているケース､ もしくはその逆 (変動崖→浸食崖) の事例が少なからずあるであろうことを示唆できた｡

キーワード：深谷断層､ 副次断層､ 河成段丘､ 地震の再来間隔

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