PALEOSEISMIC ACTIVE FAULT STUDYIN KINKI TRIANGLE
Ehsanullah*
MEE08160

Supervisor: Harutaka SAKAI**

ABSTRACT
Kohistan-Chilas area in northern Pakistan has experienced few damaging earthquakes for recent few
decades that showed the current active crustal movement.
This study is mainly aimed to understand the relation of tectonic stress field and active faults in
Kinki Triangle and to compare it with those of Kohistan-Chilas area. It is also aimed to confirm therelationship between geomorphologic evidence detected by the interpretation of aerial photographs andstructural behavior of the geological units surveyed in the field.
Throughout the study of eleven active faults in Kinki-Triangle this relationship has been confirmed.
It is revealed also that the active faults there are grouped into three: those along Median Tectonic Line
(MTL), those of left lateral motion having NW(NNW)-SE(SSE) strike and those of right lateral havingNE(NNE)-SW(SSW) strike. The latter two can be summarized by the tectonic stress of E-W compression.
Namely they are conjugate shear faults. It has been said for long time that this regional stress might becaused by subducting Pacific Plate. The recent studies, however, show a possible alternative cause, i.e.,
the irregularly shaped subducting Philippine Sea Plate.
These two areas show a similarity in the tectonic frame work of the compressive regional tectonic
stress and the accompanying conjugate shear faults systems. The existence of other tectonic units,
however, makes them different each other.
Therefore, it is necessary to conduct systematic studies of active faults in Kohistan, northern
Pakistan not only by the conventional geological survey but also by the geomorphologic interpretation of
aerial photographs combined with field survey including trench observation.
Keywords: Paleoseismology, Active Fault, Aerial Photograph, Kohistan Island Arc, Kinki Triangle

1. INTRODUCTION
Pakistan is located mainly along the active boundary between Eurasian plate and Indian plate except thecoastal area along the Arabian Sea. Kohistan-Chilas area in the northern Pakistan is the ancient Island Arc
in Ceno-Tethys and today located among these plates. Today a big dam in Chilas along Indus River is inplanning stage. Moreover, the damaging earthquakes that took place in recent few decades in Kohistanshow that the crustal movement in this area is still active and that it is necessary to take the earthquake
hazard in this area into account for the safety of the dam. This is the reason why it is necessary to conduct
studies on the current crustal activities there.
My overall goal in future is a clear understanding of the tectonic framework and crustal activities ofKohistan-Chilas area and surroundings for estimation of earthquake hazard. This study is the first step forthis overall goal and mainly aimed to understand the relation of tectonic stress field and active faults in
Kinki Triangle and to compare it with those of Kohistan-Chilas area. It is also aimed to confirm the*Water and Power Development Authority (WAPDA), Pakistan
Professor, Department of Geology and Mineralogy, Kyoto University**

relationship between geomorphological evidence detected by the interpretation of aerial photographs andstructural behavior of the geological units surveyed in the field.

2. Method
In order to confirm the relationship between geomorphologic evidence detected by the interpretation of
aerial photographs and structural behavior of the geological units surveyed in the field. I selected elevenmajor active faults from the Kinki Triangle.
+ Okamura, Hatano, Ikeda faults (MTL in Shikoku area, right-lateral)+ Neo-dani, Yamasaki, Yoro-Kuwana faults (NW(NNW)-SE(SSE) strike, left-lateral)+ Hanaore, Katata, Aibano, Rokko, Ikoma faults (NE(NNE)-SW(SSW) strike, right-lateral)
Then I tried to detect displaced landform and identify active faults on aerial photographs of
1:20,000 by means of stereoscope. After finding of offset of topographic references, I interpreted the senseof displacement and drew lines of active faults on the topographic maps of 1:25,000 published byGeographical Survey Japan. Total number of aerial photographs which I interpreted was about 360 photos.
After recognition of active faults on the aerial photographs, I conducted literature survey for eachfault and compiled the information about the selected active faults.
Lastly, I visited the areas where active landform and fault exposures can be observed in order toconfirm, in the field, the reliability of geomorphological evidences for identification of active faults.

3. STUDY ON ACTIVE FAULTS IN KINKI TRIANGLE, JAPANKinki district is located in the central part of Japan, and Kyoto and Nara of ancient capital of Japan andmodern mega-city, Osaka and Kobe, are main cities in this district. Those cities are developed in basinssurrounded by narrow mountain ranges (e.g., Huzita, 1974; Research Group for Active Faults of Japan,
1991). The basins and ranges are bounded by thrust faults or strike-slip faults, many of which are active atpresent and occasionally cause earthquakes.
Central part of the Kinki district is characterized by young basins and ranges structure formed byfolding of basement rocks accompanied by many thrusts that initiated their activity in Quaternary. The
area characterized by such topography and geologic structure forms a triangular shape connecting theBays of Wakasa, Osaka and Ise. Huzita (1962) proposed to name this area as “Kinki Triangle”.
From the view points of active fault tectonics, the Kinki Triangle is bordered by three tectonic lines:
the bottom of triangle is MTL, the eastern side is the Tsuruga Bay-Ise Bay Tectonic Line (TITL), thewestern side is the Niigata-Kobe tectonic zone (NKTZ). This neotectonic province is characterized bydense concentration of active faults running three directions: NW(NNW) - SE(SSE) trending left-lateraland thrust faults, NE(NNE) - SW(SSW) trending right-lateral and thrust faults and E-W trending rightlateral faults of the MTL.

4. MAJOR ACTIVE FAULTS SYSTEMSMedian Tectonic Line (MTL) in Central Shikoku
MTL is the first class fault that divides Southwest Japan into the Inner Zone and the Outer Zone. The
present movement of the MTL in Central Shikoku is predominantly arc-parallel right-lateral strike-slip
fault, which is related to subduction of the Philippine Sea plate beneath the Eurasian plate along the
Nankai Trough. Its slip rate during the Quaternary is 5~10 mm/yr in Shikoku (Okada, 1973). The verticaldisplacement during the Quaternary was estimated to be more than 1500 m in Central Shikoku, and form

Figure 1 Left) Aerial photographs of the Hatano fault, Doi area, MTL. Right) The Road
constructed on the scarp of the Hatano fault which shows prominent offset of ridge andvalley. Topography is artificially changed and a substation was constructed.
the Ishizuchi fault scarp though its average displacement rate is one digit smaller than that of lateral-slip(Okada, 1980).
I selected three faults from the MTL fault system: Okamura fault in Niihama area; Hatano fault in
Doi area (Figure 1), Ikeda fault in Awa-Ikeda area. In Niihama and Doi, I could convince Quaternary
right-lateral movement from offset of ridge and stream, fault scarp, vertical displacement of terrace
surface. The maximum lateral displacement is more than 230 m after 29,000 yr BP, and its displacement
rate is estimated to be 7~8 mm/yr (Okada, 1973; 1980). In Awa-Ikeda town, I could confirm that the riverterrace of ancient Yoshino River was tilted and cut by active faults. Okada (1968) estimated vertical slip
rate of 1mm/yr based on 14 C dating of wood fragments contained in the terrace gravels, and reported~200m right-lateral displacement.
NW (NNW) – SE (SSE) left-lateral faults
One of the prominent topographic features of active faults in the Kinki Triangle and its neighboring area isa set of NW(NNW)-SE(SSE) trending left-lateral faults and NE(ENE)-SW(WSW) trending right-lateral
faults, both of which are considered to be a set of conjugate shear faults. They were recognized as very
sharp lineaments in satellite images and aerial photographs, and historical earthquake occurred in somefaults.
Among them, most famous left-lateral fault is the Neo-dani fault, which caused the Nobi earthquakeof M8.0 in 1891 (Matsuda, 1974). I have studied its fault topography both in the aerial photographs and in
the field. Especially in the field, I could convince about 6 to 7 m vertical displacement along theearthquake fault in 1891, both in the field and the Museum (Figure 2).
I selected the Yamasaki fault, because its fault topography as a strike-slip fault is prominent, though
it is located 50km to the west of the western margin of the Kinki Triangle. I detected many offset ridge
and offset stream and valley, misfit river, shutter ridge and pond along the linear valley. All indicators ofdisplacement show right-lateral movement and its maximum offset attains 400 m and verticaldisplacement reaches 200 m. After 14 C dating and discovery of tephra layers from several trenches

Figure 2 Left) Aerial photograph of the Neo-dani fault around Midori village. Right) A
scene of the fault scarp of the Neo-dani fault formed by Nobi Earthquake, 1891.
crossing the fault, the lateral displacement rate was estimated to be 7 mm/yr and vertical one to be morethan 1mm/yr (Okada et al., 1987).
In the southeastern part of the Kinki Triangle, NNW-SSE trending faults are thrust faults rather thanstrike-slip faults (Ota and Sangawa, 1984; Awata and Yoshida, 1991). The Kuwana fault is located in thefrontal deformation zone in the Suzuka Mountain marginal fault system. On the aerial photograph, I could
recognize various kinds of fault topography: tilting, flexure and folding of terrace surfaces, depression
zone bounded by linear fault scarps. The vertical displacement of the Kuwana fault is 5 to 25 m, andgenerally increasing from S to N. The average displacement rate is estimated to be 0.1~0.3 mm/yr (Suzuki
et al., 1996).
NE(NNE) - SW(SSW) right-lateral faults
Another fault of conjugate shear set is NE(NNE) - SW(SSW) trending right-lateral fault. One of the fault
running in the western part of the Awaji Island called Nojima fault, caused Hyogoken-Nanbu earthquake
in 1995. Its maximum net slip was 2.5 m, and the right-lateral displacement was 2.0~2.1 m, whereas
vertical displacement was 1.1~1.4 m. The NE trending Nojima fault has fault scarps higher than 250m forabout 10km. The total right-lateral displacement is greater than 300m, almost equal to total verticaldisplacement.
The northeast continuation of the Nojima fault in mainland Honshu is the Rokko fault system,
which is composed of a series of right-lateral fault for about 30 km. At the surface, the fault systemappears as the high-angle thrust faults. The Rokko Mountains north of the Kobe-Nishinomiya area isinterpreted to have been uplifted by neotectonic movement, called the “Rokko Movements” (Huzita, 1962;
Huzita et al. 1971; Huzita and Kasama, 1982; 1983). The average vertical slip rate is greater than 0.5mm/yr based on the elevation difference of the first marine bed called Ma 1 in the Osaka Group, whereas
the right-lateral slip rate is not well defined by the filed data.
Another big right-lateral strike-slip active fault is the Hanaore fault, which is traceable for 48 km
from the northeastern margin of Kyoto basin to Kutsuki to the north of the Hira Mountain. It shows NNESSW trending very sharp lineament with a narrow valley bottom in mountain area, but has a thrust
component of east-side up. Offset of ridge and stream indicates maximum lateral displacement of 120 m,
but its slip rate is not well defined. The middle Pleistocene terrace surface shows ~30m verticaldisplacement along the fault, and its slip rate is estimated about 0.3 mm/yr.
To the east of the Hanaore fault beyond the Hira Mountain, the Biwako-seigan (west coast) fault
system is running along the western margin of the Lake Biwa (Sangawa and Tsukuda, 1987). They areFigure 3 Left) Aerial photographs of the Aibano fault around Oomi-Imazu station of JR. Right)
Terrace gravels change their attitude to perpendicular after drag by faulting. The digging tool isabout 15-20 cm long.
thrust faults, along which the Hira Mountain thrusts onto the Oomi sedimentary basin including the Lake
Biwa. I selected two faults for my study: one is the Katata fault and another is the Aibano fault (Figure 3).
The Pleistocene beds of the Kobiwako Group are uplifted about 800 m along the Katata fault during thelast 0.85 m/yr, and its vertical slip rate is estimated about 1 mm/yr. The Ikoma fault is also NNE-SSW
trending thrust fault running along the margin of the Osaka plain, and the Ikoma Mountain thrusts onto the
Osaka plain. Based on the elevation difference of basement rocks and the base of the Osaka Group on theseismic profile, total vertical displacement of 600m is estimated.

3. CONCLUSION & RECOMENDATIONS
Active faults in northern Pakistan are mainly grouped into two: one is left-lateral strike-slip fault along thewestern margin of northward drifting Indian subcontinent and another is thrust fault caused by collision ofIndian plate. There are several right-lateral strike-slip faults that form a set of conjugate shear fault with
left-lateral one. The fault which caused a big earthquake (Mw 7.6) of October 8, 2005 in Pakistan, called
the Balakot-Garhi fault or Muzaffarabad fault, is one of the right-lateral faults. Around northern Pakistan,a typical pattern of the conjugate shear faults is shown by a big scale active fault systems. These also arecaused by N-S compressional tectonic stress field due to the collision of Indian plate to Eurasian plate.
The study in Kinki-Triangle confirms the relationship between geomorphological evidence detectedby the interpretation of aerial photographs and structural behavior of the geological units surveyed in thefield and reveals that the active faults there are grouped into three: those along MTL, those of left lateralmotion having NW-SE strike and those of right lateral one having NE-SW strike. The latter two can besummarized by the tectonic stress (E-W compression). Namely they are conjugate shear faults. It has been
said for long time that this regional stress might be caused by subducting Pacific plate. The recent studies,
however, show a possible alternative cause, i.e., the irregularly shaped subducting Philippine Sea plate.
These two regions show a similarity in the framework of the compressive regional tectonic stress andthe accompanying conjugate shear faults systems. The existence of other tectonic unit, however, makes
them different each other. That is Philippine Sea plate with irregular shape for Kinki Triangle that makes
Osaka Basin and Lake Biwa subsiding. For northern Pakistan, uplifting Nanga-Parbat Syntaxis and Hazara

Syntaxis may be caused by the collision of two continental plates and also disturb it with Kohistan Island
Arc that lies between them.
Based on the above summarized tectonics, seismicity, active faults and recent damaging earthquakes
in Kinki Triangle, it is necessary to conduct systematic studies of active fault in Pakistan not only by theconventional geological survey but also by the topographical interpretation of aerial photographs
combined with field survey including trench observation. In addition, systematic dating of carbonaceous
material contained in sediments and establishment of tephra stratigraphy in Pakistan are important toevaluate activity of the active faults and earthquake hazard estimation.

ACKNOWLEDGEMENTS
I would like to express special thanks to Prof. Takashi Nakata, Hiroshima Institute of Technology and to
Prof. Hiroyuki Tsutsumi, Kyoto University, for their helps during my aerial photographs study. I would
like to thank greatly also to Dr. R. Fujii, Kyoto University who arranged maps and the related papers forme and to Mr. Ishimura, Kyoto University for his kind helps in field survey.
I would like to thank to my advisor Dr. T. Yokoi, IISEE, BRI especially for his keen interest in mymaster study encouraging me for this work.

REFERENCES
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Huzita, K., 1962, Jour. of Geosciences, Osaka City Univ., 6, 103-144.
Huzita, K. and Kasama, T., 1971, Geology in and around Rokko Mountain Range, Kobe City Office.
Huzita, K. (ed.), 1974, Neotectonic Map “Kinki”, Geol. Surv. Japan.
Huzita, K. and Kasama, T.,1982, Geology of Osaka-Seihokubu district, Quadrangle series, 1:50,000.
Huzita, K. and Kasama, T., 1983, Geology of Kobe district, Quadrangle series, 1:50,000.
Ikebe, N., 1969, Jour. Geosciences, Osaka City Univ., 12, 45-51.
Matsuda, T., 1974, Special Bull. Earthq. Res. Inst., University of Tokyo, 13, 85-126.
Okada, T., 1968, Quat, Res., 7, 15-26.
Okada, A., 1973, in R. Sugiyama (ed.), the Median Tectonic Line, Univ. Tokai Press.
Okada, A., 1980, Mem, Geol. Soc. Jpn., 18, 79-108.
Okada, A., Ando, M. and Tsukuda, T., 1987, Jour. Geography, 96, 81-97
Ota, Y. and Sangawa, A., 1984, Geogr. Rev. Jpn., 57, 237-262 (in Japanese with English abstract).
Research Group for Active Faults of Japan, 1991, University of Tokyo Press.
Sangawa, A. and Tsukuda, E., 1987, Chishitsu News, 390, 6-12 (in Japanese)
Suzuki, K., et al., 1996, Active Fault Map in Urban Area 1：25,000 Kuwana, Geograph. Surv. Of Japan.