LETTERSPUBLISHED ONLINE: 18 AUGUST 2013 | DOI: 10.1038/NGEO1905

Subduction-zone earthquake complexity relatedto frictional anisotropy in antigorite

Marcello Campione* and Gian Carlo Capitani

Earthquakes generated in subduction zones are caused byunstable movements along faults. This fault-slip instabilityis determined by frictional forces that depend on the tem-perature, pressure, morphology and deformation state of thefault rocks. Fault friction may also be influenced by preferredmineral orientations. Over-thrusting of rocks at the interfacebetween a subducting slab and the overlying mantle wedgegenerates shear deformation that causes minerals to align1–3,and this preferred mineral orientation affects the propagationof shear seismic waves4–6. Here we use laboratory experimentsto simulate fault slip in antigorite, the most abundant hydrousmineral phase within Earth’s upper mantle7. Using atomic forcemicroscopy, we show that antigorite single crystals possessstrong frictional anisotropy on their basal slip surface andthat preferred mineral alignment extends this property to aregional scale. Depending on the alignment, fault movementscan occur along a high-friction direction, creating stick-slipbehaviour that generates earthquakes. In contrast, if move-ments occur along a low-friction direction, the mantle wedgewill deform aseismically. Our results imply that mantle rocksin subduction-zone thrust faults can exhibit two opposite fric-tional behaviours, seismic and aseismic.

Strain and slip partitioning have been recognized to have adominant role in the dynamics of fault systems8,9. With the term‘slip partitioning’ the scientific community indicates, in relationto subduction zones, the subdivision of the oblique motion alongthe fault system into a horizontal and a convergent componentaccommodated by different faults10. In the most general geometryof a thrust fault, the slab subducts with an obliquity angle φ,which is the angle between the trench-normal (T) and the plateconvergence vector (P; Fig. 1). It is known that oblique subductioncauses along-strike deformation at the subduction zone, whichmight evolve towards an along-trench translation of the frontalwedge of the overriding plate through one or several strike-slipfaults11. This slip partitioning is revealed by a deviation (γ ) of theslip vectors of earthquakes from the plate motion direction towardsthe trench normal. Also other factors, such as backarc spreading12and slab pull force, have been demonstrated to be able to induce achange of the slabmotion in a direction pointing towards the trenchnormal13. The seismic degree of slip partitioning can be expressedas κ = 1−(φ−γ )/φ. κ lies in the range between 0 and 1 in the caseof null partitioning and complete partitioning, respectively. Valuesof κ < 0 and κ > 1 cannot be accounted for in the framework ofan isotropic slip partitioning model and therefore, when registered,they are considered anomalous or deriving from uncertainty ofthe data. However, the literature data show that they are not anexception and, moreover, a high deviation γ is sometimes revealedalso in subduction zoneswhere no strike-slip faults are present.

Department of Earth and Environmental Sciences, Università degli Studi di Milano Bicocca, Milano I-20126, Italy. *e-mail: marcello.campione@unimib.it

Trench

Subducting plate

MantleMantle wedge

E

E'T'

P'

T

P

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φ

γ

Figure 1 | General scheme of plate motions in an oblique subduction zone.P, T, and E are plate convergence, trench normal and slip vectors,respectively. Their projections on the upper plate are indicated with primesuperscripts. φ is the obliquity angle. The red curve represents a polar plotof anisotropic longitudinal friction between the subducting slab and themantle wedge. The declination γ of E’ with respect to P’, when lower orequal to φ, can be explained by slip partitioning with horizontal motionaccommodated by a strike-slip fault. However, it may arise from frictionalanisotropy, which can admit also γ >φ (as in the scheme) and γ <0.

Frictional anisotropy has its intrinsic foundation in the chemicalstructure of rock minerals. Among minerals with a prominentrole in defining the rheological behaviour of faulted regions atsubduction zones, antigorite is recognized as the most important(Supplementary Fig. S1)7. To investigate the relation between thesurface crystal structure of antigorite and its frictional response,we performed a thorough nanotribological characterization withan atomic force microscope (AFM; Fig. 2a). The prerogative ofthis nanoscale approach is based on the possibility to considerour conclusions independent of the history of the serpentine-bearing rock, as deformation processes do not alter the physicalproperties of individual rock mineral phases. For this reason,we selected single crystalline regions of our antigorite sample(Fig. 2b). After having identified an atomically flat region of theorder of some tens of nanometres (Fig. 2b,c), we performed thefrictional mapping by collecting topographic and lateral forceloops14. The results of thismeasurement are reported in Fig. 2e,f, foran antigorite crystal (Antrona Valley provenance, SupplementaryFig. S2) oriented as in Fig. 2c. Consistently with the orthotropicprojected symmetry of antigorite(001) (refs 15–17), and verifiedfor this sample through transmission electron diffraction (Fig. 2d),to model its frictional response we employed an orthotropic linearconstitutive relation of anisotropic friction18: t = −NCv, wheret is the friction force vector, N is the normal load, C is anorthotropic second-rank friction tensor, and v is the slip velocity

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LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO1905

βθtp

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Figure 2 | Nanotribological characterization of antigorite(001). a, Experimental geometry (see text for details). b, Atomic force microscope (AFM)microscale morphology of antigorite(001). c, High-resolution image of the shaded region in b and 2D Fourier transform (inset). d, Electron diffractionpattern of the same sample. e,f, x- and y-components, respectively, of friction as a function of θ deduced from hysteretic height and torsion AFM data.Error bars represent standard deviations over 64 estimates. g, Longitudinal component of the friction force and orthotropic best-fit (red dashed curve).h,i, Transverse component of friction (h) and the β angle (i). The curves shown in blue are calculated with the best-fit tensor obtained in g.

versor. By fitting the tangential friction force tp deduced fromthe data in Fig. 2e,f (Fig. 2g), one obtains the coefficients C11 =

0.078±0.003, C12 =C21 = 0.029±0.004, and C22 = 0.063±0.003,corresponding to an applied load of 31 nN (effective load: 4 nN,adhesion: 27 nN). With these friction coefficients, we calculatedthe transverse friction force ts as a function of scan angle, whichis reported in Fig. 2h as a blue dashed curve together with theexperimental data deduced from Fig. 2e,f. Similarly, we reportin Fig. 2i the experimental and calculated dependence on scanangle of the angle β between the friction force vector andthe slip direction. The experimental friction hodograph deducedfrom the data in Fig. 2e,f is reported in Fig. 3a, with the blackellipse representing the orthotropic hodograph obtained from thefitted friction coefficients. The red circle represents the isotropichodograph obtained under the same conditions on an amorphoussilicon dioxide surface (Supplementary Fig. S3). Polar plots ofthe data shown in Fig. 2g,h are reported in Fig. 3b,c, respectively.By diagonalizing the tensor C, one obtains the principal frictioncoefficients C1 = 0.041± 0.007 and C2 = 0.100± 0.007, and theprincipal directions of friction at θ =−52± 7◦ and θ = 38± 5◦,which are reported as dashed lines in Fig. 3.

This nanotribological characterization allows one to draw thefollowing conclusions: orthotropy is a good approximation for

the symmetry of friction of antigorite(001), with an anisotropyof 85% (A% = 200|C1 −C2|/(C1 +C2)); the a and b axes of thesurface unit cell of antigorite(001) are parallel to the direction ofmaximum andminimumprincipal friction, respectively (Fig. 3b,c).These conclusions can be explained by analysing the peculiarsurface corrugations of antigorite (Supplementary Fig. S1). Indeed,the presence of tetrahedral reversals in the crystal structureconfers a puckering of the cleaved surface17 which is revealedas parallel straight grooves spaced by 4.40 ± 0.28 nm in AFMimages and appearing as enhanced intensities in the 2D Fouriertransform (Fig. 2c). This corrugation gives a major contributionto the AFM detected friction, as friction symmetry reflects thepresence of two orthogonal symmetry planes19. On the otherhand, friction anisotropy reflects the lower friction coefficientwhen sliding parallel to the direction of the grooves and a highercoefficient when sliding transversally to the grooves20,21. Similarresults were obtained on an antigorite sample from Valmalenco(Supplementary Fig. S4).

The interaction between the AFM tip and the antigorite singlecrystal represents a single elemental junction between an isotropicsurface and an anisotropic (orthotropic) surface, respectively. Theslab–mantle interface can be modelled as the interface separatingtwo sliding surfaces: that of the deformed serpentine in the mantle

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NATURE GEOSCIENCE DOI: 10.1038/NGEO1905 LETTERS(anisotropic) and that of the oceanic crust constituting the upperportion of the downgoing slab (Fig. 1). This latter surface isbasaltic in composition, and texturally isotropic. It may transformin eclogite at depths below 40–45 km, a massive metamorphicrock mostly composed of equant garnet and pyroxene grains22,thus reasonably assumable as isotropic. On the macroscale, theinterface can be considered as composed of myriad elementaljunctions23. Under this assumption, the anisotropic characterpersists if the junctions involve anisotropic surface domains witha preferential orientation. In this regard, we point out how theresults of nanoscale friction anisotropy of antigorite (Fig. 3b) closelyreflect the anisotropic distribution of shear strength of rock jointsmeasured on serpentinite on the laboratory scale (Figs 5, 12 ofref. 24), showing a clear orthotropic symmetry with anisotropylevels reaching 90%.

The high degree of crystal preferred orientation (CPO) exhibitedby antigorite crystal domains in foliated rocks as well as afterdeformation experiments1–4 is a fundamental requisite for the onsetof the anisotropic frictional behaviour of thrust faults. Experimentalevidence substantiates the occurrence of two types of strain-inducedCPO for antigorite. In both types, the antigorite(001) plane exhibitsa preferred orientation parallel to the shear plane. However, thea-axis concentrates either subparallel to the shear direction (CPO-I;refs 1,4) or orthogonal to it (CPO-II; refs 2,3). In the CPO-I case,the a-axis being associated with the direction of maximum friction(Fig. 3b), junctions are characterized by a high strength. As theCPO-I increases as a consequence of strain, we infer a progressivestrengthening of the fault (strain-hardening). In contrast, in theCPO-II case, the b-axis being associated with the direction ofminimum friction (Fig. 3b), we infer a strain-softening behaviour.Let us consider a thrust fault with plates converging at 30◦ obliquityand an isotropic frictional response (Fig. 4a,b, black circle). Slipalong P is mechanically stable because any projection of the dragforce (equating the friction force) to any other direction resultsin a force lower than friction. This is true also when CPO-I setsin, provided the anisotropy remains lower than 67% (Fig. 4a, blueplot). If anisotropy exceeds 67%, the projection of the drag forceequals the friction force at two specular directions. In the caseof orthotropy, these directions are at γe =±cos−1(C2/C1−C2)=±cos−1[(100/A%)−0.5)]. A plot of γe as a function of anisotropy isreported in Fig. 4c, and friction plots corresponding toA%=86 and100 are reported in Fig. 4a as green and red lines, respectively.Underthese conditions, on the basis of an equilibrium principle, slip alongP ismechanically unstable towards slip along directions deflected byγe. For an anisotropy close to that measured on antigorite crystals,this angle is as large as 48◦ (green arrows in Fig. 4a). This deflectionangle would correspond to κ =±1.6. As a limiting case, we reportin Fig. 4a the deflection predicted for A%= 100, which would causea slip exactly parallel to the trench (κ=−2.0, right red arrow).

On the basis of data assembled from the Harvard CentroidMoment Tensor catalogue, earthquakes with κ > 1 and κ < 0 fora large number of subduction zone segments are more the rulethan the exception13 (Supplementary Fig. S5). These anomalousvalues concentrate at obliquities lower than 20◦. As this latterangle is considered as a threshold for the initiation of the arcparallel strike-slip faulting10, this observation corroborates the roleof an intrinsic property of the thrust fault, for example frictionalanisotropy, in determining deviations of the slip vector. The seismicanisotropy of some of these regions (for example, Ryukyu) has beenrecently attributed to CPO of serpentine4, thus substantiating thepresence of deformed serpentine at the slab–mantle interface.

As far as the CPO-II mechanism is concerned (Fig. 4b), thissupports stability of the slip direction because friction is always atitsminimumvalue. Furthermore, the strain process softens the faultstrength. In oblique subduction settings, this softening inhibits thetriggering of transcurrent motion in the forearc region, as the shear

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Figure 3 | Frictional anisotropy of antigorite crystals. a, Data of Fig. 2e,freported in friction space, representing the friction hodograph measured onthe antigorite(001) surface. The best-fit orthotropic hodograph isrepresented by the black ellipse, where dashed lines indicate the principaldirections; the red circle is the best-fit hodograph as measured on thereference silicon dioxide surface. b,c, Polar plots of the longitudinal andtransverse friction forces, respectively (force scale is the same as in a); theinset in b is the same image reported in Fig. 2c; dashed lines indicate theprincipal directions of friction.

force projected parallel to the trench (Fig. 1) might never reach theforce required tomove the strike-slip fault. This behaviourmight beresponsible for the marked non-partitioned character (κ(φ)= 1) ofcertain subduction zone segments such as north-eastern Japan and

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LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO1905

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Figure 4 | Modelling of frictional anisotropy between the upper mantle and the subducting slab at thrust faults. a,b, Polar plots of orthotropiclongitudinal friction with 0% (black), 67% (blue), 86% (green) and 100% (red) anisotropy and identical average friction. Strain-hardening (a) andstrain-softening (b) behaviours due to CPO-I and CPO-II of serpentine, respectively (φ= 30◦), are shown. Dashed lines indicate orthogonal projections ofthe friction force. c, Dependence of the equilibrium deflection angle γe on frictional anisotropy. Equilibrium slip directions are visualized in the polar plotsby coloured arrows (γ =0◦, γ =±48◦ and γ =±60◦ for the blue, green and red plots, respectively).

central Chile13 (Supplementary Fig. S5).Otherwise, this observationsupports the postulated aseismic character of the forearcmantle25.

Frictional anisotropy provides a straightforward interpretationof slip events occurring in a large number of subduction segmentswhich, in the context of isotropic slip partitioning models, are notexplainable. In the intrinsic crystal structure of rock minerals suchas antigorite we identified the source of frictional anisotropy, which,under proper conditions, is shown to potentially cause anomaliesin slip trajectories. Frictional anisotropy could be enhanced orsmeared out by extrinsic factors such as a surface morphologystructured on different scales and the presence of fault gouges. Thestudy of the interplay of all these factors in a multiscale approachwill be of fundamental interest for a thorough description of thedynamics of subduction zones.

MethodsSamples. Two antigorite samples were selected for the present investigation, onesample from Antrona Valley (Italian Alps) and one sample from Valmalenco(Italian Alps). The former was collected on a foliated ophicalcite and struckthe present investigators for the very large dimensions of the crystals, with apronounced anisotropy and parallel orientation, which helped during preparationand AFM measurements. The latter comes from a thermo-metamorphosedserpentinite and was chosen because it has a proven good crystal quality and is wellcharacterized15,16 (Supplementary Fig. S1).

AFM. Fragments of rock were glued with their (001) face on the sample-holderusing a bicomponent resin and then cleaved with a glaucoma knife using aprocedure similar to that described in ref. 17. Measurements were carried out attemperature of 28 ◦C and 30% relative humidity. Image processing was carriedout with WSXM (ref. 26).

We used a Nanoscope V (Bruker) equipped with J-type and E-type scanners.The probes were silicon nitride probes with a force constant of 0.09Nm−1. Thelateral force constant was calibrated on amorphous silicon dioxide surfaces usingthe method described in ref. 27. Tomeasure a friction hodograph, one has to collectfriction shear force components acting parallel to the cantilever axis (causingbuckling of the cantilever) and orthogonal to it (causing torsion of the cantilever),while spanning the scan angle over the round angle. Buckling and torsion aredetected as hysteretic height and lateral-force loops, which are translated intofriction forces by virtue of a calibration procedure that we performed on isotropicsilicon dioxide surfaces14 (Supplementary Fig. S3). Hodographs were constructedby collecting the trace and retrace topographic (height, nm) signal simultaneouslywith the trace and retrace lateral force (friction, mV) signal as a function of the scanangle. For each scan line, the trace-minus-retrace values (TMR-H and TMR-T,respectively) are considered to be proportional to the x- and y-component offriction force, respectively. For each scan angle, lateral force and height data areaveraged from a set of 64 scan lines of 512 points each. The fit function used

in Fig. 2g was tp =−N [C11 cos2 θ +2C12 cosθ sinθ +C22 sin2 θ ], correspondingto a linear orthotropic constitutive law of friction18. The best fit was obtainedstarting with friction coefficients deduced by an anisotropic friction fitting ofthe curves in Fig. 2e,f.

After data collection, a topographic scan was performed over the analysed areato verify that the surface suffered no wear and plastic deformation. The results ofthis procedure performed on the isotropic reference substrate of silicon dioxideobtained by an optimized wet-method28 are reported in Supplementary Fig. S3,corresponding to a normal load of 9 nN (effective load: 4 nN, adhesion: 5 nN). Thehodograph measured for an antigorite sample of the Antrona Valley is reported inFigs 2 and 3. The same set of measurements for an antigorite sample of Valmalencois reported in Supplementary Fig. S4.

X-ray diffraction. The sample from Antrona Valley, as it had not been subjected toany previous crystallographic characterization, was investigated by X-ray powderdiffraction (XRPD) and transmission electron microscopy (TEM). Both methodsconfirm the Pm symmetry and m= 17 polysome. Specular scans on powderedsamples were performed with a PANalytical X’Pert Pro powder diffractometer withCuKα radiation. The spectrum along with some indexed reflections is reportedin Supplementary Fig. S2. Cell refinement gives a= 43.3067(25), b= 9.2347(5),c = 7.2683(2), β = 91.61(1).

TEM. A small amount of the same powder used for XRPD was dispersed inethanol and pipetted on a holey-carbon-coated Cu-grid. TEM investigations wereperformed at the Earth Sciences Department ‘Ardito Desio’ of the University ofMilan with a FEI Tecnai F20 microscope and selected area electron diffractionpatterns acquired on a Gatan 794 CCD along the [001] direction. TEM resultsconfirm the symmetry and cell parameters found with XRPD and show limitedpolysomatic disorder. In particular, the modulation vector was found alwaysrigorously parallel to the subcell [100] direction.

Received 28 November 2012; accepted 3 July 2013;published online 18 August 2013

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AcknowledgementsThe microscopic characterization was performed at the Atomic Force MicroscopyLaboratory of the Department of Materials Science, Università degli Studi di MilanoBicocca, thanks to the support of A. Sassella. We thank N. Malaspina for discussions andcomments that improved this manuscript.

Author contributionsM.C. performed the nanotribological analysis and developed the geophysical model,G.C.C. prepared natural samples and performed structural analysis. Both authorsdiscussed the results and implications.

Additional informationSupplementary information is available in the online version of the paper. Reprints andpermissions information is available online at www.nature.com/reprints. Correspondenceand requests for materials should be addressed toM.C.

Competing financial interestsThe authors declare no competing financial interests.

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