ARTICLES
PUBLISHED ONLINE: 18 MAY 2015 | DOI: 10.1038/NGEO2436
Phase transformation and nanometric flow cause
extreme weakening during fault slip
H. W. Green II1,2*, F. Shi1,3, K. Bozhilov2, G. Xia1† and Z. Reches4
Earthquake instability requires fault weakening during slip. The mechanism of this weakening is central to understanding
earthquake sliding and, in many cases, has been attributed to fluids. It is also unclear why major faults such as the San Andreas
Fault do not exhibit significant thermal anomalies due to shear heating during sliding and whether or not fault rocks that have
been melted—pseudotachylytes—are rare. High-speed friction experiments on a wide variety of rock types have shown that
they all exhibit extreme weakening and that the sliding surface is nanometric and contains phases not present at the start. Here
we use electron microscopy to examine these two key observations in high-speed friction experiments and compare them with
high-pressure faulting experiments. We show that phase transformations occur in both cases and that they are associated
with profound weakening. However, fluid is not necessary for such weakening; the nanometric fault filling is inherently
weak at seismic sliding rates and it flows by grain boundary sliding. These observations suggest that pseudotachylytes
are rare in nature because shear-heating-induced endothermic reactions in fault zones prevent temperature rise to melting.
Microstructures preserved in the Punchbowl Fault, an ancestral branch of the San Andreas Fault, suggest similar processes
during natural faulting and offer an explanation for the lack of a thermal aureole around major faults.
F
or the past ∼100 years, there has been a broad consensus
among earthquake physicists that shallow earthquakes
(< ∼30 km) initiate on pre-existing faults when accumulation
of tectonic elastic strain raises the stress locally to a value exceeding
static friction1 . This basic model of earthquake instability requires
dynamic but relatively small weakening of the fault zone. However,
over the past ∼20 years, experiments at earthquake sliding rates
simulating very shallow depths in Earth have found that shear
heating rapidly follows the onset of sliding and is accompanied by a
large and rapid decrease of frictional resistance2–12 . Microstructural
examination of experimental sliding surfaces shows that shear
heating has been sufficient to trigger phase transformations that
facilitate sliding. In the case of quartz4,5 , a gelification reaction in
a humid environment is the transformation, whereas in the case
of gabbro2 and some granites (Supplementary Information), the
transformation is melting, and in all other cases volatile-containing
phases (serpentine, clays, gypsum, carbonates) are broken down
within seconds of sliding initiation2,5–12 , yielding a nanocrystalline
aggregate of solid reaction products.
The stress magnitude for initiation of earthquake sliding
increases rapidly with depth because of the strong normal stress
dependence of static friction (∼18 MPa km−1 ). Simultaneously,
increasing temperature with depth exponentially lowers the stress
needed for flow. Thus, at depths >30–50 km, brittle failure
becomes impossible and rocks flow rather than fail by faulting.
Nevertheless, earthquakes occur in subduction zones to ∼700 km
depth. In laboratory experiments under high-pressure conditions,
the magnitude of normal stress is no longer the controlling
factor; shear failure occurs at shear stresses significantly lower
than the value necessary to overcome static friction as predicted
from the normal stress13,14 . This shear failure occurs only when
phase transformation generates a small amount of low-viscosity
nanometric material that enables initiation of the instability and
lubricates sliding15 . As in most high-speed friction experiments,
high-pressure faulting results in a nanocrystalline gouge. Thus,
in faulting experiments simulating depths greater than 30–50 km
(refs 13–21), phase transformation under stress is the cause of
failure, rather than a consequence of failure; but in both highpressure experiments and most high-speed experiments, the result
is a nanocrystalline sliding zone composed of the product phase(s).
On the basis of the microstructural similarity between highpressure faulting and high-speed sliding, and other information
gained from high-pressure experiments (Supplementary
Information), we hypothesize the existence of an earthquake
sliding mechanism that is controlled by phase transformation, and
which can operate at the full depth range of earthquake occurrence,
from crustal depths to ∼700 km (ref. 22). To test this hypothesis,
we analyse the nanoscale structures developed during high-speed
friction experiments on dolomite sheared under moderate normal
stress, compare those nanoscale structures with the fault ‘gouge’
produced in high-pressure faulting imaged here for the first time,
demonstrate that high-pressure faults have extraordinarily low
frictional resistance, and identify the mechanism of sliding. We
then show relics of a nanocrystalline matrix within the principal
slip surface of the Punchbowl Fault, California. This fault has
been exhumed from a depth of several kilometres by erosion,
and is surmised to have slipped tens of kilometres on a gouge of
a few millimetres in thickness23–25 . We speculate that the sliding
mechanism described here may have operated in this fault zone.
High-speed frictional sliding
Figure 1a shows the friction coefficient and slip velocity evolution
during high-speed shear in a rotary apparatus11 . The apparatus
incorporates a flywheel that was spun to a desired velocity, and
1 Department
of Earth Sciences, University of California, Riverside, California 92521, USA. 2 Central Facility for Advanced Microscopy and Microanalysis,
University of California, Riverside, California 92521, USA. 3 State Key Laboratory of Geological Processes and Mineral Resources, China University of
Geosciences, Wuhan 430074, China. 4 School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma 73019, USA. †Present address:
School of Earth Sciences, University of Queensland, Queensland 4072, Australia. *e-mail: [email protected]
484
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NATURE GEOSCIENCE DOI: 10.1038/NGEO2436
a
ARTICLES
b
Flywheel energy = 3.3 × 106 J m−2
Friction; slip velocity (m s−1)
1.0
Kasota dolomite
Friction
0.8
Slip velocity
dc = 0.006 m
0.6
dt = 0.56 m
0.4
0.2
5 µm
0.0
10−2
10−1
Slip distance (m)
c
d
20 nm
10 nm
500 nm
150 nm
Figure 1 | Kasota dolomite sliding experiment. a, Slip and velocity versus distance. Friction (red) rises rapidly, followed by a rapid decrease continuing
down to ∼0.2; friction rises again as velocity (blue) drops to zero. b, SEM image of an area of a sliding surface partially modified by post-sliding growth of
an interlocking ‘pavement’ (see Supplementary Methods and Supplementary Fig. 1). Arrows show regions where grooves in the sliding surface are covered
or muted by pavement. c, Very high magnification of a sliding surface shows that most grains are rounded and have a diameter of ≤50 nm. d, Higher
resolution shows many grains < 10 nm; image slightly blurry because the uncoated specimen was drifting slowly.
then rapidly coupled to an experimental fault that is under a
prescribed normal stress. The flywheel abruptly applied its finite
kinetic energy, causing an extremely rapid acceleration of the
experimental fault12 . The experiment was conducted on dolomite,
CaMg(CO3 )2 , at a normal stress of 28.4 MPa, which is lower than the
normal stress at nucleation depths of significant crustal earthquakes,
but much higher than most high-speed friction experiments2 . The
fault slid for 0.6 s at ∼1 m s−1 , yielding a dark, mirror-like sliding
surface26 (Fig. 1b–d). Figure 1b shows slip-parallel striations with
a spacing of ∼0.5 µm, and higher magnification reveals no grains
larger than ∼50 nm (Fig. 1c) and a median diameter of ∼10 nm
(Fig. 1d). Further, as well as the mirror-like areas, a significant
fraction of the surface is covered with small, post-sliding, carbonate
plates (averaging ∼100–200 nm across) that fit together like a
stone pavement (Fig. 1b). This observation, although not directly
relevant to the sliding process, potentially provides insight into
post-earthquake processes (Supplementary Information).
Chemical analysis of the sliding (Fig. 2a) and paved
(Supplementary Fig. 1a) surfaces by energy-dispersive spectroscopy
(EDS) shows that the former consists of oxides (MgO and CaO)
and the latter is carbonate (Ca, Mg)CO3 . Thus, the dolomite was
decomposed to oxides during sliding (requiring a temperature of
>1,000 K) and was partially reconstituted as magnesian calcite on
the surface after sliding ceased, replacing a thickness of ∼100 nm
of the oxide sliding surface (see Supplementary Fig. 1 and detailed
discussion in Supplementary Information).
Foils for transmission electron microscopy (TEM) were cut
normal to the sliding surface by focused ion beam (FIB; Methods).
The sliding surface and the first ∼200 nm below it consist of a
dense, randomly oriented solid composed of equant nanocrystalline
oxide grains (Fig. 2b). A central question here is ‘What is the
process that generated these nanocrystalline oxides which make
up the sliding zone?’. Figure 2c,d provides the answer. Figure 2c
shows a ‘holly-leaf ’-shaped fragment of dolomite, located about
400 nm from the sliding surface, which was quenched while in the
process of decomposing when sliding stopped. Each of the recesses
between the cusps on the border of this crystal houses a single
oxide grain that is ‘budding off’ from the parent carbonate. Arrows
point to two irregular holes in the TEM foil. Whether they are part
of the plumbing system by which CO2 escapes from the sample
(Supplementary Information) or simply defects in the TEM foil
is not clear, but the foil is otherwise fully dense in this region.
Figure 2d was obtained by tilting in the TEM to an orientation where
moiré fringes are formed on most of the grain boundaries. The
regularity of those patterns shows that grain boundaries are tight,
with no voids or bubbles, or anything else on them. We performed
many further tilting experiments in the TEM, in which lattice
fringes were imaged in adjacent grains (analogous to those shown
in Fig. 2g of ref. 11). These images show that the material is fully
dense and crystalline, confirming that the sliding process does not
involve dilation. The crystals are generally separated by high-angle
grain boundaries.
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NATURE GEOSCIENCE DOI: 10.1038/NGEO2436
ARTICLES
a
b
Mg
O
Fe
C
Fe
Al
Pt
Si
Pt Pt
K K
Ca
100 nm
Ca
c
Sliding surface
d
Oxide
Dolomite
100 nm
20 nm
Figure 2 | Composition of a sliding surface and TEM microstructures of a cross-section from the high-speed dolomite experiment. a, EDS analysis of a
bare sliding surface. Note the lack of a carbon peak. b, TEM cross-section of the specimen of Fig. 1 showing a nanocrystalline oxide surficial layer about
200 nm in thickness. c, ‘Holly-leaf’ dolomite fragment (dark) located about 400 nm from the sliding surface. Small grains around this fragment and
between cusps are nanometric oxides being formed as the decomposition reaction proceeds. Arrows indicate holes in the foil. d, Detail of c in the region of
the star, tilted to show moiré interference patterns on grain boundaries illustrating tight boundaries.
High-pressure faulting
The physics of high-pressure faulting is driven by phase
transformation. We demonstrate this failure mechanism by
experiments on magnesium orthogermanate (Mg2 GeO4 ), which
undergoes an exothermic polymorphic transformation from an
olivine structure to a spinel structure at elevated temperatures
and moderately high pressures (1–5 GPa). This transformation
is an analogue of the transformations that occur at much higher
pressures (13–23 GPa equivalent to 400–700 km depth) in silicate
olivine16,17 , the most abundant mineral in Earth’s upper mantle.
The defining characteristic is that the instability can occur only
during an exothermic polymorphic phase transformation with a
volume change. Under these conditions, nucleation of the new
phase becomes just possible kinetically, but growth is still essentially
impossible. We show below that faulting due to this instability yields
a very weak nanometric ‘gouge’. The potential relationship between
this transformation-induced faulting and deep-focus earthquakes
has been discussed extensively elsewhere13–17,27,28 and is summarized
in the Supplementary Information.
Figure 3 shows images of a high-pressure fault in a specimen
of Mg2 GeO4 olivine that was deformed in the stability field of
Mg2 GeO4 spinel (T = 1,200 K, P = 1.3 GPa). This experiment was
quickly stopped after the stress drop by cutting power to the
furnace, causing T to fall to <800 K within 6 s. Optical examination
showed a fault (∼5 mm long) cutting through the entire specimen.
SEM imaging (Fig. 3a) shows an extremely thin (1 µm) fault
with displacement of ∼3 µm and largely untransformed olivine
surrounding the fault zone. A thin foil normal to the image
in Fig. 3a was prepared by FIB (Methods) and the fault crosssection examined by TEM (Fig. 3b,c). Despite numerous attempts
in H.W.G.’s laboratory for 25 years, this is the first entire fault zone
preserved and imaged. The fault gouge, ∼70 nm thick, consists of
a fully dense, randomly oriented, polycrystalline solid of equant
nanocrystals of the spinel phase. Tilting in the microscope revealed a
grain-size distribution of <5 nm to ∼15 nm. A large crack observed
486
along the fault is not part of the faulting process. It formed during
decompression, presumably because of stresses associated with the
8% volume reduction during transformation to spinel. Fault gouge
is observed sometimes on both sides of the crack.
Using the microstructural information from Fig. 3, the
P, T conditions of the experiment and the assumption of
adiabatic conditions, we calculated an upper bound on the
frictional resistance of this high-pressure fault using the relation:
1T = µσn d/ρch (ref. 29), where µ is the effective friction
coefficient, σn is the normal stress (1.5 GPa), d is the coseismic slip
distance (3 µm), ρc is the specific heat capacity (∼3 MPa K−1 ) and h
is the fault zone thickness (70 nm; see Supplementary Information
and Supplementary Fig. 2 for our constraints on 1T ). Using these
parameters and solving the above equation for µ yields µ < 0.01.
Relaxing the adiabatic constraint doubles the estimated upper
bound, still a value much lower than that exhibited in high-speed
friction studies, as shown in Fig. 1a and elsewhere2 .
It is important to note that this type of faulting requires an
exothermic phase transformation, and the extremely fine-grained
nature of the fault zone provides a sink for some of the energy
released during faulting. In the Supplementary Information, we
quantitatively examine these energetics and find that the amount
of latent heat released by the transformation is comparable to the
grain-boundary energy of the gouge. This suggests that the critical
step in the instability may be the very high local stress environment
where the nanocrystals are created (for example, anticrack tips15
and/or dislocation pileups30 in olivine); this process also may be
responsible for the wide variation of failure stresses under otherwise
similar conditions (Supplementary Fig. 2)13,14 .
In summary, the fault zone exhibits a fully dense, nanocrystalline
solid composed of equant, randomly oriented crystals of the
spinel phase (grain size ≤15 nm) that has slipped 3 µm under
a normal stress of 1.5 GPa and very low shear stress. The only
known physical mechanism that can satisfy all of these parameters
is flow by grain-boundary sliding (gbs). It has been known for
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NATURE GEOSCIENCE DOI: 10.1038/NGEO2436
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ARTICLES
b
px
FIB foil
∗
ol+sp
Di
sp
lac
em
Crack along fault is a ent
depressurization
feature
10 μm
This
side
towards
you
ol
50 nm
c
roll. Rather, the grains may describe primarily rolling-like motions,
but all of them must slide against their neighbours during rolling,
hence sliding must occur with very low resistance.
Comparing the high-speed sliding zone (Figs 1 and 2) with the
high-pressure fault zone (Fig. 3), we find that both experiments:
involve mineral phase transformations; lack amorphous material
or evidence of melting; have equant, rounded nanograins lacking
preferred orientation in sliding zones; show no porosity in sliding
zones; and exhibit very low frictional resistance. Flow by grainboundary sliding (‘superplasticity’) is the only known mechanism
that can explain all these observations.
We predict that similar processes are likely to be active in natural
faulting, whether driven by rapid shear heating in the crust (Figs 1
and 2) or by an exothermic solid state phase transformation in the
mantle (Fig. 3). The key step in both cases in laboratory experiments
is a nanocrystalline solid generated by runaway nucleation in the
process zone of propagating faults or slip patches. If no subsolidus
phase transformation is possible—for example, in rocks such as
gabbro and some granites that have negligible volatile-bearing
phases—shear heating under high normal stress may be expected
to drive the temperature to the melting point, resulting in a meltcovered shear surface (Supplementary Fig. 3). Last, some materials
may undergo a mineral-specific phase transformation such as
quartz does; in a humid environment, high-speed sliding of quartz
induces loss of crystal structure and incorporation of H2 O, yielding
a low-viscosity thixotropic gel5 .
Natural fault gouge
∗
50 nm
Figure 3 | Fault in Mg2 GeO4 olivine (1.3 GPa, 1,200 K). a, Backscatter SEM
image of polished section shows the sense of shear, displacement and
location of FIB-cut foil. White, MgGeO3 pyroxene (px); mottled regions,
(ol+sp) partially transformed before faulting. b, TEM image of FIB
cross-section. Fault zone (dashed lines) ∼70 nm thick, grain size ≤ 15 nm.
Wall rock both sides of the fault (black) is single large, deformed olivine
crystal oriented for strong diffraction. c, Detail of b tilted to a slightly
different orientation, showing fault boundaries. Diffraction pattern (inset)
shows olivine from the fault wall (arrow) and rings of spinel. Asterisks
in b,c identify the same location. See Supplementary Information and
Supplementary Fig. 2.
more than 20 years that nanomaterials can flow by gbs even at
seismic strain rates (102 –103 s−1 ; refs 31,32). Nanomaterials also
exhibit decreasing strength with decreasing grain size (reverse
Hall–Petch effect), in contrast to microcrystalline materials33,34 .
We interpret this as reflecting the rapidly decreasing ability of
nanograins to accommodate dislocations into their structure,
requiring accommodation of gbs by glide and climb of grainboundary dislocations, bulk rotation, and/or diffusion in volumes
with increasingly small crystals33 . Thus, gbs of nanometric solids at
elevated temperature can yield an extremely low shearing resistance
at seismic sliding rates, even under extraordinary normal stress. We
note that the nanocrystalline oxides that make up the sliding surface
in both high-pressure faulting and high-speed sliding do not consist
of a powder for which the individual grains are isolated and free to
There is emerging recognition that at least some deeply eroded
natural fault zones exhibit a remarkably thin principal slip zone
along which much of the slip was localized. These fault zones also
show remnants of gouge with exceedingly small grain size23–25,35 .
Given the experimental results here, we searched for evidence of the
nanometric slip mechanism in the well-studied Punchbowl Fault,
an extinct branch of the San Andreas Fault system that has been
exhumed from a depth of several kilometres. This fault contains
an extremely thin (a few millimetres) principal slip zone, along
which there seems to have been at least several kilometres of slip,
and contains nanometric crystals in its gouge25 (Supplementary
Information). We collected gouge samples from the principal slip
zone (Supplementary Fig. 4) and examined them using the same
high-resolution methods we used to study the experimental faults.
The gouge contains extensive evidence of fluid-induced alteration
that has occurred since the last time the fault moved. Broken
fragments ‘float’ in very fine-grained matrix. Most of the matrix has
been heavily altered to clays, but a few ‘large’ sieve-textured, porous,
authigenic potassium feldspar crystals grew before the hydrous
alteration (potentially under conditions close to those of active
faulting36,37 ) and have preserved very large numbers of nanoparticles
within them (Fig. 4 and Supplementary Figs 4 and 5). Larger gouge
fragments often show partial inhibition of post-sliding grain growth
by nanoparticles (Supplementary Fig. 4).
Thus, the Punchbowl Fault contains abundant remnants of a
nanometric gouge that may have been similar to the experimental
gouges figured above. The experimental studies here, high-speed
experiments elsewhere, shearing experiments on nanometric MgO
(refs 38,39), and the abundance and variety of nanocrystals in
natural gouge suggest that nanocrystals also may serve as lubricant
during natural sliding. Nanocrystals also can be the direct product
of comminution rather than phase transformation40,41 , but to our
knowledge comminution has not been shown to yield a fully dense
nanometric solid that would be required for several of the processes
discussed here to operate.
Many of the individual observations we present on high-speed
friction have been described by previous workers, but without
the overarching concepts of control by phase transformation and
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NATURE GEOSCIENCE DOI: 10.1038/NGEO2436
ARTICLES
a
everything driven by shear heating happen much faster, and our
high-pressure failure experiments show that extreme weakening
does not require a fluid and that the weakening can only be
explained by grain-boundary sliding, because no other mechanism
is possible at the higher pressures, temperatures and small grain
sizes. Crustal earthquake hypocentres are at much higher normal
stresses than any high-speed friction experiments so far, thereby
making it essentially certain that shear heating will be even more
extreme than reported here. Why, then, are pseudotachylytes rarely
observed? Our model proposes that feldspars and other minerals
in crustal silicate rocks decompose to hydrous phases during
interseismic periods (confirmed here for the Punchbowl Fault). The
endothermic reactions involved in breakdown of these alteration
products in most cases inhibit heating to the melting temperature;
only in fault zones (or conceivably only in newly created faults)
deficient in volatile-containing minerals does the temperature reach
the melting point, yielding pseudotachylyte.
These results present a potential explanation for the lack of
thermal aureole around large, mature faults (San Andreas Fault
heat-flow paradox). They also reconcile the apparent rarity of
pseudotachylytes as originally postulated from experiments on
gypsum42 . We are at present pursuing a critical test of this
hypothesis—measurement of the rheology of nanometric solids—
to identify mechanism(s) by which frictional resistance to sliding
is rapidly reduced as sliding accelerates, and why it is recovered as
sliding slows.
3,000 nm
b
Methods
Methods and any associated references are available in the online
version of the paper.
Received 1 October 2014; accepted 9 April 2015;
published online 18 May 2015
References
1,200 nm
Figure 4 | Punchbowl Fault. a, Backscatter SEM image of gouge, showing
part of a large, porous, post-seismic potassium feldspar crystal (medium
grey) that engulfed large numbers of bright nanometric particles of original
gouge, protecting them from subsequent ubiquitous hydrous alteration
(arrow). b, Detail of the boxed area in a. Black arrows (lower left and right)
highlight post-seismic titanite (sphene) crystals that have pushed outwards
bright nanometric grains (iron oxides?) as they grew; white arrows highlight
overgrowths on albite grain (upper left) and pyroxene (centre right)
similarly excluding grains of lesser brightness. See Supplementary
Information for images of fault outcrop and gouge composition.
low-viscosity flow by grain-boundary sliding. So far, we have not
discussed processes that may operate during the initiation of sliding,
except for shear heating. We agree with De Paola et al.11 that,
immediately on initiation of sliding, flash heating of asperities
will take place and transient thermal pressurization of any fluids
present is likely to occur. Large-scale shear heating as discussed
above then produces the thermal spike that drives the temperature
to the level necessary to activate and sustain bulk endothermic
reactions. The primary differences between our results and previous
workers is that the normal stress in our dolomite experiment is
generally roughly ten times that of their experiments, making
488
1. Scholz, C. H. The Mechanics of Earthquakes and Faulting 2nd edn, 496
(Cambridge Univ. Press, 2002).
2. Di Toro, G. et al. Fault lubrication during earthquakes. Nature 471,
494–498 (2011).
3. Tullis, T. E. in Treatise on Geophysics, v.4, Earthquake Seismology
(ed. Kanamori, H.) Ch. 5 (Elsevier, 2014, in the press).
4. Goldsby, D. L. & Tullis, T. E. Low frictional strength of quartz rocks at
subseismic slip rates. Geophys. Res. Lett. 29, 1844 (2002).
5. Di Toro, G., Goldsby, D. L. & Tullis, T. E. Friction falls towards zero in
quartz rock as slip velocity approaches seismic rates. Nature 427,
436–439 (2004).
6. Han, R., Shimamoto, T., Hirose, T., Ree, J-H. & Ando, J-I. Ultralow friction of
carbonate faults caused by thermal decomposition. Science 316,
878–881 (2007).
7. Hirose, T. & Bystricky, M. Extreme dynamic weakening of faults during
dehydration by coseismic shear heating. Geophys. Res. Lett. 34, L14311 (2007).
8. Brantut, N., Schubnel, A., Rouzaud, J-N., Brunet, F. & Shimamoto, T.
High-velocity frictional properties of a clay-bearing fault gouge and
implications for earthquake mechanics. J. Geophys. Res. 113, B10401 (2008).
9. Han, R., Hirose, T. & Shimamoto, T. Strong velocity weakening and powder
lubrication of simulated carbonate faults at seismic slip rates. J. Geophys. Res.
115, B03412 (2010).
10. Reches, Z. & Lockner, D. A. Fault weakening and earthquake instability by
powder lubrication. Nature 467, 452–455 (2010).
11. De Paola, N. et al. Fault lubrication and earthquake propagation in thermally
unstable rocks. Geology 39, 35–38 (2011).
12. Chang, J. C., Lockner, D. A. & Reches, Z. Rapid acceleration leads to rapid
weakening in earthquake-like laboratory experiments. Science 338,
101–105 (2012).
13. Burnley, P. C., Green, H. W. II & Prior, D. Faulting associated with the olivine to
spinel transformation in Mg2 GeO4 and its implications for deep-focus
earthquakes. J. Geophys. Res. 96, 425–443 (1991).
14. Tingle, T. N., Green, H. W. II, Scholz, C. H. & Koczynski, T. A. The rheology of
faults triggered by the olivine-spinel transformation in Mg2 GeO4 and its
implications for the mechanism of deep-focus earthquakes. J. Struct. Geol. 15,
1249–1256 (1993).
NATURE GEOSCIENCE | VOL 8 | JUNE 2015 | www.nature.com/naturegeoscience
© 2015 Macmillan Publishers Limited. All rights reserved
NATURE GEOSCIENCE DOI: 10.1038/NGEO2436
15. Green, H. W. II & Burnley, P. C. A new self-organizing mechanism for
deep-focus earthquakes. Nature 341, 733–737 (1989).
16. Green, H. W. II & Houston, H. The mechanics of deep earthquakes. Annu. Rev.
Earth Planet. Sci. 23, 169–213 (1995).
17. Green, H. W. II Shearing instabilities accompanying high-pressure phase
transformations and the mechanics of deep earthquakes. Proc. Natl Acad. Sci.
USA 104, 9133–9138 (2007).
18. Jung, H., Green, H. W. II & Dobrzhinetskaya, L. F. Intermediate-depth
earthquake faulting by dehydration embrittlement with negative volume
change. Nature 428, 545–549 (2004).
19. Xia, G. Experimental studies on dehydration embrittlement of serpentinized
peridotite and effect of pressure on creep of olivine. PhD thesis, Univ.
California, 134 (2014).
20. Green, H. W. II, Young, T. E., Walker, D. & Scholz, C. H. Anticrack-associated
faulting at very high pressure in natural olivine. Nature 348, 720–722 (1990).
21. Zhang, J., Green, H. W. II, Bozhilov, K. & Jin, Z-M. Faulting induced by
precipitation of water at grain boundaries in hot subducting oceanic crust.
Nature 428, 633–636 (2004).
22. Green, H. W. et al. Nanometric Gouge in High-Speed Shearing Experiments:
Superplasticity? Abs. #T31D-08 (Amer. Geophys. Union Fall Meeting San
Francisco, 2010).
23. Chester, F. M. & Logan, J. M. Implications for mechanical properties of brittle
faults from observations of the Punchbowl Fault zone, California. PAGEOPH
124, 79–106 (1986).
24. Chester, F. M. & Chester, J. S. Ultracataclasite structure and friction processes of
the Punchbowl Fault, San Andreas system, California. Tectonophysics 295,
199–221 (1998).
25. Chester, J. S., Chester, F. M. & Kronenberg, A. K. Fracture surface energy of the
Punchbowl Fault, San Andreas system. Nature 437, 133–136 (2005).
26. Chen, X., Madden, A. S., Bickmore, B. R. & Reches, Z. E. Dynamic weakening
by nanoscale smoothing during high-velocity fault slip. Geology 41,
739–742 (2013).
27. Green, H. W. II, Scholz, C. H., Tingle, T. N., Young, T. E. & Koczynski, T.
Acoustic emissions produced by anticrack faulting during the olivine–spinel
transformation. Geophys. Res. Lett. 19, 789–792 (1992).
28. Schubnel, A. et al. Deep focus earthquake analogs recorded at high pressure
and temperature in the laboratory. Science 341, 1377–1380 (2013).
29. Rice, J. R. Heating and weakening of faults during earthquake slip. J. Geophys.
Res. 111, B05311 (2006).
30. Riggs, E. & Green, H. W. II A new class of microstructures which lead to
transformation-induced faulting in magnesium germanate. J. Geophys. Res.
110, B03202 (2005).
31. Chokshi, A. H., Mukherjee, A. K. & Langdon, T. G. Superplasticity in advanced
materials. Mater. Sci. Eng. R10, 237–274 (1993).
32. Padmanabhan, K. A. & Basariya, M. R. Mesoscopic grain boundary sliding as
the rate controlling process for high strain rate superplastic deformation.
Mater. Sci. Eng. A 527, 225–234 (2009).
33. Mohamed, F. A. Deformation mechanism maps for micro-grained,
ultrafine-grained, and nano-grained materials. Mater. Sci. Eng. A 528,
1431–1435 (2011).
34. Ovid’ko, I. A. & Sheinerman, A. G. Kinetics of grain boundary sliding and
rotational deformation in nanocrystalline materials. Rev. Adv. Mater. Sci. 35,
48–58 (2013).
ARTICLES
35. Niemeijer, A. et al. Inferring earthquake physics and chemistry using an
integrated field and laboratory approach (Review). J. Struct. Geol. 39,
2–36 (2012).
36. Rice, C. M. et al. A Devonian auriferous hot spring system, Rhynie, Scotland.
J. Geol. Soc. 152, 229–250 (2013).
37. Meyer C. & Hemley J. Hydrothermal alteration of some granodiorites.
Sixth National Conference On Clays and Clay Minerals 89–100 (Pergamon
Press, 1959).
38. Han, R., Hirose, T., Shimamoto, T., Lee, Y. & Ando, J. Granular nanoparticles
lubricate faults during seismic slip. Geology 39, 599–602 (2011).
39. Dominguez-Rodriguez, A., Gomez-Garcia, D., Zapata-Solvas, E., Shen, J. Z. &
Chaim, R. Making ceramics ductile at low homologous temperatures. Scr.
Mater. 56, 89–91 (2007).
40. Yund, R. A., Blanpied, M. L., Tullis, T. E. & Weeks, J. D. Amorphous
material in high strain experimental fault gouges. J. Geophys. Res. 95,
15589–15602 (1990).
41. Wilson, B., Dewers, T., Reches, Z. & Brune, J. Particle size and energetics of
gouge from earthquake rupture zones. Nature 434, 749–752 (2005).
42. Brantut, N., Han, R., Shimamoto, T., Findling, N. & Schubnel, A. Fast slip with
inhibited temperature rise due to mineral dehydration: Evidence from
experiments on gypsum. Geology 39, 59–62 (2011).
Acknowledgements
Discussions with D. Lockner and N. Beeler over several years provided important
suggestions that significantly contributed to the evolving ideas now presented here.
J. Zhang contributed helpful comments on experimental techniques. We also thank FEI
Corporation for cutting FIB foils and for assistance with the highest-resolution scanning
electron microscopy. In particular, Fig. 3d was obtained on the Magellan microscope at
the FEI research facility in Portland, Oregon. F.S. acknowledges the China University of
Geosciences and China Scholarship Council for a fellowship to pursue his Ph.D research
at UC Riverside. Formal reviews by D. Moore and T. Tullis greatly improved the
manuscript. This paper is based on work supported by the National Science Foundation
under Grant #1247951 to H.W.G. II and Z.R. and #1015264 to H.W.G. II. The study was
also supported by the NSF Geosciences, Equipment and Facilities, Grant No. 0732715,
and partial support of NSF, Geosciences, Geophysics, Grant No. 1045414, both to Z.R.
Author contributions
H.W.G. II conceived the project, contributed the primary ideas and wrote the
manuscript. F.S. conducted the specific high-pressure faulting experiment and succeeded
in preserving fault contents intact (Fig. 3). K.B contributed critical SEM and TEM
imaging and analysis. G.X participated in electron microscopy (Supplementary Fig. 3c)
and in the hunt for critical images of the Punchbowl Fault. Z.R. conducted the
high-speed experiments (Figs 1 and 2 and Supplementary Fig. 3a,b) and contributed to
the development of the ideas. All authors contributed to manuscript preparation.
Additional information
Supplementary information is available in the online version of the paper. Reprints and
permissions information is available online at www.nature.com/reprints.
Correspondence and requests for materials should be addressed to H.W.G. II.
Competing financial interests
The authors declare no competing financial interests.
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489
NATURE GEOSCIENCE DOI: 10.1038/NGEO2436
ARTICLES
Methods
High-speed experiments were conducted on solid blocks of Kasota dolomite
(quarried at Mankato, Minnesota, XRD analysis indicates 97.3% dolomite, 2.6%
quartz and traces of feldspar). The samples include two cylindrical blocks of
101.6 mm diameter and 50.8 mm height. The upper, stationary block has a
raised-ring structure with inner and outer diameters of 63.2 mm and 82.3 mm,
respectively. The blocks were pressed against each other along the raised ring.
Thermocouples were cemented into holes drilled 3 mm away from the sliding
surfaces. The normal stress was kept constant during the experiment. The
experimental system has the capability to apply normal stress up to 35 MPa, a slip
velocity of 0.001 to 2 m s−1 , fast rise to full velocity (<0.1 s), unlimited slip distance,
and high frequency, continuous monitoring of the experimental data. The
apparatus power system includes a 100 HP electric motor with a torque controller
up to 3,000 N m. The control system is based on National Instruments components
and a dedicated LabView program.
Specimens for high-pressure experiments were cored from a commercially
hot-pressed block and encapsulated in Pt. After placing specimens into the
high-pressure assembly, the entire assembly was stored in a vacuum oven at 110 ◦ C
until loaded into the Griggs-type deformation apparatus43 . All high-pressure
experiments were conducted at 1,200 K using alkali chlorides (NaCl or CsCl) as
confining medium; temperature was measured by two B-type thermocouples
located near the top and bottom of the specimen. Experiments were conducted in
the following manner. The pressure was increased first to a level desired for each
experiment, followed by the temperature until the desired run conditions (1,200 K)
were obtained. The samples were loaded at a constant strain rate of 10−4 s−1 . Stress
was measured with a load cell external to the apparatus; total friction in the system
was measured by the force level on the moving (deformation) piston just before it
encountered the specimen. Detailed procedures of the experiments using this
apparatus are presented in ref. 18.
Scanning electron microscopy (SEM) enables images of surfaces at much
higher magnification than optical microscopy because of the much
smaller wavelength of electrons compared to visible light. Imaging can be
optimized for surface topography, by capturing secondary electrons (SE)
emanated from the surface, or optimized for atomic weight contrast, by
capturing backscattered electrons (BSE). Here, SEM imaging and
energy-dispersive X-ray spectroscopy (EDS) analysis were performed on an FEI
NNS450-FEG SEM at the Central Facility for Advanced Microscopy and
Microanalysis (CFAMM) at the UC Riverside, equipped with an Oxford Aztec EDS
system and a 50Max SDD detector with a resolution of 127 nm at Mn Kα. The
imaging in SE and BSE modes and EDS analyses were performed at an accelerating
voltage of 15 kV in the SEM. High-resolution SEM imaging was done at the FEI
NanoPort facility in Hillsboro, Oregon on a Magellan SEM at an accelerating
voltage of 15 kV. The samples from the high-speed friction experiments and the
Punchbowl Fault zone were examined on pristine surfaces after gentle cleaning
with alcohol and coating with thin carbon film. The samples from the
high-pressure experiments were prepared by cutting and polishing cross-sections
to reveal the fault plane, and were carbon coated as well for SEM imaging
and analysis.
Transmission electron microscope (TEM) bright field images are created from
the population of electrons that pass through a very thin specimen. Images can also
be created by capturing only the electrons diffracted by a particular lattice
reflection of the specimen (dark field imaging). TEM images, diffraction and
energy-dispersive spectroscopy were performed on an FEI CM300 TEM equipped
with a LaB6 cathode and an EDAX Genesis EDS system with a 30 mm2 Si(Li)
detector with a resolution of 129 nm at Mn Kα at 300 kV in the CFAMM at the UC
Riverside. The thin-foil TEM samples of thickness (∼80 nm) were cut
perpendicular to the surfaces shown by SEM in Figs 1–3 by the FIB technique44 at
the FEI NanoPort facility in Hillsboro, Oregon on a Quanta 3D DualBeam FIB at
20 kV, with final low-angle polishing at 5 kV.
References
43. Green, H. W. II & Borch, R. S. A new molten salt cell for precision stress
measurement at high pressure. Eur. J. Mineral. 1, 213–219 (1989).
44. Dobrzhinetskaya, L. F. et al. Focused ion beam technique and transmission
electron microscope studies of microdiamonds from the Saxonian Erzgebirge,
Germany. Earth Planet. Sci. Lett. 210, 399–410 (2003).
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