Ujiie and Kimura Progress in Earth and Planetary Science 2014, 1:7
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REVIEW

Open Access

Earthquake faulting in subduction zones: insightsfrom fault rocks in accretionary prisms
Kohtaro Ujiie1,2* and Gaku Kimura3

Abstract
Subduction earthquakes on plate-boundary megathrusts accommodate most of the global seismic moment release,frequently resulting in devastating damage by ground shaking and tsunamis. As many earthquakes occur indeep-sea regions, the dynamics of earthquake faulting in subduction zones is poorly understood. However, theIntegrated Ocean Drilling Program (IODP) Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) and fault
rock studies in accretionary prisms exhumed from source depths of subduction earthquakes have greatly improved
our understanding of earthquake faulting in subduction zones. Here, we review key advances that have been madeover the last decade in the studies of fault rocks and in laboratory experiments using fault zone materials, with aparticular focus on the Nankai Trough subduction zone and its on-land analog, the Shimanto accretionary complex
in Japan. New insights into earthquake faulting in subduction zones are summarized in terms of the following:(1) the occurrence of seismic slip along velocity-strengthening materials both at shallow and deep depths;(2) dynamic weakening of faults by melt lubrication and fluidization, and possible factors controlling coseismic
deformation mechanisms; (3) fluid-rock interactions and mineralogical and geochemical changes during
earthquakes; and (4) geological and experimental aspects of slow earthquakes.
Keywords: Subduction earthquake; Accretionary prism; Fault rock; NanTroSEIZE

Review
Introduction

Great subduction earthquakes (Mw > 8.0) commonly occur
along plate-boundary thrusts beneath accretionary prisms.
Ground shaking and tsunamis generated in association
with earthquake faulting in subduction zones inflict agreat natural hazard to humans, and yet our understanding of how subduction earthquakes and tsunamis
occur remains poor.
Thermal models suggest that subduction earthquakes
occur in a temperature range of approximately 100°C to
150°C to approximately 350°C to 450°C (Hyndman et al.
1997; Oleskevich et al. 1999). It was proposed that the
updip limit of the seismogenic subduction zone may becontrolled by diagenetic to low-grade metamorphic processes such as quartz cementation and pressure solution* Correspondence: kujiie@geol.tsukuba.ac.jp
1
Graduate School of Life and Environmental Sciences, University of Tsukuba,
1-1-1 Tennodai, Tsukuba 305-0006, Japan
2
Research and Development Center for Ocean Drilling Science, Japan
Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho,
Yokosuka 237-0061, Japan
Full list of author information is available at the end of the article(Moore and Saffer 2001), while the downdip limit could
correspond with the onset of plastic deformation or aserpentinized mantle (Hyndman et al. 1997). It wasconventionally thought that the shallow portions of
the plate-boundary thrusts were aseismic, owing to thevelocity-strengthening frictional behavior of unconsolidated materials, while the seismogenic portions ofplate boundaries exhibited a velocity-weakening behavior(Marone and Scholz 1988; Moore and Saffer 2001). Vrolijk(1990) proposed that the transformation of smectite toillite may control the updip limit of seismogenic subduction zones because it occurs in the temperature
range of 100°C to 150°C. However, both smectite and illite commonly show velocity-strengthening frictional
behavior at low slip rates; thus, it is unlikely that thesmectite to illite transformation controls the updip limitof the seismogenic zone (Saffer and Marone 2003; Ikari
et al. 2007).
The 2011 Tohoku-Oki earthquake (Mw9.0) has provided
an invaluable opportunity to rethink the updip limit of theseismogenic subduction zone. Seafloor global positioning
system measurements, repeated bathymetry and seismic© 2014 Ujiie and Kimura; licensee Springer. This is an Open Access article distributed under the terms of the Creative
Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, andreproduction in any medium, provided the original work is properly credited.

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surveys, and numerous coseismic slip distribution models
indicate that the coseismic slip extended all the way to theJapan Trench axis with an unprecedented slip displacement of approximately 50 m (e.g., Fujiwara et al. 2011; Ideet al. 2011; Sato et al. 2011; Kodaira et al. 2012). The Integrated Ocean Drilling Program (IODP) Expedition 343
and 343 T, Japan Trench Fast Drilling Project (JFAST),
successfully detected the residual heat associated with theearthquake on the shallow plate-boundary thrust near theJapan Trench axis (Fulton et al. 2013). Logging while drilling and observations of the recovered cores indicated thatthe plate-boundary faulting is localized along a pelagic
clay layer less than 5 m thick (Chester et al. 2013).
High-velocity friction experiments on the plate-boundary
thrust material suggest that the large coseismic slip can be
explained by a very low coseismic shear stress, which canbe attributed to both the abundance of smectite in the pelagic clay and thermal pressurization effects (Ujiie et al.
2013). Notably, JFAST confirmed that the large coseismic
slip occurred in the shallow subduction thrust where
earthquakes are unlikely to nucleate.
Seismological and geodetic observations made in thetwenty-first century have revealed that seismic and aseismic slips are the end members of various fault slip behaviors. Slow earthquakes in subduction zones, detected by
the widespread development of seismic observation networks and GPS stations, can be categorized according totheir predominant frequency or period, which comprise
low frequency tremors, low frequency and very low frequency earthquakes, and slow slip events (e.g. Obara 2002;
Rogers and Dragert 2003; Shelly et al. 2006; Ito and Obara
2006a; Ito et al. 2007). Slow earthquakes reflect transientmodes of fault slip with longer characteristic durations,and they radiate much less seismic energy than regularearthquakes (Ide et al. 2007). The spatial distribution ofslow earthquakes suggests that transient slip appears tooccur in the transition zone between seismic and aseismic
slip zones (Ito and Obara 2006a; Ito et al. 2007; Sugioka
et al. 2012). Thus, a locked seismogenic zone withaseismic zone above and below is an oversimplified
conceptual view.
Concurrently with an increased understanding of thecomplex modes of fault slip in subduction zones, ourunderstanding of plate-boundary faulting associated withdynamic earthquake rupture and slow earthquakes has
progressed. In recent years, active fault drilling has been
conducted by the IODP Nankai Trough Seismogenic
Zone Experiment (NanTroSEIZE) project, focusing on
the Nankai accretionary margin offshore the Kii Peninsula,
southwest Japan, where great subduction earthquakes
have repeatedly occurred (Figure 1a,b). The coseismic
slip distribution of the 1944 Tonankai earthquake (Mw8.1)
inferred from tsunami and seismic waveform inversions
suggests that the earthquake rupture propagated along a

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megasplay fault (or an out-of-sequence thrust) thatbranches from the plate-boundary megathrust and cuts
through the prism (Tanioka and Satake 2001; Kikuchi
et al. 2003). The ultimate goals of the NanTroSEIZE
deep drilling program are to sample the plate-boundary
megathrust and megasplay fault materials at an approximately 6-km depth and to instrument an observatory
system across the plate-boundary fault system (Tobin andKinoshita 2006). Although this complex ocean drilling
project is ongoing, fault rocks have been obtained fromthe shallow portions of the megasplay fault and plateboundary thrust (Figure 1b). Moreover, field studies ofexhumed subduction thrusts and out-of-sequence thrusts,
microstructural and chemical analyses of fault rocks, andlaboratory experiments on fault zone materials have been
vigorously conducted over the past decade because fault
rocks in exhumed accretionary prisms have provided awindow to examine earthquake processes in subduction
zones that are not accessible or observable using ocean
drilling.
In this review, we summarize and discuss new findings
and major progresses that have been made in fault rockstudies of accretionary prisms. In particular, we focus on
identification of past seismic slips from fault rocks, coseismic deformation mechanisms, physical and chemical processes that occur during earthquake faulting, and theestimation of earthquake source parameters through fault
rocks and laboratory experiments. First, we describe thestructure of the megasplay fault and active plate-boundary
thrust in the Nankai subduction zone and show how shallow earthquake ruptures were recorded in fault gouges.
Then, we review earthquake faulting and earthquake- or
earthquake cycle-related fluid-rock interactions recorded
in various fault rocks in exhumed accretionary complexes
with an emphasis on but not limited to research resultsfrom the Shimanto accretionary complex in Japan, the onland analog of the Nankai accretionary prism (Figure 1c).
We also review progress made in the geological andexperimental investigations of slow earthquakes. Finally,
we make concluding remarks related to new insights
into earthquake faulting in subduction zones.
Fault rocks in the Nankai accretionary prism

Fault rocks in the shallow portions of the megasplay fault
and plate-boundary thrust of the Nankai accretionary
prism were collected during IODP NanTroSEIZE Expedition 316 (Figures 2 and 3). In the following subsections,
we document primary results of fault rock studies thatwere obtained during Expedition 316 (Kinoshita et al.
2009) and post-expedition research.
Fault zone structure

Megasplay fault The shallow portion of the megasplay
fault zone was drilled at site C0004 (Figure 1b), where itUjiie and Kimura Progress in Earth and Planetary Science 2014, 1:7
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Figure 1 Location maps of the Nankai Trough and the Shimanto accretionary complex. (a) Map of the Nankai accretionary margin offshore
southwest Japan. The orange area and red line indicate the rupture area during the 1944 Tonankai earthquake (Mw8.1) and the location of theseismic reflection line in (b), respectively. (b) Seismic reflection profile of the Nankai accretionary prism showing drill sites C0004 and C0007 at theshallow portions of a megasplay fault and plate-boundary thrust, respectively (modified from Moore et al. 2009). (c) Distribution of the Shimanto
accretionary complex along the Pacific side of southwest Japan and the Ryukyu Arc.

was defined at 256 to 315 m below the sea floor (mbsf )based on the abrupt occurrence of brecciated fragmentswith polished and slickenlined surfaces at 256 mbsf andthe absence of brecciated and fractured rocks below 315
mbsf. The fault zone is developed in the middle Pliocene
hemipelagic mudstone in which volcanic ash layers are
frequently distributed (Figure 2). Two biostratigraphic age

reversals occur in the fault zone and are consistent withthrust faulting along the megasplay fault. There is noclear evidence of fluid-rock interactions having takenplace in the fault zone, such as mineralized veins oralteration.
Structural elements found in the megasplay fault
zone include fractured rocks, fault breccia, microbreccia,

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Figure 2 Lithologic units and distribution of fault rocks in the megasplay fault zone at site C0004.

drilling-induced breccia, and fault gouge (Figure 2). Borehole resistivity images obtained from logging while drillingindicate the presence of multiple conductive fractures in
the megasplay fault zone, which may inhibit core recovery(Kinoshita et al. 2009). Based on recovered cores, there isa wide distribution of fractured rocks and drilling-induced
breccia in the fault zone, while the other abovementioned structural elements show a relatively limited spatial

distribution. Fractured rocks are defined by the presenceof centimeter- to decimeter-sized trapezoidal broken rockfragments that occur along sets of polished and striatedfracture surfaces. Drilling-induced breccia is derived fromnatural fractured and brecciated rocks that were enhancedby the drilling processes (see Kinoshita et al. 2009 for additional detail about drilling-induced breccia). Fault breccia
and microbreccia are characterized by the predominanceUjiie and Kimura Progress in Earth and Planetary Science 2014, 1:7
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Figure 3 Lithologic units and distribution of fault rocks in the plate-boundary fault zone at site C0007.

of centimeter- to millimeter-sized and millimeter- to
submillimeter-sized, angular to subangular fragments, respectively. Brecciated fragments are commonly polishedwith variable slip directions. Some fractured rocks andfault breccia show foliations defined by the alignment offragments along inclined fracture surfaces. Shear displacements along individual fractures and slip surfaces
are considered to be small because the original layeringof hemipelagic mudstone and ash is mostly preserved
despite intense fracturing and brecciation.
Fault gouges develop in microbreccia as approximately
10-mm-thick dark zones deficient in visible fragments(Figure 4a). At site C0004, one fault gouge smoothly cuts

the microbreccia, while the other gouge is undulated
and fragmented, indicating the recurrence of microbrecciation and shear localization along the thin gouge zones.
On X-ray computed tomography (CT) images, the gouges
are expressed as bright zones with relatively high CT
numbers, suggesting that they are denser than the surrounding microbreccia.
Plate-boundary thrust The plate-boundary thrust was
drilled at site C0007 (Figure 1b), with the plate-boundary
fault zone defined at 398.5 to 446 mbsf based on the spacing of fractures and the preservation of fault breccia andfault gouge. The fault zone and its hanging wall occur inUjiie and Kimura Progress in Earth and Planetary Science 2014, 1:7
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Figure 4 Occurrence of fault gouges in recovered cores. Fault gouges are shown from the (a) megasplay fault zone and (b) plate-boundary
fault zone. See Figures 2 and 3 for the locations of fault gouges.

Pliocene hemipelagic mudstone with volcanic ash layers,
while the cores recovered from the footwall show sanddominated sediments that are possibly of Pleistocene age,suggesting that temporal and lithological changes are associated with thrust faulting (Figure 3).
The fault zone is characterized by the heterogeneous
distribution of fractured and brecciated rocks and a very
limited distribution of fault gouges (Figure 3). Fracturing and brecciation in the fault zone show similar

characteristics to those in the megasplay fault zone.
Microbreccia is absent in the fault zone associated withthe plate-boundary thrust.
Fault gouges are developed in two intervals. The fault
gouge at 418.83 to 418.94 mbsf shows foliations definedby both the alignment of submillimeter fragments in theclay matrix and color banding. Foliations are cut by the
anastomosing slip surfaces that surround fragments. The
foliations and slip surfaces locally exhibit a composite

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planar fabric such as a P-Y fabric (Chester and Logan
1987), with the shear sense consistent with thrust faulting. Fault gouges are also found in the lowermost part of
the fault zone at 438.28 to 438.57 mbsf where hemipelagic mudstone underwent intense brecciation into
approximately 1- to 10-mm-sized fragments along ananastomosing network of polished and striated surfaces.
The gouges are also intensely brecciated, resulting infragmentation of gouge or irregular gouge boundaries.
However, the 2-mm-thick dark fault gouge at the base of
the interval exhibits planar upper and lower boundaries
and sharply separates the intensely brecciated hemipelagic mudstone above from the unbroken hemipelagic
mudstone and ash below (Figure 4b). A biostratigraphic
age reversal that occurs across the interval records an estimated time difference of approximately 1.67 Myr, which

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suggests that this thin, dark gouge represents an extreme
localization of slip during thrust faulting.

Earthquake faulting recorded in fault gouges

Both the shallow portions of the megasplay fault andthe plate-boundary thrust show repeated localization ofshear along approximately 2- to 10-mm-thick dark
gouges that occur within broader zones undergoing distributed fracture and brecciation (Figure 4). These
gouges are marked by the preferred orientation of veryfine grained clay particles that are parallel and oblique
to gouge boundaries (Figure 5). This contrasts with thesurrounding brecciated materials, which show an anastomosing network or relatively widely spaced clay foliations. The relative concentration of clay particles within

Figure 5 Microstructures of fault gouges. Fault gouges in (a) and (b) are from the megasplay fault zone, with white arrows indicating their
upper boundaries. Fault gouges in (c) and (d) are from the plate-boundary fault zone, with a white double and single arrow indicating the gougezone and the upper boundary of the fault gouge, respectively. All photomicrographs were taken under cross-polarized light.

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the gouges is consistent with the higher bulk density of
gouges compared to the surrounding breccias.
The gouges in the megasplay fault and the plateboundary thrust appear to record evidence for frictional
heating associated with coseismic slip even at shallow
depths of 271 and 438 mbsf, respectively, as indicatedby optical reflectance anomalies in vitrinite (Sakaguchi
et al. 2011). Vitrinite is a constituent of bituminouscoal. Vitrinite reflectance (Ro) is an indicator of thethermal maturation of coal, which increases with increasing temperature and heating duration. Since anincrease in Ro is an irreversible reaction, it is expected
to record the maximum temperature. Sakaguchi et al.
(2011) measured Ro across the dark gouges in the megasplay fault and the plate-boundary thrust and identified
zones of anomalous Ro values symmetrically distributed
around the gouges (Figure 6). Ro values were seen to belargest within the gouges and decreased with distanceaway. The maximum temperatures estimated from themeasured Ro anomalies were 390 ± 50°C in the megasplay
fault and 330 ± 50°C in the plate-boundary thrust, bothof which are much higher than the ambient temperaturesof <20 ± 15°C. It is likely that these high temperatures
are caused by the generation of frictional heat associatedPage 8 of 30

with earthquake faulting along the thin (approximately 2
to 10 mm) gouges, while high Ro values adjacent to thegouges could record the effects of heat diffusion awayfrom the gouge zones (Sakaguchi et al. 2011).
Hirono et al. (2009) examined other temperature proxies(i.e., fluid-mobile trace elements, strontium isotopes, magnetic minerals, inorganic carbon content, and Raman
spectra of carbonaceous material) for the same gouge inthe megasplay fault zone. These proxies successfully
identified previous seismic slip on the Chelungpu thrust,
including that attributed to the 1999 Chi-Chi earthquake(Mw7.6) in Taiwan (Hirono et al. 2006; Ishikawa et al.
2008). However, Hirono et al. (2009) did not detect any
thermal anomaly. No detection of thermal anomaly may
simply reflect the fact that vitrinite is more sensitive to increased heat than other temperature proxies. Alternatively,
it is possible that the coseismic temperature does not exceed 300°C (Hirono et al. 2009), and/or the maximum
temperature estimated from Ro may have been overestimated (Kitamura et al. 2012; Furuichi et al. 2013). The
kinetic model of Sweeney and Burnham (1990), which
considers a first-order Arrhenius law, was used to calculate the maximum temperature from Ro values (Sakaguchi
et al. 2011). In this model, Ro is related to the extent of the

Figure 6 Distribution of vitrinite reflectance (Ro) across fault gouges (modified from Sakaguchi et al. 2011). (a) From the megasplay fault
zone and (b) from the plate-boundary fault zone. Black dots on core surfaces indicate coal particles used for Ro measurement. Red lines showsmoothed Ro curves. See Figures 2 and 3 for locations of fault gouges.

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parallel chemical reactions (F) associated with thermal maturation, which is described by the empirical
relationship:
R0 % ¼ eð−1:6þ3:7F Þð1Þ

However, it is not possible to apply this chemical kinetic
model to short-lived thermal events such as frictional
heating on faults. In fact, recent high-velocity (1.3 m/s)
friction experiments on a mixture of 95 wt% clay-rich
powder and 5 wt% coal grains from the host rock of themegasplay fault gouge and the Kumano forearc basin in
the Nankai subduction zone, respectively, demonstrated
that Ro can be increased by high-velocity shearing, but the
Ro determined from the kinetic model of Sweeney and
Burnham (1990) is greater than the Ro measured after
the experiments (Furuichi et al. 2013). Fulton and Harris(2012) conducted numerical modeling to explain thespatial distribution of Ro across the fault gouges using
the thermal model for frictional heat generation and diffusion with finite thickness, coupled with the kinetic
model of Sweeney and Burnham (1990). Their results
indicate that very large coseismic slip of several tens ofmeters is necessary to account for both the peak andspatial extent of anomalous Ro found in and around thefault gouges. Thus, estimation of coseismic temperatureusing the kinetic model of Sweeney and Burnham (1990)
is problematic and requires further investigation. However,
the increase in Ro in the fault gouges even at shallow
depths is evident, and several experimental studies havedemonstrated that Ro values can increase during highvelocity shearing on short timescales comparable to an approximately 10-s earthquake rise time (O'Hara et al. 2006;
Kitamura et al. 2012; Furuichi et al. 2013).
Yamaguchi et al. (2011b) reported geochemical andmineralogical anomalies in the same fault gouge in the

Figure 7 Geochemical and mineralogical data showing transformation of Ca-smectite into illite in the fault gouge. Modified from
Yamaguchi et al. (2011b). See Figure 2 for the location of the fault gouge. (a) Photograph of the fault gouge in the megasplay fault zone
showing the XRF mapping area in (b) (white rectangle). Red triangles indicate fault gouge (upper) and fragmented gouge (lower) in
microbreccia. (b) Compositional maps across fault gouges (solid triangles). (c) XRD patterns for fault gouge and microbreccia shown in (a).

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megasplay fault zone where Sakaguchi et al. (2011)detected anomalous Ro values. They conducted a nondestructive X-ray fluorescence (XRF) analysis using a
core-imaging scanner and X-ray diffraction (XRD) analysis on the fault gouge and surrounding microbreccia.
Their compositional maps indicate that the fault gouge
is chemically enriched in Al and K, and depleted in Ca
and Sr relative to the surrounding breccias, which isconsistent with the XRD data showing that the fault
gouge has a higher illite content in illite-smectite mixedlayer clays (Figure 7). These results suggest that the transformation of Ca-smectite into illite has progressed in thefault gouge. It has been proposed that the transformationof smectite to illite in the shear zone occurs by dissolution
and recrystallization processes, shear-related deformations, and frictional heating (Yan et al. 2001; Hirono et al.
2008; Casciello et al. 2011). The coexistent anomalies ofRo and the clay mineralogy anomalies in the gouges suggest that frictional heating is the most likely cause for theillitization within the gouges (Yamaguchi et al. 2011b).
Fault zone structures and analyses of approximately
2- to 10-mm-thick dark gouges in the megasplay fault
and the plate-boundary thrust suggest that earthquake
rupture propagated to shallow depths. Clay-rich materials taken from the shallow megasplay fault and shallow
plate-boundary thrust show velocity-strengthening frictional behavior at low slip rates, suggesting that earthquake
nucleation is unlikely to occur at shallow depths (Ikari and
Saffer 2011). However, the high-velocity (1.3 m/s), wet(water-saturated) friction experiments on clay-rich materials taken from gouge host rocks and pure clays
demonstrated that earthquake rupture propagates easily through clay-rich fault gouge by high-velocity weakening associated with thermal pressurization, which ismarked by low peak and steady-state friction, and avery small slip weakening distance and fracture energy(Ujiie and Tsutsumi 2010; Faulkner et al. 2011). The
permeabilities of the fault zone materials in the megasplay
fault and the plate-boundary thrust are in the ordersof 10−20 to 10−18 m2 (Ikari et al. 2009; Tanikawa et al.
2012). Unless dilatancy hardening operates in lowpermeability gouges (Rudnicki and Chen 1988; Segall
and Rice 1995), water is not expected to be able to escape from a shear zone during earthquake faulting(Lachenbruch 1980; Mase and Smith 1987; Rice 2006).
Therefore, when frictional heat is generated along narrow gouges approximately 2- to 10-mm thick, thermalpressurization is likely to occur even at shallow depths.
This suggests that a shallow fault exhibiting stable, velocitystrengthening behavior at low slip rates may be weakened
by thermal pressurization. Recent numerical modeling considering the rate- and state-dependent friction constitutive
law, and high-velocity weakening due to rapid shear heatingof pore fluids, has revealed that unstable slip occurs even in

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fault segments where velocity-strengthening occurs at low
slip rates (Shibazaki et al. 2011; Noda and Lapusta 2013).
In summary, fault rock analyses and experimental investigations using fault zone materials highlight that the
propagation of seismic slip to shallow portions of themegasplay fault and the plate-boundary thrust is likely tohave occurred during subduction earthquakes (Figure 8),
which would then have facilitated the generation of tsunamis in the Nankai subduction zone.
Fault rocks in exhumed accretionary complexes

Examination of fault rocks exhumed from various depths
in subduction zones is essential in order to understandfault slip behaviors such as dynamic earthquake rupture,
low-frequency tremor and earthquakes, slow slip events,
and post-seismic and interseismic creep. In particular,
tectonic mélanges in exhumed accretionary complexes
can be regarded as fault rocks that formed along subduction plate boundaries and are thus expected to record
earthquake faulting in subduction zones (Kimura et al.
2012). Mélanges are characterized by competent blocks(e.g., sandstone) in an incompetent (e.g., mudstone)
matrix. Fagereng and Sibson (2010) pointed out thatvariation in the ratio of competent to incompetent material causes spatial variations in shear strength, viscosity,
shear strain rate, and fault slip behavior. They suggested
that localization of shear occurs along lithological boundaries or within competent blocks, while distributed shear
is accommodated by foliated phyllosilicates in the incompetent matrix. The competent blocks composed ofstrong minerals tend to show high frictional strengthand velocity-weakening behavior, whereas the foliatedphyllosilicates commonly exhibit low frictional strengthand velocity-strengthening behavior (Collettini et al. 2011;
Ikari et al. 2011). It is likely that the heterogeneity ofmélange affects aseismic slip and earthquake nucleation(Fagereng and Sibson 2010). Recently, Rowe et al. (2013)
compiled data on the thicknesses of subduction thrusts at
depths of <1 to 15 km. Although mélanges can be several
hundreds of meters thick, a simultaneously active part of a
subduction plate boundary consists of one or multiple
fault strands with a thickness of 5 to 35 m, which are
sharply cut by discrete slip surfaces with thicknesses of <1
to 20 cm. These observations suggest that dynamic earthquake rupture occurs in highly localized slip zones withthickness of <1 to 20 cm, while post-seismic and interseismic creep is generated in single to several strands withthicknesses of 5 to 35 m (Rowe et al. 2013). In the following
subsections, we show representative results of fault rocks
studies conducted in exhumed accretionary complexes.
Frictional melting

Pseudotachylytes Pseudotachylytes (i.e., solidified frictional melts produced during seismic slip) have beenUjiie and Kimura Progress in Earth and Planetary Science 2014, 1:7
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Figure 8 Schematic profile of the Nankai accretionary prism offshore the Kii Peninsula, southwest Japan. Orange dots indicate locations offault gouges in which coal maturation and illitization were found by drilling into the shallow portions of the megasplay fault and the plate-boundary
thrust. The tectonic setting of the duplex underplating of the tectonic mélange and basalt is shown on the profile. Tectonic settings ofpseudotachylytes (red dots) and fluidized fault rocks (blue dots), which are identified in the Shimanto accretionary complex (the on-land analog
of the Nankai accretionary prism), are shown in the profile.

found in exhumed accretionary complexes (Ikesawa et al.
2003; Kitamura et al. 2005; Rowe et al. 2005; Mukoyoshi
et al. 2006; Okamoto et al. 2006; Ujiie et al. 2007b;
Meneghini et al. 2010; Hashimoto et al. 2012). Geological settings associated with pseudotachylyte-bearing
faults are plate-boundary thrusts, roof thrusts of duplex
structures, and megasplay faults (or out-of-sequencethrusts) (Figure 8) that were active at depths of 2.5 to
14 km and temperatures of 160°C to 320°C (Ikesawa
et al. 2003; Kitamura et al. 2005; Rowe et al. 2005;
Mukoyoshi et al. 2006; Okamoto et al. 2006; Hashimoto
et al. 2012). In many cases, pseudotachylyte-bearing
faults develop near the boundary between argillaceousmélange and relatively coherent sandstone, and pseudotachylytes are hosted in either foliated or non-foliatedmélange-derived cataclasites (Figure 9a). Pseudotachylytebearing fault zones can range in thickness from less than
1 m to tens of meters. Pseudotachylyte-bearing fault zones
in the exhumed accretionary complexes commonly contain quartz, calcite, ankerite, and laumontite veins andtheir fragments, which are consistent with fluid-infiltrated
conditions (Ikesawa et al. 2003; Mukoyoshi et al. 2006;
Okamoto et al. 2006; Ujiie et al. 2007b) and are in contrast
to pseudotachylytes that form in crystalline rocks under
dry conditions (Sibson and Toy 2006). One explanation
for the occurrence of pseudotachylytes in fluid-saturated
fault zones is that seismic slip occurred along a thin slip
zone with a high permeability/compressibility. In such acase, fluid could escape from the slip zone during earthquake faulting, generating temperatures high enough for
frictional melting. Pseudotachylyte-bearing dark veinsin the exhumed accretionary complexes are commonly
thinner than a few millimeters (typically 1 mm), whichis consistent with the presence of a thin slip zone during
earthquake faulting (Figure 9b).
The pseudotachylyte-bearing dark veins at outcrop arecontinuous and straight, but in places show the appearance of both fault and injection veins. The length of theinjection veins is less than 10 mm. Based on the following
observations, the dark veins are thought to have originallybeen pseudotachylyte veins that subsequently sufferedhydrothermal alteration: the dark veins locally preserve
the embayed boundaries (Figure 9c) and are mostly
composed of illite, which was possibly derived fromhydrothermal alteration; mineralized veins are present
along the dark veins (Figure 9c); and some portions of
the dark veins and injection veins survived alteration
and are preserved as evidence for frictional melting(Figure 9d). Under plane-polarized light, the unaltered
pseudotachylytes are characterized by the presence ofsubangular to subrounded grains of quartz and feldspar,and small dark grains in the transparent matrix. Undercross-polarized light, the matrix is dark and opticallyisotropic.
Examination under a scanning electron microscope(SEM) and transmission electron microscope (TEM) has

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Figure 9 Occurrences of the pseudotachylyte-bearing dark veins in the Shimanto accretionary complex. (a) Dark veins (red arrows)
sharply bounding the foliated cataclasite. (b) Enlarged view of the dark vein (red arrow). (c) Dark and injection veins under plane-polarized light.
(d) The pseudotachylyte surviving alteration, which is recognized as the transparent material under plane-polarized light (red arrow). The red
double arrow indicates the dark vein.

revealed the following evidence for frictional melting(Figure 10 and see also Mukoyoshi et al. 2006; Okamoto
et al. 2006; Ujiie et al. 2007b; Meneghini et al. 2010): a
glassy matrix, grains with irregular and embayed margins,
vesicles and amygdules, Fe-rich spherules, idiomorphic
acicular microlites of mullite or muscovite, and tabular
zoned microlites of plagioclase. Acicular and tabular
microlites were reported from the 1-mm and 10- to
100-mm-thick pseudotachylytes, respectively (Rowe et al.
2005; Mukoyoshi et al. 2006; Ujiie et al. 2007b; Meneghini

et al. 2010), with the latter being exceptionally thickcompared to typical pseudotachylytes (Sibson 1975,
2003; Di Toro and Pennacchioni 2004). Different crystal
morphologies may be the result of variations in the cooling rate of the melt, although other parameters (e.g., thepresence and abundance of nuclei for crystallization) may
also contribute (Lofgren 1980; Corrigan 1982). The tabularmorphology of plagioclase microlites is consistent withslow cooling in an exceptionally thick melt (Meneghini
et al. 2010). The presence of embayed grains, vesicles, andUjiie and Kimura Progress in Earth and Planetary Science 2014, 1:7
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Figure 10 (See legend on next page.)

Page 13 of 30

Ujiie and Kimura Progress in Earth and Planetary Science 2014, 1:7
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Page 14 of 30(See figure on previous page.)
Figure 10 Microstructures of pseudotachylytes. (a-e) Pseudotachylytes from the Shimanto accretionary complex. (f) Pseudotachylyte from theKodiak accretionary complex (modified from Meneghini et al. 2010). (a) Back-scattered electron image of pseudotachylyte showing fragments in a
glassy matrix resulting from rapid cooling of the frictional melt. Albite grains (Ab) with irregular and embayed margins are present in the homogeneous
matrix. (b) Vesicles and idiomorphic acicular microlites of muscovite in the matrix. Location of the image is shown in (a). (c) Back-scattered electron
image of pseudotachylyte showing an embayed K-feldspar grain (Kf) in the homogeneous matrix. (d) Enlarged view of the embayed K-feldspar grain(Kf), suggesting marginal melting. Vesicles are visible in the matrix. Location of the image is shown in (c). (e) TEM image of pseudotachylyte showing
idiomorphic acicular microlites of mullite in a homogeneous, glassy matrix. (f) Back-scattered electron image of pseudotachylyte showing tabular zoned
microlites of plagioclase.

Fe-rich spherules in the homogeneous, glassy matrix was
successfully reproduced in the pseudotachylytes producedby high-velocity (1.3 m/s) friction experiments on argillite
from the pseudotachylyte host rock in the Shimanto accretionary complex (Ujiie et al. 2009).
The chemical composition of the pseudotachylyte
matrix determined by an electron probe microanalyzer(EPMA) is similar to that of illite (Table 1), which is consistent with the selective melting of phyllosilicates due totheir low shear yield strength, fracture toughness, andthermal conductivity (Spray 1992). Heating experiments
on illite reveal that illite melts at approximately 950°C to
1,200°C and that crystallization and melting of mullite
occur at 1,100°C and ≥1,400°C, respectively (Grim andBradley 1940; Moecher and Brearley 2004; McConvilleand Lee 2005). The rounded and embayed shapes of albite
and K-feldspar grains (Figure 10) are attributed to disequilibrium marginal melting, which has occurred due to adecrease in the melting temperature of feldspar causedby the release of OH− from hydrous phases or by viscousshear heating at >1,100°C (Spray 1992). It is therefore

considered that the frictional melts reached a minimum
temperature of 1,100°C.
The common feature of frictional melting is that hydrous
minerals melt preferentially, whereas refractory mineralssuch as quartz survive as clasts (Allen 1979; Maddock
1992; Magloughlin 1992; Magloughlin and Spray 1992;
Spray 1992; Otsuki et al. 2003). As frictional melting exhibits disequilibrium characteristics, a higher degree ofmelting leads to a greater degree of melting of refractoryminerals. Thus, the chemical composition of a pseudotachylyte matrix would be expected to change as the degreeof melting increases. However, the chemical compositionof the pseudotachylyte matrix from the Shimanto accretionary complex samples remained constant (i.e., similar
to that of illite), despite the volume fraction (ϕ) of unmelted grains ranging from 9% to 26% (Table 1). Thesevariations in volume fraction of unmelted grains cannot
be explained by differences in the degree of melting andinstead may reflect differences in the initial volume
fraction of illite prior to frictional melting (Ujiie et al.
2007b). It is likely that the illite was either derived from

Table 1 Average chemical composition of pseudotachylyte matrix, and volume fraction and average aspect ratio of
unmelted grains
Mugi area

Okitsu area

Mg-1

Mg-2

Mg-3

Mg-4

Ok-1

Ok-2

Ok-3

ϕ

0.09

0.26

0.26

0.26

0.13

0.18

0.12

R

1.58

1.56

1.55

1.54

1.39

1.5

1.41

n

11

6

13

5

13

8

10

SiO2

52.55 ± 0.85

51.80 ± 1.50

52.79 ± 0.31

51.65 ± 1.24

53.40 ± 0.92

53.05 ± 0.66

52.46 ± 1.13

TiO2

0.27 ± 0.08

0.16 ± 0.11

0.08 ± 0.01

0.13 ± 0.03

0.14 ± 0.07

0.29 ± 0.05

0.21 ± 0.02

Al2O3

30.22 ± 0.65

27.88 ± 1.15

27.97 ± 0.00

27.03 ± 0.58

29.25 ± 0.80

29.03 ± 0.37

28.92 ± 0.36

FeO

2.55 ± 0.48

4.68 ± 1.88

3.72 ± 0.12

5.32 ± 0.39

2.40 ± 0.49

2.32 ± 0.09

2.28 ± 0.11

MnO

0.21 ± 0.29

0.16 ± 0.04

0.13 ± 0.01

0.19 ± 0.02

0.03 ± 0.02

0.05 ± 0.02

0.02 ± 0.01

MgO

1.63 ± 0.08

1.96 ± 0.29

1.60 ± 0.08

2.15 ± 0.15

2.51 ± 0.14

2.52 ± 0.14

2.51 ± 0.05

CaO

0.55 ± 0.08

0.66 ± 0.10

0.94 ± 0.49

0.57 ± 0.02

0.45 ± 0.40

0.33 ± 0.04

0.42 ± 0.33

Na2O

0.13 ± 0.02

0.13 ± 0.01

0.11 ± 0.03

0.15 ± 0.01

0.06 ± 0.02

0.06 ± 0.01

0.06 ± 0.01

K2O

7.82 ± 0.35

6.92 ± 0.51

6.97 ± 0.08

6.88 ± 0.14

6.90 ± 0.81

8.38 ± 0.52

8.23 ± 0.19

Total

95.93

94.34

94.30

94.11

95.15

96.04

95.12

Missing mass

4.07

5.66

5.7

5.89

4.85

3.96

4.88

Modified from Ujiie et al. (2007b). ϕ, volume fraction of unmelted grains; R, average aspect ratio of unmelted grains; n, numbers of analyses.

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Page 15 of 30hydrothermal alteration, which may be a precursor tofrictional melting (Magloughlin 1992), or from the alteration of the pseudotachylyte matrix. The presence of
illite and chlorite was also reported in pseudotachylyte
layers from the Kodiak accretionary complex, Alaska, andwas interpreted as representing the alteration of the pseudotachylyte matrix (Meneghini et al. 2010). Pseudotachylytes are rarely preserved in a recognizable form due torecrystallization, alteration, and subsequent deformation
by cataclastic and crystal-plastic mechanisms (Kirkpatrick
and Rowe 2013). In accretionary complexes, the primarycontrol on the scarcity of pseudotachylytes appears toresult from hydrothermal alteration in fluid-saturated
fault zones.
Effects of frictional melting on earthquake faulting
The estimation of shear resistance during frictional melting
is crucial to evaluate coseismic fault behavior. Earthquake
faulting associated with frictional melting is controlled byfactors such as strain rate, melt thickness, fault roughness,melt escape from the slip interface, and melt viscosity(Fialko and Khazan 2005; Nielsen et al. 2008). In this section, we show how shear resistance was determined frompseudotachylytes in exhumed accretionary complexes.
By assuming the Arrhenian temperature dependence ofmelt viscosity, the matrix viscosity of frictional melts wascalculated using the Shaw (1972) model (Spray 1993; Ujiie
et al. 2007b; Kirkpatrick et al. 2012):
 BηðT Þ ¼ Aexp
T

ð2Þ

where T is the absolute temperature, and A and B are
the reference viscosity (temperature-independent prefactor) and the activation temperature, respectively.
This model is able to predict the melt viscosity fromthe melt composition and has been used for calculationof the temperature dependence of melt viscosity underanhydrous and hydrous conditions. In the case of thefrictional melt formed in accretionary complexes, thechemical composition of the pseudotachylyte matrix
listed in Table 1 was used for the calculation of thematrix viscosity. It is considered likely that the frictional melting of illite produced the hydrous melt. The
water content in the frictional melt estimated from themissing mass in the pseudotachylyte matrix is 3.96% to5.89% (Table 1), which is comparable to the content ofstructural water from the OH− ions in illite (Deer et al.
1992). Since most of the adsorbed and interlayer waterof the illite would be driven off under the temperature
conditions in pseudotachylyte-bearing fault zones (160°C
to 320°C), the estimated water content appears to be reasonable. The calculated matrix viscosity at a minimum

melting temperature of 1,100°C is low, ranging from 21 to
130 Pa s (Figure 11).
In contrast, frictional melts may show non-Arrhenian
Newtonian viscosity (Dingwell and Webb 1989; Dingwell
et al. 1993). The non-Arrhenian temperature dependenceof the melt viscosity was expressed by the Vogel-FulcherTammann (VFT) equation based on >1,770 measurementsof viscosity on multicomponent anhydrous and hydrous
silicate melts:logη ¼ A þ

B
T −C

ð3Þ

where A, B, and C are adjustable parameters (Giordano
et al. 2008). We calculated the non-Arrhenian temperaturedependence of the matrix viscosity of frictional meltusing the viscosity calculator of Giordano et al. (2008)
and adopted the same chemical composition of thepseudotachylyte matrix and water content as used for
the Arrhenian temperature dependence of the matrix
viscosity of frictional melt (Table 1). The fluorine content is assumed to be zero. The matrix viscosity at
1,100°C is in the order of 20 to 111 Pa s, which is of thesame order as that used to consider the Arrhenian
temperature dependence (Figure 11).
Unmelted grains and bubbles influence the viscosityof a melt (Metzner 1985; Mueller et al. 2010); for example, the viscosity of silicate melts becomes nonNewtonian when the volume fraction of the crystals inmelts exceeds 40% (Lejeune and Richet 1995). The relative viscosity (i.e., the ratio of the viscosity with solid

Figure 11 Matrix viscosity versus temperature for sevenpseudotachylyte samples from the Shimanto accretionary
complex. Both Arrhenian (blue lines) and non-Arrhenian (red lines)
temperature dependences of the melt are considered in thecalculation of the matrix viscosity.

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grains to the inclusion-free fluid viscosity) can be expressed
as (Kitano et al. 1981):
  −2ϕηr ¼ 1−
ð4Þ
A
with A = 0.54 − 0.0125R, where A is the parameter related to the packing geometry of solid grains, and R isthe average aspect ratio of the unmelted grains determined from SEM back-scattered electron images (Table 1).
The Arrhenian and non-Arrhenian temperature dependence of the viscosity of the frictional melt at 1,100°C wasestimated to be in the range of 85 to 290 and 79 to
258 Pa s, respectively. The volume fraction of vesicles in
the pseudotachylyte is less than 5%; therefore, the effect of
the bubbles on the bulk viscosity would be negligible.
The viscosity of the frictional melt in accretionary
complexes is low and is independent of the Arrhenianand non-Arrhenian temperature dependence. This low
viscosity is attributed to the high melting temperatureof ≥1,100°C, the water content of illite, and the small
volume fraction of unmelted grains. The seismic slip along
the thin (mostly less than a few millimeters in thickness)layer would induce the high heat production rate andtemperature, which in turn would reduce the viscosityof the frictional melt. Assuming the Arrhenian Newtonianviscosity of the frictional melt, the shear resistance (τ)
along the melt layer is expressed as:
τ¼η

dγ
dt

ð5Þ

where η is the viscosity of the frictional melt and dγ/dt isthe shear strain rate. Considering τ along a 1-mm-thickmelt layer at a slip rate of 1 m/s and η at a minimum melting temperature of 1,100°C, τ is in the range of 0.1 to
0.3 MPa. The estimated shear resistance could provide alower bound of the melt strength because the calculation
assumes a Newtonian rheology.
Provided that most of the mechanical work during
earthquake faulting is converted into heat, the shear resistance of the frictional melt is estimated as (Sibson
1975; Di Toro et al. 2005):w
τ ¼ ρ C p ðT m −T hr Þ þ ð1−ϕ ÞHð6Þdwhere ρ = 2,500 kg/m3 is the rock density, Cp =1,000 J kg−1°C−1 is the specific heat at constant pressure,
Tm = 1,100°C is the melting temperature, Thr = 180°C or
250°C is the host rock temperature, ϕ = 0.09 to 0.26 is thevolume fraction of unmelted grains (Table 1), H = 3.2 ×
105 J kg−1 is the latent heat of fusion, w = 1 mm is the typical thickness of the melt layer, and d = 0.1 to 4 m is thefault displacement during earthquakes of magnitudes M5
to M8 (Sibson 1989). The estimated shear resistance inthe presence of the melt layer is in the order of 0.7 to
30.3 MPa (Table 2), and the shear resistance calculated
from Equation 5 is in the same order of that during an M8
earthquake, which occurs in subduction zones.
The high-velocity friction experiments on argillite(typical host rock of pseudotachylytes in exhumed accretionary complexes) show the slip-strengthening behavior during the growth (thickening) of the melt layer(Ujiie et al. 2009), which is in contrast to the typicalslip-weakening behavior during the formation of the melt
layer in crystalline rocks (Tsutsumi and Shimamoto 1997;
Hirose and Shimamoto 2005; Di Toro et al. 2006). Thisslip strengthening is attributed to dehydration increasing
the viscosity of the melt layer, which may be specific toclay mineral-dominated argillite (Ujiie et al. 2009). On theother hand, the apparent coefficient of friction (the ratioof shear to normal stress) during slip strengthening progressively decreases from 1.2-1.4 to 0.4-0.6 when normal
stress (σn) increases from 2.7 to 13.3 MPa (Ujiie et al.
2009), which is consistent with theoretical predictions
showing that viscous shear stress is almost independent of
σn (Fialko and Khazan 2005). These experimental results
on argillite imply that suppression of seismic slip due toviscous braking is expected at shallow depths in accretionary complexes if substantial melt dehydration occurs on
the timescale of seismic slip. However, melt lubrication
may occur at greater depths because the ratio of viscousshear stress to σn progressively decreases with depth.
Assuming the Newtonian rheology of the frictionalmelt, there may be a critical depth at which the frictional strength overcomes the viscous strength (Fialko
and Khazan 2005):ηdγ< μσ ndt

ð7Þ

Table 2 Shear resistance of frictional melt at given fault displacement during earthquake of magnitudes M5 to M8
Shear resistance, τ (MPa)Magnitude

Fault displacement

M

d (m)

Mg-1

Mg-2

Mg-3

Mg-4

Ok-1

Ok-2

Ok-3

5

0.1

30.3

28.9

28.9

28.9

28.2

27.8

28.3

6

0.4

7.6

7.2

7.2

7.2

7.1

7.0

7.1

7

1

3.0

2.9

2.9

2.9

2.8

2.8

2.8

8

4

0.8

0.7

0.7

0.7

0.7

0.7

0.7

Modified from Ujiie et al. (2007b).

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where dγ/dt is the shear strain rate, and μ is the friction
coefficient. This may explain why pseudotachylytes inexhumed accretionary complexes were found at depthsof ≥2.5 km. Field observations and high-velocity friction
experiments suggest that there is a transition from melt
lubrication at greater depths to viscous braking at shallow
depths. This could be one of the factors controlling the
updip limit of the earthquake rupture zone in subductionaccretion complexes.
Fluidization

Fluidized fault rocks Fluidized fault rocks that arethought to be associated with earthquake faulting havebeen reported from various geological settings including
normal faults (Smith et al. 2008, 2011), strike-slip faults(Monzawa and Otsuki 2003; Otsuki et al. 2003), andthrusts in the exhumed accretionary complexes (Roweet al. 2005; Ujiie et al. 2007a; Boullier et al. 2009; Brodsky
et al. 2009; Meneghini et al. 2010). Fluidization is aphenomenon by which granular materials in a suspension
move freely, akin to molecules in a gas. Fluidization of
granular materials occurs at high shear rates in a graininertia regime in which stress is transmitted by intergranular collisions (Bagnold 1954; Hanes and Inman
1985). Fluidized fault rocks may be then cohesive andare composed of extremely comminuted granular materials, which could be classified as ultracataclasite. Grainsin the ultrafine-grained matrix are subangular to rounded,suggesting preferential abrasion and wearing of the grain
corners. In the exhumed accretionary complexes, fluidized fault rocks were typically found in the argillaceousmélange and the underplated basalt (Figure 8).
In the early Tertiary accretionary complex of Kodiak
Island, Alaska, the argillaceous mélange continuously
develops along strike and has been interpreted as representing a paleo-subduction thrust that was subducted at
depths of 12 to 14 km and temperatures of 230°C to
260°C (Byrne 1984; Fisher and Byrne 1987; Vrolijk et al.
1988). The approximately 0.3-m-thick black and aphanitic fault rocks have developed in the 15- to 20-m-thickcataclasite that is derived from the argillaceous mélange(Rowe et al. 2005; Meneghini et al. 2010). The cataclasite
is considered to represent episodes of localized deformation during subduction (Rowe et al. 2013). At outcrops,
the black fault rock commonly exhibits deformationalfeatures consistent with fluidization (Rowe et al. 2005;
Brodsky et al. 2009; Meneghini et al. 2010). While thelower boundary of the black rock is smooth and subplanar, the upper boundary is marked by flame-like intrusions of black rock into the cataclasite or flow structuresand in places the black rock mixes with the overlying
cataclasite (Figure 12a). The smoothly curved forms of
the upper boundary and the ductilely deformed cataclasite adjacent to the black rock suggest the mobilization

Page 17 of 30

of black rock and cataclasite during intrusions. The cuspate form of the black rock intrusions suggests that it isless viscous than the overlying cataclasite. When the upperboundary is composed of a sandstone-rich mélange, theblack rock shows sharp-walled injection veins into theoverlying sandstone.
At the microscale scale, the black fault rock displays
internal layering composed of ultracomminuted granularand crystalline materials, with the latter interpreted torepresent tabular zoned microlites of plagioclase in pseudotachylytes (Meneghini et al. 2010). Flow, intrusion, andfolding structures are observed along the boundary between granular and crystalline microlayers, suggestingthat granular material and frictional melt deformed together in a ductile manner (Figure 12b). The granularmaterial shows a homogeneous texture without any indication of slip localization. The intimate associationof granular and crystalline microlayers suggests thatfluidization is synchronous with frictional melting, butthat it occurred prior to solidification of the frictional
melt. In another case, clasts of granular and crystalline
materials are found in the crystalline and granularmicrolayers, respectively, showing mutual brittle truncation and incorporation into the surrounding granularmaterial (Figure 12c). This suggests that each earthquake faulting event occurred after the solidification offrictional melt and fluidized material.
In the Late Cretaceous to early Tertiary Shimanto accretionary complex of eastern Shikoku, the basalt slices andmélange (blocks of hemipelagic red shale, acidic tuff, andsandstone in a sheared argillaceous matrix) are imbricatedalong thrusts, representing a duplex underplating associated with the downward step of the subduction thrust at
depths of 4 to 6 km and temperatures of 130°C to 150°C(Ikesawa et al. 2005) (Figure 8). One well-exposed duplex
fault zone is a few tens of meters thick and is composedof basalt and basalt-derived fault rocks, in which slip
was concentrated along a few-centimeters-thick ultracataclasite at mélange and basalt/basalt-derived foliatedcataclasite boundary. The ultracataclasite is thought tohave recorded the fluidization of comminuted granularmaterials (Ujiie et al. 2007a) given that the basalt-derived
comminuted material is commonly injected into the overlying mélange, suggesting the mobilization of the fluidizedmaterial (Figure 12d); basalt- and mélange-derived comminuted materials are locally intermixed without shear
surfaces, suggesting non-laminar (or turbulent) flow associated with fluidization (Figure 12e); fragmented counterparts (i.e., clast fragments that were originally part of alarger clast) are rarely identified in the injected materials,
indicating that most of the fragmented counterparts
moved away from one another in relation to fluidization(Monzawa and Otsuki 2003; Otsuki et al. 2003); and thehomogeneous texture without discrete slip surfaces

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Figure 12 (See legend on next page.)

Page 18 of 30

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Page 19 of 30(See figure on previous page.)
Figure 12 Macroscopic and microscopic occurrences of fluidized fault rocks. (a-c) Fluidized fault rocks from the Kodiak accretionary
complex (modified from Brodsky et al. 2009; Meneghini et al. 2010). (d-f) Fluidized fault rocks from the Shimanto accretionary complex (modified
from Ujiie et al. 2007a). (a) Flame-like intrusions of the black fault rock into the overlying cataclasite, and mixing flow between the black fault rockand the cataclasite. (b) Flow deformation developed in granular (Gr) and crystalline (Cy) microlayers (plane-polarized light). (c) Back-scattered electron
image showing clasts of crystalline material (white arrow) in the granular (Gr) microlayer. (d) Injection of the basalt-derived comminuted material into
the overlying mélange. Black double arrows indicate the ultracataclasite showing injection structures. (e) Zone of mixing flow between basalt- andmélange-derived comminuted materials (black double arrow). (f) Homogeneous texture typically observed in the slipping zone showing
injection structures (plane-polarized light).

could reflect that fluidization homogenized the internal
texture of the slipping zone (Figure 12f ). The very similar grain size distribution between the tip and the rootof the injection vein indicates that settlement of thecoarser grains by gravity is unlikely to have occurred
during the injection, suggesting a rapid injection of
fluidized granular material during faulting (Ujiie et al.
2007a).
The fluidized rock and the basalt slice immediately
below (≤1 m) the fluidized rock constitute the altered zone
where the magnetic susceptibilities are 1 or 2 orders ofmagnitude lower than those of basalt and basalt-derived
foliated cataclasite in the fault zone due to a loss ofmagnetite and maghemite, and where color changes
from dark green/dark red to light green/light brown are
observed due to increased amounts of alteration products
such as chlorite, mixed-layer chlorite/smectite, corrensite,and calcite (Ujiie et al. 2007a; Kameda et al. 2011). In
addition, calcite and laumontite veins are also concentrated in and around the altered zone, showing a mutuallycrosscutting relationship between veins and the fluidizedzone. These features suggest that fluidization occurredunder fluid-infiltrated conditions.
The fluidized rock in the underplated basalt preserves
evidence for frictional heating (Ujiie et al. 2008; Kameda
et al. 2011). Fluid inclusions trapped in calcite can reequilibrate and increase in volume (i.e., stretching) whenheated to temperatures higher than those of the initialentrapment at the entrapment pressure, resulting in an
increase in homogenization temperatures (Roedder 1984).
Ujiie et al. (2008) examined a distribution of homogenization temperatures across fluidized rock and revealed
that the asymmetric, skewed frequency distributions of
the homogenization temperatures due to stretching of
fluid inclusions in calcite only occur in the fluidized
rock; this feature was absent in the surrounding rocks
including the altered basalt immediately below the fluidized rock (Figure 13a). Based on rapid overheating
experiments on fluid inclusions in calcite, those authors also showed that histograms of homogenization
temperatures from the fluidized rocks could be reproduced by a 50°C to 150°C temperature rise along thefluidized rock with respect to the ambient temperaturesof 130°C to 150°C.

Kameda et al. (2011) examined the clay mineralogy
across the fluidized rock where the stretching of fluidinclusions in calcite due to frictional heating was detected by Ujiie et al. (2008). They found that the fluidized rock shows a markedly higher chlorite content in
the mixed-layer chlorite/smectite with respect to thesurrounding rocks (Figure 13b), which cannot be explained by hydrothermal alteration or regional very low
grade metamorphism, but is consistent with frictional
heating associated with localized slip. The estimated temperatures during the increased heating determined fromchlorite geothermometry and the presence of small quantities of corrensite range from 185°C to 260°C (Kameda
et al. 2011). This suggests that the temperature rise during
frictional heating is in the range of 35°C to 150°C, consistent with that determined from the stretching of fluid inclusions in calcite. Fluidization requires a reduction of
normal stress on the fault, and several driving mechanisms
have been proposed for fluidization in faults, including
strong, short-wavelength and normal interface vibrations(Brune et al. 1993; Melosh 1996) and fluid pressurization
and reduction of effective normal stress due to frictional
heating (i.e., thermal pressurization) (Sibson, 1973; Rice
2006). Theoretical models of thermal pressurizationsuggest that the temperature rise is <200°C when pore
dilation and loss of heat and fluid are negligible during
seismic slip (e.g., Lachenbruch 1980; Mase and Smith
1987). Therefore, fluidization at a small temperaturerise of less than 150°C under fluid-infiltrated conditions
most likely occurred under thermal pressurization.
Effects of fluidization on earthquake faulting Compared to a grain friction regime in which grain friction andcomminution are the dominant deformation mechanisms,
fluidization in a grain-inertia regime is expected to reduce
the shear strength of faults. In fact, high-velocity (1.3 m/s)
friction experiments on natural fault gouges successively
reproduced fluidization-related microstructures found innature, during which the apparent coefficient of friction(ratio of shear to normal stress) decreased to approximately 0.1 to 0.2 (Mizoguchi et al. 2009; Kitajima et al.
2010; Ujiie and Tsutsumi 2010).
Mizoguchi et al. (2009) reported folding and fluttering structures in the gouge after high-velocity friction

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Figure 13 Evidence for frictional heating preserved in fluidized rock. (a) Histograms of homogenization temperatures of fluid inclusions incalcite (modified from Ujiie et al. 2008). (b) XRD patterns for oriented samples of the <2-μm size fractions in ethylene-glycolated state (modified
from Kameda et al. 2011).

experiments under dry (room humidity) conditions. Suchmicrostructures suggest that the Reynolds number waslarge enough for the generation of turbulent flow, possibly
due to a decrease in viscosity and/or an increase in theeffect of inertial flow.

Ujiie and Tsutsumi (2010) demonstrated that the steadystate shear stress at high slip rate is almost independent of
normal stress under wet (water-saturated) conditions, withthe resultant microstructure showing grain size segregation associated with granular collision in the fluidized flow

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Page 21 of 30(Figure 14). A similar microstructure was reported fromthe fault gouge that is interpreted to be the principal slip
zone during the 1999 Chi-Chi earthquake in Taiwan(Boullier et al. 2009). The dispersive pressure (P) related
to granular collision in the fluidized flow is proportional
to the square of the shear rate (dγ/dt) (Bagnold 1954;
Hanes and Inman 1985):
 2
 2
dγ
2 V
P∝ρðλDÞ
≈ρðλDÞ
dtw
2

ð8Þ

where ρ is the density of the granular fluid materials, λis the linear grain concentration defined by the ratio of
the grain diameter to the mean free separation distance
between the grains, D is the grain diameter, V is the slip
rate, and w is the thickness of the slip zone. Equation 8
indicates that the difference in P between the large andsmall grains is greater at high V, which results in the formation of voids around the larger grains. Small grains
have a higher probability of filling these voids than thelarger ones do, and so the larger grains migrate upwards
as the smaller grains fill the voids through downward
movement under the influence of gravity. Such grain size
segregation is comparable to the Brazil-nut effect: whengrain mixtures are shaken or collided, the larger grains
rise to the top, whereas smaller grains move downwards
under gravity (Rosato et al. 1987; Breu et al. 2003). Grainsize segregation is observed in the gouge sheared at slip
rates of ≥0.62 m/s, possibly reflecting the fact that theBrazil-nut effect is caused by the large difference in P
between the large and the small grains at high V (Ujiie
and Tsutsumi 2010). These experimental and microstructural features suggest that the gouge behaved like a
fluid during fluidization at high slip rates. As the shear
stress of gouge exhibiting fluid-like behavior is nearly independent of the normal stress, the apparent coefficient

Figure 14 Grain size segregation seen in the gouge after
high-velocity friction experiments under wet conditions. This
microstructure is absent at low slip rates and is potentially a new
microstructural indicator of fluidization at high shear rates(back-scattered electron image).

of friction would be very low, particularly at larger
normal stresses.
Analysis of the geometry of intrusive structures may be
useful in estimating the shear resistance during earthquake
faulting. As shown in Figure 12a, cusped intrusions extending upward from the black fault rock could represent
gravitational (Rayleigh-Taylor) instability that occurs whena lower density layer is overlain by a higher density layer,
and the denser layer flows to accommodate intrusions
from the less dense layer (Allen 1985). Brodsky et al.
(2009) measured both the spacing of intrusions and thedensity of layered materials, and then conducted a linear
stability analysis using the fluid dynamics of the RayleighTaylor instability. Their results show that the spacing between intrusions is best explained by a moderately inertial
flow with fluid velocities in the order of 0.1 m/s. The
viscosity determined from the Reynolds number of thebuoyant flow is less than 1 Pa s, which requires a very
low shear resistance of less than 20 Pa (Brodsky et al.
2009).
There are limited studies on dynamic processes occurring during fluidization in faults. However, all results currently available suggest that the fault was lubricated due to
fluidization.
Fluid-rock interactions in fault zones

Fault zones in exhumed accretionary complexes record
fluid-rock interaction processes, many of which are interpreted in terms of a seismic cycle or coseismic fluid-rockinteractions (Okamoto et al. 2006; Meneghini and Moore
2007; Ujiie et al. 2007a; Ishikawa et al. 2008; Hamada et al.
2011; Yamaguchi et al. 2011a, 2012; Kimura et al. 2013).
The Taiwan Chelungpu Fault Drilling Project successfully recovered the fault zone associated with the Chi-Chi
earthquake, for which a large coseismic displacement ofapproximately 8.3 m was recorded near the drill sites(Ma et al. 2006). Ishikawa et al. (2008) analyzed thetrace element concentrations and isotope ratios acrossthe active Chelungpu Fault. The results show that there
are marked enrichments of Sr; depletions in Li, Rb, andCs; and a decrease of strontium isotope in the 2- to 15cm-thick fault gouges (Figure 15). Indeed, one of gouges
at a depth of 1,136 m is very likely to have slipped during
the Chi-Chi earthquake (Ma et al. 2006). Model calculations reveal that the observed patterns of fluid-mobile
trace elements and isotope ratios in the gouges are well
reproduced when solid-fluid interactions occur at temperatures higher than 350°C. Abrupt changes of fluid-mobile
elements and of strontium isotopes in the gouges are suggested to represent the interaction of high-temperature
pore fluid and gouge materials during earthquake faulting(Ishikawa et al. 2008). A very similar feature was found in
the 5- to 10-mm-thick slip zone in the thrust of the Emi
Group, Boso Peninsula, Japan, where the accretionary

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Figure 15 Trace-element concentrations and isotope ratios across the possible rupture zone during the 1999 Chi-Chi earthquake.
Modified from Ishikawa et al. (2008).

prism has been exhumed from 1- to 4-km depth following
the arc-arc collision in central Japan (Hamada et al. 2011).
These studies demonstrate that fluid-mobile trace elements and isotope ratios are useful tools for the detection of the recent and past coseismic presence ofhigh-temperature fluid (>350°C) in the slip zones.
The Rodeo Cove thrust in the Marine Headlands
terrane, Franciscan complex, which imbricates basaltchert-sandstone sequences, is considered to have been
formed by a duplex underplating at depths of 8 to 10 km
and temperatures of 200°C to 250°C (Meneghini andMoore 2007). Thrust-related deformations mainly occur
along the basalt, which shows repeated brittle shear,carbonate-filled vein formation, and pressure solution.
The veins are oriented parallel to foliations that formedin association with brittle shear and pressure solution,
which can be explained by either a switching of the principal stress orientations between the intervals of veining
and the pressure solution, or a fixed stress orientation witha difference in tensional strength parallel to and perpendicular to the foliation. In either case, very high fluid pressure is required. Meneghini and Moore (2007) interpreted
the veining as representing an immediately post-seismic
fluid pressure drop and consequent carbonate precipitation, while shear- and pressure solution-related foliations
reflect slow interseismic deformations.
The underplating-related thrust in the Late Cretaceous
to early Tertiary Shimanto accretionary complex ofeastern Shikoku, which imbricates basalt-mélange sequences (Figure 8), shows different aspects of veining(Ujiie et al. 2007a; Yamaguchi et al. 2012). Here, both network and fault-filled veins are recognized (Yamaguchi
et al. 2012). Network veins develop in the mélange adjacent to the basalt-derived ultracataclasite that records

fluidization, and many are extensional veins that areoriented perpendicular to the thrust. The vein-fillingminerals are primarily calcite and secondarily laumontite/
quartz. In some cases, the network veins show a mutuallycrosscutting relationship with the ultracataclasite, andin other cases, the mélange, in which the network veinsare intensely developed, is fragmented and incorporated
into the basalt-derived foliated cataclasite. These features indicate repeated episodes of faulting and veining.
The orientation of extension veins suggests that σ1 wasperpendicular to the thrust between faulting episodes,
suggesting the presence of a weak fault due to fluidpressurization. The fault-filling veins are restricted to thegently dipping segment of the thrust with a length shorter
than a few tens of meters. This type of vein is marked byangular to subangular fragments of ultracataclasites andaltered basalts in calcite matrix. The non-planar geometryof the thrust and the spatial distribution and texturalfeatures of fault-filling veins are suggested to represent
implosion brecciation (Sibson 1986), which could occur
in association with a rapid depressurization at dilatantjogs in a thrust system during earthquake faulting (Ujiie
et al. 2007a; Yamaguchi et al. 2012). Yamaguchi et al.
(2012) revealed that the rare earth element patterns aredifferent between the two vein types; while the source
fluid of network veins was affected by a sorption process,that of fault-filling veins was affected neither by sorption
nor by complexation processes. This suggests a temporalchange in the physico-chemical behavior of the fluid along
the thrust, which may occur in response to a seismic
cycle.
Possible implosion breccia was also reported from thesubsidiary fault in the hanging wall of the major out-ofsequence thrust (Nobeoka Thrust) in the Cretaceous-Ujiie and Kimura Progress in Earth and Planetary Science 2014, 1:7
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Tertiary Shimanto accretionary complex of eastern Kyushu(Okamoto et al. 2006; Kimura et al. 2013). The resultsof fluid-inclusion microthermobarometry indicate that
the Nobeoka Thrust was active at temperatures of 180°C
to 300°C and fluid pressures of 140 to 250 MPa, which
may be analogous to the deeper part of the megasplay
fault in the Nankai Trough (Kondo et al. 2005) (Figure 8).
The breccia is marked by fragments of host rock (phyllite)in the carbonate matrix and is found in dilational jogs (orreleasing bends along the undulatory fault plane) of lessthan a few tens of centimeters in length. The 1-mm-thickpseudotachylyte, found along the fault, cuts the breccias.
Okamoto et al. (2006) and Kimura et al. (2013) interpreted
the breccias as representing implosion brecciation at
dilational jogs, while the pseudotachylyte resulted fromfrictional melting under relatively high effective normal
stress after depressurization.
The fault zone associated with the Nobeoka Thrust
contains mutually crosscutting faults and extension veins.
Yamaguchi et al. (2011a) conducted major element mapping and trace element analyses of these veins and found atemporal change in fluid redox state. Here, quartz-calcite
extension veins were derived from relatively oxidized
fluids, while ankerite fault veins with a positive europium
anomaly originated from relatively reduced fluids. Such atemporal change in fluid redox state may be linked to theseismic cycle (Yamaguchi et al. 2011a). Relatively oxidized
fluids likely induced hydraulic fracturing at the pre-failure
stage, resulting in the formation of extension veins. In
contrast, the relatively reduced fluids precipitated along
the faults at the immediate post-failure stage, which could
have been caused by the advection of an exotic reducing
fluid from serpentinized mantle peridotites or by the generation of hydrogen from coseismic mechanochemical
reactions.
Geological and experimental aspects of slow earthquakes

Studies using seismic reflection, ocean bottom seismometers, and seismic tomography suggest that the source
regions of low-frequency seismic events correspond tozones of elevated fluid pressure, suggesting a very low
stress drop during these events (Ito and Obara 2006b;
Shelly et al. 2006; Matsubara et al. 2009; Kitajima and
Saffer 2012). Low-frequency tremors and earthquakes at
depths of 30 to 45 km and temperatures of 400°C to
550°C under increased fluid pressures are generally assumed to result from dehydration of the subducting
oceanic plate (Obara 2002; Shelly et al. 2006; Matsubara
et al. 2009). However, the geological aspects and physicalmechanisms controlling slow earthquakes remain elusive(Schwartz and Rokosky 2007). Recently, geological records
of slow earthquakes were examined in accretionary complexes that were exhumed from hypocentral depths andtemperatures of slow earthquakes (Fagereng et al. 2011;

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Hayman and Lavier 2014). In addition, experimental investigations of slow earthquakes have been conductedusing possible representative rocks from their hypocentral
regions (Burlini et al. 2009; Katayama et al. 2012; Ikari
et al. 2013; Saito et al. 2013).
The Triassic accretionary mélange in the Chrystalls
Beach Complex, South Island, New Zealand is considered to have formed during subduction at pumpellyiteactinolite facies metamorphic conditions at temperaturesof 220°C to 350°C (Fagereng and Sibson 2010). In thismélange, the shear and extension veins developed contemporaneously, and the shear veins developed at angles
subperpendicular to σ1, consistent with shear slip at elevated fluid pressure during subduction (Fagereng et al.
2010). The shear veins are subparallel to cleavage in the
mudstone matrix and are marked by incrementally developed quartz slickenfibers (Figure 16). Fagereng et al.
(2011) revealed shear slip increments of 10 to 100 μm
based on measurements of inclusion band spacing. The
quartz-coated shear surfaces are traceable for meters totens of meters; thus, the ratio of slip to potential rupture
length (i.e., u/L) is in the range of 10−6 to 10−5. Considering u/L = Δτ/G (Wells and Coppersmith 1994), whereΔτ is the stress drop and G is the shear modulus for
subducting sediments (3 GPa), Δτ can be estimated in
the range of 3 to 30 kPa (Fagereng et al. 2011). It is likelythat the tectonic setting of the mélange and the very low
stress drop during a single shear slip event are comparable
to the low-frequency earthquakes observed in swarms
during episodic tremors and slow slip events along subduction thrusts. Thus, shear veins with incrementallydeveloped slickenfibers (Figure 16) may be a product oftremor and slow slip (Fagereng et al. 2011).
Saito et al. (2013) examined the fault rocks and frictional
properties of thrusts in the Eocene Kayo Formation in
the Shimanto accretionary complex of Okinawa Island,
Ryukyu Arc. The Kayo Formation consists of coherent
turbidites that were scraped off and accreted to theoverriding plate by subduction of the young oceanic
plate, during which folds and thrusts were buried at
depths of 5 to 6 km (Ujiie 1997). The geological settingof the Kayo Formation is comparable to that of shallow
very low frequency earthquakes (VLFE) in accretionary
prisms (Ito and Obara 2006a; Ando et al. 2012; Sugioka
et al. 2012). Two types of deformations are recognizedin thrusts: a localized slip along the quartz-coated discrete
slip surface and a distributed shear along clay foliations.
The frictional velocity dependence of thrust materials
indicates that quartz-rich thrust material shows velocity
weakening at 0.0028 to 0.28 mm/s but velocity strengthening at 0.28 to 2.8 mm/s, which is favorable for theoccurrence of VLFE. In contrast, the thrust material
with clay foliations only exhibits velocity strengthening
at 0.0028 to 2.8 mm/s. These data suggest that theUjiie and Kimura Progress in Earth and Planetary Science 2014, 1:7
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Figure 16 Shear veins developed in the mélange (modified from Fagereng et al. 2010). Pairs of half arrows indicate sense of shear.
(a) Mesoscopic appearance of a shear vein with slickenfibers. (b) Microscopic appearance of a shear vein cut parallel to the slickenfibers andperpendicular to the shear surface, showing the small angle between the slickenfibers and the vein margin, and the solid inclusion bands
subperpendicular to the slickenfibers (cross-polarized light). (c) Schematic diagram showing incremental growth of slickenfibers.

Ujiie and Kimura Progress in Earth and Planetary Science 2014, 1:7
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occurrence and spatial distribution of VLFE may becontrolled by the quartz content and development ofclay foliations along thrusts. Quartz-rich thrusts derived
from sandy turbidites (quartzose arenite) and quartzcoated slip surfaces (or quartz fibrous shear veins) arepotential sources of VLFE, whereas foliated thrust materials are frictionally stable, thereby arresting the generationof VLFE (Saito et al. 2013).
On the basis of friction experiments on brecciatedhemipelagic silty claystone taken from shallow portionsof the megasplay fault and the plate-boundary thrust in
the Nankai subduction zone, Ikari et al. (2013) proposedthat slip-weakening behavior may be a viable mechanism
for the generation of VLFE, rather than velocity-weakening
behavior. They measured the slip dependence of friction
and the change in friction associated with velocity steps in
the range of 0.03 to 100 μm/s. Although all samples
showed velocity strengthening, the effect of slip weakening
overcomes the initial velocity-strengthening behavior, withthis effect being greatest in the velocity range of 0.3 to
3 μm/s. The maximum effect of slip weakening in thisvelocity range can facilitate slip acceleration, but the effect of velocity-strengthening behavior becomes dominant at high slip velocities, which likely generates slow
slip events or VLFE. Ikari et al. (2013) also reported that
nucleation patch sizes estimated from physical property
measurements on the Nankai samples are comparable
to those expected for the VLFE in the Nankai subduction zone.
Katayama et al. (2012) shed light on the inferred fluid
overpressure in regions of episodic tremor and slip (ETS)at depths of around 30 km. Hydrous minerals in subducting oceanic plates release water into the overlying
mantle wedge due to dehydration reactions, resulting inserpentinization at the plate interface. The serpentinite
has a strong foliation associated with the preferred orientation of highly anisotropic minerals parallel to the plateinterface (Katayama et al. 2009). Kawano et al. (2011)
demonstrated that fluid flow parallel to serpentinite foliation is at least 1 order of magnitude faster than thatperpendicular to foliation based on permeability measurements. It is likely that this permeability anisotropyof serpentinite induces preferential fluid migration
along the subducting plate interface. However, migrating fluid could become trapped at the Moho, because
the gabbro in the lower crust is 2 orders of magnitudeless permeable than the serpentinite in the underlying
mantle (Katayama et al. 2012). This could lead to anincrease in fluid pressure at the corner of the mantle
wedge that overlies the subducting plate, thereby triggering ETS at a depth of about 30 km. Katayama et al.
(2012) proposed that slow earthquakes under elevatedfluid pressure are likely to occur when the velocity of
upward fluid migration is faster than the subduction

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velocity, potentially causing regional variations in lowfrequency seismic events.
The generation of seismic tremor was confirmed fromdehydration experiments on serpentinite (Burlini et al.
2009). The experiments were conducted in a Paterson
apparatus at isostatic stresses of 200 or 320 MPa andtemperatures up to 1,000°C, during which the acoustic
emission (AE) output was recorded by piezoelectric transducers. The results show that tremor and swarms of AEs
were generated when the expulsion of water occurredfrom the dehydration of serpentinite (the timing of dehydration was detected by transient sample cooling due
to the endothermic dehydration reaction of serpentinite).
Seismic signals in the form of AEs are closely similar to
those in natural tremor, and frequency scaling can beestablished between experimental and natural tremors(Burlini et al. 2009). Dehydration of serpentinite, thetypical rock of a subducting oceanic mantle, is expected
to occur at the temperatures in the hypocentral regionof ETS, which is located in the transition zone between
the locked section of the subduction thrust and theaseismically/plastically deforming zone (Obara 2002;
Shelly et al. 2006; Matsubara et al. 2009). The experiments demonstrated that dehydration triggers tremor,
even in the absence of an external differential stress.
Dehydration-derived water may increase the fluid pressure and makes the transition zone highly unstable undera small differential stress, which could trigger ETS and resultant increase in stress acting on the locked section(Burlini et al. 2009).

Conclusions
We have reviewed recent advances in the study of fault
rocks in accretionary prisms. Compared to other tectonic
settings (for example fault zones in the continental crust,as recently reviewed by Faulkner et al. 2010), studies offault rocks in subduction zones are still immature. Studiesof the physical and chemical processes that occur duringsubduction earthquakes also remain in their infancy (seethe detailed review of earthquake physics and chemistrybased on the integrated field and laboratory approach by
Niemeijer et al. 2012). Nevertheless, in the last decade,our understanding of earthquake faulting in subduction
zones has greatly improved through fault rock studies
in accretionary prisms. These new insights gained into
earthquake faulting in subduction zones are summarized
below.
The major result from fault rock studies stemming fromthe NanTroSEIZE project is the detection of past frictional
heating along 2- to 10-mm-thick, dark, clay-rich fault
gouges of the megasplay fault and the plate-boundary
thrust, even at shallow depths (<450 mbsf). High-velocity
friction experiments and permeability measurements conducted on fault zone materials have demonstrated that

Ujiie and Kimura Progress in Earth and Planetary Science 2014, 1:7
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earthquake rupture propagates easily through clay-richfault gouge by dynamic weakening at high slip rates (e.g.,thermal pressurization). On the other hand, the clay-richmaterials commonly exhibit velocity-strengthening behavior at low slip rates. The NanTroSEIZE project hasclarified that once high-velocity weakening mechanisms
begin to operate, the propagation of seismic slip could
occur even in initially velocity-strengthening materials
at shallow depths (Figure 17). This fault behavior would
therefore enhance tsunami generation during subduction earthquakes.
Propagation of seismic slip along velocity-strengthening
materials may also occur at deeper depths in subduction
zones. Pseudotachylytes in the Shimanto accretionary
complex are derived from frictional melting of illite-richslip zones that commonly show velocity-strengthening
behavior at low slip rates (Saffer and Marone 2003), although some low-velocity friction experiments on illite
gouge exhibit stick-slip behavior (Moore et al. 1989).
The estimated viscosity and shear resistance during frictional melting are very low, suggesting the enhancementof earthquake rupture propagation due to melt lubrication. However, high-velocity friction experiments on thehost rock of pseudotachylytes (argillite) suggest that viscous braking associated with the dehydration increasing
viscosity of the melt layer could occur at shallow depths.
If frictional melting is a dominant coseismic deformation

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mechanism along the fault in a subduction zone, then thetransition from melt lubrication at deep depths to viscousbraking at shallow depths may be one of the factors controlling the updip limit of the earthquake rupture zone.
An important implication of this is that the updip limit of
the earthquake rupture zone should be viewed in terms ofdynamic processes that operate at high slip rates, rather
than the change from velocity-weakening to velocitystrengthening frictional behavior (Figure 17).
A further discovery made using fault rock studies in theexhumed accretionary complexes is the fluidized rocks
within or along the margin of the tectonic mélanges,
which are thought to represent paleo-subduction thrusts.
The fluidized rock-bearing fault zones commonly record
fluid-related phenomenon, and some of fluidized rocks
preserve evidence for a small temperature rise of less than
150°C associated with frictional heating. These features
suggest that fluidization may be caused by the restraintof a temperature rise by thermal pressurization. In the
Shimanto accretionary complex, the thickness of thefluidized zones is typically a few centimeters, which is 1
order of magnitude thicker than that of molten zones.
In contrast to frictional melting, it is probable that fluid
does not escape from the slipping zone during earthquake
faulting, which results in thermal pressurization-induced
fluidization. In addition to transport properties such aspermeability, the thickness of the seismic slip zone appears

Figure 17 Concept of earthquake faulting in subduction zones shown in schematic profile of the Nankai accretionary prism. (a) Previous
view of seismogenic zone. Seismogenic zone shows velocity-weakening frictional behavior, whereas aseismic zone exhibits velocity-strengthening
frictional behavior. (b) Improved view of earthquake faulting in subduction zones after fault rock studies in the NanTroSEIZE and the Shimanto
accretionary complex. Fault segments showing velocity-strengthening behavior at low slip rates could be weakened as a result of dynamic
weakening. This could occur at various depths in the subduction zone, in some cases resulting in shallow slip. In this conceptual model, theupdip limit of the earthquake rupture zone corresponds to the location where dynamic weakening is no longer effective. Possible causes ofearthquake rupture termination include viscous braking due to an increase in melt viscosity and ineffective thermal pressurization/fluidization
due to increased effective normal stress associated with decreased fluid pressure.

Ujiie and Kimura Progress in Earth and Planetary Science 2014, 1:7
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to be an important factor controlling coseismic deformation mechanisms. The estimated viscosity and apparent
coefficient of friction (or shear stress) during fluidization
are very low, showing that fluidization is also a dynamic
weakening mechanism of faults during subduction
earthquakes.
The fluid-rock interaction recorded in fault zones ofexhumed accretionary complexes has been mostly interpreted in terms of a seismic cycle or coseismic fluid-rockinteractions. Geological and experimental investigationsof subduction zone rocks suggest that fluid plays a keyrole in slow slip processes in subduction zones, which isconsistent with the results of geophysical measurements. However, there has been limited research in thisarea, and future study is required to examine the frictional
properties of various lithologies in subduction zones.
On-land accretionary complexes and metamorphic rocks
exhumed from source depths of slow earthquakes are expected to record the slow slip processes in subduction
zones. Future geological studies are required to shed lighton slow earthquakes.
Finally, multi-disciplinary and integrated approaches
are crucial to gain a better understanding of the dynamics
and physico-chemical processes operating during earthquakes in subduction zones. They include investigationsof fault rocks in accretionary prisms, laboratory experiments using subduction zone materials, numerical modeling incorporating the results of fault rock studies andlaboratory measurements, geophysical surveys in subduction zones, and sampling and monitoring of active fault
zones by deep-ocean drilling. In particular, geophysicalsurveys and deep-ocean drilling are considered the mostpromising and only avenues for investigating earthquake
processes in non-accreted, erosive margins.
Abbreviations
ETS: episodic tremor and slip; IODP: Integrated Ocean Drilling Program;
mbsf: meters below sea floor; NanTroSEIZE: Nankai Trough Seismogenic
Zone Experiment; VLFE: very-low-frequency earthquakes.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
KU and GK wrote the manuscript and prepared the figures. KU carried out
the experimental and microstructural studies. Both authors read andapproved the final manuscript.
Acknowledgements
We would like to thank all of our collaborators for their insightful andthoughtful discussions. The members of the ‘IODP NanTroSEIZE Expedition
316 fault rock team’ (Frederick Chester, Daniel Curewitz, Olivier Fabbri, David
Goldsby, Chun-Feng Li, Arito Sakaguchi, Akito Tsutsumi, and Asuka Yamaguchi)
greatly contributed to the shipboard data collection and post-cruise sample
treatment and measurement presented here. We would like to give special
thanks to Akito Tsutsumi, Asuka Yamaguchi, Arito Sakaguchi, Yuri Fialko, Jun
Kameda, Yoshitaka Hashimoto, Haruka Yamaguchi, and Shoichi Toh for theircollaborative research in the field and laboratory. We owe thanks to Arito
Sakaguchi and Asuka Yamaguchi for providing original graphics in Figures 6
and 7, respectively; to Francesca Meneghini for original photomicrographs

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in Figures 10f, 12b, and c; Christie Rowe for an original outcrop photo in
Figure 12a; Jun Kameda and Tsuyoshi Ishikawa for original graphics inFigures 13b and 15, respectively; and to Ake Fagereng for original photos
and graphics in Figure 16. We thank two anonymous reviewers, Francesca
Meneghini, and Casey Moore for their helpful comments and editing. This
research was funded by the Ministry of Education, Culture, Sports, Scienceand Technology of Japan through grant no. 21107005.
Author details
1
Graduate School of Life and Environmental Sciences, University of Tsukuba,
1-1-1 Tennodai, Tsukuba 305-0006, Japan. 2Research and Development
Center for Ocean Drilling Science, Japan Agency for Marine-Earth Scienceand Technology, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan.
3
Department of Earth and Planetary Science, The University of Tokyo, 7-3-1
Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
Received: 29 November 2013 Accepted: 6 April 2014
Published: 9 May 2014

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