Journal of Structural Geology xxx (2012) 1e12
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Journal of Structural Geology
journal homepage: www.elsevier.com/locate/jsg
Surface roughness evolution on experimentally simulated faults
François Renard a, b, *, Karen Mair b, c, Olav Gundersen b
a
ISTerre, University of Grenoble I, CNRS, BP 53, F-38041 Grenoble, France
Physics of Geological Processes, University of Oslo, Norway
c
Department of Geosciences, University of Oslo, Norway
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 October 2011
Received in revised form
17 February 2012
Accepted 20 March 2012
Available online xxx
To investigate the physical processes operating in active fault zones, we conduct analogue laboratory
experiments where we track the morphological and mechanical evolution of an interface during slip. Our
laboratory friction experiments consist of a halite (NaCl) slider held under constant normal load that is
dragged across a coarse sandpaper substrate. This set-up is a surrogate for a fault surface, where brittle
and plastic deformation mechanisms operate simultaneously during sliding. Surface morphology
evolution, frictional resistance and infra-red emission are recorded with cumulative slip. After experiments, we characterize the roughness developed on slid surfaces, to nanometer resolution, using white
light interferometry. We directly observe the formation of deformation features, such as slip parallel
linear striations, as well as deformation products or gouge. The striations are often associated with
marginal ridges of positive relief suggesting sideways transport of gouge products in the plane of the slip
surface in a snow-plough-like fashion. Deeper striations are commonly bounded by triangular brittle
fractures that fragment the salt surface and efficiently generate a breccia or gouge. Experiments with an
abundance of gouge at the sliding interface have reduced shear resistance compared to bare surfaces and
we show that friction is reduced with cumulative slip as gouge accumulates from initially bare surfaces.
The relative importance of these deformation mechanisms may influence gouge production rate, fault
surface roughness evolution, as well as mechanical behavior. Finally, our experimental results are linked
to Nature by comparing the experimental surfaces to an actual fault surface, whose striated morphology
has been characterized to centimeter resolution using a laser scanner. It is observed that both the stress
field and the energy dissipation are heterogeneous at all scales during the maturation of the interface
with cumulative slip. Importantly, we show that the formation of striations on fault planes by mechanical
abrasion involves transport of gouge products in the fault plane not only along the slip direction, but also
perpendicular to it.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Faulting
Fracture
Frictional sliding
Earthquake mechanics
Heat dissipation
1. Introduction
Earthquake traces appear as more or less linear trends on the
Earth’s surface, however a closer examination reveals that these
ruptures in fact display a rich morphological complexity. Similarly,
exhumed fault planes have been observed to show striations at all
scales (Brown and Scholz, 1985), that can be described using a selfsimilar or self-affine (i.e. fractal) approximation (Power et al., 1987;
Renard et al., 2006; Sagy et al., 2007; Bistacchi et al., 2011). Such
striations have been used to characterize directions of slip either
right after an earthquake or to characterize several slip events on the
* Corresponding author. ISTerre, University of Grenoble I, CNRS, BP 53, F-38041
Grenoble, France.
E-mail address: francois.renard@ujf-grenoble.fr (F. Renard).
same fault (Jackson and McKenzie, 1999; Liu-Zeng et al., 2010). They
represent kinematic indicators to infer large scale tectonic processes
(Petit, 1987; Marrett and Allmendinger, 1990; Mercier et al., 1992;
Yin et al., 1999; Zeilinger et al., 2000) and may be used to calculate
paleostress tensors (Angelier, 1979; Fry, 1999). They are also found in
other geological systems associated with large scale friction, for
example bedrock below an ice-sheet (Eyles and Boyce, 1998) or the
imprints left by submarine landslides on the sea floor (Gee et al.,
2005). The complex morphology of striated fault surface records
a plethora of mechanical processes, such as abrasion, damage and
crack interactions through branching, all at work during faulting and
rupture propagation both at seismic and low velocity. Since fault
roughness contributes to the heterogeneity observed in fault
properties, this parameter has been proposed to account for the
variability of earthquake source parameters (Venkataraman and
Kanamori, 2004; Choy and Kirby, 2004; Schmittbuhl et al., 2006),
0191-8141/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jsg.2012.03.009
Please cite this article in press as: Renard, F., et al., Surface roughness evolution on experimentally simulated faults, Journal of Structural Geology
(2012), doi:10.1016/j.jsg.2012.03.009
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F. Renard et al. / Journal of Structural Geology xxx (2012) 1e12
such as static stress drop (Candela et al., 2011a), and final slip
distribution during an earthquake (Candela et al., 2011b).
Since faults are non-planar, stress concentrations occur at
topographic heights and valleys during slip. Numerical simulations
of slip on fractal faults have shown that the stress can reach the
yield limit at geometric irregularities, leading to secondary off-fault
damage whose amount depends non-linearly on the initial
roughness of the fault (Dieterich and Smith, 2009). This leads to
non-linear scaling between slip distance and fault length, and
importantly, damage is produced all over the fault and not only at
its propagating tips (Griffith et al., 2010). As a consequence, off-fault
damage represents an efficient way to dissipate plastic energy out
of the fault plane. However, part of the energy can also clearly be
dissipated along the fault plane, through melting or grain comminution of the gouge material. In the present study we show that an
additional dissipation mechanism may be at work: the production
of the gouge particles at the origin of surface striation, coupled to
grain reworking through a ploughing effect can also dissipate
energy along the fault plane (in directions parallel and perpendicular to slip), and not only off-fault.
Several mechanisms are responsible for the maturation of a fault
and the development of well-developed fault zones, e. g. (Caine
et al., 1996): fragmentation of particles into smaller pieces,
branching of cracks on the fault plane, wear due to the drag of hard
particles in the fault zone, transient stress concentrations at the
rupture tip, and damage due to the propagation of seismic waves (i.
e. dynamic pulverization, see for example (Dor et al., 2006; Doan
and Gary, 2009)). All these mechanisms contribute to the development of the complex morphology of slip surfaces and may
control how the frictional resistance evolves with cumulative slip.
They also lead to the formation of a fault gouge, observed to have
a scale-free particle size distribution (Storti et al., 2003). Such
processes have been reproduced experimentally on natural rocks
(Wang and Scholz, 1994; Amitrano and Schmittbuhl, 2002),
however, it has been difficult to observe in situ both the developing
microstructures and the dissipation during deformation.
In this study we investigate energy dissipation during frictional
sliding on analogue models of geological faults. We focus on the
development of roughness on simulated faults, demonstrating that
the roughness produced on a mature fault not only involves the
formation of striations parallel to slip but also results in a lateral
component of the roughness pattern. This indicates that lateral
ploughing and fracturing processes in the direction perpendicular
to sliding (but still in the fault plane) are also important in
accommodating slip. Moreover, roughness development is
accompanied by gouge formation, that modifies the shear strength
of the interface.
2. Experimental method
2.1. Experimental set-up
We have built a shear loading apparatus, where a 3 3 cm2
cleaved monocrystal of halite (NaCl), a surrogate for an active fault
rock, was in contact with a rough sandpaper substrate (Struer #80
grit, with an average sand grain size in the range 190e201 microns)
or a gouge layer (Fig. 1). We have chosen halite (i.e. salt) because it is
both brittle and plastic at room temperature, depending on the
intensity and localization of stress. The presence of cleavages in
halite partly controls fracture propagation and initial damage
production at the onset of sliding. During experiments, the halite
slider bore a constant normal load of 12 kg (0.13 MPa normal
stress). Then a horizontal sliding velocity of 1.7, 0.9 or 0.6 mm/s was
applied to the sandpaper substrate (Table 1) using a step motor and
an endless screw connected to the plate that supports the sandpaper. An Omega load cell (LCL-040) connected to the plate allows
measurement of the shear force during sliding with a resolution
close to 0.25% of the measured force. This apparatus is similar to
that used in a previous study where the heat dissipation of a sliding
interface was analyzed (Mair et al., 2006).
We conducted series of experiments either on bare cleaved or
roughened halite surfaces or where a thin layer of granular gouge,
infra-red objective
dead
weight
VD
step motor
HD
V
sand paper
ball bearing
load cell
stiff aluminum frame
NaCl slider
30 mm
Fig. 1. Shear loading apparatus to measure heat dissipation during frictional sliding. A halite (NaCl) slider bonded to a stiff aluminum frame and subjected to a constant (dead
weight) load is in contact with a coarse sandpaper substrate (or gouge layer). The substrate is pulled at a constant velocity (V) by a stepper motor to generate shear at the slider sandpaper (or gouge) interface. Horizontal (HD) and vertical (VD) displacements are monitored using displacements sensors and shear force is recorded using a load cell. An infrared camera, focused at the interface between the slider and the sand paper (or gouge), records the heat dissipation.
Please cite this article in press as: Renard, F., et al., Surface roughness evolution on experimentally simulated faults, Journal of Structural Geology
(2012), doi:10.1016/j.jsg.2012.03.009
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Table 1
List of experiments. B: bare surface, Ro: initial rough surface, GB: salt gouge, GS:
glass beads gouge. Friction: mss. Hurst exponent in the direction parallel to slip (Hpa)
or perpendicular (Hpe). Root-mean square roughness parallel/perpendicular to slip:
rmspa, rmspe. Slip velocity (V) and cumulative slip (d).
Experiment
Type
V mm/s
mss
d
mm
Hpe
Hpa
Rmspe
mm
Rmspa
mm
cleaved
os014
os017
os021a
os021b
os025
os026
os027
os028
os029
os031
os032
os033
os034
os048
os049
os041
os042
os039
os040
os047
os052
os053
os054
os055
os056
B
B
B
B
B
B
B
GB
GB
GB
Ro
Ro
Ro
Ro
Ro
Ro
Ro
Ro
Ro
Ro
Ro
GS
GS
GS
GS
Ro
0
1.7
1.7
0.6
0.6
1.7
1.7
1.7
1.7
1.7
1.7
1.7
1.7
1.7
1.7
1.7
0.9
0.9
0.6
0.6
0.6
1.7
1.7
0.9
0.6
1.7
0
e
e
e
e
1.08
1.09
0.27
0.28
0.28
1.03
1.00
0.98
0.99
0.92
0.93
0.84
0.84
0.90
0.96
0.81
0.61
0.60
0.53
0.59
0.94
0
50
50
50
100
53
50
27
44
35
111
167
213
250
662
709
454
477
410
422
615
831
873
897
913
957
e
0.35
0.44
0.45
0.50
0.12
0.22
0.33
0.26
0.35
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
0.44
e
0.15
0.08
0.09
0.09
0.01
0.13
0.03
0.33
0.40
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
0.26
0.008
0.212
0.170
0.107
0.147
0.119
0.050
0.035
0.008
0.021
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
0.179
0.002
0.045
0.032
0.041
0.057
0.038
0.050
0.015
0.0008
0.022
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
0.045
3
a thermistor onto a black body. The resolution is 0.06 K for
temperature differences, and the accuracy to absolute temperature
is 0.5 K. In the current experiments a field of view of 7 9 mm was
imaged. Due to low normal stress, the range of temperature
increase was modest (e.g. usually less than 1 ) and the produced
heat did not modify the frictional properties of the interface during
sliding. Therefore, infra-red imaging is used here for identifying
plastic deformation (Mair et al., 2006).
2.3. Topography measurements
After the friction experiments were conducted, optical images of
the interface were taken with a binocular microscope. The topography of several samples was measured using white light interferometer micrography (Wyko 2000 Surface Profiler from Veeco).
This involves a microscope using a broad-band white light source
coupled to a Michelson interferometer. A reference arm creates
interference fringes with maximum intensity at equal optical path
lengths of the imaging beam and reference beam. By vertical
movement of the sample and simultaneous image capturing, the
interference intensity envelope and thereby the relative height of
the imaged surface at each pixel is determined with a vertical
resolution of 3 nm. The horizontal resolution depends on the lens
used and with the highest magnification it is at the diffraction limit
of white light, of about 0.5 mm. In the present study, a horizontal
resolution of 1.57 mm was selected. The height fields were processed to remove the planar tilt of the surface.
3. Results
initially w1 mm thick, separated the interfaces (Table 1). The
hardness of the gouge grains was either equal to (sieved halite
grains, 180e250 microns) or greater than (SiO2 sand grains
Aldrich 50 þ 70 mesh, 211e297 microns) the slider hardness. We
monitored horizontal and vertical displacements using linear
variable displacement transducers (LVDT) with sub-micron resolution and measured shear resistance to imposed sliding with the
load cell. These parameters were recorded at 2 kHz and tests were
conducted at ambient room temperature and humidity. In the
experiments with initially bare surfaces, gouge products were
produced during slip and a gouge layer several grains thick developed. For gouge experiments, even if the system was not confined
laterally, a continuous layer of grains remained trapped at the
interface (negligible amounts of gouge escaped from the sides of
the slider). Finally, we have presented experiments where a single
indenter (glass bead, 1 mm diameter) was dragged onto a cleaved
halite surface, to produce a single groove, with associated damage.
Two experiments of this kind were performed, varying the orientation of the produced groove with respect to the main cleavage
directions of the salt crystal (Sarwar, 2008).
2.2. Infra-red light imaging of the interface
Since halite is transparent to infra-red light, we can monitor
heat emission at the sliding surface by looking through the slider
using a high resolution infra-red camera located above the halite
sample and focused at the slider interface undergoing shear (Fig. 1).
We used a Phoenix infra-red camera with a 12-bit InSb matrix
sensor that captures infra-red emissions in the 3e5 mm range. The
result is a time-lapse movie of the infra-red radiations emitted
during frictional sliding, with 256 320 pixels images at a pixel
resolution of 28.7 micron and a capturing rate of 50 frames per
second. The infra-red signal is transformed into approximate
temperature values by calibration using a Pelletier element and
3.1. Friction, infra-red emission, and damage
The typical mechanical behavior and infra-red emission
observed during sliding experiments is presented in Fig. 2. Friction
(shear force/normal load) increases initially during the loading
phase before reaching a peak friction at the onset of sliding. Then
friction slightly decreases and fluctuates about a relatively constant
steady-state value. At the onset of sliding a small dilatant behavior
is observed, usually less than 100 mm. The different experimental
conditions e bare surface and sand gouge e show distinct behaviors, the friction level being systematically higher for bare surface
experiments than those containing a layer of gouge material.
Average temperature change is calculated for a 7 9 mm area
on sliding surface using the infra-red emissions. Both types of
experiments exhibit a moderate increase in average temperature
during sliding (Fig. 2b), always smaller than 1 K. We can therefore
neglect the effect of temperature increase on any change in shear
resistance during sliding. The average temperature rise in the bare
surface experiments increases monotonically with slip, whereas
the gouge experiments show an initial increase followed by
a leveling off or saturation after some characteristic time. This
leveling off is thought (Mair et al., 2006) to reflect a plateau where
heat production and thermal diffusion through the gouge are
equivalent. Thermal diffusion through bare surfaces is likely to be
less and hence the heating term dominates throughout.
Close up thermal images (Fig. 3) highlight the heterogeneity of
the thermal signal associated with the development of a deformation groove in the halite slider. The signal is dominated by a ‘fat
head’ region, where the highest infra-red emission is concentrated,
and a ‘narrow tail’ where heat emission is elevated compared to the
undeformed slider but less than at the head. The width of the
‘head’, where most of the plastic strain is located, is 3e4 times
larger than for the ‘tail’. Each head corresponds to an asperity of the
sandpaper that is dragged along the salt surface, damages it, and
produces a striation. The fact that infra-red emission extends 34
Please cite this article in press as: Renard, F., et al., Surface roughness evolution on experimentally simulated faults, Journal of Structural Geology
(2012), doi:10.1016/j.jsg.2012.03.009
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F. Renard et al. / Journal of Structural Geology xxx (2012) 1e12
shear load / normal load
a
secondary fractures, 3e4 times the groove diameter, develops
(Fig. 5aeb); whereas the scratch is oriented in the direction of the
cleavages, the width of the damage zone is 1e2 times the groove
diameter (Fig. 5c).
1.5
bare surface
gouge
1
3.2. Topography
The topography of a freshly cleaved surface and several striated
surfaces that had undergone various amount of slip were measured
using white light interferometry (Table 1). The data obtained, for
each sample, is a 2D topographic height field of the roughness over
a selected area, from which 1D roughness profiles were extracted
(Fig. 6). For each surface, 1D profiles were characterized by their
root-mean-square roughness xrms defined as:
0.5
0
0
10
20
30
time (s)
temperature increase (°C)
b
xrms
0.5
bare surface
gouge
0.4
0.3
0.2
0.1
0
0
10
20
30
time (s)
Fig. 2. Time evolution of sliding friction (a) and heat emission (b) during experiments
for imposed velocity V ¼ 1.7 mm/s. Data are presented for bare surface experiments,
where the halite slider directly contacts sand paper, and sand gouge experiments
where the slider contacts the sandpaper via a thin (initially 1 mm thick) layer of sand.
Heat emission is presented as average temperature change occurring on a 7 9 mm
area imaged by the infra-red camera and located at the center of the sample sliding
surface.
times the width of the produced striae indicates that dissipation
processes (wear, grain fragmentation, fracturing) occur in a volume
much larger than the size of the ploughing asperity.
This is verified on optical images of a slip surface (Fig. 4), where
striae and gouge products are clearly visible. The grooves have
developed through a ‘ploughing’ effect of the sandpaper asperities.
Interestingly, on the sides of many grooves we observe oblique
fractures (labeled F in Fig. 4aeb) that have damaged the surface,
tearing out fragments that become part of the gouge. Each sandpaper asperity thus inflicts damage both at the actual contact with
the salt, but also in its vicinity. The particles produced are then
presumably dragged and accumulate at the bottom of each groove
(Fig. 4c) or are evacuated sideways. A sketch of this process (Fig. 4d)
shows that volume of the whole damage area is 3e4 times larger
than the diameter of the ploughing asperities.
To better image the damage around a single striae, we have also
performed experiments where a single asperity (glass bead, 1 mm
diameter) was dragged onto a fresh cleaved halite surface,
producing a single scratch (Sarwar, 2008). The resulting damage
(Fig. 5) shows the development of secondary fractures that propagate in a volume around the main scratch. Depending on the
orientation of the scratch with respect to the cleavage directions of
the salt slider, the damage is more or less pronounced: if the scratch
is oriented at 45 to the main cleavage directions, a wide zone of
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ðxi xÞ2
¼
n
(1)
where xi is the height at the pixel i, and x is the average height of the
profile. When the average height of the profile is set to zero, xrms is
equal to the standard deviation, and quantifies the importance of
height variations in the profile. To calculate xrms , several 1D parallel
profiles were extracted from the 2D surface. For each profile, the
mean trend was removed, before xrms was calculated according to
Eq. (1). Then, the xrms of all parallel profiles were averaged (see
Table 1). The freshly cleaved halite surface is almost flat, with xrms
less than 10 mm (Fig. 6a). Striated surfaces exhibit larger xrms,
between 0.03 and 0.2 mm. Moreover, as striated surfaces are
strongly anisotropic, xrms is two to five times larger perpendicular
to the direction of slip, than parallel to it (see Fig. 6b, c). During all
experiments, the vertical displacement measured was small, in the
range of the depth of the grooves produced. For experiments with
gouge, a slight compaction was observed, on the order of 1e2 gouge
grain diameters. This is interpreted to be due to the evolution of the
gouge towards a more compacted state, as gouge loss on the sides
of the slider was found to be negligible. Importantly, the formation
of this roughness involves several processes: formation of grooves
along direction of slip, fracturing on the sides of these grooves, and
transport of grains from the grooves to their sides in a snow-plough
like manner (Fig. 7).
We have also characterized the scaling properties of the rough
surfaces. For this, we have extracted several hundreds of profiles in
the direction of slip and perpendicular to it to analyze the statistical
properties of the slider surface. A Fourier power spectrum method,
which has been shown to be robust and reliable, was used to
characterize scaling properties of rough surfaces and search for
self-affine properties, that are often observed on natural faults
(Renard et al., 2006; Sagy et al., 2007; Candela et al., 2009; Bistacchi
et al., 2011). A 2-D rough profile is self-affine if it remains statistiH
cally invariant under the scaling transformation dx/ldx; dz/l dz,
where x is the coordinate along the 2-D profile, z the roughness
amplitude and H the so-called Hurst exponent. In such a system,
the roughness amplitude depends on the magnification l,
following a power law relationship. If the power law scaling
exponent H lies in the range 0 < H < 1, different magnification
factors will be needed in the directions parallel and perpendicular
to the profile for a small portion of the profile to appear statistically
similar to the entire profile. Then, the profile is called self-affine. If
H ¼ 1, the profile is called self-similar.
For all white light interferometry data of rough slip surfaces, the
topography shows a self-affine geometry. The different slip surfaces
analyzed cover approximately 2 decades of length scales and show
a scaling relationship with a Hurst exponent H that varies with
cumulative slip (Fig. 9). H is close to 0.5 for profiles perpendicular to
the slip direction and slightly smaller along slip (see Table 1).
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5
slip
temperature (°C)
2 mm
0.4
0.3
0.2
1 mm
left-over
groove
active damage
zone
temperature (°C)
0.1
0.35
0.3
0.25
active asperity
Fig. 3. (Top) Infra-red imaging of heat spots localized at the tip of grooves at the frictional interface of the slider. The striations produced correspond to sand paper asperities that
induce permanent damage on the slider surface. The temperature increase due to friction is less than 0.5 C. The slip direction is indicated by the arrow. (Bottom) A zoom on the
active damage zone, which concentrates most of the heat dissipation, shows it is 3e4 times wider than the produced striation. The size of the sand paper asperity is indicated.
3.3. Effects of cumulative slip
The effects of cumulative slip are presented in terms of mean
steady state friction and root-mean square roughness in Fig. 8.
Each point on the graph is a single experiment. In some cases an
individual halite sample was used for a series of experiments,
therefore total cumulative slips of 1000 mm were possible on
a single sample although the slip of an individual experiment was
w50 mm. The data sets are split into three groups: bare surface;
sand gouge; and halite gouge. Mean steady state friction level
(calculated for the portion of the friction curve considered to be
steady state, e.g. Fig. 2) is highest, w0.8e1.0, for bare rough
samples and bare cleaved surfaces. Experiments having a layer of
sand gouge result in intermediate levels of friction w 0.6, whereas
those with halite gouge yielded lower friction w 0.4. A slight
mechanical weakening, characterized by a reduction in steady
state friction coefficient, is observed with increasing amounts of
slip for bare rough surfaces.
Interestingly, the Hurst exponent evolves with slip from lower
values to a more or less constant value reached after around 50 mm
cumulated slip. Here, the initial increase of Hurst exponent (Fig. 9)
is due to the fact that the slip starts on an initial bare surface with
H ¼ 0 and the roughness evolves after a small cumulative slip only
towards a more or less constant value.
For an initially bare surface, the amplitude of the roughness
(calculated as the root-mean-square) increases during the first
w50 mm of slip, then appears to stabilize at larger slips (Fig. 8b) at
a value that stays constant within the error bar of measurements.
For this value of slip, all the initially flat surface is covered with
striations. The root-mean-square roughness values for a 2D surface
and profiles parallel and perpendicular to slip direction are
compared. The roughness of a profile measured perpendicular to
slip is highest, the profile parallel to slip direction is lowest and
perhaps unsurprisingly, the roughness of the 2D surface falls inbetween.
In the experiments, presented in Fig. 9, the Hurst exponent,
calculated in the direction perpendicular to slip is always larger
than for the slip direction. This is also what is generally observed
on natural faults, where H is smaller in the direction of slip (e.g.
(Renard et al., 2006)) than perpendicular to it. A difference is
a smaller Hurst exponent (0.3e0.4) in the experiments, compared
to natural fractures (0.6e0.8). These experimental data of slip
surface anisotropy confirm and extend previous studies showing
the self-affine anisotropy properties of fault surfaces (Power
Please cite this article in press as: Renard, F., et al., Surface roughness evolution on experimentally simulated faults, Journal of Structural Geology
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Fig. 4. Optical images of the slip surface of sample OS017 showing the main characteristics of the grooves (aeb). On the sides of striae, damage is produced in the form of secondary
fractures (F) that wear the slip surface. c) At the bottom of each groove, particles of halite having a grain size in the range 5e20 microns are observed. These particles are interpreted
as the gouge produced. d) Sketch showing active asperities at the extremity of grooves. Most of these grooves show a well-defined striae and secondary fractures, the whole volume
of the damage zone being 3e4 times larger than the asperity diameter.
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7
Fig. 5. Optical images of a single scratch on cleaved halite surface. A bead of silica was dragged on the surface, creating a groove and secondary fractures (labeled F). The amount
of damage depends on the crystallographic orientation. aeb) Groove along the main crystallographic (100) direction, with fractures forming at 45 , along cleavage directions.
c) Groove along the cleavage direction (at 45 from Fig. a), showing much less secondary fractures (Sarwar, 2008).
et al., 1987; Renard et al., 2006; Sagy et al., 2007; Bistacchi et al.,
2011).
4. Discussion
4.1. Striations on natural faults
Fault roughness development represents an important issue in
terms of how stresses are distributed, concentrated, and dissipated
during the seismic cycle (Power et al., 1988). Fault roughness,
which can be measured accurately using high resolution profilometers (Power et al., 1987) or laser distance meters (Power et al.,
1987; Renard et al., 2006; Sagy et al., 2007; Candela et al., 2009;
Bistacchi et al., 2011) exhibit scale invariance properties over
several decades of spatial scales. The rough interface can be
interpreted either as a limit between the core of the fault zone
that contains a gouge and fault rocks (Caine et al., 1996) and the
wider damage zone, or between two adjoining fault blocks. The
latter interpretation applies to those cases where the thickness of
the fault core is much smaller than the length scale at which the
fault is characterized. Several mechanisms have been proposed to
explain the formation of gouge particles during sliding, such as
dynamic pulverization (Reches and Dewers, 1987; Doan and Gary,
2009), fault branching (Power et al., 1988), grain fragmentation
(e.g. Abe and Mair (2005) and references therein), or asperity
ploughing (see Fig. 10). The latter mechanism can be observed on
natural faults when a hard object is dragged along the fault plane
leaving a permanent damage imprint. This is observed, for
Please cite this article in press as: Renard, F., et al., Surface roughness evolution on experimentally simulated faults, Journal of Structural Geology
(2012), doi:10.1016/j.jsg.2012.03.009
Fig. 6. White light interferometry technique is demonstrated by a surface topography image (left) and linear topography transects (right) in two perpendicular directions (X along
slip, Y perpendicular to slip). The black arrow indicates the direction of sliding. (a) Transects on a flat portion of the interface without damage (i.e. fresh surface). (b) Transects at the
bottom of a groove. (c) Transects on the side of a groove where a marginal ridge is observed.
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F. Renard et al. / Journal of Structural Geology xxx (2012) 1e12
9
Fig. 7. Topography images, obtained using white light interferometry, highlight different features developed after increasing amounts of cumulative slip. (aee) Bare salt surface over
sandpaper, with the initial cleaved surface indicated (C). The formation of striations sub-parallel to the slip direction occurred by fragmentation of the salt along grooves (G),
fractures (F), and plastic deformation of the salt that was pushed on the sides of the grooves, producing marginal ridges (R). (f) Topography of an interface where a gouge was
initially present during shearing, demonstrating the lower level of damage production.
example, on the Vuache Fault (French Alps), where high resolution
topography measurements have been acquired using laser
surveying (Renard et al., 2006). On this left-lateral slip fault in
limestone, the direction of slip is indicated by striations at all
scales that form a self-affine morphology of the slip surface. In
Fig. 10 the topography of a 11 8 m2 area of the fault, showing
meter wide striations is displayed. Making the analogy with our
experiments, we infer that some of these striations correspond to
grooves produced by the ploughing effect of an asperity that was
harder than the material lying at the sliding surface. In our
experiments we observed that the ploughing effect created
damage not only inside the groove, but also in a region 3e4 times
wider than the groove itself. Moreover, the wear products are
pushed not only in the direction of slip, but also sideways (i.e.
lateral to slip) hence potentially amplifying the generation of
roughness in that direction by forming marginal ridges.
4.2. Faulting processes in laboratory experiments
To reproduce slip and faulting in laboratory experiments, one
can use conditions of stress and temperature similar to those along
a fault plane at depth and study the quasi-static growth of microcracks, leading to fracturing (e.g. Lockner et al. (1991). To study
dynamic fracture, one can visualize 2D ruptures using photoelasticity (Xia et al., 2004; Nielsen et al., 2010) or 3D ruptures using
ultrafast ultrasonic imaging in analogue materials (Latour et al.,
2011). However, in such experiments, direct imaging of damage
processes in the fault plane itself are generally not possible. In the
present study, damage processes along the slip surface can be
visualized and energy dissipation measured through the use of
infra-red emission. This provides additional information on the
localization of fracturing and plastic deformation processes that
may operate in natural systems.
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F. Renard et al. / Journal of Structural Geology xxx (2012) 1e12
H, perpendicular to slip
H, along slip
0.9
0.8
0.7
1.2
Hurst exponent
steady−state friction coefficient
a
1
0.8
0.6
0.5
0.4
0.3
0.6
0.2
0.4
0.1
0.2
0
0
0
0
500
1000
cumulative slip (mm)
250
200
150
100
50
0
0
500
100
200
300
400 500 600 700
cumulated slip (mm)
800
900 1000
Fig. 9. The Hurst exponent (see text) is calculated and plotted parallel and perpendicular to slip as a function of cumulative slip.
2D surface
1D perpendicular to slip
1D along slip
b
rms of surface roughness (µm)
1
Bare cleaved surface
Bare rough surface
Rough surface + salt gouge
Cleaved surface + salt gouge
Rough surface + sand gouge
1000
cumulative slip (mm)
Fig. 8. a) Steady-state friction of the sliding interface is plotted as a function of
increasing cumulative slip for different initial conditions. b) Root-mean square of the
roughness, for initial flat slider, as a function of cumulative slip. The data are presented
in the direction of slip, perpendicular to it, and for the whole 2D surface. These data
show that the roughness increases from zero (flat surface) to a value that remains more
or less constant after a small amount of cumulative slip (less than 100 mm).
The experimental conditions we apply are clearly far from
conditions at depth in a fault, however the experiments do share
common properties with faults: the presence of a material that can
behave in both a brittle and ductile manner, the formation of gouge
particles, the presence of strong asperities that can pin the interface
during sliding, and a slight dilatancy at the onset of sliding. Such
dilatancy is measured experimentally when faulting rocks are
deformed under upper crust conditions (see Scholz (2002),
Figure 1.24) and slowly vanishes when reaching the ductile regime.
Finally, our experiments are dry, whereas fluids are known to play
an important role during slip in natural faults, due to variations of
pore pressure or chemical effects.
The experimental data indicate that the roughness of an initially
flat interface develops rapidly with slip. Both root-mean-square
roughness and Hurst exponent reach a more or less constant
value after less than 50 mm of cumulative slip, which corresponds
to the distance necessary to cover the initial flat surface with striations. In the experiments, the initial surfaces are flat, and hence
somewhat like natural joints in rocks. The amplitude of striations
reach a steady state that we interpret as imposed by the constant
grain size of the sand paper or the initial sand gouge. In the
experiments the friction coefficient continues to decrease (Fig. 8a)
even if the surface roughness does not evolve much. A possible
explanation would be related to the production of gouge particles,
whose rolling effect could decrease the resistance to shear (Oda
et al., 1982). Another explanation is that the striations that form
fit the sand paper asperities, such that erosion of the sand grains
becomes less efficient.
The evolution of fault roughness under the effect of asperity
ploughing has two consequences on damage, as evidenced by the
experimental results. Firstly, the damage zone around the asperity
is much larger than the produced groove, as demonstrated by the
infra-red emissions (Fig. 3) and we see the formation of fractures
that grow sideways, i.e. oblique to slip (Figs. 4 and 7). In the
experiments, the orientation of these fractures is partly controlled
by the crystallographic orientation (i.e. the fractures labeled on
Fig. 4aeb corresponds to directions of cleavage) of the salt slider
and by the strong elastic contrast between the sandpaper and the
slider. In Nature, these fractures are controlled both by variations in
local stress orientation and by the presence of heterogeneities in
material properties. There too, a strong elastic contrast may exist
between damaged fault rocks and a strong asperity dragged into
the fault zone. Secondly, the gouge material produced is pushed on
each side of the groove, forming a marginal ridge. This indicates
that, during wear, gouge material is transported not only along the
slip direction, but also perpendicularly.
The mechanics of scratching indenters is well-studied in the
engineering community for now more than 40 years (Bowden and
Please cite this article in press as: Renard, F., et al., Surface roughness evolution on experimentally simulated faults, Journal of Structural Geology
(2012), doi:10.1016/j.jsg.2012.03.009
F. Renard et al. / Journal of Structural Geology xxx (2012) 1e12
11
Fig. 10. Topographic images of the Vuache fault surface (from the French Alps) are presented. The fault plane has been measured using a 3D laser scanner to obtain the Digital
Elevation Model of the fault roughness shown. This fault shows clear striations at all scales. Some of these grooves (G) appear to have been created by a ploughing effect of an
asperity that was dragged into the fault zone during slip.
Tabor, 1966; Gee, 2001), and has been, for example, applied to
machine drill bits (Beste et al., 2004). For example, scratch tests are
performed on various materials to measure the erosion, friction and
wear produced. Several experimental results on various materials
have shown the presence of positive ridges developing on the sides
of a striation (Li et al., 1996). The presence of ridges is observed and
numerically simulated in ductile materials (Barge et al., 2003),
whereas secondary fractures also develop in more brittle materials
such as soda-lime glass (Li et al., 1998), ceramics (Lee et al., 2000),
or minerals (Broz et al., 2006). The formation of these cracks was
successfully modeled using finite elements (Holmberg et al., 2003)
for scratches forming on steel coated with TiN ceramic and showing
that tensile stress is generated around the contact, allowing for
fracturing. This secondary damage therefore apparently plays an
important role in dissipating strain within, but also outside of the
main zone of wear production. Such structures appear to be analogous to those we observe in our halite experiments.
5. Conclusions
Our experiments demonstrate that wear production and plastic
dissipation (monitored by the infra-red emission) along a rough
sliding interface is highly heterogeneous when strong asperities
produce striations on slip surfaces. Heterogeneities, both structural
features and stress concentrators, can strongly damage the fault
surface through various deformation processes (gouge production,
asperity ploughing, fracturing), that represent a significant contribution to the overall friction coefficient. Importantly, the fault
interface is not only damaged parallel to slip direction, but also
perpendicularly to slip due to fracturing and ploughing effects that
transport material at various angles to the sliding direction. With
cumulative slip, these combined damage effects appear to result in
a gradual decrease in the friction coefficient due to the development of striations themselves and the accumulation of gouge
particles that dissipate strain by a rolling effect.
Acknowledgments
This work was supported by a Center of Excellence grant from
the Norwegian Research Council to PGP (Physics of Geological
Processes) at the University of Oslo and the grant ANR-09-JCJC0011-01 from Agence Nationale de la Recherche. The authors
acknowledge the contribution of Munib Sarwar and Stephane
Santucci, whose single indentor scratch experiments (presented in
Fig. 5) along with those conducted by the authors, gave additional
insights into the roughness evolution we present in this paper.
Appendix A. Supplementary material
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.jsg.2012.03.009.
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