CSIS Discussion Paper #49

Identification of an active fault in the Japanese Alps from DEM-based hill shading

Takashi OGUCHIa *, Tatsuto AOKIb and Nobuhisa MATSUTAcaCenter for Spatial Information Science, the University of Tokyo, c/o Department ofGeography, Faculty of Science, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japanb

Department of Geography, Faculty of Letters, Kanazawa University, Kakuma-cho,Kanazawa 920-1192, Japanc

Department of Earth and Planetary Science, Graduate School of Science, the Universityof Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan* Corresponding author. E-mail address: oguchi@csis.u-tokyo.ac.jp (T. Oguchi)

Abstract
Shaded-relief images created from digital elevation models (DEMs) are helpful in identifying faults
in rugged mountains, because, unlike conventional airphoto interpretation, the method can enhancelineaments at different orientations by simulating topographic illumination under varied light
directions. Interpretation of shaded-relief images of the Japanese Alps led to the discovery of alineament unrelated to bedrock structure. Field surveys and detailed analysis of large-scale maps andairphotos revealed the lineament to be an active fault with high rates of vertical and lateral slip. The
new fault can be regarded as the southernmost segment or branch of a known adjacent fault, and therate and direction of its slip provide fresh insight into late Quaternary activities of the fault system.
Because previous research mistook the fault scarp for a terrace scarp, discovery of the fault also led
to revision of river-terrace correlation in the Northern Japanese Alps. This correction affects
Pleistocene glacial chronology in the upstream area.
Key words: DEM, hill shading, active fault, surface displacement, Japan

Introduction
Faults are a major geomorphic focus in tectonically active areas. Based on extensive air-photointerpretation and literature surveys, the 55 researchers of the Research Group for Active Faults ofJapan (1991) compiled an inventory of Japanese faults that moved at least once in the Quaternary.
The inventory contains information on ca. 1,800 active faults, such as degree of certainty, activity,1and age of movement. Distribution of the faults is shown in 123 map sheets. The inventory probably
is one of the most comprehensive lists of active faults in a given country or region of the world.
The RGAFJ (1991), however, has not fully investigated faults within rugged mountains, althoughsteep mountains occupy much of the Japanese Islands. Attention was paid mostly to faults inpiedmont areas, which often separate mountains from lowlands, and urbanized lowlands which are
directly related to earthquake hazards in cities. In general, faults in mountain areas often remain
undiscovered because of infrequent investigation and the lack of broad flat surfaces that clearlyrecord surface displacement. The nature of the airphotos also restricts fault identification in
mountains, since it is relatively difficult to identify faults with a north-south orientation in Japan.
Because most airphotos were taken around noon to reduce shaded areas, the east- or west-facing
scarps located along north-south-striking faults are not clearly illuminated or shadowed.
Simulated topography can be illuminated from an arbitrary direction and mapped using grid
DEMs. This technique, analytical hill-shading, results in shaded-relief images (e.g., Yoeli, 1965;
Thelin and Pike, 1991). These images are useful not only for the representation of terrain features but
also for the identification of lineaments, because shaded relief shows bare-ground surfaces
unobscured by other surface cover such as vegetation and land use. In addition, shaded-relief images
under a light direction different from the conventional sun location may aid in identifying lineaments
or faults that are hard to see by the usual methods of airphoto interpretation.
This paper describes how DEM-generated shaded-relief images contributed to discovering anactive fault in the Northern Japanese Alps. The existence of the fault, confirmed by field surveys anddetailed airphoto interpretation, has geomorphological and paleoenvironmental implications.

Shaded-relief images and fault identification
The Japanese Alps, located in central Honshu Island, are the highest non-volcanic mountains inJapan. The range consists of Northern, Central, and Southern components (Fig. 1). Summit altitudes
are around 3,000 m, and the mountains are underlain mainly by Cretaceous granitic rocks, Paleozoic
to Mesozoic sedimentary rocks, and Paleozoic to Mesozoic metamorphic rocks. The ranges arecharacterized by steep slopes with a dominant modal angle of ca. 35 degrees (Katsube and Oguchi,1999).
Shaded-relief images for the Japanese Alps were created from 2.25"×1.5" DEMs provided by the
Geographical Survey Institute of Japan. The grid interval of the DEMs at the center of the Japan
Alps is nominally 50 m (ca. 56.5 m in the E-W direction and 46.2 m in the N-S direction). The DEM
data were imported into SURFER (Golden Software) to make shaded-relief images using Lambertianreflection (Thelin and Pike, 1991; Keckler, 1995). Various light directions were used to create
images; Fig. 1 is an example of northwest illumination.
Visual interpretation of the shaded-relief images revealed lineaments not identified by the RGAFJ(1991). Comparisons between the distribution of the lineaments and geological maps suggest thatmost lineaments are related to bedrock structure rather than recent fault displacement. Somelineaments, however, appear unrelated to bedrock structure, implying that they are active faults. We

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Fig. 1 Shaded-relief image of the Japanese Alps, showing N: Northern, C: Central, and S: Southerndivisions and detailed area (a) imaged in Figs. 2 to 4. Source: 2.25"×1.5" (50 m) DEM provided
by the Geographical Survey Institute of Japan (GSIJ).

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Fig. 2 Shaded-relief image of the Karasu River Basin and its surrounding, Northern Japanese Alps,
under E illumination and 25° sun angle. Source: 0.45"×0.3" (10 m) DEM interpolated from GSIJ
50 m DEM.
X-X’-Y-Y’-Y”: Jonendake Fault, X’-X”: newly discovered active fault, X’-Y-Z: older fault.
Image is ca. 16 km × 20 km.

4Fig. 3

Shaded-relief image of the same area as Fig. 2, but under S illumination and 54° sun angle.

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observed one especially clear lineament unidentified by the RGAFJ (1991) in the eastern part of theNorthern Japanese Alps (a in Fig. 1). The lineament is ca. 9 km long, strikes nearly north-south, andmost of it is located in the catchment of the River Karasu, where fluvial and hillslope
geomorphology was earlier investigated by Oguchi (1988, 1996).
To examine the lineament in detail, we prepared shaded-relief images for a smaller area aroundcontaining 286×427 grid points of the 50 m DEM. As noted by Yoeli (1965), shaded-relief images
with pixels larger than 0.25 mm are inappropriate for representing terrain characteristics. If pixel size
is set to 0.2 mm, the resulting image for the area has a dimension of ca. 65 mm ×85 mm, too small
for the visual interpretation of terrain features. To solve this problem, the DEM data wereinterpolated to a finer resolution. Various methods of interpolating raster geographical data havebeen proposed (e.g., Schut, 1976; Desmet, 1997), kriging being one often applied to DEM data (e.g.,
Keckler, 1995). We generated a 0.45"×0.3" DEM with 1,430 ×2,135 grid points from the 50 m
DEMs by kriging with a linear variogram model without drift and nugget effects. Shaded-relief
images created under various light directions from the new DEM revealed that low-angle
illumination from the east at a 25° sun angle best depicts the lineament of the inferred fault (X'-X" inFig. 2). By contrast, the more conventional illumination shown in Fig. 3 (S at 54° sun angle) for thearea fails to reveal the inferred fault on the image, even after the application of the sharpening filter,
explaining why the fault remained undiscovered by conventional airphoto interpretation.

Validation and slip rates of the fault
To confirm the existence of a fault, detailed topographic features along the inferred feature were
investigated by 1:5,000 topographic maps, 1:8,000 airphotos, and field surveys that included profile
measurements. The results indicate that more than 10 locations are associated with topographic
displacement by the fault (Fig. 4). One of them is a set of fluvial terraces along the west-to-east
flowing River Karasu. Three different levels of terraces are displaced by the fault with left-lateralslip and the relative lowering of the eastern block (Figs. 5 and 6). The highest deformed terrace canbe correlated to the Q2 surface defined by Oguchi (1996), and the lower two terraces can becorrelated to the Q3 surfaces. Field surveys and the longitudinal profiles of terraces show that thevertical displacement of the Q2 surface is about 17 m and that of the Q3 surfaces is about 10 m (Figs.
6 and 7). Fig. 5 shows that the terrace scarp between Q2 and Q3 terraces is horizontally displaced by
ca. 25 m. Some ridges along the fault also are subjected to left-lateral displacement and/or the
lowering of the eastern block (Figs. 4 and 8). We also found the fault to outcrop at a(x) in Fig. 4,
revealing a nearly vertical fault plane (Fig. 9). Therefore, the fault is confirmed to be active.
Tephrochronological surveys in and around the Northern Japanese Alps (Oguchi, 1988) andrelative height of the terraces indicate that the Q2 terrace was formed at 40 to 60 ka BP, and Q3
terrace surfaces cut by the fault were formed at 25 to 35 ka BP. These ages and the amount of terrace
displacement yield a horizontal slip rate of 0.7 to 1.0 mm/yr and a vertical slip rate of 0.3 to 0.4mm/yr. Therefore, the net slip rate amounts to 0.8 to 1.1 mm/yr. The RGAFJ (1991) classified fault
activities in Japan into three levels: A class (1 to 10 m / 1000 yr), B class (0.1 to 1 m / 1000 yr) and6

Fig. 4Map showing locations of topographic features displaced by the newly discovered fault.

a: ridge displacement by left-lateral strike slip and relative lowering of the eastern block; b: ridge displacement
by left-lateral strike slip; c: ridge displacement with relative lowering of the eastern block; x: fault outcrop
shown in Fig. 9; y: ridge displacement shown in Fig. 8.
Image is ca. 16 km × 20 km.

7Fig. 5Airphoto showing displaced terraces in the middle reach of the Karasu River.
A-A’: Location of topographic profile in Fig. 7.

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Fig. 6

Fig. 7

Longitudinal profiles of river terraces along the Karasu River.

Topographic profile surveyed in the field showing ca. 17 m vertical displacement of Q2
terrace by the newly discovered fault.
Location of profile is shown in Fig. 5.

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Fig. 8

Photo of faulted ridge at y in Fig. 4, looking north. Arrow shows position of fault.
Left-lateral slip and downfaulting of eastern block are evident.

Fig. 9Sketch of fault outcrop observed at x in Fig. 4, showing nearly vertical fault plane.

10C class (0.01 to 0.1 m / 1000 yr). Class A faults are relatively few and regarded as very active. The
slip rate of the newly discovered fault corresponds to the border between A and B classes, indicatingthat the fault has relatively high movement among Japanese active faults.
Although the displaced topographic features in Fig. 4 are located at various altitudes and theirmaximum height difference is about 850 m, the fault trace is linear, indicating a nearly vertical slip
plane. This inference is consistent with the observed dip of the fault plane at the outcrop (Fig. 9).

Geomorphological and paleoenvironmental implications of the fault
Discovery of the fault provides some new insights into tectonic geomorphology of the Northern
Japanese Alps. The northern end of the fault (X' in Fig. 2) seems to converge into the Jyonendake
Fault (X-X'-Y-Y'-Y" in Fig. 2) which already has been described by the RGAFJ (1991). Ito (1983)
and Oguchi (1990) noted that the highest river-terrace surface in the region (Q1 surface in Oguchi,
1996) was displaced by the Jyonendake Fault (Y'-Y" in Fig. 2), but lower surfaces including Q2 andQ3 were not clearly displaced. The gravel of Q1 terrace is almost directly overlain by the A-Pm
tephra (Ito, 1983), indicating that the terrace formed around 300 to 350 ka BP. Because the Q2terrace formed around 50 ka BP, the Y'-Y" segment of the fault must have been active at some time
between 50 and 350 ka, although it has been inactive at least since ca. 50 ka.
The shaded relief image in Fig. 2 shows another clear lineament (Y-Z) near the southern end of
the Jyonendake Fault. This lineament converges with the Jyonendake Fault at location Y, and its
strike is similar to that of the Jyonendake Fault north of the convergence. The lineament is probably
older than the segment of the Jyonendake Fault between locations Y and Y", because terrace surfaces
to the south of location Z were not displaced. These observations indicate that the activity along thefault south of location X' shifted from east to west in the following order: X'-Y-Z, X'-Y-Y" andX'-X". Accordingly, the newly discovered fault can be regarded as a currently active segment or
branch of the Jyonendake Fault.
The RGAFJ (1991) suggested that the slip rate of the Jyonendake Fault lies between Classes B
and C. However, this study has revealed that the current southern branch of the fault has a higher rate,
which may reach Class A. Furthermore, the fault branch has a high horizontal slip rate, although the
RGAFJ (1991) notes that the Jyonendake Fault induced only vertical displacement. The area in andaround the Japanese Alps is under the compressive stress of WNW-ESE direction (Tsukahara and
Ikeda, 1991; Nakagawa et al., 2001). Because this stress can cause left-lateral slip of N-S oriented
faults, the lateral slip of the southern part of the Jyonendake Fault can be attributed to regional
tectonism. The vertical slip of the fault may reflect rapid granite intrusion beneath the western block(Harayama, 1992).
The discovery of the fault in the catchment of the Karasu River also forces revision of glacialchronology along the upper reach. Ito (1983) suggested that fluvial terraces of ca. 100 ka widely
occur in the upper reach of the river, whereas, terraces of ca. 50 ka (Q2) and younger ones are
dominant in the lower reach. He also suggested that glaciers developed in cirques near the top of therange around 100 ka, because the 100-ka terraces are topographically continuous to a theater-headed

11valley in the uppermost reach (Figs. 4 and 6). Although Momose et al. (1984) and Oguchi (1988)
cast doubt on the existence of wide 100-ka terraces, Ito's (1983) correlation and glacial chronology
have been used in later studies (e.g., Sakaguchi, 1988; Ono, 1990; Hasegawa, 1996). Most of the
100-ka terraces identified by Ito (1983) occur only in areas upstream from the newly discovered fault,
however, and the boundary of 100-ka and Q2 terraces suggested by Ito (1983) is located where theterraces are displaced by the fault. These observations indicate that Ito (1983) misinterpreted thefault scarp as a terrace scarp. Consequently, Ito’s (1983) 100-ka terraces should be reclassified as Q2
terraces formed around 50 ka, and his glacial chronology needs to be revised. Moreover, it remains
undetermined whether the theater-headed valley is a glacial cirque (Momose et al., 1984).

Conclusions
DEM-based hill shading has led to the discovery of an active fault in the Japanese Alps, which
had not been found by conventional airphoto interpretation. Only unconventional illumination could
reveal the existence of the fault-related lineament. The fault discovery has provided new insights into
not only regional tectonics but also river-terrace correlation and Pleistocene glacial chronology. We
conclude that creation and careful interpretation of shaded-relief images can contribute significantly
to solving various problems in geomorphology and tectonics.

Acknowledgment
We would like to thank Richard Pike, U.S. Geological Survey, for reading an early draft of thispaper and constructive comments. We also like to thank Keiichi Katsube, University of Tokyo, for
his help with making a figure.

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