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Peer Review File
REVIEWER COMMENTS
Reviewer #1 (Remarks to the Author)
:
In this interesting manuscript, by means of newly
-
conceived experiments, it is shown that small
-
scale mineral heterogeneity in fault slip zones controls both fault strength (or the maximum stress
at which a fault fails producing earthquakes or aseismic cre
ep) and the response of the fault to
perturbations in the loading conditions (e.g., far field velocity, stress). This latter response of the
fault is relevant: if, during slip, the fault weakens (or the dynamic friction decreases) faster that
the rate at w
hich the elastic strain energy stored in the fault wall rocks is released, this will result
in a frictional instability (= earthquake ruptures); if the dynamic friction increases or, alternatively,
decreases slower than the rate at which the elastic strain
energy is released, this will result in
aseismic slow slip transients)
.
But what is “fault rock heterogeneity”? Typically, laboratory experiments are conducted on (1)
synthetic fault gouges made by one mineral, (2) synthetic fault gouges made by mixtures
of two
or more minerals and (3) natural fault gouges retrieved from outcrops or fault drilling projects
(almost all natural gouges consist of an assemblage of grains with different composition: clay,
quartz, feldspar, calcite, dolomite, etc.). In cases (2
) and (3) the experimental fault is
heterogeneous in composition (in this manuscript, this mineral assemblage is called
“homogeneously mixed” fault gouge). There are several thousands of published data about friction
experiments conducted on these mixtures
. Instead, there are very few laboratory studies which
reproduce natural slip zones where “fault rock heterogeneity” is related to the spatial arrangement
of two or more minerals in the experimental fault. For instance, motivated by the spatial
arrangement
of different minerals found in natural slip zones of active seismogenic faults,
Smeraglia et al. (Scientific Reports, 2017) considered the “fault rock heterogeneity” as sub
-
parallel
to the fault slip surface. In this case, heterogeneity consists in a comp
ositional layering with clays
next to the slip surface and calcite grains beneath
.
The novelty of this study submitted to Nature Communication is that, for the first time to my
knowledge, the “fault rock heterogeneity” (i.e., layers of clays alternated to
quartz
-
built gouges) is
perpendicular to the slip surface (Fig. 1b). At the millimetre to sub
-
millimetre scale such spatial
organization can be found in faults which cut across foliated rocks (e.g., quartzites, micaschists).
For instance, in the case of i
mpure greenschists facies quartzites, mm
-
cm thick layers made of
quartz grains are alternated to sub
-
millimetre
-
thick layers made of chlorite and white micas
.
Thanks to this newly conceived experimental configuration, the authors show that the small scale
spatial mineralogical heterogeneity (layering) has profound effects on the frictional behaviour of
the experimental fault. In particular, (1) the friction coefficient of the layered heterogeneous
gouges decreases with increasing fault slip (this slip weak
ening behaviour is not observed in
experiments performed on “homogeneously mixed” fault gouges, Fig. 1c
-
d) and (2), layered
heterogeneous fault gouges are, regarding the response of the friction coefficient to loading
perturbations, more unstable; that is,
they are more prone to trigger frictional instabilities and
laboratory earthquakes than “homogeneously mixed” fault gouges (Fig. 3)
.
I find the description of the experimental configuration and of the results well
-
written and the
conclusions based on sol
id experimental evidence. The figures are well
-
designed. I also suggest to
keep the supplemental material as it is (i.e., I really enjoyed the discussion about the effect of
differential compaction on interface weakening). In conclusion, the manuscript is
nicely written and
surely deserves to be published in Nature Communications after considering or discussing the
following minor point
:
My field and experimental experience is that at the scale of natural slip zones (< 2 cm thick, e.g.,
Sibson 2003, Bullet
in of the Seismological Society of America) the compositional heterogeneity is
parallel to the fault slip surfaces (e.g., clay
-
rich layers sub
-
parallel to calcite
-
rich layer in the case
of exhumed seismogenic faults in the Central Apennines, see Smeraglia
et al., Scientific Reports
2017) or, very common as the authors of this manuscript well know, that the mineral composition
in the slip zone is a mixture of minerals (homogeneously mixed fault gouges). This spatial or
mineralogical heterogeneity is easily e
xplained by the smearing and dragging of the wall rock
materials and their mixing in the slip zone, as also shown in Fig. 2a of this manuscript, or by other
processes occurring in natural faults (incongruent pressure
-
solution
-
precipitation, neo
-
formation o
f
clay minerals in natural fault cores, etc.)
.
In this manuscript is discussed a “fault rock heterogeneity” (i.e., layers of clays alternated to
quartz
-
built gouges) perpendicular to the fault slip surface (Fig. 1b). However, as explained above,
such init
ial “micro
-
scale” spatial heterogeneity or compositional layering is lost during initial
shearing of the gouges (Fig. 2a). As a consequence, I suggest the authors to reconsider (or
discuss by introducing some natural examples) the last statement of the abs
tract (lines 20
-
24)
:
“The results demonstrate that small
-
scale geological heterogeneity has pronounced effects on fault
strength and stability, and by extension on the occurrence of slow
-
slip transients versus
earthquake ruptures and the characteristics o
f the resulting events, and should be incorporated in
lab experiments, fault friction laws, and earthquake source modeling.
In fact, the mechanical data of the experiments discussed in this manuscript are representative of
the very first mm or cm of slip
(or slip initiation, depending on the thickness of the layered rocks)
in natural faults cutting layered rocks. Such small
-
scale heterogeneity perpendicular to the fault
slip surface will be lost after few millimetres or tens of centimetres at most of slip
in natural faults
and, as a consequence, these experiments cannot reproduce the conditions that lead to slow
-
slip
transients versus earthquake ruptures in large displacement faults. Instead, if the authors consider
their experimental configuration as an a
nalogue, to be scaled, of large
-
scale spatial heterogeneity
(see their figure 4) associated to large displacement faults which put in contact rocks with different
mineral composition and mechanical properties, I do agree with their conclusion
.
Congratulat
ions to the authors for this nice piece of work
,
Giulio Di Tor
o
Reviewer #2 (Remarks to the Author)
:
This is a really interesting paper and I think it should be published in Nature Communications, but
I have some reservations that would need to be add
ressed
.
The topic is very appropriate for Nature Communications. This paper addresses interesting
questions that are topical and cutting
-
edge in earthquake physics: what causes unstable frictional
sliding and what causes the spectrum of slip behaviors on t
ectonic faults. The experiments are
elegant, the results are clear and the lab work is very well described. But my sense is that some of
the interpretation needs to be reoriented
.
My main concern is that I'm not sure I agree with the title. Does heterogene
ity really promote
unstable slip? The experiments shown indicate that stability is dictated mainly by the clay/qtz
fraction, right? I appreciate the results in Fig 3 but it seems like the main effect there is simply the
result of incomplete mixing given th
e available shear displacement. The data of Fig 3c show that
the effect decreases with shear displacement. Doesn't this imply that the main factor causing a
reduction in a
-
b (as seen in Fig3a/b) is the role of the quartz patch? The quartz is velocity
weake
ning so the fault has more of a tendency for velocity weakening when the fault has a
separate quartz patch. As the quartz patch becomes increasingly mixed the homogeneous and
heterogeneous faults have similar behavior. So, I'm not sure I agree with the ide
a that
heterogeneity is the key ingredient. Another way to make this point is: if heterogeneity itself
promotes unstable slip shouldn't I expect that to happen for a heterogeneous mixture of weak,
velocity strengthening materials
?
A related point involves
the data of Figure 3a. My sense is that these are a key part of the story
but when I look at the raw data of Figure 1c (or supplementary Fig 3) I only see velocity
weakening for clay fractions of 0 and 20%. The raw data for 30 and 40% clay are velocity
str
engthening at all displacements and the a
-
b values for the velocity step at 1.5 mm would be
even larger if the strain weakening trend from 1
-
1.5 mm were accounted for. This doesn't negate
the importance of Fig 3a but it reduces it. In any case it's importa
nt to make sure readers can
follow your analysis
.
Minor points in no particular orde
r
1. How do you measure layer thickness during shear? Hopefully this could be added to the
supplement. The overall thinning seems small; the layer goes from 1 mm to 0.85 mm
after a
shear strain of 10
?
2. How do you discourage or eliminate boundary shear at the edge of the forcing block
?
3. Please provide a bit more detail on the jacket. PVC can be used for rigid plumbing pipe. I
assume you're using another type and it would
be good to provide this information in the
supplement
.
4. Your references are very good. You might also consider commenting on the comparison of your
work with that of Niemeijer et al. GRL 2010 (Fabric induced weakness of tectonic faults)
.
5. A useful addi
tion to your discussion of Figure 4 would be to connect spatial heterogeneity to the
slip patch size for rupture nucleation/propagation. One way to produce slow slip is to limit the size
of the velocity weakening region (for example as shown by Liu & Rice
2005/2007) and your data
provide some nice connections to these ideas
.
6. I like the idea of the color bars for strength and seismic potential in Fig. 4 but I'm not sure I see
the argument for low strength and high seismogenic potential. Maybe this is just
the same point I
raised above, but here you seem to be going beyond the idea of a relative change (heterogeneity
promotes unstable slip) to a more definite statement
.
7. It would be useful to comment on how the kind of heterogeneity you envision would be
sustained in a tectonic fault zone with sizable offset. For example, one could imagine that a fault
zone with initial heterogeneity of the sort you examine would become homogeneously mixed after
a relatively small offset
.
Chris Maron
e
Reviewer #3 (Remar
ks to the Author)
:
Review of 315765
-
0, “Fault rock heterogeneity produces faul
t
weakness and promotes unstable slip” by Bedford, Faulkner and Lapsuta
.
By Terry E. Tulli
s
General Comment
s
My comments are both for the editor and the authors. This is a goo
d paper and could be published
as is. It is clear and succinctly written and the noteworthy results are made clear in the paper,
namely that heterogeneous distribution of rock types along a fault contribute to the strength and
stability of the fault in non
-
intuitive ways and that this is important in modeling earthquakes. It is
a significant contribution to the field. The work and methodology is sound and supports the
conclusions. Enough detail is presented that the results could be reproduced. However, I h
ave
some suggestions for the authors to consider that should not involve much additional work and
should improve an already fine paper. I have included these as comments in both the Word file for
the article and the Word file for the supplement, using Word
’s Comments feature under Word’s
Review tab
.
I have copied and pasted below all my marginal comments, but they are better understood in the
context of where they appear in the Word doc. Although there are a number of comments
scattered among my marginal c
omments, perhaps the very last one on the Supplement file is the
most important, as it affects the reader’s understanding of what the authors conclude about the
correct explanation for their experimental observations
.
Comments in the Article file
:
Line 2
8: mu should be ta
u
Line 129
-
130: However, if the clay is supporting more of the normal stress and therefore is
contributing more than its volumetric proportion to the overall measured properties, why is the
velocity dependence seem to be skewed toward th
e rate weakening quartz? Have you tried to do a
quantitative model of a
-
b as you do for mu? Of course it’s not clear how one should to that
I
suppose just treat a
-
b as a parameter with values that go with each phase and that sum up
linearly in proportion
to their contact area along the two bounding faults as you do for the friction.
Not sure if that makes sense or not
!
Line 135
-
136: Have you seen any evidence of this in thin sections? Seems as if there might be
shear/damage zones in the quartz connecting
the tips of the smeared clay on opposite sides of the
overall layer. In any case, some comment as to whether this has been observed directly would be
helpful as it seems a likely process. Related to this, see my note commenting on Figure 2
.
Line 174
-
177: T
his overall problem sort of reminds me of the complexities of deformation of a
poly
-
phase aggregate, where neither the uniform stress or uniform strain models adequately
describe the behaviour and some more detailed modelling is needed. Your experimental g
eometry
nominally simpler that that, but even so the simple model is predictions don’t seem to adequately
explain the results. With patches on a fault plane rather than simple strips perpendicular to the slip
direction as in the experiments, the situation
will clearly be even more complex
.
Line 343
-
344: I’m thinking that it would be helpful to show calculated curves for 60, 70, and 80
percent clay. Given that the length of the quartz patch along the fault is progressively smaller in
those cases, the decline
of the calculated curves as a function of slip would occur sooner and
sooner as the clay content goes up. This is seen in the data. So looking at this might help to better
understand the proportion of the weakening due to clay smearing
.
This, combined wit
h localized stress concentrations as you discuss on lines 135
-
136, could help
explain the observations
.
Comments in the Supplement file
:
Caption to Supplementary Figure 1: It would help if you give the length of the sample so one is
able to think about h
ow the amount of displacement as the experiments progress compares with
that length. Without that information one is unable to reproduce your calculations in Figure 2c.
Perhaps your figure is to scale and the 20 mm diam. platens is all you need, but the fi
gure could
be more schematic
.
“Similar weakening” in caption for Supplementary Figure 3: I note however that the value of a
-
b
seems to be more positive than in the configuration with the quartz patch in the middle and in fact
is similar to the homogeneous
gouge (Figure 1d). I presume you have no explanation for this
difference, assuming it is significant, which appears to be the case. Either say that you have no
explanation for it or maybe you can come up with one. One value of making a comment to this
effe
ct is that when some reader notices it you will not seem to be unaware of it! I suggest you
include the data for these experiments in Table S1
.
First line of very last paragraph: In spite of these caveats I think it is worth considering an attempt
to add another component to Figure 2, i.e. Figure 2d, that includes an attempt to calculate the
curves for one or more cases of clay fraction as you do
in Figure 2c. I assume you could take the
data at each displacement and run through the calculation as a function of displacement. It is
always advisable for your figures to tell your story since that is what many readers will focus on.
As the paper now st
ands, Figure 2c leads one to conclude that you don’t really understand the
extra observed weakening. Reading the lines near the end of the previous paragraph in the
supplement is needed to realize that you actually do have a viable explanation for the weak
ening.
This location hides that understanding too much! The new figure could, as does part c now, only
focus on each effect by itself, leaving to words the statement that one looks larger than the other
and together they could both clearly do the job
.
1
Response to reviewers’ comments on “Fault rock heterogeneity produces fault weakness and reduces
fault stability”
In this document, we outline our response to the re
viewers’ comments on the original manuscript. We
would like to sincerely thank all three reviewers fo
r their constructive comments; the manuscript has
been significantly improved by their suggestions. In part, we have added additional text into the
discussion of the manuscript to clarify how our result
s relate to heterogeneity observed in natural fault
zones and to emphasize the importance of further st
udy of fault heterogeneity and its evolution. We
have modified the title and added extra discussion on the role of heterogeneity in modifying stability
and the requirement of having sufficient rate-weakening material in the fault. We have also added an
additional panel into Fig. 2 to show the potential
magnitude of the weakening effect from differential
compaction and a supplementary Figure (Fig. S4) to illustrate that the effects of the differential
compaction are compatible with the friction stabilit
y changes. Our main results and conclusions remain
the same.
Please find below our detailed point-by-point respon
se to the reviewers’ comments on the original
manuscript. Our responses are written in black font and any line numbers to which we refer to are for
the new redlined version of the manuscript where our changes are documented.
Blue: reviewer comment.
Black: authors’ response.
2
Reviewer 1 comments
In this interesting manuscript, by means of newly-co
nceived experiments, it is
shown that small-scale
mineral heterogeneity in fault slip zones controls bo
th fault strength (or the maximum stress at which a
fault fails producing earthquakes or aseismic creep) a
nd the response of the fault to perturbations in the
loading conditions (e.g., far field velocity, stress). This latter response of the fault is relevant: if, during
slip, the fault weakens (or the dynamic friction decrease
s) faster that the rate at which the elastic strain
energy stored in the fault wall rocks is released, this
will result in a frictional instability (= earthquake
ruptures); if the dynamic friction increases or, altern
atively, decreases slower th
an the rate at which the
elastic strain energy is released, this will
result in aseismic sl
ow slip transients).
But what is “fault rock heterogeneity”? Typically,
laboratory experiments are conducted on (1) synthetic
fault gouges made by one mineral, (2) synthetic faul
t gouges made by mixtures of two or more minerals
and (3) natural fault gouges retrieved from outcrops
or fault drilling projects
(almost all natural gouges
consist of an assemblage of grains with different co
mposition: clay, quartz, feldspar, calcite, dolomite,
etc.). In cases (2) and (3) the experimental fault is
heterogeneous in composition (in this manuscript, this
mineral assemblage is called “homogeneously mixe
d” fault gouge). There are several thousands of
published data about friction experiments conducted
on these mixtures. Instead, there are very few
laboratory studies which reproduce natural slip zones
where “fault rock heterogeneity” is related to the
spatial arrangement of two or more minerals in the
experimental fault. For instance, motivated by the
spatial arrangement of different minerals found in
natural slip zones of active seismogenic faults,
Smeraglia et al. (Scientific Reports, 2017) considered
the “fault rock heterogeneity” as sub-parallel to
the fault slip surface. In this case, heterogeneity co
nsists in a compositional la
yering with clays next to
the slip surface and calcite grains beneath.
The novelty of this study submitted to Nature Communica
tion is that, for the first time to my knowledge,
the “fault rock heterogeneity” (i.e.,
layers of clays alternated to quartz-built gouges) is perpendicular to
the slip surface (Fig. 1b). At the m
illimetre to sub-millimetre scale such
spatial organization can be found
in faults which cut across foliated rocks (e.g., quartzite
s, micaschists). For instan
ce, in the case of impure
greenschists facies quartzites, mm-
cm thick layers made of quartz
grains are alternated to sub-
millimetre-thick layers made of chlorite and white micas.
Thanks to this newly conceived experimental configur
ation, the authors show th
at the small scale spatial
mineralogical heterogeneity (layering) has profound effects on the frictional behaviour of the
experimental fault. In particular, (1) the friction coefficient of the layered heterogeneous gouges
decreases with increasing fault slip (this slip weakening behaviour is not observed in experiments
performed on “homogeneously mixed” fault gouges,
Fig. 1c-d) and (2), layered heterogeneous fault
gouges are, regarding the response of the friction co
efficient to loading perturbations, more unstable;
that is, they are more prone to trigger frictional instabilities and laboratory earthquakes than
“homogeneously mixed” fault gouges (Fig. 3).
I find the description of the experimental configuration and of the results well-written and the
conclusions based on solid experime
ntal evidence. The figures are well-
designed. I also suggest to keep
the supplemental material as it is (i.e., I really en
joyed the discussion about the effect of differential
compaction on interface weakening).
In conclusion, the manuscript is nicely written and surely deserves
to be published in Nature Communications after co
nsidering or discussing the following minor point:
3
We are grateful to the reviewer for the positive co
mments on our manuscript, and also for detailing the
importance of our work and how it fits into the bigger picture of fault mechanics. We also thank the
reviewer for bringing the reference of Smeraglia et al.,
(2017) to our attention; it
is very relevant to our
work and we have included it in the manuscript (ref. number 44).
My field and experimental experience is that at the sc
ale of natural slip zones (< 2 cm thick, e.g., Sibson
2003, Bulletin of the Seismological Society of America)
the compositional heterogeneity is parallel to the
fault slip surfaces (e.g., clay-rich layers sub-para
llel to calcite-rich layer in the case of exhumed
seismogenic faults in the Central
Apennines, see Smeraglia et al.,
Scientific Reports 2017) or, very
common as the authors of this manuscript well know, th
at the mineral composition
in the slip zone is a
mixture of minerals (homogeneously mixed fault gouges
). This spatial or mineralogical heterogeneity is
easily explained by the smearing and dragging of the wa
ll rock materials and their mixing in the slip zone,
as also shown in Fig. 2a of this manuscript, or by
other processes occurring in natural faults (incongruent
pressure-solution-precipitation, neo-formation of
clay minerals in natural fault cores, etc.).
In this manuscript is discussed a “fault rock heterogene
ity” (i.e., layers of clays alternated to quartz-built
gouges) perpendicular to the fault slip surface (Fig
. 1b). However, as explained above, such initial
“micro-scale” spatial heterogeneity or compositional la
yering is lost during initial shearing of the gouges
(Fig. 2a). As a consequence, I sugg
est the authors to reconsider (or di
scuss by introducing some natural
examples) the last statement of
the abstract (lines 20-24):
“The results demonstrate that small-scale geologic
al heterogeneity has pronounced effects on fault
strength and stability, and by extension on the occu
rrence of slow-slip transients versus earthquake
ruptures and the characteristics of the resulting even
ts, and should be incorporated in lab experiments,
fault friction laws, and ea
rthquake source modeling.”
In fact, the mechanical data of the experiments discussed in this manuscript are representative of the
very first mm or cm of slip (or s
lip initiation, depending on the thickness of the layered rocks) in natural
faults cutting layered rocks. Such small-scale heterogene
ity perpendicular to the fault slip surface will be
lost after few millimetres or tens of centimetres at mo
st of slip in natural faul
ts and, as a consequence,
these experiments cannot reproduce the conditions th
at lead to slow-slip transients versus earthquake
ruptures in large displacement faults. Instead, if
the authors consider their experimental configuration
as an analogue, to be scaled, of large-scale spatial
heterogeneity (see their figure 4) associated to large
displacement faults which put in contact rocks wi
th different mineral composition and mechanical
properties, I do agree with their conclusion.
Congratulations to the authors for this nice piece of work,
Giulio Di Toro
We agree that, in our simple experimental setup, if the layer could be taken to greater shear
displacement, clay smearing would encapsulate the
quartz patch after a few centimeters of shear and
thus the heterogeneity would essentially be lost.
However, this view only considers the loss of
heterogeneity parallel to the overall,
large-scale, slip direction. In a non-planar fault zone with a complex
internal structure, slip may also
locally occur in directions differe
nt from the slip vector, allowing
heterogeneity to potentially persist
even for large displacement. Hence,
in natural fault zones, we would
expect heterogeneity to persist over different sc
ales. While some fault-zone studies indeed find
localized through-going layers that are relatively ho
mogeneously mixed, as discussed by the reviewer,
4
complex compositional heterogeneity is still commonly
observed in natural fault zones, such as the
Carboneras Fault (e.g., Figure 5a of Rutter et al
., 2012, JSG) and the Punchbowl Fault where there is
heterogeneity on either side of the principal slip surface from both the different types of ultracataclasite
and the different host rocks (e.g., Fig. 4 of Chester and Chester, 1998, Tectonophysics).
One of the main outcomes of our study is that it demonstrates that fault strength is not just an average
of the respective frictional properties of the different
patches (Fig. 2c), but rather there are a series of
additional weakening effects that are caused by having a spatially heterogeneous distribution of fault
materials. Of course, more work
is required to document how different types of spatial heterogeneity
affect the mechanical behaviour of faults (e.g., Ni
emeijer et al., 2010). However, we think that our
statement at the end of the abstract is still valid, over
all. At the same time, to highlight this discussion
and complexity of the issue in the manuscript, we have slightly modified the abstract and have added
additional text to the manuscript.
The end of the abstract now reads (changes italicized):
“The results demonstrate that geological heterogeneity
and its evolution
can have pronounced effects
on fault strength and stability and, by extension,
on the occurrence of slow-slip transients versus
earthquake ruptures and the characteristics
of the resulting events, and should be
further studied in lab
experiments and earthquake source modelling
.”
On lines 160-173, we now discuss ho
w heterogeneity might be altered
and preserved in natural fault
zones:
---------------------------------------------------
In our experiments, if the gouge layers could be take
n to greater shear displacements, the clay smearing
we observe along the edges of the quartz patch (Fig. 2a) would ultimately form a through-going layer of
interconnected weak material after a few centimet
res of slip. Previous work has shown that such
through-going layers can lead to a reduction in the frictional strength at slow slip velocities
11
and also
increase the efficiency of dynamic weakening at seismic slip velocities (1 m/s)
44
. Although weak phase
smearing would, to some extent, homogenize the faul
t in the overall direction of shear, heterogeneity
would likely always be prevalent in natural faults, particularly perpendicular to the slip direction and also
at scales larger than investigated in this
study, as observed in natural fault zones
25,45
. Our results show
that the average frictional strength of laterally
heterogeneous faults is not just an average of the
respective friction properties (Fig. 2c), and that co
mpetency contrasts can substantially reduce the fault
strength, even when structural foliations are in their infancy and unconnected (Fig. 2a). They also
highlight the need to investigate further how different types of fault heterogeneity, including fault-
parallel and fault-normal heterogeneity, and its evolut
ion, affect the frictional behaviour of faults.
---------------------------------------------------
Following the suggestion of the reviewer, we have in
cluded, on lines 200-207, a mention of the potential
analogy between our experiments and effects of larg
e-scale heterogeneity; we have also highlighted
that the small-scale heterogeneity can more easily evolve:
5
---------------------------------------------------
There are similarities between the slip behaviour we observe in our small-scale heterogeneous
experiments and how large-scale heterogeneities are thought to control the behaviour of natural faults.
For example, decreasing the size of the rate-weakening patch makes the response more stable in both
our experiments and numerical modelling
53
, as can be intuitivel
y expected and consistent with stability
studies of rate-and-state faults that slip instabili
ty can only result from large enough rate-weakening
patches
39
. At the same time, small-scale fault zone heterogeneity would more readily evolve with shear,
and hence may depend on the fault maturity, healing processes, and spatio-temporal history of fault slip.
---------------------------------------------------
Finally, we modified the end of the manus
cript to state (cha
nges italicized):
“These considerations, together with our findings
, necessitates further laboratory experiments and
modelling to study the effects
and evolution of fault rock heterogeneity within complex fault zones
, to
enable
the quantification and
inclusion of the small-scale hetero
geneity effects into larger-scale
constitutive laws for modelling fault processes of so
cietal interest, such as nucleation of natural and
induced earthquakes.”
Reviewer 2 comments
This is a really interesting paper and I think it should be published in Nature Communications, but I have
some reservations that would need to be addressed.
The topic is very appropriate for
Nature Communications. This paper addresses interesting questions
that are topical and cutting-edge in earthquake physics
: what causes unstable frictional sliding and what
causes the spectrum of slip behavi
ors on tectonic faults. The experiments are elegant, the results are
clear and the lab work is very well
described. But my sense is that some of the interpretation needs to
be reoriented.
My main concern is that I'm not sure I agree with
the title. Does heterogeneity really promote unstable
slip? The experiments shown indicate that stability is
dictated mainly by the clay/qtz fraction, right? I
appreciate the results in Fig 3 but it seems like the ma
in effect there is simply the result of incomplete
mixing given the available shear displacement. The data of Fig 3c show that the effect decreases with
shear displacement. Doesn't this imply that the main
factor causing a reduction in a-b (as seen in
Fig3a/b) is the role of the quartz patch? The quartz
is velocity weakening so the fault has more of a
tendency for velocity weakening when the fault ha
s a separate quartz patch. As the quartz patch
becomes increasingly mixed the homogeneous and hete
rogeneous faults have similar behavior. So, I'm
not sure I agree with the idea that heterogeneity is
the key ingredient. Another way to make this point
is: if heterogeneity itself promotes unstable slip sh
ouldn't I expect that to happen for a heterogeneous
mixture of weak, velocity strengthening materials?
A related point involves the data of Figure 3a. My
sense is that these are a key part of the story but
when I look at the raw data of Figure 1c (or supplementary Fig 3) I only see velocity weakening for clay
fractions of 0 and 20%. The raw data for 30 and 40% cl
ay are velocity strengthen
ing at all displacements
and the a-b values for the velocity step at 1.5 mm
would be even larger if the strain weakening trend
from 1-1.5 mm were accounted for. This doesn't negate
the importance of Fig 3a but it reduces it. In any
6
case it's important to make sure
readers can follow your analysis.
We thank the reviewer for raising this important po
int. Indeed, heterogeneity may not always promote
instability or reduce stability, as the reviewer’s exam
ple of mixed rate-strengthening gouges indicates.
Similarly, heterogeneity may not always result in fa
ult weakness. Hence we have modified the title to
state
“Fault rock heterogeneity can produce fault weakness and reduce fault stability.”
Further, we
agree that unstable behaviour require
s a significant fraction of rate-weakening material (e.g., quartz) to
be present in the fault, regardless of whether it ha
s a heterogeneous structure or not. We have added
additional text to the discussion section of the
manuscript on lines 177-187 to
emphasize this point in
more detail:
---------------------------------------------------
Our experiments show that heteroge
neity produces an overall reduction in stability when compared to
homogeneous faults (Fig. 3). It should be noted that
a sufficient amount of rate-weakening material is
still required to promote unstable slip. In our experi
ments, when the proportion of the rate-weakening
material is
70%, the heterogeneous faults are stable overall, with positive (a – b) values, although the
values are closer to zero (and hence rate-neutral behaviour) than those of their homogeneous
counterparts (Fig. 3); however the behaviour remains ra
te-strengthening, instabilities do not initiate and
aseismic slip prevails. Only when the strong rate-weakening patch comprises
80% of the layer do stick-
slip instabilities occur (Fig. 1c).
---------------------------------------------------
Minor points in no particular order
1. How do you measure layer thickness during shear? Hopefully this could be added to the supplement.
The overall thinning seems small; the layer goes fr
om 1 mm to 0.85 mm after a shear strain of 10?
We use pore volumometry to track the layer thickness evolution during shear; we assume that the
sliding area remains constant and that all volumetr
ic strain is accommodated by a change in layer
thickness. The layer thickness values in Supplementary Fig. 2b were calculated by measuring the gouge
layer thickness at the end of the experiment using a micrometer and then back-calculating the layer
thickness evolution during the experiment using the pore
volume data, as was also done by Faulkner et
al., (2018). We have now stated in
the caption of Supplementary Fi
gure 2 how the layer thickness was
measured:
---------------------------------------------------
At the end of the experiment, the
layer thicknesses were measured using a micrometer to be almost
identical. The thickness evolution during the experiment was then back-calculated using the pore volume
data, assuming the sliding area remains constant and that all volumetric strain is accommodated by a
change in layer thickness
33
(see Methods).
---------------------------------------------------
We also now give more detail on the pore volumo
metry in the methods section (lines 240-245 of the
main article):
7
---------------------------------------------------
Linear variable differential transformers (LVDTs) are
attached to the pistons of the servo-control pumps,
meaning that the volume of fluid expelled from the sample as it compacts during shearing can be
monitored as the pressure is held constant. We
therefore use the pore pressure pump as a pore
volumometer to track the evolution
of layer thickness during our experiments (Supplementary Fig. 2); we
assume that sliding area remains constant and that all volumetric strain is accommodated by a change
in layer thickness.
---------------------------------------------------
Most of the layer thinning actually occurs during
pressurization of the gouge layer to the starting
conditions (effective normal stress of 40 MPa). The layer thickness prior to pressurization is about 1.3
mm, which we now clarify on line 225. The 1 mm initial
thickness quoted in the ma
in text is actually the
layer thickness at the onset of shear, after initial
pressurization, which we now clarify on line 60. The
magnitude of layer thinning we observe during our experiments is comparable to layer thinning
observed in previous studies using a direct
-shear setup (e.g., Faulkner et al., 2018).
2. How do you discourage or eliminate boundary shear at the edge of the forcing block?
To discourage boundary shear at the edges of the go
uge layer, the direct-shear forcing blocks contain
grooves cut perpendicular to the sliding direction. We
now include details of this in the methods section
(lines 228-230):
---------------------------------------------------
To discourage boundary shear at the edges of the gouge layer, the sliding area (50 × 20 mm) on the
forcing blocks contains grooves cut perpendicular
to the sliding direction
(200 μm deep with 400 μm
spacing).
---------------------------------------------------
3. Please provide a bit more detail on the jacket. PVC can be used for rigid plumbing pipe. I assume
you're using another type and it would be good
to provide this information in the supplement.
We use a soft PVC jacket (Nalgene 180 clear tubing
) and we have added this detail to the methods
section (Line 233):
---------------------------------------------------
...placed into a soft, 3 mm thick, PV
C jacket (Nalgene 180 clear tubing).
---------------------------------------------------
4. Your references are very good. You might also co
nsider commenting on the comparison of your work
with that of Niemeijer et al. GRL 2010 (Fab
ric induced weakness of
tectonic faults).
We thank the reviewer for suggesting this reference
as it is relevant to our manuscript, particularly
regarding the role of weak phase localization on fa
ulting. We have therefore added the reference (ref.
number 11) on line 34 and also on line 164.
8
5. A useful addition to your discussion of Figure 4
would be to connect spatial heterogeneity to the slip
patch size for rupture nucleation/prop
agation. One way to produce slow slip is to limit the size of the
velocity weakening region (for example as shown by
Liu & Rice 2005/2007) and your data provide some
nice connections
to these ideas.
Thank you for highlighting that our results relate well to the idea that relative patch size controls the
fault slip behaviour in nature. We have added the following text to our discussion of Figure 4 on lines
200-207, although we cited a different example fr
om Liu and Rice (2005/2007)
since their studies
modified the properties of the velocity-weakening patches more so than the size:
---------------------------------------------------
There are similarities between the slip behaviour
we observe in our small-scale heterogeneous
experiments and how large-scale heterogeneities are th
ought to control the behaviour of natural faults.
For example, decreasing the size of the rate-weakening patch makes the response more stable in both
our experiments and numerical modelling
53
, as can be intuitively expected
and consistent with stability
studies of rate-and-state faults that slip instability
can only result from large enough rate-weakening
patches
39
. At the same time, small-scale fault zone hete
rogeneity would more readily evolve with shear,
and hence may depend on the fault maturity, healing proc
esses, and spatio-temporal history of fault slip.
---------------------------------------------------
6. I like the idea of the color bars
for strength and seismic potential in Fig. 4 but I'm not sure I see the
argument for low strength and high seismogenic potent
ial. Maybe this is just
the same point I raised
above, but here you seem to be going beyond the id
ea of a relative change (heterogeneity promotes
unstable slip) to a more definite statement.
We agree with the reviewer and have adjusted the fi
gure so that colour bar now represents “relative
seismogenic potential” between heterogeneous and
homogeneous faults. We also now state on the
figure that unstable slip re
quires rate-weakening material:
9
7. It would be useful to comment on how the kind of
heterogeneity you envision would be sustained in a
tectonic fault zone with sizable offset. For example,
one could imagine that a fault zone with initial
heterogeneity of the sort you examine would beco
me homogeneously mixed after a relatively small
offset.
Chris Marone
This point was also raised by Reviewer 1 and we ha
ve responded in detail there. In part, we have now
added additional text on lines 160-173 to discuss th
e preservation of heterogeneity in natural fault
zones:
---------------------------------------------------
In our experiments, if the gouge layers could be take
n to greater shear displacements, the clay smearing
we observe along the edges of the quartz patch (Fig. 2a) would ultimately form a through-going layer of
interconnected weak material after a few centimet
res of slip. Previous work has shown that such
through-going layers can lead to a reduction in the frictional strength at slow slip velocities
11
and also
increase the efficiency of dynamic weakening at seismic slip velocities (1 m/s)
44
. Although weak phase
smearing would, to some extent, homogenize the faul
t in the overall direction of shear, heterogeneity
would likely always be prevalent in natural faults, particularly perpendicular to the slip direction and also
at scales larger than investigated in this
study, as observed in natural fault zones
25,45
. Our results show
that the average frictional strength of laterally
heterogeneous faults is not just an average of the
respective friction properties (Fig. 2c), and that co
mpetency contrasts can substantially reduce the fault
strength, even when structural foliations are in their infancy and unconnected (Fig. 2a). They also
highlight the need to investigate further how different types of fault heterogeneity, including fault-
parallel and fault-normal heterogeneity, and its evolut
ion, affect the frictional behaviour of faults.
---------------------------------------------------
Reviewer 3 comments
Review of 315765-0, “Fault rock heterogeneity prod
uces fault weakness and promotes unstable slip” by
Bedford, Faulkner and Lapsuta.
By Terry E. Tullis
General Comments
My comments are both for the editor and the authors.
This is a good paper and could be published as is.
It is clear and succinctly written and the noteworthy
results are made clear in the paper, namely that
heterogeneous distribution of rock types along a fault contribute to the strength and stability of the
fault in non-intuitive ways and that this is impo
rtant in modeling earthquakes. It is a significant
contribution to the field. The work and methodol
ogy is sound and supports the conclusions. Enough
detail is presented that the results could be repr
oduced. However, I have some suggestions for the
authors to consider that should no
t involve much additional work a
nd should improve an already fine
paper. I have included these as comm
ents in both the Word file for the article and the Word file for the
supplement, using Word’s Comments feature under Word’s Review tab.
10
I have copied and pasted below all my marginal
comments, but they are better understood in the
context of where they appear in the Word doc.
Although there are a number of comments scattered
among my marginal comments, perhaps the very last
one on the Supplement file is the most important,
as it affects the reader’s understa
nding of what the authors conclude about the correct explanation for
their experimental observations.
Comments in the Article file:
Line 28: mu should be tau
Thank you for finding this mistake, it has now been corrected (Line 29).
Line 129-130: However, if the clay is supporting more
of the normal stress and therefore is contributing
more than its volumetric proportion to the overall
measured properties, why is the velocity dependence
seem to be skewed toward the rate
weakening quartz? Have you tried to do a quantitative model of a-b
as you do for mu? Of course it’s not clear how on
e should to that – I suppose just treat a-b as a
parameter with values that go with each phase and th
at sum up linearly in prop
ortion to their contact
area along the two bounding faults as you do for the
friction. Not sure if that makes sense or not!
This is an interesting idea and we have now made a quantitative model of (a-b) like we did for mu, by
taking the (a-b) values from the endmember gouges (at a displacement of 1.5 mm) and then calculating
how these would evolve with displacement as a greater
portion of the sliding surface is occupied by clay
gouge due to smearing. The (a-b) data are more scatte
red than the mu data so caution should be taken
when interpreting the results, however, the data actually show that clay smearing underpredicts the
evolution of (a-b) and that the observed (a-b) values are higher than the predicted values. The data are
therefore slightly skewed towards the rate-strengthe
ning clay rather than the rate-weakening quartz.
This actually supports our hypothesis that clay is
supporting more of the normal stress with increasing
displacement, as we hypothesize from the pore vo
lume data. We have now included this quantitative
model of (a-b) as a new figure in the supplementary material (Supplementary Fig. 4):