New Perspective of Subduction Zone Earthquake
Site C0009
2.0

a
0

Depth [m]

0

km

2.4

Kii Peninsula

Kumano basin

C0009

33 30'N

fault-splay

Mega

nt

n fro

atio

rm
Defo

33 00'N

Su

bd

gh

Trou
ankai

N
32 30'N
135 30'E

136 00'E

2.6

Philippine
Sea Plate1944 Tonankai
earthquake

Depth [km]34 00'N

2.2

Eurasian
Plate

50

b

136 30'E

137 00'E

2.8

3.0

3.2

3.4

uct

ion

dir

ect

ion

137 30'E

3.6

2960m

908mbsf

3190m

1137mbsf

Editorial Board (Oct. 2015)
Editor-in-Chief
Yasuo Ogawa, Tokyo Institute of Technology, Japan

Editorial Board
Yosuke Aoki, University of Tokyo, Japan
Stephen Bannister, GNS Science, New Zealand
Benjamin Fong Chao, Academia Sinica, Taiwan
Nobuo Geshi, Geological Survey of Japan, AIST,Japan
Lin-Ni Hau, National Central University, Taiwan
Hauke Hussmann, German Aerospace Center,
Germany
Alexey Kuvshinov, Institute for Geophysics,
Switzerland
Hiroo Kanamori, California Institute ofTechnology, U.S.A.
Kristine M. Larson, University of Colorado, U.S.A.
Takuto Maeda, University of Tokyo, Japan
Koji Matsumoto, National Astronomical
Observatory, Japan
Yasuhiro Nariyuki, University of Toyama, Japan

Nozomu Nishitani, Nagoya University, Japan
Azusa Nishizawa, Japan Coast Guard, Japan
Keiji Ohtsuki, Kobe University, Japan
Taku Ozawa, National Research Institute for Earth
Science and Disaster Prevention, Japan
Takeshi Sakanoi, Tohoku University, Japan
Martha Savage, Victoria University of Wellington,
New Zealand
Shanaka de Silva, Oregon State University, U.S.A.
Hiroshi Takenaka, Okayama University, Japan
Hiroaki Toh, Kyoto University, Japan
Yih-Min Wu, National Taiwan University, Taiwan
Zhongliang Wu, China Earthquake Administration,
China
Toshitsugu Yamazaki, University of Tokyo, Japan
Akimasa Yoshikawa, Kyushu University, Japan

Journal Scope
Earth, Planets and Space (EPS) is the official journal of the Society of Geomagnetism and Earth, Planetary
and Space Sciences, The Seismological Society of Japan, The Volcanological Society of Japan, The
Geodetic Society of Japan, and The Japanese Society for Planetary Sciences.
EPS is a peer-reviewed, open-access journal published under SpringerOpen. It is an international journal
covering scientific articles in the entire field of earth and planetary sciences, particularly geomagnetism,aeronomy, space science, seismology, volcanology, geodesy, and planetary science. EPS also welcomesarticles in new and interdisciplinary subjects, and technical reports on instrumentation and software.
The journal was launched in 1998 and has since published over 2700 articles. All of them are available forfree on SpringerLink:
http://link.springer.com/journal/40623
More information about the journal, its article collections, and the submission process is available on thejournal homepage:
http://www.earth-planets-space.com/
Submit your next research article to Earth, Planets and Space via:
https://www.editorialmanager.com/epsp
Yours sincerely,
Prof. Yasuo Ogawa
Editor-in-Chief, Earth, Planets and SpaceEarth, Planets and Space
Kimura et al. Earth, Planets and Space (2015) 67:123
DOI 10.1186/s40623-015-0236-9

PREFACEOpen AccessPreface for the special issue of “NewPerspective of Subduction Zone Earthquakes”
Gaku Kimura1*, Juichiro Ashi2, Masataka Kinoshita3, Christie Rowe4 and David Shelly5
construction and verification of a comprehensive model
for pre- and co-seismic processes.
On March 11, 2011, during the middle phase of the
KANAME program, the Tohoku-oki earthquake andtsunami took place. Chikyu was damaged by the tsunami in the Hachinohe harbor in northeast Japan, andall programs that had been planned for implementation
using Chikyu had to be delayed or canceled, including
NantroSEIZE and KANAME. On the other hand, IODP
immediately planned and conducted drilling at theJapan Trench, at the site of the Tohoku-oki earthquake,after Chikyu was repaired. We modified the KANAME
project to include research on the Japan Trench anddrilling at other subduction zones, such as Costa Rica.
Many new findings and papers have been produced as aresult of the KANAME project. The project was completed in 2015, but the NantroSEIZE effort is ongoing.
This special issue presents some of the results of the
KANAME project, mainly by scientists who took part in
the project, although many papers have already publishedin other journals. Drs. Shelly and Rowe were invited asguest editors in addition to our internal guest editors(Kimura, Kinoshita, and Ashi).
This issue includes 33 papers:

Giant subduction zone earthquakes such as the recent
2004 Sumatra and 2011 Tohoku-oki earthquakes and theiraccompanying tsunamis cause significant damage to human society, and it is our earnest desire to scientificallyelucidate how they occur. In the Nankai Trough, Japan,
great earthquakes and tsunamis have occurred repeatedly,
and more are predicted for this site in the near future.
Super-deep drilling by the drilling vessel Chikyu will
enable us to directly sample seismogenic faults as well as
conduct analyses, experiments, and in situ boreholemeasurements. These efforts should significantly improve
our understanding of pre- and co-seismic processes ofgreat subduction zone earthquakes. The Integrated Ocean
Drilling Program (IODP, 2003–2012) and International
Ocean Discovery Program (2013–2022) have decided topromote the Nankai Trough Seismogenic Zone experiments (NantroSEIZE) using Chikyu, although the deeptarget of the plate boundary fault has not yet been reached
as of 2015.
We created a Japanese partnership program with
NantroSEIZE (new perspective on great subduction
zone earthquakes from super deep drilling: Kakenhi for
the Nankai trough megaquakes, KANAME program, withfinancial support from the Ministry of Education, Culture,
Sports, Science, and Technology from 2008–2015). The
scientific objective of the program is to improve ourunderstanding of pre- and co-seismic processes of great
subduction zone earthquakes by directly sampling seismogenic faults and conducting analyses, experiments,
and in situ borehole measurements in the Nankai
Trough region. To realize this goal, our strategy consists of the following three schemes: (1) understanding
the overall framework of the Nankai Trough seismogenic zone, (2) revealing the materials and mechanicaland hydrologic properties of seismogenic faults, and (3)(1)The Nankai trough (14 papers; Sugihara et al. 2014,
Yamano et al. 2014, Tsuji et al. 2014, Idehara et al.
2014, Akuhara and Mochizuki 2014, Hyodo et al.
2014, Ashi et al. 2014, Tanikawa et al. 2014, Takahashi
et al. 2014, Yamada and Shibanuma 2015, Hamada
et al. 2015, Takeshita et al. 2014, Hino et al. 2015, andToki et al. 2014)(2)The Japan trench (6 papers; Aochi and Ide 2014,
Nakamura et al. 2014, Koge et al. 2014, Lin et al.
2014, Sawai et al. 2014, and Boston et al. 2014)(3)Other trenches and fault zones (4 papers; Maekawa
et al. 2014, Namiki et al. 2014, Ishikawa et al. 2014,
and Yabe et al. 2014) and ancient accretionary
complexes and faults on land (7 papers; Schumann
et al. 2014, Hamahashi et al. 2015, Fukuchi et al. 2014,* Correspondence: gaku@eps.s.u-tokyo.ac.jp
1
Department of Earth and Planetary Science, University of Tokyo, 7-3-1
Hongo, Bunkyo-ku 113-0033, Tokyo, Japan
Full list of author information is available at the end of the article© 2015 Kimura et al. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made.

1

New Perspective of Subduction Zone Earthquake
Kimura et al. Earth, Planets and Space (2015) 67:123

Page 2 of 3

Kameda et al. 2014, Yamaguchi et al. 2014, Kogure
et al. 2014, and Hashimoto and Yamano 2014)(4)Theoretical treatments of fracture and earthquake(2 papers; Kame et al. 2014 and Nishiyama et al. 2014)

Chiba 277-8564, Japan. 3Kochi institute for Core Sample Research, Japan
Agency for Marine-Earth Science and Technology, 200 Monobe Otsu,
Nankoku City 783-8502, Kochi, Japan. 4Earth and Planetary Sciences
Department, McGill University, Montreal, QC H3A 0E8, Canada. 5US
Geological Survey, 345 Middlefield Road MS910, Menlo Park, CA 94025, USA.
Received: 19 April 2015 Accepted: 19 April 2015

The papers are also categorized into three scientificissues, focusing on different regions within each topic:

References
Akuhara T, Mochizuki K (2014) Application of cluster analysis based on waveform
cross-correlation coefficients to data recorded by ocean-bottom seismometers:
results from off the Kii Peninsula. Earth, Planets and Space 66:80
Aochi H, Ide S (2014) Ground motions characterized by a multi-scale
heterogeneous earthquake model. Earth, Planets and Space 66:42
Ashi J, Sawada R, Omura A, Ikehara K (2014) Accumulation of an earthquake-induced
extremely turbid layer in a terminal basin of the Nankai accretionary prism.
Earth, Planets and Space 66:51
Boston B, Moore GF, Nakamura Y, Kodaira S (2014) Outer-rise normal fault
development and influence on near-trench décollement propagation
along the Japan Trench, off Tohoku. Earth, Planets and Space 66:135
Fukuchi R, Fujimoto K, Kameda J, Hamahashi M, Yamaguchi A, Kimura G, Hamada
Y, Hashimoto Y, Kitamura Y, Saito S (2014) Changes in illite crystallinity within
an ancient tectonic boundary thrust caused by thermal, mechanical, andhydrothermal effects: an example from the Nobeoka Thrust, southwest
Japan. Earth, Planets and Space 66:116
Hamada Y, Sakaguchi A, Tanikawa W, Yamaguchi A, Kameda J, Kimura G (2015)
Estimation of slip rate and fault displacement during shallow earthquake
rupture in the Nankai subduction zone. Earth, Planets and Space 67:39
Hamahashi M, Hamada Y, Yamaguchi A, Kimura G, Fukuchi R, Saito S, Kameda J,
Kitamura Y, Fujimoto K, Hashimoto Y (2015) Multiple damage zone structureof an exhumed seismogenic megasplay fault in a subduction zone—a study
from the Nobeoka Thrust Drilling Project. Earth, Planets and Space 67:30
Hashimoto Y, Yamano N (2014) Geological evidence for shallow ductile-brittle
transition zones along subduction interfaces: example from the Shimanto
Belt, SW Japan. Earth, Planets and Space 66:141
Hino R, Tsuji T, Bangs NL, Sanada Y, Park J-O, Rv H, Moore GF, Araki E, Kinoshita
M (2015) Q P structure of the accretionary wedge in the Kumano Basin,Nankai Trough, Japan, revealed by long-offset walk-away VSP. Earth, Planets
and Space 67:7
Hyodo M, Hori T, Ando K, Baba T (2014) The possibility of deeper or shallower
extent of the source area of Nankai Trough earthquakes based on the 1707
Hoei tsunami heights along the Pacific and Seto Inland Sea coasts, southwest
Japan. Earth, Planets and Space 66:123
Idehara K, Yabe S, Ide S (2014) Regional and global variations in the temporalclustering of tectonic tremor activity. Earth, Planets and Space 66:66
Ishikawa T, Hirono T, Matsuta N, Kawamoto K, Fujimoto K, Kameda J, Nishio Y,
Maekawa Y, Honda G (2014) Geochemical and mineralogical characteristics
of fault gouge in the Median Tectonic Line, Japan: evidence for earthquake
slip. Earth, Planets and Space 66:36
Kame N, Nagata K, Nakatani M, Kusakabe T (2014) Feasibility of acoustic
monitoring of strength drop precursory to earthquake occurrence. Earth,Planets and Space 66:41
Kameda J, Kawabata K, Hamada Y, Yamaguchi A, Kimura G (2014) Quartz deposition
and its influence on the deformation process of megathrusts in subduction
zones. Earth, Planets and Space 66:13
Koge H, Fujiwara T, Kodaira S, Sasaki T, Kameda J, Kitamura Y, Hamahashi M, Fukuchi
R, Yamaguchi A, Hamada Y, Ashi J, Kimura G (2014) Friction properties of theplate boundary megathrust beneath the frontal wedge near the Japan Trench:
an inference from topographic variation. Earth, Planets and Space 66:153
Kogure T, Raimbourg H, Kumamoto A, Fujii E, Ikuhara Y (2014) Subgrainboundary analyses in deformed orthopyroxene by TEM/STEM with EBSD-FIB
sample preparation technique. Earth, Planets and Space 66:84
Lin W, Fulton PM, Harris RN, Tadai O, Matsubayashi O, Tanikawa W, Kinoshita M(2014) Thermal conductivities, thermal diffusivities, and volumetric heat
capacities of core samples obtained from the Japan Trench Fast Drilling
Project (JFAST). Earth, Planets and Space 66:48
Maekawa Y, Hirono T, Yabuta H, Mukoyoshi H, Kitamura M, Ikehara M, Tanikawa
W, Ishikawa T (2014) Estimation of slip parameters associated with frictional
heating during the 1999 Taiwan Chi-Chi earthquake by vitrinite reflectance
geothermometry. Earth, Planets and Space 66:28(1)Framework and setting of the seismogenic zone
from the view point of seismicity in the Nankai
trough (Idehara et al. 2014, Akuhara andMochizuki 2014), Japan Trench (Aochi and Ide2014), Hikurangi margin (Yabe et al. 2014), or fromthe geological and geophysical points of view for
the Nankai Trough (Tsuji et al. 2014, Ashi et al.
2014, Sugihara et al. 2014, Yamano et al. 2014) andJapan Trench (Boston et al. 2014, Koge et al. 2014,
Nakamura et al. 2014, Lin et al. 2014)(2)Material and physical properties or conditions of thefault for the Nankai Trough (Takahashi et al. 2014,
Tanikawa et al. 2014, Hamada et al. 2015), the Japan
Trench (Sawai et al. 2014), and the Costa Rica
margin (Namiki et al. 2014)(3)Analysis of water–rock interaction with faulting inan ancient accretion complex and fault in the
Shimanto Belt, Japan (Schumann et al. 2014,
Hamahashi et al. 2015, Fukuchi et al. 2014, Kameda
et al. 2014), Chelunpu Fault, Taiwan (Maekawa et al.
2014), Alaska (Yamaguchi et al. 2014), Median
Tectonic Line, Japan (Ishikawa et al. 2014), crustal
rocks on land (Kogure et al. 2014), and theoreticaltreatment of fracturing, friction, and earthquakes(Nishiyama et al. 2014, Kame et al. 2014)
Traditional research on earthquakes is physics-based,
but physico-chemical processes and their geological record in fault rocks, both in active and inactive fossilized
rocks, are of recent concern. Many studies of natural
records of seismic faulting have been conducted in termsof their physico-chemical processes. The Japanese islands
are composed mainly of ancient and modern accretionary
complexes, some of which were exhumed from the seismogenic depth of the plate boundary megathrust and yield
the best analog of seismogenic fault rocks in the subduction zone. This special issue will present such examples.
Authors’ contributions
GK wrote the draft of the paper and others confirmed the contents. All
authors read and approved the final manuscript.
Acknowledgements
We are grateful to MEXT for their financial support of this project 21107001.
Author details
1
Department of Earth and Planetary Science, University of Tokyo, 7-3-1
Hongo, Bunkyo-ku 113-0033, Tokyo, Japan. 2Atmosphere and Ocean
Research Institute, the University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa,

2

Earth, Planets and Space
Kimura et al. Earth, Planets and Space (2015) 67:123

Page 3 of 3

Nakamura Y, Kodaira S, Cook BJ, Jeppson T, Kasaya T, Yamamoto Y, Hashimoto Y,
Yamaguchi M, Obana K, Fujie G (2014) Seismic imaging and velocity
structure around the JFAST drill site in the Japan Trench: low Vp, high Vp/Vs
in the transparent frontal prism. Earth, Planets and Space 66:121
Namiki Y, Tsutsumi A, Ujiie K, Kameda J (2014) Frictional properties of sediments
entering the Costa Rica subduction zone offshore the Osa Peninsula: implications
for fault slip in shallow subduction zones. Earth, Planets and Space 66:72
Nishiyama S, Ohnishi Y, Ito H, Yano T (2014) Mechanical and hydraulic behaviorof a rock fracture under shear deformation. Earth, Planets and Space 66:108
Sawai M, Hirose T, Kameda J (2014) Frictional properties of incoming pelagic
sediments at the Japan Trench: implications for large slip at a shallow plateboundary during the 2011 Tohoku earthquake. Earth, Planets and Space 66:65
Schumann K, Behrmann JH, Stipp M, Yamamoto Y, Kitamura Y, Lempp C (2014)
Geotechnical behavior of mudstones from the Shimanto and Boso accretionary
complexes, and implications for the Nankai accretionary prism. Earth, Planets
and Space 66:129
Sugihara T, Kinoshita M, Araki E, Kimura T, Kyo M, Namba Y, Kido Y, Sanada Y, Thu
M (2014) Re-evaluation of temperature at the updip limit of locked portionof Nankai megasplay inferred from IODP Site C0002 temperature observatory.
Earth, Planets and Space 66:107
Takahashi M, Azuma S, Ito H, Kanagawa K, Inoue A (2014) Frictional properties ofthe shallow Nankai Trough accretionary sediments dependent on the content
of clay minerals. Earth, Planets and Space 66:75
Takeshita T, Yamaguchi A, Shigematsu N (2014) Stress reversal recorded in calcite
vein cuttings from the Nankai accretionary prism, southwest Japan. Earth,Planets and Space 66:144
Tanikawa W, Mukoyoshi H, Lin W, Hirose T, Tsutsumi A (2014) Pressure
dependence of fluid transport properties of shallow fault systems in the
Nankai subduction zone. Earth, Planets and Space 66:90
Toki T, Higa R, Ijiri A, Tsunogai U, Ashi J (2014) Origin and transport of pore fluids
in the Nankai accretionary prism inferred from chemical and isotopic
compositions of pore water at cold seep sites off Kumano. Earth, Planets andSpace 66:137
Tsuji T, Ashi J, Ikeda Y (2014) Strike-slip motion of a mega-splay fault system in
the Nankai oblique subduction zone. Earth, Planets and Space 66:120
Yabe S, Ide S, Yoshioka S (2014) Along-strike variations in temperature andtectonic tremor activity along the Hikurangi subduction zone, New Zealand.
Earth, Planets and Space 66:142
Yamada Y, Shibanuma J (2015) Small-scale stress fluctuations in borehole
breakouts and their implication in identifying potential active faults aroundthe seismogenic megasplay fault, Nankai Trough, SW Japan. Earth, Planets
and Space 67:17
Yamaguchi A, Ishikawa T, Kato Y, Nozaki T, Meneghini F, Rowe CD, Moore J,
Tsutsumi A, Kimura G (2014) Fluid-rock interaction recorded in black fault
rocks in the Kodiak accretionary complex, Alaska. Earth, Planets and Space 66:58
Yamano M, Kawada Y, Hamamoto H (2014) Heat flow survey in the vicinity of the
branches of the megasplay fault in the Nankai accretionary prism. Earth,Planets and Space 66:126

3

New Perspective of Subduction Zone Earthquake

FRONTIER LETTER

Open AccessRegional and global variations in the temporal clustering oftectonic tremor activity
Koki Idehara*, Suguru Yabe and Satoshi Ide
Earth, Planets and Space 2014, 66:66 doi:10.1186/1880-5981-66-66
Received: 30 December 2013, Accepted: 17 June 2014, Published: 7 July 2014

Abstract
The temporal distribution of tremor activity exhibits a highly non-Poissonian behavior, and its maximum period of nonPoissonian clustering statistically describes the recurrence interval of major tremor bursts. Here, we examine variations in thetemporal clustering properties of tremor activity by assessing their characteristic times, which are determined by the
maximum period of the non-Poissonian distribution. By applying a two-point correlation integral to some of the world’s major
tremor zones, including Shikoku, Kii-Tokai, and Kyushu in Japan; Cascadia, Jalisco, and Guerrero in Mexico; southern Chile;
Taiwan; and Manawatu in New Zealand, we reveal local spatial variations in the temporal clustering properties in each tremorzone and show global-scale variations in tremor activity. The spatial variation in local tremor activity is characterized by agradual transition in the along-dip direction and shorter-wavelength heterogeneities in the along-strike direction, possibly
associated with a spatial change in frictional conditions at the plate interface andrheological conditions in the surrounding materials. The characteristic time correlates
positively with locally measured median tremor duration, implying an inherent
Cascadia
Kii-Tokaicorrelation between the moment release rate and the recurrence interval of tremors.
60˚

30˚

Keywords: Tectonic tremor; Non-Poissonian clustering; Episodicity; Recurrence
interval

Kyushu Shikoku
TaiwanJalisco

Guerrero0˚-30˚

Manawatu
S.Chile
120˚

150˚

180˚

-150˚

-120˚

-90˚

-60˚

Figure 1*Corresponding author: Koki Idehara, koki.idhr@gmail.com

FRONTIER LETTER

Open Access

Frictional properties of sediments entering the Costa Rica subduction
zone offshore the Osa Peninsula: implications for fault slip in shallow
subduction zones
Yuka Namiki, Akito Tsutsumi*, Kohtaro Ujiie and Jun Kameda
Earth, Planets and Space 2014, 66:72 doi:10.1186/1880-5981-66-72
Received: 31 December 2013, Accepted: 4 July 2014, Published: 16 July 2014

Abstract
We examined the frictional properties of sediments on the Cocos plate offshore the Osa Peninsula, Costa Rica, and explored variationsin the intrinsic frictional properties of the sediment inputs to the Costa Rica subduction zone. Sediment samples were collected at Site
U1381A during the Integrated Ocean Drilling Program Expedition 334, and include hemipelagic clay to silty clay material (Unit I) andpelagic silicic to calcareous ooze (Unit II). The frictional properties of the samples were tested at a normal stress of 5 MPa under watersaturated conditions and with slip velocities ranging from 0.0028 to 2.8 mm/s for up to 340 mm of displacement. The experimental
results reveal that the steady-state friction coefficient values of clay to silty clay samples are as low as ~0.2, whereas those of silicic tocalcareous ooze samples are as high as 0.6 to 0.8. The clay to silty clay samples show a positive dependence of friction on velocity for all
tested slip velocities. In contrast, the silicic to calcareous ooze samples show a negative dependence of friction on velocity at velocities
of 0.0028 to 0.28 mm/s and either neutral or positive dependence at velocities higher than
0.28 mm/s. Given the low frictional coefficient values observed for the clay to silty clay
samples of Unit I, the décollement at the Costa Rica Seismogenesis Project transect offshore
the Osa Peninsula likely initiates in Unit I and is initially very weak. In addition, the velocitystrengthening behavior of the clay to silty clay suggests that faults in the very shallow
portion of the Costa Rica subduction zone are stable and thus behave as creeping
segments. In contrast, the velocity-weakening behavior of the silicic to calcareous ooze
favors unstable slip along faults. The shallow seismicity occurred at a depth as shallow as~9 km along the Costa Rica margin offshore the Osa Peninsula (Mw 6.4, June 2002),
indicating that materials characterized by velocity-weakening behavior constitute the fault
zone at the depth of the seismicity. Fault slip nucleating along a fault in Unit II would be alikely candidate for the source of the shallow earthquake event.

Keywords: Friction; Costa Rica; Earthquake; Subduction zone; a −b
*Corresponding author: Akito Tsutsumi, tsutsumi@kueps.kyoto-u.ac.jp
4

Figure 1

Earth, Planets and SpaceFRONTIER LETTER

Open Access

QP structure of the accretionary wedge in the Kumano Basin,Nankai Trough, Japan, revealed by long-offset walk-away VSP
Ryota Hino*, Takeshi Tsuji, Nathan L Bangs, Yoshinori Sanada, Jin-Oh Park, Roland von Huene, Gregory F Moore,
Eiichiro Araki and Masataka Kinoshita
Earth, Planets and Space 2015, 67:7 doi:10.1186/s40623-014-0175-x
Received: 28 January 2014, Accepted: 17 December 2014, Published: 17 January 2015

Abstract
We determined the seismic attenuation structure of the Kumano Basin, a forearc basin in the central part of the Nankai subduction
zone. Despite its importance for understanding the physical condition of the Earth’s interior and seismic wave propagation processes,
the attenuation factor Q has been poorly estimated in the crustal layers of the offshore areas of Nankai because severe attenuation
occurring in the seafloor sediments prevents the reliable estimation of Q from conventional active source seismic surveys. In thepresent study, we derive Q values from the diminishing rate of the high-frequency contents of seismic energy during propagation
through sub-seafloor layers. The records of vertical seismic profiling acquired at approximately
1,000 m below the seafloor, which have fewer effects from shallow attenuation, enabled us toaelucidate depth variation of Q of P waves (QP), the attenuation factor of P waves, down toapproximately 8 km below the seafloor. Assuming that the frequency dependence of Q is small
EurasianPlateand using a previously obtained P-wave velocity structure model for the basin, we inverted the50
0km
fall-off rate of the spectral ratios at various shot-receiver distances to obtain QP in the three
Kii Peninsula
sub-bottom layers. The QP values for the upper two layers with P-wave velocity (VP) < 2.7 km/s
1944 Tonankai
are 34 and 57. These values are almost identical to those obtained in the North Atlantic,
earthquake
Kumano basinsuggesting the broad consistency of QP within seafloor sediment. The basement layer (VP
C0009
approximately 4 km/s) has a much higher QP value of 349, which is comparable to the value
estimated for crustal layers exposed onshore. This QP value is higher than the value previously
fault-splayassumed in a simulation of strong ground motion associated with megathrust earthquakes
t
Mega
fron
tion
along the Nankai margin. We interpret that the high QP, low seismic attenuation in the
rma
Defo
Su
basement layer reflects tectonic stability of the inner wedge of the accretionary margin. Our
bd
ucti
on
dir
ugh
ecti
first estimates of QP in the present study provide a strong basis for future studies of seismiconai TroknaNstructure and strong ground motion prediction.

Site C0009
2.00Depth [m]

b

2.2

2.4

34 00'N

2.6

33 30'N

Depth [km]Philippine
Sea Plate

2.8

3.0

3.2

33 00'N

32 30'N
135 30'E

136 00'E

Keywords: Seismic attenuation; Q; Nankai Trough; VSP; IODP

136 30'E

137 00'E

137 30'E

Figure 1*Corresponding author: Ryota Hino, hino@irides.tohoku.ac.jp

FULL PAPER

Open Access

Quartz deposition and its influence on the deformation process ofmegathrusts in subduction zones
Jun Kameda*, Kuniyo Kawabata, Yohei Hamada, Asuka Yamaguchi and Gaku Kimura
Earth, Planets and Space 2014, 66:13 doi:10.1186/1880-5981-66-13
Received: 30 October 2013, Accepted: 24 March 2014, Published: 16 April 2014

Abstract
We present a quantitative examination of the liberation and subsequent deposition of silica at the subduction zone plateinterface in the Mugi mélange, an exhumed accretionary complex in the Shimanto Belt of southwest Japan. Frequency andthickness measurements indicate that mineralized veins hosted in deformed shales make up approximately 0.4% of thevolume of this exposure. In addition, whole-rock geochemical evidence suggests that the net volume of SiO2 liberated fromthe mélange at temperatures of < 200°C was as much as 35%, with up to 40% of the SiO2 loss related to the smectite-illite (S-I)
conversion reaction, and the rest attributable to the pressure solution of detrital quartz and feldspar. Kinetic modeling of the
S-I reaction indicates active liberation of SiO2 at approximately 70°C to 200°C, with peak SiO2 loss at around 100°C, althoughthese estimates should be slightly shifted toward lower temperature conditions
132
130
136
134
138
140
40
40
based on X-ray diffraction (XRD) analyses of mixed-layer S-I in the Mugi mélange. Theaonset of pressure solution was not fully constrained, but has been documented to36
occur at around 150°C in the study area. The deposition in deformed shales of quartz 36
liberated by pressure solution and the S-I reaction is probably linked to seismogenic
Kii
34
behavior along the plate interface by (1) progressively enhanced velocity-weakening 34
ku
iko
h
S
shu
Kyu
properties, which are favorable for unstable seismogenic faulting, including very-lowbhougfrequency earthquakes and (2) increasing intrinsic frictional strength, which leads to
i Tr
32
32
nka
a
N
a step-down of the plate boundary décollement into oceanic basalt.
Northern Shimanto Belt

Southern Shimanto Belt

Philippine Sea Plate

30

Keywords: Subduction zone; Diagenesis; Veining; VLFE

130

132

134

136

Figure 1*Corresponding author: Jun Kameda, kameda@mail.sci.hokudai.ac.jp
5

138

140

30

3.4

3.6

2960m

908m

3190m

1137m

New Perspective of Subduction Zone Earthquake

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Open Access

Geochemical and mineralogical characteristics of fault gouge in
the Median Tectonic Line, Japan: evidence for earthquake slip
Tsuyoshi Ishikawa*, Tetsuro Hirono, Noriko Matsuta, Kazuro Kawamoto, Koichiro Fujimoto, Jun Kameda,
Yoshiro Nishio, Yuka Maekawa and Go Honda
Earth, Planets and Space 2014, 66:36 doi:10.1186/1880-5981-66-36
Received: 13 November 2013, Accepted: 30 April 2014, Published: 20 May 2014

Abstract
We carried out geochemical and mineralogical analyses on fault-zone rocks from the Anko section of the Median Tectonic Linein Nagano Prefecture, Japan, to investigate coseismic physicochemical processes in the fault zone. The latest fault zone in the
Anko section contains cataclasite, fault breccia, and fault gouge of granitic composition, and brecciated basic schist. Protolithsof the granitic composition are from the Ryoke metamorphic belt and those of the basic schist from the Sambagawa
metamorphic belt. X-ray diffraction analyses show a selective decrease of clay minerals coupled with an increase ofamorphous phase in an intensely deformed layer of black gouge (5- to 10-cm thick). SEM observation reveals that the black
gouge is characterized by a drastic reduction of grain size and abundant ultrafine particles of submicrometer to several tens ofnanometers with well-rounded spheroidal shapes. These observations for the black gouge are indicative of strong mineral
lattice distortion and granulation associated with earthquake slip. Geochemically, theblack gouge is characterized by distinctly higher Li content and 87Sr/86Sr isotope ratio
135 ˚E(a) 130 ˚Ethan surrounding cataclasites, breccias, and gouges, which have similar major
NAM
EUR
N
element compositions. Model analysis reveals that the trace element composition of
3 6 ˚N
the black gouge is consistent with high-temperature (up to 250°C) coseismic fluidrock interactions. Thermal and kinetic constraints indicate that there have been
PAC
repeated slips on the fault at moderate depths (e.g., 600 m), although the tectonic
PHS
Ryoke Belt
process by which the fault zone has been uplifted and exposed in this area is not well
3 2 ˚NSambagawa Belt
understood.
MTL
100 km

Keywords: Active fault; Frictional heat; Fault gouge; Trace elements; Isotope ratios

Figure 1*Corresponding author: Tsuyoshi Ishikawa, t-ishik@jamstec.go.jp

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Open Access

Feasibility of acoustic monitoring of strength drop precursory toearthquake occurrence
Nobuki Kame*, Kohei Nagata, Masao Nakatani and Tetsuya Kusakabe
Earth, Planets and Space 2014, 66:41 doi:10.1186/1880-5981-66-41
Received: 3 January 2014, Accepted: 16 May 2014, Published: 28 May 2014

Abstract
Rate- and state-dependent friction law (RSF), proposed on the basis of laboratory experiments, has been extensively applied
to modeling of earthquake stick-slip cycles. A simple spring-slider model obeying RSF predicts a significant decrease of thefrictional strength Φ (the state of contact) that is localized within a few years preceding the earthquake occurrence. On theother hand, recent laboratory experiments successfully monitored the history of the strength by simultaneously measuring
the P-wave transmissivity |T| across the frictional interface using a 1-MHz transducer. This suggests a possibility of earthquake
forecast by monitoring the strength of a natural fault by acoustic methods. The present paper explores the feasibility of suchmonitoring in the field on the basis of the physics of RSF combined with the linear slip model (LSM) employed in the classicalacoustic methodology for monitoring an imperfectly welded interface. The characteristic frequency fc, around which |T| (orreflectivity |R|) has a good sensitivity to the interface strength, is shown to be
proportional to the strength and inversely proportional to the representative scale ofreal contacts. For natural faults, fc is estimated to be 1 to 100 Hz, which is practicablein the field. The changes of |T| and |R| depend on the ratio of the strength drop to theabsolute strength level, the latter of which is not constrained by RSF simulations.
Expected changes in wave amplitude in the preslip period would be several percent
for strong faults and several tens percent for weak faults, which may be detectable byacoustic methods such as seismic reflection surveys.

Keywords: Fault strength; Earthquake cycle; Rate- and state-dependent friction;
Precursor; Linear slip model; Acoustic monitoring; Reflection survey*Corresponding author: Nobuki Kame, kame@eri.u-tokyo.ac.jp
6Figure 1

Earth, Planets and Space

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Ground motions characterized by a multi-scale heterogeneous
earthquake model
Hideo Aochi* and Satoshi Ide
Earth, Planets and Space 2014, 66:42 doi:10.1186/1880-5981-66-42
Received: 22 January 2014, Accepted: 30 April 2014, Published: 29 May 2014

Abstract
We have carried out numerical simulations of seismic ground motion radiating from a mega-earthquake whose rupture
process is governed by a multi-scale heterogeneous distribution of fracture energy. The observed complexity of the Mw 9.0
2011 Tohoku-Oki earthquake can be explained by such heterogeneities with fractal patches (size and number), even without
introducing any heterogeneity in the stress state. In our model, scale dependency in fracture energy (i.e., the slip-weakening
distance Dc) on patch size is essential. Our results indicate that wave radiation is generally governed by the largest patch ateach moment and that the contribution from small patches is minor. We then conducted parametric studies on the frictionalparameters of peak (τp) and residual (τr) friction to produce the case where the effect of the small patches is evident during the
progress of the main rupture. We found that heterogeneity in τr has a greater influence on the ground motions than does
heterogeneity in τp. As such, local heterogeneity in the static stress drop (Δτ) influences the rupture process more than that in
the stress excess (Δτexcess). The effect of small patches is particularly evident when these are almost geometrically isolated andnot simultaneously involved in the rupture of larger patches. In other cases, the wave
radiation from small patches is probably hidden by the major contributions fromlarge patches. Small patches may play a role in strong motion generation areas withlow τr (high Δτ), particularly during slow average rupture propagation. This effect canbe identified from the differences in the spatial distributions of peak ground
velocities for different frequency ranges.

Figure 1*Corresponding author: Hideo Aochi, h.aochi@brgm.fr

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Open Access

Frictional properties of the shallow Nankai Trough accretionary
sediments dependent on the content of clay minerals
Miki Takahashi, Shuhei Azuma, Hidenori Ito, Kyuichi Kanagawa* and Atsuyuki Inoue
Earth, Planets and Space 2014, 66:75 doi:10.1186/1880-5981-66-75
Received: 2 December 2013, Accepted: 30 June 2014, Published: 23 July 2014

Abstract
We conducted triaxial friction experiments on the shallow Nankai Trough accretionary sediments at confining pressures, porewater pressures, temperatures close to their in situ conditions, and axial displacement rates (Vaxial) changed stepwise among
0.1, 1, and 10 µm/s. The results revealed that their frictional properties change systematically according to the content of clay
minerals, smectite in particular. The steady-state friction coefficient (μss) at Vaxial = 1 µm/s decreases with increasing clay mineralcontent, shown in weight percent, from 0.82 for a sandstone sample (6 wt%), through 0.71 for a tuff sample (≈17 wt%), and0.53 to 0.56 for siltstone samples (29 to 34 wt%), to 0.25 for a claystone sample (42 wt%). Slip-dependent frictional behavior
changes accordingly from slip hardening for the sandstone sample, through quasi steady-state slip for the tuff and siltstone
samples, to distinct slip weakening for the claystone sample. Although all samples(a)exhibit velocity-strengthening behavior upon stepwise changes in sliding velocity,
the ratio of the (a − b) value to the velocity dependence of steady-state friction(∆μss/∆lnVsliding) decreases with increasing clay mineral content, which implies that thefriction component decreases while the flow component increases accordingly. Thus,
faulting in the shallow Nankai Trough accretionary prism is likely controlled by theclay mineral content, in particular the smectite content, in the sediments as well as in
the fault zones.
0

35°N

N

50 km

NAP

100 kmEurasian Plate

Kii Peninsula

PP

PSP

00

10

-

34°

00

20

2000

1000

in

as

oB

an

X Kum

2000

3000

00

40

3000

C0009

C0002

-

C0004

2000

4000

00

30

Keywords: Accretionary sediments; Nankai Trough; Friction; Clay mineral content

C0007
Philippine Sea plate

ugh

ro
kai T

Nan

4000

4.1–6.5 cm/y

136°

137°

X (b)
2

7

C0002

C0009
Kumano Basin sediments

138°

Figure 1
C0004

0

139°E

5 km
C0007

Depth (kmbsl)*Corresponding author: Kyuichi Kanagawa, kyu_kanagawa@faculty.chiba-u.jp

33°

4000

Accretionary prism

5

Décollement
sediments
Subducting

Older accretionary prism

splay

Mega

10

Top of

fault
eanic

the occrust

Shikoku Basin
sediments

New Perspective of Subduction Zone Earthquake

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Application of cluster analysis based on waveform cross-correlation
coefficients to data recorded by ocean-bottom seismometers: resultsfrom off the Kii Peninsula
Takeshi Akuhara* and Kimihiro Mochizuki
Earth, Planets and Space 2014, 66:80 doi:10.1186/1880-5981-66-80
Received: 7 March 2014, Accepted: 18 July 2014, Published: 30 July 2014

Abstract
Waveform cross-correlation coefficients (CCs) have often been used to investigate clustering of earthquakes. Such techniques,
however, have rarely been applied to waveform data recorded by ocean-bottom seismometers (OBSs). The data recorded bysome OBSs are strongly influenced by site effects such as the existence of unconsolidated sediment layers. The resulting
waveforms tend to have a monotone frequency so that the calculated CCs stay artificially high, and false delay times are
obtained at certain multiples of the period. This effect also varies from place to place. To overcome such problems, CC
measurements were first performed using multiple time windows with different(a)lengths in order to reject unstable results. Station-specific CC thresholds were thenobjectively determined based on the CC distributions. This method was applied toseismic measurements obtained by OBSs deployed off the Kii Peninsula from 2003 toKii Peninsula
2007 in order to identify characteristic hypocenter patterns. Two types of earthquake 34˚N Shikoku Island
LS23
clusters were found. The first occurred within the oceanic mantle, and the
LS21
LS06
component events were linearly aligned along the NNE direction. Such earthquakes
LS03
LS17 LS22
LS02 LS05 LS09
LS20
are considered to occur on preexisting faults due to dehydration of the serpentinized
LS11 LS13 LS16
LS08
LS01 LS04
LS10
LS12 LS15 LS19
mantle. The second type of cluster consisted of interplate earthquakes that occurred
33˚N
LS07
LS36
LS32
LS14 LS18
at the southern tip of the Kii Peninsula. These are thought to be the result of stress
LS35
LS33
LS37 LS38
LS34
caused by local structural heterogeneity of the dense rock body.
LS39 LS40
PA

Keywords: Cross-correlation coefficients; Ocean-bottom seismometers;

PH

134˚E

Subduction zone; Earthquake cluster

Nankai Trough

135˚E

136˚E

Figure 1*Corresponding author: Takeshi Akuhara, akuhara@eri.u-tokyo.ac.jp

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Open Access

Subgrain boundary analyses in deformed orthopyroxene by
TEM/STEM with EBSD-FIB sample preparation technique
Toshihiro Kogure*, Hugues Raimbourg, Akihito Kumamoto, Eiko Fujii and Yuichi Ikuhara
Earth, Planets and Space 2014, 66:84 doi:10.1186/1880-5981-66-84
Received: 10 January 2014, Accepted: 23 July 2014, Published: 4 August 2014

Abstract
High-resolution structure analyses using electron beam techniques have been performed for the investigation of subgrainboundaries (SGBs) in deformed orthopyroxene (Opx) in mylonite from Hidaka Metamorphic Belt, Hokkaido, Japan, to understandductile deformation mechanism of silicate minerals in shear zones. Scanning electron microscopy (SEM) and electron backscatter
diffraction (EBSD) analysis of Opx porphyroclasts in the mylonitic rock indicated that the crystal orientation inside the Opx crystals
gradually changes by rotation about the b-axis by SGBs and crystal folding. In order to observe the SGBs along the b-axis bytransmission electron microscopy (TEM) or scanning TEM (STEM), the following sample preparation protocol was adopted. First,
petrographic thin sections were slightly etched with hydrofluoric acid to identify SGBs in SEM. The Opx crystals whose b-axes wereoriented close to the normal of the surface were identified by EBSD, and the areas containing SGBs were picked and thinned for (S)TEM analysis with a focused ion beam instrument with micro-sampling system. Highresolution TEM imaging of the SGBs in Opx revealed various boundary structures from aperiodic array of dissociated (100) [001] edge dislocations to partially or completely
incoherent crystals, depending on the misorientation angle. Atomic-resolution STEM
imaging clearly confirmed the formation of clinopyroxene (Cpx) structure between the
dissociated partial dislocations. Moreover, X-ray microanalysis in STEM revealed that theCpx contains a considerable amount of calcium replacing iron. Such chemical
inhomogeneity may limit glide motion of the dislocation and eventually the plastic
deformation of the Opx porphyroclasts at a low temperature. Chemical profiles across thehigh-angle incoherent SGB also showed an enrichment of the latter in calcium at theboundary, suggesting that SGBs are an efficient diffusion pathway of calcium out of hostOpx grain during cooling.

Keywords: Deformation; Dislocation; Pyroxene; Subgrain boundary; TEM/STEM*Corresponding author: Toshihiro Kogure, kogure@eps.s.u-tokyo.ac.jp
8

Figure 1

Earth, Planets and Space

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Open Access

Re-evaluation of temperature at the updip limit of locked portion of Nankai
megasplay inferred from IODP Site C0002 temperature observatory
Takamitsu Sugihara*, Masataka Kinoshita, Eichiro Araki, Toshinori Kimura, Masanori Kyo, Yasuhiro Namba,
Yukari Kido, Yoshinori Sanada and Moe Kyaw Thu
Earth, Planets and Space 2014, 66:107 doi:10.1186/1880-5981-66-107
Received: 7 February 2014, Accepted: 21 August 2014, Published: 4 September 2014

Abstract
In 2010, the first long-term borehole monitoring system was deployed at approximately 900 m below the sea floor (mbsf) and was assumed tobe situated above the updip limit of the seismogenic zone in the Nankai Trough off Kumano (Site C0002). Four temperature records show thatthe effect of drilling diminished in less than 2 years. Based on in situ temperatures and thermal conductivities measured on core samples, thetemperature measurements and heat flow at 900 mbsf are estimated to be 37.9°C and 56 ± 1 mW/m2, respectively. This heat flow value is inexcellent agreement with that from the shallow borehole temperature corrected for rapid sedimentation in the Kumano Basin. We use thesevalues in the present study to extrapolate the temperature below 900 mbsf for a megasplay fault at approximately 5,200 mbsf and a plateboundary fault at approximately 7,000 mbsf. To extrapolate the temperature downward, we use logging-while-drilling (LWD) bit resistivity
data as a proxy for porosity and estimate thermal conductivity from this porosity using a geometrical mean model. The one-dimensional (1-D)
thermal conduction model used for the extrapolation includes radioactive heat and frictional heat production at the plate boundary fault. The
estimated temperature at the megasplay ranges from 132°C to 149°C, depending on the assumed thermal
conductivity and radioactive heat production values. These values are significantly higher, by up to 40°C,
than some of previous two-dimensional (2-D) numerical model predictions that can account for the highheat flow seaward of the deformation front, including a hydrothermal circulation within the subductedigneous oceanic crust. However, our results are in good agreement with those of the 2-D model, which
does not include the advection cooling effect. The results imply that 2-D geometrical effects as well as theinfluence of the advective cooling may be critical and should be evaluated more quantitatively. Revision of
2-D simulation by introducing our new boundary conditions (37.9°C of in situ temperature at 900 mbsf andapproximately 56 mW/m2 heat flow) will be essential. Ultimately, in situ temperature measurements at themegasplay fault are required to understand seismogenesis in the Nankai subduction zone.

Keywords: Seismogenic zone; Scientific drilling; Nankai Trough; Thermal regime; Heat flow;
Thermal conductivity; Logging while drilling; Long-term bore hole observatory;Integrated Ocean Drilling Program (IODP)

Figure 1*Corresponding author: Takamitsu Sugihara, sugiharat@jamstec.go.jp

FULL PAPER

Open AccessMechanical and hydraulic behavior of a rock fracture under shear
deformation
Satoshi Nishiyama*, Yuzo Ohnishi, Hisao Ito and Takao Yano
Earth, Planets and Space 2014, 66:108 doi:10.1186/1880-5981-66-108
Received: 27 February 2014, Accepted: 22 August 2014, Published: 4 September 2014

Abstract

120.0 mmWith regard to crystalline rock that constitutes deep geology, attempts have been made to explore its hydraulic characteristics
by focusing on the network of numerous fractures within. As the hydraulic characteristics of a rock are the accumulation of
hydraulic characteristics of each fracture, it is necessary to develop the hydraulic model of a single fracture to predict thelarge-scale hydraulic behavior. To this end, a simultaneous permeability and shear test device is developed, and shear-flow
coupling tests are conducted on specimens having fractures with varied levels of surface roughness
in the constant normal stiffness conditions. The results show that the permeability characteristics in(a) Specimen J055(c) Specimen J122
(JRC=5.5)(JRC=12.2)
the relation between shear displacement and transmissivity change greatly at the point where thestress path reaches the Mohr-Coulomb failure curve. It is also found that there exists a range inwhich transmissivity is not proportional to the cube of mechanical aperture width, which seems tobe because of the occurrence of channeling phenomenon at small mechanical aperture widths. This
channeling flow disappears with increasing shear and is transformed into a uniform flow. We(b) Specimen J107(d) Specimen J200
develop a simulation technique to evaluate the macroscopic permeability characteristics by the(JRC=10.7)(JRC=20.0)
lattice gas cellular automaton method, considering the microstructure of fracture, namely thefracture surface roughness. With this technique, it is shown that the formation of the HagenPoiseuille flow is affected by the fracture microstructure under shear, which as a result determines
the relationship between the mechanical aperture width and transmissivity.
80.0 mm

Keywords: Shear-flow coupling test; Constant normal stiffness; Fracture surface roughness;
Mohr-Coulomb failure; Lattice gas cellular automaton; Modified cubic law;
Hagen-Poiseuille flow

*Corresponding author: Satoshi Nishiyama, nishiyama.satoshi@okayama-u.ac.jp
9

shear and permeation direction

30.00

32.50

35.00 37.50 40.00
Height (mm)Figure 1

42.50

45.00

New Perspective of Subduction Zone Earthquake

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Open Access

Changes in illite crystallinity within an ancient tectonic boundary
thrust caused by thermal, mechanical, and hydrothermal effects:
an example from the Nobeoka Thrust, southwest Japan
Rina Fukuchi*, Koichiro Fujimoto, Jun Kameda, Mari Hamahashi, Asuka Yamaguchi, Gaku Kimura, Yohei Hamada,
Yoshitaka Hashimoto, Yujin Kitamura and Saneatsu Saito
Earth, Planets and Space 2014, 66:116 doi:10.1186/1880-5981-66-116
Received: 10 March 2014, Accepted: 29 August 2014, Published: 11 September 2014

Abstract
Illite crystallinity (IC), the full width at half maximum of the illite (001) peak in clay-fraction X-ray diffraction (XRD), is a common
geothermometer widely applied to various tectonic settings. Paleotemperature estimation using IC presents methodological ambiguity
because IC is not only affected by background temperature but also by mechanical, hydrothermal, and surface weathering effects. To clarify
the influences of these effects on IC in the fault zone, we analyzed the IC and the illite 001 peak intensity of continuous borehole core samples
from the Nobeoka Thrust, a fossilized tectonic boundary thrust in the Shimanto Belt, the Cretaceous-Paleogene Shimanto accretionary
complex in southwest Japan. We also carried out grinding experiments on borehole coresamples and sericite standard samples as starting materials and investigated the effect of
mechanical comminution on the IC and illite peak intensity of the experimental products. We
observed the following: (1) the paleotemperatures of the hanging wall and footwall of theNobeoka Thrust are estimated to be 288°C to 299°C and 198°C to 249°C, respectively, which areapproximately 20°C to 30°C lower than their previously reported temperatures estimated byvitrinite reflectance; (2) the fault core of the Nobeoka Thrust does not exhibit IC decrease; (3) the
correlation of IC and illite peak intensity in the hanging wall damage zone were well reproducedby the grinding experiment, suggesting that the effect of mechanical comminution increases
toward the fault core and; (4) the abrupt increase in IC value accompanied by high illite peakintensity is explained by hydrothermal alterations including plagioclase breakdown and theformation of white micas. Our results indicate that IC has potential for quantifying the effects ofmechanical comminution and hydrothermal alteration within a fault zone.

Keywords: Illite crystallinity; Nobeoka thrust; X-ray diffraction; Comminution;
Hydrothermal alteration; Tectonic boundary thrust

Figure 1*Corresponding author: Rina Fukuchi, rfukuchi@aori.u-tokyo.ac.jp

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Open Access

Strike-slip motion of a mega-splay fault system in the Nankai obliquesubduction zone
Takeshi Tsuji*, Juichiro Ashi and Yasutaka Ikeda
Earth, Planets and Space 2014, 66:120 doi:10.1186/1880-5981-66-120
Received: 26 February 2014, Accepted: 9 September 2014, Published: 19 September 2014

Abstract
We evaluated the influence of the trench-parallel component of plate motion on the active fault system within the Nankai
accretionary wedge from reflection seismic profiles, high-resolution seafloor bathymetry, and deep-towed sub-bottom
profiles. Our study demonstrated that a large portion of the trench-parallel component of oblique plate subduction isreleased by strike-slip motion along a fault located just landward of and merging down-dip with a mega-splay fault. The
shallow portion of the splay fault system, forming a flower structure, seems to accommodate dominant strike-slip motion,
while most of the dip-slip motion could propagate to the trenchward décollement. Numerous fractures developed around thestrike-slip fault release overpressured pore fluid trapped beneath the mega-splay fault. The well-developed fractures could be
related to the change in stress orientation within the accretionary wedge. Therefore,
the strike-slip fault located at the boundary between the inner and outer wedges is a
key structure controlling the stress state (including pore pressure) within theaccretionary prism. In addition, the strike-slip motion contributes to enhancing thecontinuous mega-splay fault system (outer ridge), which extends for approximately
200 km parallel to the Nankai Trough.

Keywords: Strike-slip fault; Mega-splay fault; Nankai Trough; Oblique subduction;
Stress boundary

Figure 1*Corresponding author: Takeshi Tsuji, tsuji@i2cner.kyushu-u.ac.jp
10Earth, Planets and Space

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Open Access

Seismic imaging and velocity structure around the JFAST drill site in
the Japan Trench: low Vp, high Vp/Vs in the transparent frontal prism
Yasuyuki Nakamura*, Shuichi Kodaira, Becky J Cook, Tamara Jeppson, Takafumi Kasaya, Yojiro Yamamoto,
Yoshitaka Hashimoto, Mika Yamaguchi, Koichiro Obana and Gou Fujie
Earth, Planets and Space 2014, 66:121 doi:10.1186/1880-5981-66-121
Received: 10 March 2014, Accepted: 13 September 2014, Published: 22 September 2014

Abstract
Seismic image and velocity models were obtained from a newly conducted seismic survey around the Integrated Ocean
Drilling Program (IODP) Japan Trench Fast Drilling Project (JFAST) drill site in the Japan Trench. Pre-stack depth migration(PSDM) analysis was applied to the multichannel seismic reflection data to produce an accurate depth seismic profile together
with a P wave velocity model along a line that crosses the JFAST site location. The seismic profile images the subduction zone
at a regional scale. The frontal prism where the drill site is located corresponds to a typically seismically transparent (orchaotic) zone with several landward-dipping semi-continuous reflections. The boundary between the Cretaceous backstop
and the frontal prism is marked by a prominent landward-dipping reflection. The P wave velocity model derived from the
PSDM analysis shows low velocity in the frontal prism and velocity reversal across thebackstop interface. The PSDM velocity model around the drill site is similar to the P
wave velocity model calculated from the ocean bottom seismograph (OBS) data andagrees with the P wave velocities measured from the core experiments. The average
Vp/Vs in the hanging wall sediments around the drill site, as derived from OBS data, issignificantly larger than that obtained from core sample measurements.

Keywords: Japan Trench; Seismic image; P and S wave velocities; JFAST

Figure 1*Corresponding author: Yasuyuki Nakamura, yasu@jamstec.go.jp

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The possibility of deeper or shallower extent of the source area of
Nankai Trough earthquakes based on the 1707 Hoei tsunami heights
along the Pacific and Seto Inland Sea coasts, southwest Japan
Mamoru Hyodo*, Takane Hori, Kazuto Ando and Toshitaka Baba
Earth, Planets and Space 2014, 66:123 doi:10.1186/1880-5981-66-123
Received: 10 March 2014, Accepted: 17 September 2014, Published: 25 September 2014

Abstract
To validate the abundance of scenarios of large earthquakes in the Nankai Trough, we examined the effects of both lateral andvertical expansions of the source areas on maximum tsunami heights along the Pacific coast and Seto Inland Sea. The recently
proposed Nankai Trough earthquake scenario (Mw = 9) has a maximum slip of 20 m near the trough axis. However, thepredicted tsunami heights exceeded those obtained from historical records of damage caused by
the 1707 Hoei tsunami event at Tosa Bay and along the Pacific coastlines near the Kii Channel,
owing to the large slip on the up-dip extension of fault segments off Shikoku Island. Suchdiscrepancy indicates that for segments off Shikoku Island, the slip near the trough axis was
unremarkable, even for the 1707 Hoei earthquake event, which is considered one of the larger
historical Nankai Trough earthquake events. For segments east of the Kii Peninsula, the large slipon the up-dip end might be ineffective. While the proposed Mw9-class scenario also includes large
slip of several meters on the down-dip side (down to about 35-km depth), coseismic crustal
subsidence reached further landward than is usual for Nankai Trough earthquakes. For the Seto
Inland Sea region, this resulted in maximum subsidence of about 1 m, and such crustal subsidence
effectively increased the height of the tsunamis. Furthermore, simulated tsunami heights,
corrected for crustal subsidence, were in good agreement with those obtained from historical
records of the damage caused in the Seto Inland Sea region.

Keywords: Nankai Trough earthquake; Historical tsunami; Earthquake scenario*Corresponding author: Mamoru Hyodo, hyodo@jamstec.go.jp
11

Figure 1

New Perspective of Subduction Zone Earthquake

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Geotechnical behavior of mudstones from the Shimanto and Boso
accretionary complexes, and implications for the Nankai accretionary
prism
Kai Schumann*, Jan H Behrmann, Michael Stipp, Yuzuru Yamamoto, Yujin Kitamura and Christof Lempp
Earth, Planets and Space 2014, 66:129 doi:10.1186/1880-5981-66-129
Received: 22 November 2013, Accepted: 17 September 2014, Published: 16 October 2014

Abstract
Triaxial shear tests on mudstone samples from the Shimanto Belt and the Boso accretionary complexes (SW Japan and central Japan)
were carried out. Pre-exhumation burial depths in the two paleo-accretionary prisms were up to 9,000 m and about 1,000 m for the
Shimanto and the Boso samples, respectively. Three methods were applied: (1) pressure stepping tests at increasing confiningpressures between 25 and 65 MPa and pore pressures between 20 and 52 MPa; (2) constant confining pressure tests at 55 and 65
MPa, with stepwise pore pressure decrease from 80% to 50% and 25%, and from 90% to 60% and 30% of the confining pressure; and(3) a cyclic loading test on one sample from Boso (19 cycles to 70-MPa differential stress). After some contraction due topressurization in the first cycles, the sample showed tendencies to creep rather than to fracture.
Effective shear parameters show that angles of internal friction between 30° and 50° are in part quite high in both sample subsets,
and ranges of cohesion are between about 2 and 6 MPa (Boso) and 13 and >30 MPa
B
AC(Shimanto). The mechanical results from these paleo-accretionary prisms are taken to
constrain the shear parameters of rocks in the deeper parts of the present Nankai
accretionary wedge and forearc. Static friction resembles results from experiments on awide range of phyllosilicate-quartz-feldspar gouges and shows that the forearc is
composed of relatively strong rock. Cohesion increase due to diagenesis and/or very low
ED
grade metamorphism is of overriding importance and probably permits stresses of up to
18 MPa to be transmitted to the updip end of the seismogenic zone at depth and 5 to 13
MPa to the backstop of the actively deforming frontal prism.
Muroto Peninsula, Shimanto

33˚30'

139˚30'Ochotsk
Plate

Eurasian Plate

36˚

Boso Peninsula

Miura
PeninsulaKii

33˚24'

Philippine
Plate

kai
Nan gh
Trou

32˚

132°

Kazusa Grp
Chikura Grp
Sagami Grp

140°

136°

Hane
Sch2011/27
Sch2011/29

Miura Grp

Hota Grp
Hayama Grp

33˚18'

Cape Gyoto
Sch2011/18
hi
as
Nar
Muroto Port
inaShi t
Sch2011/8
Faul
Sch2011/12

Mineoka Grp

Nankai33˚30'

Nabae Group

Maruyama
Intrusions

0

00

0

00

NZ-PS-4

K020

−4000

Sakamoto
MélangeMisakiAssemblage

NZ-PS-5

−4000

−4000

0

−4

139˚45'36"

K022
136°30´

137°00´

139˚48'

Miura Gr (Boso Peninsula)
Kagamigaura
Formation−40

00

32˚30'
136°00´

Lanikai

34˚57'36"(Oligocene - Miocene)(Eocene - Oligocene)

33˚00'

00

Tsuro Formation

NZ-PS-1
NZ-PS-2

0

MurotoFormation

Hioki Mélange

K004
K014
K015

00

Mudstone; Subduction zone

Sakahima
Mélange

0

00

−3

−2000

K011
K013
K016

−4

Keywords: Shimanto Belt; Boso accretionary prism; Nankai; IODP; Triaxial shear test;

Murotohanto Group

Shijuji yama
Formation

Naha rigawa
Formation

−2000

−4

134˚12'−3

134˚06'

Boso Peninsula

0

00

−2

K005

33˚12'

134˚00'140˚00'

Honshu35˚00'

Misaki
Sch2011/34

Cape Hane
Sch2011/23
Sch2011/24

137°30´

Nishizaki
Formation

Figure 1*Corresponding author: Kai Schumann, kaschumann@geomar.de

FULL PAPER

Open AccessAlong-strike variations in temperature and tectonic tremor activity
along the Hikurangi subduction zone, New ZealandSuguru Yabe*, Satoshi Ide and Shoichi Yoshioka
Earth, Planets and Space 2014, 66:142 doi:10.1186/s40623-014-0142-6
Received: 24 February 2014, Accepted: 4 October 2014, Published: 18 October 2014

Abstract
In the Hikurangi subduction zone, situated along the east coast of the North Island, New Zealand, where the old oceanic Pacific Plate is
subducting beneath the Australian Plate, several slow slip events and tectonic tremors have recently been documented. These observations
are somewhat surprising because such slow seismic phenomena tend to be common in subduction zones where relatively young oceanic
plate is subducting. The locations of tectonic tremors, down-dip limit of slow slip events and seismic coupling transition change along strikefrom greater depths in the south to shallower depths in the north, suggesting significant along-strike variations in the characteristics of theplate interface. Similar along-strike variations have been observed for other characteristic features of the Hikurangi subduction zone. Here,
we demonstrate that along-strike variations observed for tectonic tremors, slow slip events, and seismic coupling can be explained by lateraldifferences in the thermal structure of the subduction zone, which are controlled mainly by
variations in convergence rate and friction along the plate interface. To demonstrate this, wefirst confirm that tectonic tremors occur around the plate interface. Then, we calculate thethermal structure of the Hikurangi subduction zone using a two-dimensional finite difference
code. To explain the along-strike variation in the heat flow observed in the forearc region,
temperatures along the plate interface should be systematically higher in the northern region
than in the southern region, which we interpret as a consequence of higher convergence rates
and greater frictional heating in the northern region. We compare the along-strike variation ofseismic characteristics with calculated thermal structure and highlight that this along-strikevariation in temperature controls the depth of the brittle-ductile transition, which is consistent
with the observed spatial variations in tectonic tremors, down-dip limit of slow slip events andseismic coupling. Our results suggest that tectonic tremors recorded within subduction zones
reflect the transient rheology of the materials being subducted, which is controlled byvariations in temperature along the plate interface.

Keywords: New Zealand; Subduction zone; Tectonic tremor; Slow earthquake;
Thermal structure

Figure 1*Corresponding author: Suguru Yabe, yabe@eps.s.u-tokyo.ac.jp
12Earth, Planets and Space

FULL PAPER

Open AccessOrigin and transport of pore fluids in the Nankai accretionary prism
inferred from chemical and isotopic compositions of pore water atcold seep sites off Kumano
Tomohiro Toki*, Ryosaku Higa, Akira Ijiri, Urumu Tsunogai and Juichiro Ashi
Earth, Planets and Space 2014, 66:137 doi:10.1186/s40623-014-0137-3
Received: 18 December 2013, Accepted: 30 September 2014, Published: 24 October 2014

Abstract
We used push corers during manned submersible dives to obtain sediment samples of up to 30 cm from the subseafloor at the
Oomine Ridge. The concentrations of B in pore water extracted from the sediment samples from cold seep sites were higher than
could be explained by organic matter decomposition, suggesting that the seepage fluid at the site was influenced by B derived fromsmectite-illite alteration, which occurs between 50°C and 160°C. Although the negative δ18OH2O and δDH2O values of the pore fluids
cannot be explained by freshwater derived from clay mineral dehydration (CMD), we
considered the contribution of pore fluids in the shallow sediments of the accretionary
prism, which showed negative δ18OH2O and δDH2O values according to the results obtained
during Integrated Ocean Drilling Program (IODP) Expeditions 315 and 316. We calculated
the mixing ratios based on a four-end-member mixing model including freshwater
derived from CMD, pore fluids in the shallow (SPF) accretionary prism sediment, seawater(SW), and freshwater derived from methane hydrate (MH) dissociation. However, theOomine seep fluids were unable to be explained without four end members, suggestingthat deep-sourced fluids in the accretionary prism influenced the seeping fluids from thisarea. This finding presents the first evidence of deep-sourced fluids at cold seep sites in
the Oomine Ridge, indicating that a megasplay fault is a potential pathway for the deepsourced fluids.

Keywords: Cold seep; Pore fluid; Nankai Trough; Accretionary prism; Kumano;

Boron; Lithium; Clay mineral dehydration; Methane hydrate dissociation

Figure 1*Corresponding author: Tomohiro Toki, toki@sci.u-ryukyu.ac.jp

FULL PAPER

Open AccessOuter-rise normal fault development and influence on near-trenchdécollement propagation along the Japan Trench, off Tohoku
Brian Boston*, Gregory F Moore, Yasuyuki Nakamura and Shuichi Kodaira
Earth, Planets and Space 2014, 66:135 doi:10.1186/1880-5981-66-135
Received: 28 February 2014, Accepted: 25 September 2014, Published: 28 October 2014

Abstract
Multichannel seismic reflection lines image the subducting Pacific Plate to approximately 75 km seaward of the Japan Trenchand document the incoming plate sediment, faults, and deformation front near the 2011 Tohoku earthquake epicenter.
Sediment thickness of the incoming plate varies from <50 to >600 m with evidence of slumping near normal faults. We findrecent sediment deposits in normal fault footwalls and topographic lows. We studied the development of two different
classes of normal faults: faults that offset the igneous basement and faults restricted to the144˚30'145˚00'sediment section. Faults that cut the basement seaward of the Japan Trench also offset theseafloor and are therefore able to be well characterized from multiple bathymetric surveys.
A
D19
Images of 199 basement-cutting faults reveal an average throw of approximately 120 m andFig. 2D17
average fault spacing of approximately 2 km. Faults within the sediment column are poorly
documented and exhibit offsets of approximately 20 m, with densely spaced populations near
D15
the trench axis. Regional seismic lines show lateral variations in location of the Japan Trench
D13
38˚30'
D11
deformation front throughout the region, documenting the incoming plate’s influence on the
S13
Fig. 3D09
deformation front’s location. Where horst blocks are carried into the trench, seaward propagation
D08
of the deformation front is diminished compared to areas where a graben has entered the trench.
38˚00'TH03
D06
We propose that the décollement’s propagation into the trench graben may be influenced by
D05
N Tn TsFz
local stress changes or displacements due to subduction of active normal faults. The location and
TH04
M38
D03
M
geometry of the up-dip décollement at the Japan Trench is potentially controlled by theW
D02
E
Fig. 9037˚30'
incoming outer-rise faults.
144˚00' 144˚03'

HD34B

37˚54'

37˚51'

4000

Keywords: Japan Trench; Subduction; Normal faults; Outer rise; Trench; Oceanic plate*Corresponding author: Brian Boston, bboston@hawaii.edu
13

HD26B

144˚00'144˚30'

1

B

HD21A

6000
Depth (m)8000 0Figure 1

10
Slope (°)

20

C.

40˚N

38˚N
0
200
400
600
Sediment Thickness (m)New Perspective of Subduction Zone Earthquake

FULL PAPER

Open AccessStress reversal recorded in calcite vein cuttings from the Nankai
accretionary prism, southwest Japan
Toru Takeshita*, Asuka Yamaguchi and Norio Shigematsu
Earth, Planets and Space 2014, 66:144 doi:10.1186/s40623-014-0144-4
Received: 7 March 2014, Accepted: 14 October 2014, Published: 29 October 2014

Abstract
The Nankai Trough subduction zone in southwest Japan is a typical convergent margin where the Philippine Sea platesubducts in the northwest direction beneath the Eurasian plate, and devastating earthquakes have repeatedly occurred in thisregion in the past. In order to investigate the evolution of the stress state in the subduction zone, we analyzed deformation
microstructures and the preferred orientation of calcite grains in two cuttings of calcite veins from Hole C0002F that was
drilled through the inner wedge of the Nankai accretionary prism during theIntegrated Ocean Discovery Program (IODP) Expedition 338 in 2012. For bothasamples collected at depths of 1,085.5 and 1,885.5 meters below the sea floor (mbsf ),the c-axes of calcite grains are preferentially oriented perpendicular to the vein wall,
which is indicative of competitive growth of calcite during the vein opening caused
by a vein normal extension. Also, mechanical e-twins were developed in both
samples, and these are inferred to have been developed under the same stress fieldas that responsible for the formation of calcite veins based on the paleostressanalyses in grains with e-twins. For the calcite vein retrieved at the depth of 1,885.5
mbsf, kink bands were also developed by the compression in the direction
perpendicular to the vein wall, which is indicative of stress reversal after theformation of mechanical e-twins. Although we could not reach a definite conclusion
for the cause of the stress reversal, it could have occurred during either folddevelopment or seismic cycles in the Nankai accretionary prism.

Keywords: IODP expedition 338; Nankai accretionary prism; Calcite vein;
Stress reversal; Paleostress analyses

Figure 1*Corresponding author: Toru Takeshita, torutake@mail.sci.hokudai.ac.jp

FULL PAPER

Open AccessMultiple damage zone structure of an exhumed seismogenic megasplay
fault in a subduction zone - a study from the Nobeoka Thrust Drilling Project
Mari Hamahashi*, Yohei Hamada, Asuka Yamaguchi, Gaku Kimura, Rina Fukuchi, Saneatsu Saito, Jun Kameda,
Yujin Kitamura, Koichiro Fujimoto and Yoshitaka Hashimoto
Earth, Planets and Space 2015, 67:30 doi:10.1186/s40623-015-0186-2
Received: 10 March 2014, Accepted: 14 January 2015, Published: 24 February 2015

Abstract
To investigate the mechanical properties and deformation patterns of megathrusts in subduction zones, we studied damage zone structures of
the Nobeoka Thrust, an exhumed megasplay fault in the Kyushu Shimanto Belt, using drill cores and geophysical logging data obtained during
the Nobeoka Thrust Drilling Project. The hanging wall, composed of a turbiditic sequence of phyllitic shales and sandstones, and the footwall,consisting of a mélange of a shale matrix with sandstone and basaltic blocks, exhibit damage zones that include multiple sets of ‘brecciatedzones’ intensively broken in the mudstone-rich intervals, sandwiched by ‘surrounding damage zones’ in the sandstone-rich intervals withcohesive faults and mineral veins. The fracture zones are thinner (2.7 to 5.5 m) in the sandstone-rich intervals and thicker in the shale-dominantintervals (2.3 to 18.6 m), which indicates a preference of coseismic slip and velocity-weakening in the former, and aseismic deformation in thelatter. However, the surrounding damage zones observed in the current study are associated with an increase in resistivity, P-wave velocity, anddensity and a decrease in porosity, inferring densification and strain-hardening in the sandstone-rich intervals and strain-weakening in the
mudstone-rich intervals. These observations indicate that the sandstone-rich damage zones may weaken in the short term but may strengthen
in the geologically long term, contributing to a later stage of fault activity. In contrast, the mudstone-rich damage zones may strengthen in the
short term but develop weak structures through longer time periods. The observed shear zone thickness inthe hanging wall is thinner (2.3 to 18.6 m) compared to the footwall damage zones (12 to 39.9 m), possibly
because faults in the hanging wall were concentrated and partitioned between the preexisting turbiditic
sequence of alternating shale/sandstone-dominant intervals, whereas in the footwall, faults were more
sporadically distributed throughout the sandstone block-in-matrix cataclasites. A splay fault may evolveand be characterized by physical property contrasts, the lithology dependence of deformation, and thevariability of damage zone thickness due to a heterogeneous lithology distribution in the hanging wall andfootwall. The deformation patterns observed in the Nobeoka Thrust provide insights to the strainhardening/weakening behaviors of sediments along megathrusts over geological timescales.

Keywords: Subduction zone; Megasplay fault; Shimanto Belt; Physical property;
Damage zone; Nobeoka Thrust

Figure 1*Corresponding author: Mari Hamahashi, m_hamahashi@eps.s.u-tokyo.ac.jp
14Earth, Planets and Space

FULL PAPER

Open Access

Estimation of slip rate and fault displacement during shallow
earthquake rupture in the Nankai subduction zone
Yohei Hamada*, Arito Sakaguchi, Wataru Tanikawa, Asuka Yamaguchi, Jun Kameda and Gaku Kimura
Earth, Planets and Space 2015, 67:39 doi:10.1186/s40623-015-0208-0
Received: 27 February 2014, Accepted: 25 February 2015, Published: 13 March 2015

Abstract
Enormous earthquakes repeatedly occur in subduction zones, and the slips along megathrusts, in particular those
propagating to the toe of the forearc wedge, generate ruinous tsunamis. Quantitative evaluation of slip parameters (i.e., slip
velocity, rise time and slip distance) of past slip events at shallow, tsunamigenic part of the fault is critical to characterize such
earthquakes. Here, we attempt to quantify these parameters of slips that may have occurred along the shallow megasplay
fault and the plate boundary décollement in the Nankai Trough, off southwest Japan. We apply a kinetic modeling to vitrinitereflectance profiles on the two fault rock samples obtained from Integrated Ocean Drilling Program (IODP). This approach
constitutes two calculation procedures: heat generation and numerical profile fitting of vitrinite reflectance data. For the
purpose of obtaining optimal slip parameters, residue calculation is implemented to estimate fitting accuracy. As the result,the measured distribution of vitrinite reflectance is reasonably fitted with heat generation rate (Q̇) and slip duration (tr) of
16,600 J/s/m2 and 6,250 s, respectively, for the megasplay and 23,200 J/s/m2 anda2,350 s, respectively, for the frontal décollement, implying slow and long-term slips.
Tokai
Kii Peninsula
The estimated slip parameters are then compared with previous reports. Theeakmaximum temperature, Tmax, for the Nankai megasplay fault is consistent with thequtho f ara iEe
34 Ntemperature constraint suggested by a previous work. Slow slip velocity, long-terma r ka
e an
ur n
pt To
rise time, and large displacement are recognized in these fault zones (both of theRu 944 b
NAP
45 N
1
EP
megasplay, the frontal décollement). These parameters are longer and slower than
40 N
C0004
typical coseismic slip, but are rather consistent with rapid afterslip.
o

o

o

33oN

i
nka
Na ughTro

Keywords: Nankai Trough; Shallow part of megathrust; Vitrinite maturation;
Fault slip parameters

PP

C0007
135oE

PSP

145oE

Figure 1*Corresponding author: Yohei Hamada, yhamada@jamstec.go.jp

LETTER

Open Access

Estimation of slip parameters associated with frictional heating
during the 1999 Taiwan Chi-Chi earthquake by vitrinite reflectance
geothermometry
Yuka Maekawa, Tetsuro Hirono*, Hikaru Yabuta, Hideki Mukoyoshi, Manami Kitamura, Minoru Ikehara,
Wataru Tanikawa and Tsuyoshi Ishikawa
Earth, Planets and Space 2014, 66:28 doi:10.1186/1880-5981-66-28
Received: 19 November 2013, Accepted: 17 April 2014, Published: 1 May 2014

Abstract
To estimate the slip parameters and understand the fault lubrication mechanism during the 1999 Taiwan Chi-Chi earthquake,
we applied vitrinite reflectance geothermometry to samples retrieved from the Chelungpu fault. We found a marked
reflectance anomaly of 1.30% ± 0.21% in the primary slip zone of the earthquake, whereas the
TCDP
reflectances in the surrounding deformed and host rocks were 0.45% to 0.77%. By applying akinetic model of vitrinite thermal maturation together with a one-dimensional heat and thermal
diffusion equation, we determined the shear stress and peak temperature in the slip zone during
the earthquake to be 1.00 ± 0.04 MPa and 626°C ± 25°C, respectively. Taking into account theprobable overestimation of the temperature owing to a mechanochemically enhanced reaction
or flash heating at grain contacts, this temperature should be considered an upper limit. The
lower limit was previously constrained to 400°C by studies of fluid-mobile trace-element
concentrations and magnetic minerals. Therefore, we inferred that the peak temperature during
the Chi-Chi earthquake was 400°C to 626°C, corresponding to an apparent friction coefficient of
0.01 to 0.06. Such low friction and the previous evidence of a high-temperature fluid suggest that
thermal pressurization likely contributed to dynamic weakening during the Chi-Chi earthquake.
Depth
(m)

1000

1100

Changhua fault

20 km

1136

lt

gF
au

23°40’

Keywords: Carbonaceous material; Thermal maturation; Fault friction; Chelungpu fault*Corresponding author: Tetsuro Hirono, hirono@ess.sci.osaka-u.ac.jp
15

ua

ng

FZ

lung

yi F

ault

FZ

0

Che

San

1243
1250

Tohkoshan Fm.

Alluvium

Drill hole
(Hole B)

Cholan Fm.

1194
1200

120°50’Sh

epicenter

Toukoshan Fm.

FZ

1150

tun

Alluvium
Terrace Deposits
Toukoshan Fm.
Cholan Fm.
Chinshui Shale
Miocene Rocks
Oligocene Rocks

Chinshui Shale

1050

Shu

Chelung

120°10’icha

pu Fault

22°N

ngliu

Fau

lt

24°20’

Cholan Fm.

Hole B

122°E 120°30’

Cholan Fm.
ault

pu F

Chinshui Shale Keuichulin Fm. Changhua Fault

Figure 1

1 km

1
2

3
(km)

1300

Keuichulin Fm.

120°E

25°N

New Perspective of Subduction Zone Earthquake

LETTER

Open Access

Thermal conductivities, thermal diffusivities, and volumetric heat
capacities of core samples obtained from the Japan Trench Fast
Drilling Project (JFAST)
Weiren Lin*, Patrick M Fulton, Robert N Harris, Osamu Tadai, Osamu Matsubayashi, Wataru Tanikawa
and Masataka Kinoshita
Earth, Planets and Space 2014, 66:48 doi:10.1186/1880-5981-66-48
Received: 26 February 2014, Accepted: 20 May 2014, Published: 5 June 2014

Abstract
We report thermal conductivities, thermal diffusivities, and volumetric heat capacities determined by a transient plane heatsource method for four whole-round core samples obtained by the Japan Trench Fast Drilling Project/Integrated Ocean
Drilling Program Expedition 343. These thermal properties are necessary for theinterpretation of a temperature anomaly detected in the vicinity of the plateboundary fault that ruptured during the 2011 Tohoku-Oki earthquake and otherthermal processes observed within the Japan Trench Fast Drilling Project temperatureobservatory. Results of measured thermal conductivities are consistent with thoseindependently measured using a transient line source method and a divided bar
technique. Our measurements indicate no significant anisotropy in either thermal
conductivity or thermal diffusivity.

Keywords: Thermal conductivity; Thermal diffusivity; Volumetric heat capacity;
Anisotropy; Core sample measurement; JFAST

Figure 1*Corresponding author: Weiren Lin, lin@jamstec.go.jp

LETTER

Open AccessAccumulation of an earthquake-induced extremely turbid layer ina terminal basin of the Nankai accretionary prism
Juichiro Ashi*, Ritsuko Sawada, Akiko Omura and Ken Ikehara
Earth, Planets and Space 2014, 66:51 doi:10.1186/1880-5981-66-51
Received: 21 February 2014, Accepted: 30 May 2014, Published: 12 June 2014

Abstract
Seismic shaking is a major trigger for sediment redistribution in subduction zones, and clouds of dilute suspended sediment
have been reported in association with large earthquakes. Dive observations in a basin on the slope of the central Nankai
accretionary prism soon after the 2004 off-Kii Peninsula earthquakes documented a layer of suspended sediment withextremely high turbidity. We estimated the thickness of this bottom turbid layer to be more than 2.5 m by comparison ofseafloor depths between surveys in 2004 and 2010 and about 2.6 m from instrumental evidence involving the submersion ofa heat-flow probe. A high-resolution subbottom profiling survey across the basin revealed acoustically transparent layers
thicker than 2 m. Because the slope basin is a terminal basin completely enclosed by topographic highs, we examined thepossibility that the uppermost transparent layer was deposited as a consequence of the 2004 earthquakes. Considering thesediment source area and the volume of the basin fills, the mobilization and
no n
ma
i
redeposition of thin surface sediments on the prism slope can account for the
Ku c basarorefvolume of the transparent layer. We conclude that the 2004 earthquakes caused
widespread disturbance of the prism slope and concentrated surface sediments inthis terminal basin. Our results emphasize the utility of a terminal basin in a
subduction zone as an earthquake recorder.
Sept 7, 2004
0D#40,41,42,
87,95,124

0

00

−2

00

−20

Philippine Sea
Plate

−2400

33˚20'N

−2800

−3

D#33

D#35

D#43

−200

0

−4

Site C0021
Site C0018

−3200

D#36

D#29

−3600

−3600

0

−320

0

20

−3
0

60

−3

136˚30'E00

0−4

136˚40'E

Sept 5, 2004
Mj 7.1
136˚50'EkaiNan

137˚00'E

Figure 1*Corresponding author: Juichiro Ashi, ashi@aori.u-tokyo.ac.jp
16

Mj 6.5

00
0

0

D#34

0

60

−3

20

D#30,31,32

Keywords: Suspension cloud; Terminal basin; Slump; Sliding; Gravity flow

33˚30'N

−32

N

−2
00

Site NG2e

PA

Pacific Plat

JA

00

Sea of Japan

Sept 5, 2004
Mj 7.433˚10'N

gh Sept 8, 2004 isu
Mj 6.5
n e

u
Tro

10 km
137˚10'E

Ze idgR33˚00'N−3600

137˚20'E

Earth, Planets and SpaceLETTER

Open Access

Fluid-rock interaction recorded in black fault rocks in the Kodiak
accretionary complex, Alaska
Asuka Yamaguchi*, Tsuyoshi Ishikawa, Yasuhiro Kato, Tatsuo Nozaki, Francesca Meneghini, Christie D Rowe,James Casey Moore, Akito Tsutsumi and Gaku Kimura
Earth, Planets and Space 2014, 66:58 doi:10.1186/1880-5981-66-58
Received: 28 February 2014, Accepted: 4 June 2014, Published: 19 June 2014

Abstract
Ultrafine-grained black fault rocks (BFRs) in the Pasagshak Point Thrust of the Kodiak accretionary complex are examples offault rocks that have recorded seismicity along an ancient subduction plate boundary. Trace element concentrations and87
Sr/86Sr ratios of BFRs and surrounding foliated/non-foliated cataclasites were measured to explore the nature of fluid-rockinteractions along a subduction thrust. Foliated and non-foliated cataclasites do not show significant geochemical anomalies,
suggesting that they were formed by slowly distributed shear. BFRs are characterized by Li and Sr enrichment, Rb and Cs
depletion, and a low 87Sr/86Sr ratio. These geochemical signatures can be explained by fluid-rock interactions at >350°C, which
result in preferential removal of Rb and Cs and formation of plagioclase under the presence of fluids with high Li and Srconcentrations and low 87Sr/86Sr ratios. Geochemical anomalies recorded by the BFRsaindicate both frictional heating and external fluid influx into the subduction thrust.
154°W

152°W

N

58°N

50 km
70

la
n

44

Greenstones
Dip & strike offoliation

h
tT

ia
k

Is

b

25

Eocene marine sediments

57°N

st

ru

in

ak

Po

sh

g
sa

Pa

Ghost Rocks Formation

N

Paleocene intrusion

WPT009

in

t

40

Po
ha
52
42

82

70
50

gs
85

Pa

70

500 m

k

64
58

sa

85
70

b

75

75

65
40

Ko
d

Fluid-rock interactiondKeywords: Subduction zone; Seismogenic zone; Pseudotachylyte;

Ultrafine-grained
black fault rock (BFR)
Cataclasites

58

WPT014
WPT015

Pacific Ocean

Kodiak Islanda
Aleutian Trench

Figure 1*Corresponding author: Asuka Yamaguchi, asuka@aori.u-tokyo.ac.jp

LETTER

Open Access

Frictional properties of incoming pelagic sediments at the Japan
Trench: implications for large slip at a shallow plate boundary during
the 2011 Tohoku earthquake
Michiyo Sawai*, Takehiro Hirose and Jun Kameda
Earth, Planets and Space 2014, 66:65 doi:10.1186/1880-5981-66-65
Received: 30 December 2013, Accepted: 21 June 2014, Published: 7 July 2014

Abstract
The 2011 Tohoku earthquake (Mw 9.0) produced a very large slip on the shallow part of a megathrust fault that resulted in destructive
tsunamis. Although multiple causes of such large slip at shallow depths are to be expected, the frictional property of sediments
around the fault, particularly at coseismic slip velocities, may significantly contribute tolarge slip along such faults. We have thus investigated the frictional properties ofincoming pelagic sediments that will subduct along the plate boundary fault at the
Tohoku subduction zone, in order to understand the rupture processes that can cause
large slip in the shallow parts of subduction zones. Our experimental results on clayey
sediment at the base of the sedimentary section on the Pacific Plate yield a low friction
coefficient of <0.2 over a wide range of slip velocities (0.25 mm/s to 1.3 m/s), andextremely low fracture energy during slip weakening, as compared with previous
experiments of disaggregated sediments under coseismic slip conditions. Integrated
Ocean Drilling Program (IODP) Expedition 343 confirmed that the clay-rich sediment
investigated here is identical to those in the plate boundary fault zone, which ruptured
and generated the Tohoku earthquake. The present results suggest that smectite-richpelagic sediment not only accommodates cumulative plate motion over interseismic
periods but also energetically facilitates the propagation of earthquake rupture towards
the shallow part of the Tohoku subduction zone.

Keywords: 2011 Tohoku earthquake; Friction; Fracture energy; Smectite;
DSDP Leg 56; IODP Expedition 343

Figure 1*Corresponding author: Michiyo Sawai, michiyosawai@hiroshima-u.ac.jp
17New Perspective of Subduction Zone Earthquake

LETTER

Open Access

Pressure dependence of fluid transport properties of shallow fault
systems in the Nankai subduction zone
Wataru Tanikawa*, Hideki Mukoyoshi, Weiren Lin, Takehiro Hirose and Akito Tsutsumi
Earth, Planets and Space 2014, 66:90 doi:10.1186/1880-5981-66-90
Received: 25 February 2014, Accepted: 6 August 2014, Published: 13 August 2014

Abstract
We measured fluid transport properties at an effective pressure of 40 MPa in core samples of sediments and fault rocks
collected by the Integrated Ocean Drilling Program (IODP) NanTroSEIZE drilling project Expedition 316 from the megasplay
fault system (site C0004) and the frontal thrust (site C0007) in the Nankai subduction zone. Permeability decreased witheffective pressure as a power law function. Permeability values in the fault zones were 8 × 10−18 m2 at site C0004 and 9 × 10−18
m2 at site C0007. Stratigraphic variation in transport properties suggests that the megasplay fault zone may act as a barrier to
fluid flow, but the frontal thrust fault zone might not. Depth variation in permeability at site C0007 is probably controlled by
the mechanical compaction of sediment. Hydraulic diffusivity at shallow depths was approximately 1 × 10−6 m2 s−1 in both fault
zones, which is small enough to lead to pore pressure generation that can cause dynamic fault weakening. However, absenceof a very low permeable zone, which may have formed in the Japan Trench subduction zone, might prevent facilitation ofhuge shallow slips during Nankai subduction zone earthquakes. Porosity tests under dry conditions might have overestimatedthe porosity.

Keywords: Permeability; Porosity; Integrated Ocean Drilling Program; Expedition 316 (NanTroSEIZE); Hydraulic diffusivity

Figure 1*Corresponding author: Wataru Tanikawa, tanikawa@jamstec.go.jp

LETTER

Open AccessHeat flow survey in the vicinity of the branches of the megasplay fault
in the Nankai accretionary prism
Makoto Yamano*, Yoshifumi Kawada and Hideki Hamamoto
Earth, Planets and Space 2014, 66:126 doi:10.1186/1880-5981-66-126
Received: 6 March 2014, Accepted: 11 September 2014, Published: 2 October 2014

Abstract
Heat flow measurements were conducted at four sites in the Nankai accretionary prism southeast of the Kii Peninsula, aroundthe area where the megasplay fault reaches the surface, in conjunction with long-term monitoring of bottom watertemperature at nearby stations. Analysis of the obtained data showed that variations in bottom water temperature seriously
affect surface heat flow measurements in the areas with water depths of less than about 2,800 m. This effect can reach up to
20% to 30% and may have significantly contributed to a large scatter in the heat flow
values previously measured in the study area. The temperature records were also used to (a)determine heat flows from sediment temperature profiles disturbed by bottom waterKii
Peninsulatemperature variations. Results of measurements at sites deeper than 2,800 m indicate
34°N
that the regional heat flow, corrected for surface disturbances including the influence ofbathymetric relief, is about 65 mW/m2, which is consistent with the value calculated using
subduction thermal models. Local high heat flow values were obtained in the vicinity ofthe tips of the branches of the splay fault, suggesting advective heat transport by upward
pore fluid flow along the faults.

Keywords: Nankai Trough; Heat flow; Accretionary prism; Splay fault; Cold seep;

33°

Bottom water temperature; Pore fluid

kai

Nan

136°

gh

u
Tro

137°E

Figure 1*Corresponding author: Makoto Yamano, yamano@eri.u-tokyo.ac.jp
18Earth, Planets and SpaceLETTER

Open AccessGeological evidence for shallow ductile-brittle transition zones along
subduction interfaces: example from the Shimanto Belt, SW Japan
Yoshitaka Hashimoto* and Natsuko Yamano
Earth, Planets and Space 2014, 66:141 doi:10.1186/s40623-014-0141-7
Received: 26 February 2014, Accepted: 1 October 2014, Published: 16 October 2014

Abstract
Tectonic mélange zones within ancient accretionary complexes include various styles of strain accommodation along
subduction interfaces from shallow to deep. The ductile-brittle transition at shallower portions of the subduction plateboundary was identified in three tectonic mélange zones (Mugi mélange, Yokonami mélange, and Miyama formation) in the
Cretaceous Shimanto Belt, an on-land accretionary complex in southwest Japan. The transition is defined by a change in
deformation features from extension veins only in sandstone blocks with ductile matrix deformation (possibly by diffusionprecipitation creep) to shear veins (brittle failure) from shallow to deep. Although mélange fabrics represent distributed
simple to sub-simple shear deformation, localized shear veins are commonly accompanied by slickenlines and a mirror
surface. Pressure-temperature (P-T) conditions for extension veins in sandstone blocks and for shear veins are distinct on thebasis of fluid inclusion analysis. For extension veins, P-T conditions are approximately 125 to 220°C and 80 to 210 MPa. For
shear veins, P-T conditions are approximately 185 to 270°C and 110 to 300 MPa. The P-T conditions for shear veins are, on
average, higher than those for extension veins. The temperature conditions overlap
in the range of approximately 175 to 210°C, which suggests that the change from
more ductile to brittle processes occurs over a range of depths. The width of theshallow ductile-brittle transition zone can be explained by a heterogeneous
lithification state for sandstone and mudstone or high fluid pressure caused by clay
dehydration, which is controlled by the temperature conditions.

Keywords: Shallow ductile-brittle transition zone; Tectonic mélange; Shear vein;

Pressure-temperature condition; Accretionary complex; Shimanto Belt

Figure 1*Corresponding author: Yoshitaka Hashimoto, hassy@kochi-u.ac.jp

LETTER

Open Access

Friction properties of the plate boundary megathrust beneath the frontal
wedge near the Japan Trench: an inference from topographic variation
Hiroaki Koge*, Toshiya Fujiwara, Shuichi Kodaira, Tomoyuki Sasaki, Jun Kameda, Yujin Kitamura, Mari Hamahashi,
Rina Fukuchi, Asuka Yamaguchi, Yohei Hamada, Juichiro Ashi and Gaku Kimura
Earth, Planets and Space 2014, 66:153 doi:10.1186/s40623-014-0153-3
Received: 9 March 2014, Accepted: 5 November 2014, Published: 19 November 2014

Abstract
The 2011 Tohoku-Oki earthquake (Mw 9.0) produced a fault rupture that extended to the toe of the Japan Trench. The deformationand frictional properties beneath the forearc are keys that can help to elucidate this unusual event. In the present study, toinvestigate the frictional properties of the shallow part of the plate boundary, we applied the critically tapered Coulomb wedge
theory to the Japan Trench and obtained the effective coefficient of basal friction (μ′b) and Hubbert-Rubey pore fluid pressure ratio (λ)
of the wedge beneath the lower slope. We extracted the surface slope angle and décollement dip angle (which are the necessary
topographic parameters for applying the critical taper theory) from seismic reflection and refraction survey data at 12 sites in thefrontal wedges of the Japan Trench. We found that the angle between the décollement and back-stop interface generally decreases
toward the north. The measured taper angle and inferred effective friction coefficient were remarkably high at three locations. The
southernmost area, which had the highest coefficient of basal friction, coincides with the area
where the seamount is colliding offshore of Fukushima. The second area with a high effective
coefficient of basal friction coincides with the maximum slip location during the 2011 TohokuOki earthquake. The area of the 2011 earthquake rupture was topographically unique from otherforearc regions in the Japan Trench. The strain energy accumulation near the trench axis may
have proceeded because of the relatively high friction, and later this caused a large slip andcollapse of the wedge. The location off Sanriku, where there are neither seamount collisions nor
rupture propagation, also has a high coefficient of basal friction. The characteristics of the taper
angle, effective coefficient of basal friction, and pore fluid pressure ratio along the Japan Trenchpresented herein may contribute to the understanding of the relationship between thegeometry of the prism and the potential for generating seismo-tsunamigenic slips.
138˚E
42˚N

141˚E

144˚E

147˚E

1952, 2003

42˚N

Sanriku

1994

1896,1933

39˚N

shim

40 m

a

slip

0m

= 20

m

39˚N

Ibarak

36˚N

i

Fuku

2011

36˚N

1938

Inubo

Keywords: Japan Trench; Topography; Critical taper; Tohoku-Oki earthquake;
Basal friction; Surface slope angle; Seismo-tsunamigenic slip;
Taper angle; Pore fluid pressure ratio; Prism*Corresponding author: Hiroaki Koge, h.kouge@eps.s.u-tokyo.ac.jp
19

km
0
138˚E

141˚E

Figure 1

144˚E

100

200
147˚E

New Perspective of Subduction Zone Earthquake

LETTER

Open Access

Small-scale stress fluctuations in borehole breakouts andtheir implication in identifying potential active faults aroundthe seismogenic megasplay fault, Nankai Trough, SW Japan
Yasuhiro Yamada* and Jun Shibanuma
Earth, Planets and Space 2015, 67:17 doi:10.1186/s40623-014-0176-9
Received: 12 March 2014, Accepted: 20 December 2014, Published: 6 February 2015

Abstract
Borehole breakouts are enlargements and elongation of a borehole in a particular direction, caused by failure of the borehole
wall rock due to concentration of stresses around the borehole, and thus, have been widely used to determine the in situ
stress orientation. We used electrical borehole wall images obtained during offshore scientific drilling (IODP) that penetrated
through a seismogenic megasplay fault in the Nankai Trough, off SW Japan, andextracted a number of borehole breakouts. Most of the breakouts show directions thatcan be explained by the regional convergence, but some are obviously rotated by faultsand fractures in the megasplay fault zone and in its hanging wall. Stress magnitudes
estimated from the width of the breakouts also show some decrease in the horizontal
stresses, suggesting that slip along the faults and fractures release shear stress affecting
these surfaces. Since such surfaces may have the capability to reactivate where thestresses affecting the surface are geometrically appropriate, the method presented inthis paper may contribute to identifying active fault surfaces. This knowledge allows us
to identify which surfaces need to be examined in detail to assess their potential for
future activity.
Medium resistivity static

Conductive

0

N

Resistive

E

S

W

N NW

Hole location
Inline 2675

100 m

SE

Depth (m LSF)

100

200

300

Keywords: Borehole breakout; Stress rotation; Active fault identification;
Nankai Trough; IODP

400
VE=2X

Figure 1*Corresponding author: Yasuhiro Yamada, yyamada@jamstec.go.jp

20

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Contents
Preface for the special issue of “New Perspective of Subduction Zone Earthquakes”. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gaku Kimura, Juichiro Ashi, Masataka Kinoshita, Christie Rowe and David Shelly
Regional and global variations in the temporal clustering of tectonic tremor activity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Koki Idehara, Suguru Yabe and Satoshi Ide
Frictional properties of sediments entering the Costa Rica subduction zone offshore the Osa Peninsula: implications for
fault slip in shallow subduction zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yuka Namiki, Akito Tsutsumi, Kohtaro Ujiie and Jun Kameda
QP structure of the accretionary wedge in the Kumano Basin, Nankai Trough, Japan, revealed by long-offset walk-away
VSP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ryota Hino, Takeshi Tsuji, Nathan L Bangs, Yoshinori Sanada, Jin-Oh Park, Roland von Hue
Gregory F Moore, Eiichiro Araki and Masataka Kinoshita

Quartz deposition and its influence on the deformation process of megathrusts in subduction zones. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Jun Kameda, Kuniyo Kawabata, Yohei Hamada, Asuka Yamaguchi and Gaku Kimura
Geochemical and mineralogical characteristics of fault gouge in the Median Tectonic Line, Japan: evidence forearthquake slip . . . . . . . . . . . . . . . . . . . . . . . . . Tsuyoshi Ishikawa, Tetsuro Hirono, Noriko Matsuta, Kazuro Kawamoto, Koichiro Fujimoto,
Jun Kameda, Yoshiro Nishio, Yuka Maekawa and Go Honda

Feasibility of acoustic monitoring of strength drop precursory to earthquake occurrence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nobuki Kame, Kohei Nagata, Masao Nakatani and Tetsuya Kusakabe
Ground motions characterized by a multi-scale heterogeneous earthquake model . . . . . . . . . . . . . Hideo Aochi and Satoshi Ide
Frictional properties of the shallow Nankai Trough accretionary sediments dependent on the content of clay minerals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Miki Takahashi, Shuhei Azuma, Hidenori Ito, Kyuichi Kanagawa and Atsuyuki Inoue
Application of cluster analysis based on waveform cross-correlation coefficients to data recorded by ocean-bottom
seismometers: results from off the Kii Peninsula . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Takeshi Akuhara and Kimihiro Mochizuki
Subgrain boundary analyses in deformed orthopyroxene by TEM/STEM with EBSD-FIB sample preparation technique. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Toshihiro Kogure, Hugues Raimbourg, Akihito Kumamoto, Eiko Fujii and Yuichi Ikuhara
Re-evaluation of temperature at the updip limit of locked portion of Nankai megasplay inferred from IODP Site C0002
temperature observatory . . . . . . . . . . . . . . . . Takamitsu Sugihara, Masataka Kinoshita, Eichiro Araki, Toshinori Kimura, Masanori Kyo,
Yasuhiro Namba, Yukari Kido, Yoshinori Sanada and Moe Kyaw Thu

Mechanical and hydraulic behavior of a rock fracture under shear deformation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Satoshi Nishiyama, Yuzo Ohnishi, Hisao Ito and Takao Yano
Changes in illite crystallinity within an ancient tectonic boundary thrust caused by thermal, mechanical, andhydrothermal effects: an example from the Nobeoka Thrust, southwest Japan. . . . . . . . . . . . . . . . . . . . Rina Fukuchi, Koichiro Fujimoto, Jun Kameda, Mari Hamahashi, Asuka Yamaguchi, Gaku Kimura, Yohei Hamada,
Yoshitaka Hashimoto, Yujin Kitamura and Saneatsu Saito

Strike-slip motion of a mega-splay fault system in the Nankai oblique subduction zone
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Takeshi Tsuji, Juichiro Ashi and Yasutaka Ikeda
Seismic imaging and velocity structure around the JFAST drill site in the Japan Trench: low Vp, high Vp/Vs in thetransparent frontal prism . . . . . . . . . . . . . . . . Yasuyuki Nakamura, Shuichi Kodaira, Becky J Cook, Tamara Jeppson, Takafumi Kasaya,
Yojiro Yamamoto, Yoshitaka Hashimoto, Mika Yamaguchi, Koichiro Obana and Gou Fujie

The possibility of deeper or shallower extent of the source area of Nankai Trough earthquakes based on the 1707 Hoei
tsunami heights along the Pacific and Seto Inland Sea coasts, southwest Japan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mamoru Hyodo, Takane Hori, Kazuto Ando and Toshitaka Baba
Geotechnical behavior of mudstones from the Shimanto and Boso accretionary complexes, and implications for the
Nankai accretionary prism. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kai Schumann, Jan H Behrmann, Michael Stipp, Yuzuru Yamamoto, Yujin Kitamura and ChristoLempp
Along-strike variations in temperature and tectonic tremor activity along the Hikurangi subduction zone, New Zealand. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Suguru Yabe, Satoshi Ide and Shoichi Yoshioka
Origin and transport of pore fluids in the Nankai accretionary prism inferred from chemical and isotopic compositionsof pore water at cold seep sites off Kumano . . . . . . Tomohiro Toki, Ryosaku Higa, Akira Ijiri, Urumu Tsunogai and Juichiro Ashi
Outer-rise normal fault development and influence on near-trench décollement propagation along the Japan Trench,
off Tohoku . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Brian Boston, Gregory F Moore, Yasuyuki Nakamura and Shuichi KodairaStress reversal recorded in calcite vein cuttings from the Nankai accretionary prism, southwest Japan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Toru Takeshita, Asuka Yamaguchi and Norio Shigematsu
Multiple damage zone structure of an exhumed seismogenic megasplay fault in a subduction zone - a study from theNobeoka Thrust Drilling Project . . . . . . . . . . . . . . . . . . Mari Hamahashi, Yohei Hamada, Asuka Yamaguchi, Gaku Kimura, Rina Fukuchi,
Saneatsu Saito, Jun Kameda, Yujin Kitamura, Koichiro Fujimoto and Yoshitaka Hashimoto

Estimation of slip rate and fault displacement during shallow earthquake rupture in the Nankai subduction zone
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yohei Hamada, Arito Sakaguchi, Wataru Tanikawa, Asuka Yamaguchi, Jun Kameda and Gaku Kimura
Estimation of slip parameters associated with frictional heating during the 1999 Taiwan Chi-Chi earthquake by vitrinitereflectance geothermometry . . . . . . . . . . . . . Yuka Maekawa, Tetsuro Hirono, Hikaru Yabuta, Hideki Mukoyoshi, Manami Kitamura,

1
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Minoru Ikehara, Wataru Tanikawa and Tsuyoshi Ishikawa

15

Wataru Tanikawa and Masataka Kinoshita

16

Thermal conductivities, thermal diffusivities, and volumetric heat capacities of core samples obtained from the Japan
Trench Fast Drilling Project (JFAST) . . . . . . . . . Weiren Lin, Patrick M Fulton, Robert N Harris, Osamu Tadai, Osamu Matsubayashi,
Accumulation of an earthquake-induced extremely turbid layer in a terminal basin of the Nankai accretionary prism. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Juichiro Ashi, Ritsuko Sawada, Akiko Omura and Ken Ikehara
Fluid-rock interaction recorded in black fault rocks in the Kodiak accretionary complex, Alaska. . . . . . . . . . . . . . . . . . . . . . . . . . Asuka Yamaguchi, Tsuyoshi Ishikawa, Yasuhiro Kato, Tatsuo Nozaki, Francesca Meneghini, Christie D Rowe,James Casey Moore, Akito Tsutsumi and Gaku Kimura

Frictional properties of incoming pelagic sediments at the Japan Trench: implications for large slip at a shallow plateboundary during the 2011 Tohoku earthquake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Michiyo Sawai, Takehiro Hirose and Jun Kameda
Pressure dependence of fluid transport properties of shallow fault systems in the Nankai subduction zone
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wataru Tanikawa, Hideki Mukoyoshi, Weiren Lin, Takehiro Hirose and Akito Tsutsumi
Heat flow survey in the vicinity of the branches of the megasplay fault in the Nankai accretionary prism. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Makoto Yamano, Yoshifumi Kawada and Hideki Hamamoto
Geological evidence for shallow ductile-brittle transition zones along subduction interfaces: example from the Shimanto
Belt, SW Japan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yoshitaka Hashimoto and Natsuko Yamano
Friction properties of the plate boundary megathrust beneath the frontal wedge near the Japan Trench: an inference
from topographic variation . . . . Hiroaki Koge, Toshiya Fujiwara, Shuichi Kodaira, Tomoyuki Sasaki, Jun Kameda, Yujin Kitamura,Mari Hamahashi, Rina Fukuchi, Asuka Yamaguchi, Yohei Hamada, Juichiro Ashi and Gaku Kimura

Small-scale stress fluctuations in borehole breakouts and their implication in identifying potential active faults aroundthe seismogenic megasplay fault, Nankai Trough, SW Japan . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yasuhiro Yamada and Jun Shibanuma

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Cover image: Hino, R. et al., Earth, Planets and Space 2015, 67:7 doi:10.1186/s40623-014-0175-x