Proceedings of the International Symposium on Engineering Lessons Learned fromthe 2011 Great East Japan Earthquake, March 1-4, 2012, Tokyo, Japan

Active Faults along Japan Trench and Source Faults ofLarge Earthquakes
Takashi NAKATA1, Hideaki GOTO2, Mitsuhisa WATANABE3, Yasuhiro SUZUKI4Azusa NISHIZAWA5, Noriaki IZUMI6, Daishi HORIUCHI7 and Yukari KIDO8
1

Professor Emeritus, Hiroshima University, Higashi-Hiroshima, Japan, tnakata@hiroshima-u.ac.jp
2
Associate Hiroshima University, Higashi-Hiroshima, Japan, hgoto@hiroshima-u.ac.jp
3
Professor, Faculty of Sociology, Toyo University, Tokyo, Japan, manqq@toyo.jp
4Professor, Graduate School of Environmental Science, Nagoya University. Nagoya, Japan,y-sz@forest.biglobe.ne.jp
5Director for Earthquake Research, Technology Planning and International Affair Division,
Japan Coast Guard, azusa@jodc.go.jp
6
Officer, Continental Shelf Surveys Office, Japan Coast Guard, izumi-b8ijf@kaiho.mlit.go.jp
7
Officer, Continental Shelf Surveys Office, Coast Guard, horiuchi-s3har@kaiho.mlit.go.jp
8
Technical Staff, Center for Deep Earth Exploration JAMSTEC, ykido@jamstec.go.jp
ABSTRACT: Active faults observed on seafloor along Japan Trench are resultants ofrepeated large earthquakes. We discuss on relation between large earthquakes and theirsource faults based on a detailed active fault map along Japan Trench. Judging fromlocation and continuation of active fault, we presume that one of the extensive thrustfaults extends from off-Sanriku to off-Ibaraki for over 400km, is directly related to thesource fault of the 2011 off the Pacific coast of Tohoku Earthquake.
Key Words: Great East Japan earthquake, Japan Trench, submarine active fault
INTRODUCTION

Active faults on land are regarded as large earthquake source faults, and are highly considered in thelong-term earthquake prediction in Japan. However, importance of submarine active faults along theplate boundaries around Japan has been overlooked or under-estimated mainly due to lack of interest
among marine geologist as well as seismologists. They seem to believe that large earthquakes along
the plate boundaries occur on the deep-sheeted fault plane, and are not necessarily related to sea-floor
faulting along the plate boundaries such as Japan Trench (Fig.1).
We depict distribution and geometry of active faults identified by tectonic geomorphology method
similar to a method we used in stereo-pair air-photo interpretation for inland active faults byinterpretation of stereo-pair set as well as anaglyph images, and discuss on relation between tectonic
seafloor features and earthquake source faults.
SUBMARINE ACTIVE FAULTS ALONG THE JAPAN TRENCH
Identification of active faults
Japan Coast Guard and JAMSTEC have conducted narrow multi-beam bathymetric survey since 1985,
and accumulated dense raw data, but these data were not been used for detailed analysis on submarine
topography until we process for this study. We made detailed submarine topographic images based on
150m grid DEM processed from the raw data. Then we have made stereo-pair topographic images aswell as anaglyph images for stereo imaging of the heart using 3D red cyan glasses prepared by
Simple DEM Viewer®, and two images each given different shadow direction were combined on
Photoshop® in order to make clear images for interpretation of topography. The 250m topographic

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DEM in and around Japan (Kishimoto 2000) and SRTM3 data are also used to fill the data uncoveredby narrow multi-beam bathymetric survey.

Fig. 1 Index map of study area around Japan Trench
Effectiveness of an anaglyph image in interpretation of seafloor topography cannot compare with ashaded-relief image (Fig. 2). 3D image viewed through red cyan glasses enables us to findsubmarine active faults very easily, if we have certain knowledge of tectonic
geomorphology. Numerous normal faults dislocating a subsiding seamount in Fig. 1 canbe detected by shaded-relief image, but not the sides upraised along the faults.

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Fig 2. Comparison of shaded-relief image and anaglyph around Daiichi-Kashima Seamount
3D red cyan glasses are necessary to view the anaglyph image (right) correctlyFig. 3 Three dimentional image of study area
Extesive trench slope about 4000-5000m high along Japan Trench in the north is called
Sanriku Escarpment. The escarpment is concvex in cross section.
The area we discuss in this papaer is between Erimo Seamount to the north and Daiichi
Kashima Seamount to the south. As shown in fig. 3, relief along Japan Trench is much bigger
than that on land, and is composed of four major topographic units; wide undulated shelfslope dipping east about 2% up to 3000m deep, continuous gigantic trench slope about 4000m
high, trench floor between 7000 - 8000m deep, and warped outer rise to the east. The easternpart of the shelf slope is widely occupied with several elongated bulges with flexure scarps
along their eastern margins. The trench slope is of steep escarpments, a part of which iscalled Sanriku Escarpment with relative height over 3000m.

256Fig. 4 An anaglyph image of Japan Trench
3D red cyan glasses are necessary to view the image correctly257

Geometry of submarine active faults
Fig. 4 is an anaglyph image of Japan Trench with island parts of Northeast Japan. Noticethat vertical exaggeration of the figure is about 5 -10 times depending on distance from thefigure to eyes. In the anaglyph image, we can easily observe seafloor relief throughout wide area, and
it is no so difficult for most of us to see minor relief, and find faults that dislocate seafloor.
Escarpments or flexure slopes were formed by reverse faulting, whether individual fault
reaches seafloor or terminates in depth. Slope tectonically formed are convex in their crosssections, and are associated with tectonic bulge to their west. Active faults along Japan
Trench have been first mapped by Research Group for Active Faults of Japan (1991), but the faults
were identified by the rather small-scale bathymetric maps seismic reflection records available atthat time, the fault traces are intermittent and their location and extents are not always
accurate. Active faults in the study area are grouped into the following three fault zones. They arefrom east to west, (1) normal faults on Outer Rise, (2) extensive thrust faults on the trench slop, and(3) lineaments on the shelf slope (Fig. 5).

Normal faults on Outer Rise
Normal faults on Outer Rise between Erimo Seamount and Daiichi Kashima Seamount extend in N-S
direction. They are not straight but sinuous, and extend parallel each other, and are rather evenly
spaced about 5km apart, and form a basin and range type horst and graben topography. These fault
scarps are lower in the east and higher in the west, and fault scarps are relatively over 300m high near
the trench axis. They are mostly around 50km long. Some of the faults extending NE-SW to thenortheast of Joban Seamount Chain become around 100km long. One of the aftershocks of the 311
Earthquake probably took place along one of the NNE-SSW normal faults on the outer rise to thenorth of Joban Seamount Chain off Fukushima.
Extensive thrust faults on the trench slop
The most extensive thrust fault on the trench slop extends from Sanriku-Oki to Ibaraki-Oki for about
400km. This fault is associated with fault escarpments about 1500m and tectonic bulges to the west
along its northern and southern section parts, while in the area southeast of Ojika peninsula where thefault branches northward as well as southward, younger low fault scarps, yet, higher than 100m,extend on landslide mass derived from the major escarpment, suggesting that repeated faulting has
been taking place along the fault trace.
At Sanriku-Oki close to the trench axis, a 200km-long active thrust fault is identified by a steep
straight escarpment extending N-S between the above-mentioned extensive fault and Japan Trench.
This escarpment is associated with a 50km-long N-S trending sharp break in the north and several
huge landslides in the south suggesting that this fault has been growing this escarpment very rapidly.
Distribution of thrust faults along and around the Japan trench is rather simple compared with that of
the Nankai trough or/and the southwestern part of the Kuril trench.
On the eastern part of the shelf slope to the west of the extensive thrust fault, several elongatedbulges are fringed along their eastern margins by flexure scarps, They are presumably formedby subsurface reverse faulting on west-dipping fault planes.
Lineaments on the shelf slope
N-S trending lineaments are densely distributed in the eastern part of the shelf slope. They
are mainly concentrated on tectonic bulges, especially on their flexure scarps where seafloor
have been expanded due to bending on the hanging walls of subsurface thrust faults.
On the other hand, we could not find any extensive normal faults on the shelf slop, except
for the one off Fukushima. Some of the large tectonic bulges are associated with eroded
anticlinal ridges that are comonly observed along the active faults in Nakai Trough region.
Erosion of the anticlines may be due to strong shaking by faulting around the bulges.

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Fig. 5Tectonic map along Japan Trench.

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Contour inteval is 100m

ACTIVE FULTS AND EARTHQUAKES

Active fault and source fault of the 11 March 2011Tohoku-Oki earthquake earthquake
The 11 March 2011tohoku-Oki earthquake earthquake (Mw9.0) generated large tsunami with massive
pulsating pattern of waves (Maeda et al. 2011). The earthquake source fault that generated theearthquake, is beleived to have caused near-surface fault rupture reaching Japan Trench. Largedisplacement ~50m eastward and ~7 to ~10m upward was estimated between 2004 and 2011 fromcomparison of data obtained by multibeam bathymetric surveys across the trench (Fjiwara et al. 2011).
Satake et al. (2011) explained the large tsunami height by similutaneous faulting on two different
fault planes, one on subducting plate boudary and the other near the trench axis. Since most of workers
believed that the earthquake was caused by the fault ruptured up to the trench axis, exsitence ofsubmarine active fault is rather overlooked so far,although the 400km long active fault trace
coincidentally locates close to the trench in the area where Maeda et al.(2011) estimated large
displacement.
We simulated pattern of seafloor deformation associated with the earthquake using a simple
dislocation model for a single fault plane with uniform slip that dips 14 degree in depth and 33.6
degree beneth the tectonic bulge related to the extensive active fault (Fig. 6). A result shows that anarea of large uplift agrees more or less with area of tectonic bulge with width of about 20km.

Fig. 6 A simulation of sefloor deformation by simple dislocation modele.
Topographic profile abobe follows along N39 degree.

Fig. 7 Tsunami first wave recorded by GPS wave gage off Kamaishi(Modified after Port and Airport Research Institute, 2001)

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Fig. 7 is the record of tsunami first wave obtained by the GPS wave gage set on about 200m
deep seafloor off Kamaishi on southern Sanriku Coast (Port and Airport Research Institute,2011). The record suggests that after gradual sea-level rise of 2m during 6 minutes, acutesea-level rise of 4m took place within 4 minutes, and then sea-level abruptly dropped by
4m within 2 minutes. The length of pulsating tsunami wave is estimated about 17km
from tsunami propagation velocity at 200m deep sea and total duration of pulsating
pattern of tsunami, i.e. 7 minutes. This pattern resembles the extent of seafloor
deformation we calculated above. Therefore, we believe that the thrust fault that extends fromoff-Sanriku to off-Ibaraki for over 400km is closely related to the source fault of the 2011 off thePacific coast of Tohoku Earthquake.

Active fault and historical earthquake

Fig. 8Distribution of tectonic geomorphology

Hypocentral region of historical earthquakes after Earthquake Research Committee ( 1999)
Solid line oval: hypocentral region of historical earthquakes, Broken line oval: source fault of
the 2011 earthquake(after Earthquake Research Committee, 2011), Shaded area:seismic261

source fault of the 2011 earthquake proposed by this study, Solid blue line:trench axis.
We compare location of hypocentral regions of historical earthquakes (Earthquake ResearchCommittee, 1999) and tectonic landoform and distribution of submarine active faults andtectonic bulges (Fig. 8). It is noteworthy to mention that some of the hypocentral regions arelocated on tectonic bulges. This suggests that location and extent of seismic source faults in
depth will be detected from distribution of tectonic bulges. In this context, one of the M8earthquake source faults is possibly located under a large tectonic bulge with flexure scarp
extending N-S for 200km off Fukushima and Ibarak.
The source fault for Meiji Sanriku earthquake (M8.2 - 8.5) may be related to a 200km-long active
fault along the trench off Sanriku. Numerous normal faults are distributed on the outer-rise slope andthey are generally short, and may cause M7 class earthquakes. The 1933 Sanriku earthquake(M8.1-Mw8.4) is believed as one of the outer-rise earthquakes, but it is hard for us to indentify certain
around normal fault that matches to the extent of a M8 class earthquake.

CONCLUSIONS
Based on tectonic geomorphology, we claim that the submarine active faults are closely
related to earthquake source faults, and it is important to consider active faults as direct
evidence of repeated large earthquakes along Japan Trench.

ACKNOWLEDGMENTS
We are thankful to Professor Kenji Satake of University and Professor Yoshiko Yamanaka of Nagoya
University for their highly suggestive discussion with us. This study is supported by Grant-in-Aid for
Scientific Research (A)REFERENCES
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Port and Airport Research Institute (2011) GPS wave gage observation on Tsunami wave generated by2011 Tohoku Earthquake. http://www.pari.go.jp/files/items/3527/File/results.pdf
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