Earth and Planetary Science Letters 306 (2011) 105–117

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Earth and Planetary Science Letters

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Seismic anisotropy and mantle dynamics beneath China

Zhouchuan Huanga,b,⁎, Liangshu Wanga,⁎⁎, Dapeng Zhaob,⁎⁎, Ning Mia, Mingjie Xua

a School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, Chinab Department of Geophysics, Tohoku University, Sendai 980-8578, Japan

⁎ Corresponding author at: Z. Huang, School of EaNanjing University, Nanjing 210093, China.⁎⁎ Corresponding authors.

E-mail addresses: zhouchuan.huang@gmail.com (Z.(L. Wang), zhao@aob.gp.tohoku.ac.jp (D. Zhao).

0012-821X/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.epsl.2011.03.038

a b s t r a c t

a r t i c l e i n f o

Article history:Received 24 December 2010Received in revised form 29 March 2011Accepted 31 March 2011Available online 29 April 2011

Editor: P. Shearer

Keywords:Shear-wave splittingAnisotropyContinentSubduction of the Pacific plateIndia–Asia collisionAbsolute plate motion

We analyzed the shear-wave splitting at 138 permanent seismograph stations to study seismic anisotropyand mantle dynamics under Mainland China. To obtain reliable results we used three different methods tomeasure the shear-wave splitting parameters using core phases (SKS, SKKS, SKiKS and PKS) as well as thedirect S waves from regional and distant earthquakes. Our results show that the fast orientations of theanisotropy (WNW–ESE) in eastern China are generally consistent with the absolute plate motion (APM)direction of the Eurasian plate, suggesting that the anisotropy is mainly located in the asthenosphere resultingfrom the lattice-preferred orientation of olivine due to the shear deformation there. The fast axes in westernChina generally agree with the strikes of the orogens and active faults, while they are perpendicular to thedirection of the maximum horizontal stress, suggesting that the anisotropy in the lithosphere contributessignificantly to the observed shear-wave splitting. The fast axes in western China are also consistent with theAPM direction, suggesting that the APM-driven anisotropy in the asthenosphere is another source of theshear-wave splitting there. These results suggest that APM-driven anisotropy commonly exists undercontinents, similar to that under oceanic regions, even though the continental lithosphere has sufferedextensive deformation.

rth Sciences and Engineering,

Huang), lswang@nju.edu.cn

ll rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Seismic anisotropy describes the directional dependence of seismicvelocity within the Earth and it is a characteristic feature of the Earth'sinterior structure. It may exist at different depth ranges in the crust,mantle and inner core (see Mainprice, 2007 and references therein).Different factors dominate in producing the anisotropy at variousdepths,such as aligned cracks in the upper crust (e.g., Crampin, 1984) and thelattice-preferred orientation (LPO) of minerals in the lower crust andupper mantle (see Karato et al., 2008; Mainprice, 2007 for comprehen-sive reviews). The LPO (or the anisotropic fabric) in the upper mantle isgenerally considered to be the result of dislocation creep of theminerals(mainly olivine) above a depth of ~300 km (e.g., Gung et al., 2003;PanningandRomanowicz, 2006). Theanisotropic fabric dependsonboththe type and extent of strain (Savage, 1999). Many experimental studiesand theoretical models have focused on the development of differentfabrics for simple shear, pure shear, axial and uni-axial compression. Ingeneral, the fast orientation of the anisotropy (e.g., a-axis of olivine) issubparallel to the extension or shear direction in the upper mantle (e.g.,Karato et al., 2008; Nicolas, 1993; Savage, 1999; Silver and Chan, 1991;Zhang and Karato, 1995). For simple horizontal mantle flow, the fastdirection is usually parallel to the flow direction (Karato et al., 2008).

Because of the close relationship between the anisotropy and strain inthe upper mantle, observations of seismic anisotropy can in principle beused to constrain the lithospheric and sublithospheric mantle deforma-tion that produces this anisotropy (Conrad et al., 2007). Shear-wavesplitting is a popular tool for characterizing anisotropy in the Earth (e.g.,Long and Silver, 2009; Silver and Chan, 1988, 1991; Vinnik et al., 1992).Shear-wave splitting, also called seismic birefringence, is a phenomenonin which a shear wave splits into two polarized shear waves withdifferent velocities when traveling though an anisotropic medium. Twosplitting parameters (φ, δt) can be measured from seismograms, whichcorrespond to the polarization direction of the fast quasi-S phase (φ) andthe delay time (δt) between the fast and slow components, respectively(Long and Silver, 2009). By using the long-period core phases, such asSKS, SKKS, SKiKS and PKS (hereafter, we call them XKS phases), theobserved shear-wave splitting is usually considered to reflect theanisotropy in the crust anduppermantle under the seismograph stations.Hence the splitting parameters can be used to study the anisotropy anddeformation in the upper mantle.

Many researchers have used splitting parameters to constrain theglobalmantleflowand theabsoluteplatemotion(APM)(e.g., Becker et al.,2006, 2008; Conrad et al., 2007; Kreemer, 2009; Kustowski et al., 2008).Comparisons of the XKS observations with the LPO derived from thenumericalmodeling showthat thepredictionsofuppermantle anisotropymade by the global mantle circulation models match the observationswell beneath the oceans but poorly under the continents (Conrad et al.,2007; Long and Becker, 2010). The fit under the continents, however, canbe improved when considering lateral variations in the lithospheric

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thickness (Conrad et al., 2007). These results suggest that the anisotropyin the continental lithosphere, whichmay suffer extensive deformation,contributes significantly to the observed shear-wave splitting (e.g.,Fouch and Rondenay, 2006; Savage, 1999; Silver, 1996; Vinnik et al.,1992). It is far fromclear, however,whichof the sources—the anisotropicstructure in the lithosphere or the contemporary flow in the astheno-sphere—dominates in the observed anisotropic signal in the continentalregions (Long and Becker, 2010).

China provides an ideal site to estimate the contribution from thecontinental lithosphere. The Chinese continent is composed of variouslithospheric blocks formed during its long geological history (e.g., Ma,1987, 1988; Ren et al., 1999). While retaining several stable Archeanblocks, China has been suffering extensive deformation in the Cenozoic(see Yin, 2010 for a comprehensive review). The eastern and westernparts of China (Fig. 1), however, have experienced completely differenttectonic evolutions. TheCenozoic tectonicsofwesternChina (or even theentireAsia region) ismost dramatically expressed by thedevelopment ofthe Tibetan Plateau resulting from the India–Asia collision (Tapponnieret al., 1982, 2001). In theMiddle to LateMiocene (i.e., 18–8 Ma), the N–Scontraction of the early stagewas replaced by the coeval development ofconjugate strike-split faults and E–W extension which continues untiltoday (Tapponnier andMolner, 1977; Yin, 2010 and references therein).The far-field effect of the India–Asia collision is considered to havereached the Tienshan orogen and the Baikal rift zone thousands ofkilometers northward (Molnar and Tapponnier, 1975; Tapponnier andMolner, 1979). Eastern China, in contrast, is characterized by thedevelopment of the back-arc extensional system as a result of thesubduction of the Pacific and Philippine Sea plates (e.g., Ren et al., 2002;Tian et al., 1992; Zhao et al., 2011a). Debates, however, are continuing

Fig. 1. Distribution of the 138 seismograph stations used in this study. The surface topograpstations of group A with data available during 2000 to 2009, while the black triangles show tbetween different tectonic blocks (Ren et al., 1999). The bold arrows denote the motion direThe vertical dashed line shows the rough boundary between western and eastern China in

onwhether the India–Asia collisionhas influenced the tectonic evolutionof eastern China (e.g., Liu et al., 2004). The existence of various tectonicelements (blocks, orogens, faults, etc.) in China allows us to betterunderstand the origin of seismic anisotropy in the lithosphere andasthenosphere under a continental region.

In this study,we analyzed the shear-wave splitting at 138 permanentdigital seismograph stations in Mainland China. The measured splittingparameters are thenused to estimate thefirst-order anisotropic patternsin the upper mantle, including the lithosphere and asthenosphere.Although many previous studies have investigated the anisotropicstructure in various parts of China, the present work has the followingadvantages over the previous studies. (1) We analyzed the shear-wavesplitting at 138 permanent stations across China. Many of the stationshave been deployed for more than ten years (Fig. 1), and so much morehigh-quality data are available for us to better understand seismicanisotropy andmantle dynamics under the entire Chinese continent. (2)Three different methods are used to make the shear-wave splittingmeasurements, and the results by the different methods are carefullycompared andanalyzed to avoid any potential bias.While our results aregenerally consistentwithmany of the previous results, the presentworkhas provided important new insights into the seismic anisotropy anddeformation under the Chinese continental region.

2. Data and method

The 138 seismograph stations used in this study (Fig. 1) are broad-band, permanent stations operated by the China Seismic Network DataCenter. Most of the stations are equippedwith CTS-1, KS-2000 and JCZ-1seismometers, while several stations have GS-13, STS-2 and CMG-3 type

hy is shown in color with its scale shown at the bottom. The white triangles denote thehe stations of group B with data only in 2009. The dashed lines indicate the boundariesctions of the Indian, Pacific and the Philippine Sea plates relative to the Eurasian plate.vestigated in this study.

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seismometers (Table S2). These stations can be divided into two groups:Group A consists of 46 stations with data available from 2000 to 2009(white triangles in Fig. 1), while group B consists of 92 stations withavailable seismic records of one-year in 2009 (solid triangles in Fig. 1).Earthquakes with magnitude ≥6.0 at epicentral distances from 85° to140° were selected for analysis and 377 events were finally included inour data set (cross symbols in Fig. 2; 53 events in 2009). Most of theearthquakes with clear core phases (such as SKS and SKKS) occurred inthe Tonga subduction zone with back-azimuths of 110°–130°. We alsochecked many earthquakes with epicentral distances of 130°–140° andselected some of the events if their PKS phases are clearly visible, whichimproves the azimuthal coverage of the events used.

We used the SplitLab software byWüstefeld et al. (2008) tomeasurethe splitting parameters (φ, δt). The rotation cross-correlation method(RC) (e.g., Levin et al., 1999) and the transverse-component minimiza-tion method (SC) (Silver and Chan, 1991) were used simultaneously,which increases the reliability when both methods give consistentresults (e.g., Long and Silver, 2009; Vecsey et al., 2008). Both of themethods utilize a grid-search approach to identify the best-fittingsplitting parameters by rotating and time-shifting the Q–T componentsin a ray-coordinate basedL–Q–T coordinate system(Vecseyet al., 2008).The RCmethod tries to identify a pair of splitting parameters (φ, δt) thatmaximizes the cross-correlation between the corrected Q and Tcomponents, while the SC method seeks to minimize the amount ofenergy on the transverse component when the effect of splitting isaccounted for (Long and Silver, 2009). The Q and T components arerotated between−90° and 90° with a step of 1° and the time is shiftedfrom 0 s to 4 s with a step of 0.02 s. Figs. S1 and S3 show examples ofshear-wave splitting measurements on the SKS and PKS phases carriedon SplitLab, respectively, while Fig. S2 shows an example of a “null”measurement.

Themulti-channelmethod (MC) introduced by Chevrot (2000) is analternative to the single-record methods (RC and SC). The MC methodtakes advantage of the predicted variation in the amount of energy on

Fig. 2. Epicenter distribution of the earthquakes used in the study. The blue crossesdenote the 377 events with XKS (i.e., SKS, SKKS, SKiKS and PKS) phases, while the redcircles represent the 164 deep earthquakes with direct S phases. The three concentriccircles indicate the epicentral distances of 50°, 100° and 150° from the center of thestudy area. The open triangles denote the seismograph stations used. The purple dashedlines show the plate boundaries (Bird, 2003).

the uncorrected transverse component with incoming polarizationangle for a single, horizontal layer of anisotropy (Long and Silver, 2009).For the vertically propagating, long-period shear wave (δt≪T) that hastraversed a single layer of anisotropy with a horizontal axis oftransversely isotropic symmetry, the predicted transverse componentT(t) can be written as a function of the time derivative of the radialcomponent R(t) (Chevrot, 2000; Vinnik et al., 1989):

T tð Þ≈−12

δtsin2βð Þ dR tð Þdt

where β is the angle between φ and the incoming polarization direction(equivalent to the back-azimuth for XKS phases). After calculating thesplitting intensity/vector (S=δt sin 2β) for the seismograms of variousback-azimuths, we can retrieve the best-fitting splitting parameters (φ,δt) byfittinga sin 2β curve to the splitting vector (LongandSilver, 2009).The values of φ and δt can be inferred from the phase and amplitude ofthe sinusoid, respectively. The splitting intensity measurements can beeither stacked in azimuthal bins to improve the signal-to-noise ratio(Chevrot, 2000) or used individually (Long and van der Hilst, 2005;Monteiller and Chevrot, 2010).

Nevertheless, theMCmethod requires good coverage of events in theincoming polarizations (Chevrot, 2000). As shown above, the back-azimuthal coverage of events formost of the stations used in this study ispoor if only the SKS/SKKS phases are used, but it can be improved whenthe PKS phases are added. In addition, direct teleseismic S phases fromdeep earthquakes can potentially improve the back-azimuthal coverageafter their initial polarizations are measured directly from the seismo-gram (Long and van der Hilst, 2005; Vidale, 1986). The teleseismic Sphases from 164 deep events (N200 km) with epicentral distances of40°–80° (see Long and Silver, 2009 for details) were also included in ourdata set (red circles in Fig. 2; 11 events in 2009). These earthquakesmainlyoccurred in the Indonesian subductionzoneand theydramaticallyimprove the back-azimuthal coverage (Fig. 2).

3. Results

3.1. Measurements with the RC and SC methods

Wedefine thequality ofmeasurementswith theRCandMCmethodsby using the differences between the results by the two methods(Wüstefeld and Bokelmann, 2007): the angular difference of φ(Ψ=|φRC−φSC|) and the ratio of δt (ρ=δtRC/δtSC) (Fig. S4). The goodsplittingmeasurements are identified as 0.8≤ρ≤1.1 andΨ≤10°, whilethe fair measurements have 0.7≤ρ≤1.2 and Ψ≤15°. The quality ofnulls is also defined: the good nulls with 35°≤Ψ≤55° and ρ≤0.2, whilethe fair nulls with 30°≤Ψ≤60° and ρ≤0.3. The remaining ones areconsidered to be poor measurements. Following these criteria, weobtained 266 good and 152 fairmeasurements, in addition to 1457 goodand 341 fair nulls. The above criterion for the non-null measurements isvery rigorous (Fig. S4). In fact, the SC method can always yield a correctestimate of φ even if it is parallel or perpendicular to the back-azimuth(Wüstefeld and Bokelmann, 2007). We also tried to identify the goodand fair non-null measurements based on the angular differencebetween φ (with the SC method) and the back-azimuth (|φ-baz|,projected to 0°–45°) and the signal-to-noise ratio (SNR) of the originaltransverse component (Fig. S4) (e.g., Liu et al., 2008). The goodmeasurements are identified as |φ-baz|≥30° and SNR≥10, while thefair measurements have |φ-baz|≥20° and SNR≥5. The uncertaintiesof φ and δt for these measurements are constrained to be smaller than45° and 1.0 s, respectively. Following the new criterion, we obtained263 good and 459 fair non-null measurements, which contain almostall those defined with the method of Wüstefeld and Bokelmann (2007)(Fig. S4b). Note that the good and fair measurements only make upapproximately 17% of the total measurements wemade, while the nullsmake up more than 40%.

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Due to the reasonsmentioned above,we focus on the resultswith theSC method. The δt values range from 0.4 to 1.4 s with a peak at 0.8–1.2 s(Fig. 3a). As a whole, the fast directions are generally oriented E–W ineastern China, whereas they are parallel to the strikes of the orogens andactive faults in western China (Fig. 4a). The clockwise rotation of φaround the eastern Himalayan syntax and its abrupt change from N–S toE–Win the southeasternmargin of the Tibetan Plateau are also revealed,similar toprevious results (Fleschet al., 2005; Fuet al., 2008, 2011;Huanget al., 2000, 2007; Lev et al., 2006;McNamara et al., 1994; Sol et al., 2007;Wang et al., 2008).

Azimuth-dependent splitting is found atmany stations (Fig. S5) andthe corresponding splitting parameters (both φ and δt) are scattered.For example, at HIA, themeasuredφ ranges between−60° and 30° andδt between 0.5 s and 2 s (Fig. S6; Table S1), which cannot be attributedto measurement errors alone. We applied the two-layer fittingalgorithm (Silver and Savage, 1994) and found that the measurementsat HIA can be well fitted with two-layer anisotropy (Fig. S6). Two-layeranisotropy has also been identified atmanyother IRIS stations (BJT, TYA,HIA, KMI, LSA,WUS, XAN, and ULN) by this and previous studies (TableS3) (Bai et al., 2010; Barruol et al., 2008; Gao and Liu 2009; Li et al., 2010,2011), which suggests complex anisotropic structures beneath Chinaand adjacent regions.

Themeasurednulls are also summarized in Fig. 4b.However, thenullpattern is somewhat biased from the fast axis φ with the SC method,especially under eastern China. Note that many of the earthquakes usedin this work occurred in the Tonga subduction zone. At periods N8–10 s,the splitting detection limit for broadband data with typical noise levelsusing the RC and SC methods is ~0.5 s (Long and Silver, 2009). It ispossible that, with weaker anisotropy, the pattern of the nulls is biasedto reflect the earthquake distribution. At several stations of group A(with data of 10 years), the nullmeasurements cover a large or even theentire back-azimuthal range (HIA, MDJ, SSE, LSA, and WMQ) (Fig. 4b).The results may indicate isotropy beneath the stations because thesplitting is smaller than the lower detection limit of the RC and MCmethods, or they are the result of extreme lateral heterogeneity that is

Fig. 3.Histograms of δt valuesmeasured by using the SC (a) andMC (b)methods (see textfor details).

not coherent over the length scales associated with the seismicwavelengths under study (Long, 2010).

3.2. Measurements with the MC method

Asmentioned above, theMC analysis needs good azimuthal coverageof earthquakes at each station. For the stations of group A, the high-quality records of 10 years have led to many reliable splitting measure-ments (Fig. S7a), andso thesplittingparameters (φ, δt)weredeterminedfor most of the stations (Table S2). There are several stations wherethe splitting vector does not follow a sinusoid (Fig. S7d), which mayindicate theexistenceof complex structuresunder these stations, suchasazimuth-dependent or multi-layer anisotropy (Long and Silver, 2009;Long and van der Hilst, 2005). For some of the stations in group B, robustsplitting parameters are also obtained (Fig. S7b), although seismogramsare available for only one year.

For many stations in both groups, however, the XKS phasesthemselves are not enough to determine reliable splitting parameters.For those stations, the direct S phases from the teleseismic deepearthquakes could be used to better constrain the measurements(Fig. S7c; Table S2). Using the S phases, however, has a risk thatsource-side anisotropy may be mistaken for receiver-side anisotropy,although we have used only deep events (N200 km) to minimize thiseffect. Comparing the measurements from the S phases at neighboringstations canhelp to clarify this issue (Long andSilver, 2009; Longandvander Hilst, 2005). If the splitting vectors at neighboring stations from theevents in the same regions show notably different patterns, theobservations are most possibly the result of anisotropy under thestations. Otherwise, if the splitting parameters determined by using onlythe XKS phases and using both the S and XKS phases (S+XKS) for thesame station are similar, the source-side anisotropy of the teleseismic Sphases can also be excluded (Long and van der Hilst, 2005). Wecompared the results determined by using the XKS and the S+XKSphases to confirm if both approaches yield reliable solutions. Thecomparison shows that the differences between them are generally≤20° forφ and≤0.3 s for δt. These results suggest that using the Sphasesfrom the teleseismic deep earthquakes is a good choice for the MCanalysis if there are insufficient XKS phases available. But there are alsosome stations where source-side anisotropy cannot be ruled out fromthe observations. Therefore in this study, we mainly rely on measure-ments made from only the XKS phases. Only the measurements at sixstations (GUL, KSH, LYN, WZH, CN2 and HUC) are made from both theXKS and S phases because the XKS phases alone did not yield robustresults, and we have carefully checked these results to avoid possiblesource-side anisotropy.

The δt measurements with the MC method mainly range between0.4 s and 1.4 s (Fig. 3b), being consistent with that with the SC method.Thedominant valueof δt (0.6–1.0 s), however, is slightly smaller than thatfrom the SC method (0.8–1.2 s) (Fig. 3a) because individual measure-ments with the SC method can often overestimate the delay time whenthere is noise in the components (Monteiller and Chevrot, 2010). The φmeasurements with the MC method show a similar pattern to that withthe SC method, such as fault- and orogen-parallel φ in western China(Fig. 5). However, there seems a systematic discrepancy in the fast axes ineastern China: the dominant WNW–ESE φ by the MC method ascomparedwith the nearly E–Wφ by the SCmethod (Fig. 5). Note that, atstations with two-layer anisotropy, the MCmethod can also yield robustmeasurements that are the combined result of upper- and lower-layeranisotropy (Silver and Long, 2011).

3.3. Comparison with previous results

Many researchers have studied seismic anisotropy under Chinausing shear-wave splitting. The previous studies can bedivided into twotypes. One approach is to analyze the shear-wave splitting on individualseismogram using the RC or SC method (e.g., Huang et al., 2000;

Fig. 4. (a) Distribution of seismic stations with good (red) and fair (blue) measurements of shear-wave splitting with the SC method. The orientation and length of the short barsdenote the fast polarization direction (φ) and the delay time (δt), respectively. The scale for δt is shown in the inset. The curves show the epicentral distances (in degrees) from apoint (180°, 20°S) in the Tonga subduction zone. (b) Distribution of the good and fair null measurements in our data set. Nulls are plotted as crosses at each station with the barsoriented in the direction of the XKS back-azimuth and its orthogonal direction. The other labeling is the same as Fig. 1.

109Z. Huang et al. / Earth and Planetary Science Letters 306 (2011) 105–117

McNamara et al., 1994; Zhao et al., 2007). The other is to use all the dataavailable at one station to find the optimal solution with the stacking ortheMCmethod (e.g., Chen et al., 2010; Fu et al., 2011; Lev et al., 2006; Li

and Niu, 2010). In general, the results obtained by using the stacking orthe MC methods are consistent with each other, while they may showsignificant discrepancies from the results by the RC and SC methods.

Fig. 5. Distribution of the measured shear-wave splitting parameters with the MC method from the XKS phases (bold red bars) and the S+XKS phases (open red bars). Themeasurements with the SC method are also shown in blue bars, while the results of the previous studies are shown in gray bars. The splitting parameters (φ, δt) are indicated by theorientation and length of the bars with the scale shown in the upper-left inset. The orange lines denote the active faults. The other labeling is the same as Fig. 1.

110 Z. Huang et al. / Earth and Planetary Science Letters 306 (2011) 105–117

Synthetic studies show that the splitting parameters determined by theRCandSCmethods on individual seismogramsmaydeviate considerablyfrom the true values, especially when the initial polarizations (back-azimuths for theXKSphases) are parallel or perpendicular to the fast axisof the anisotropy in the media under the stations (e.g., Monteiller andChevrot, 2010; Vecsey et al., 2008; Wüstefeld and Bokelmann, 2007).The analysis also demonstrates that the stacking method (Wolfe andSilver, 1998) as well as theMCmethod can determine accurate splittingparameters (Monteiller and Chevrot, 2010).

Many of the earthquakes used in this study occur in the Tongasubduction zone with a back-azimuth of 110°–130° (Fig. 2). Because theback-azimuths of the events (the initial polarization of the XKS phases)are generally parallel with the fast axis of the anisotropy (φ by the MCmethod) in eastern China, analysis based on individual seismogramsis highly risky. For example, Zhao et al. (2007) showed that the dominantφ in South China is ENE–WSW to NE–SW, which is similar to the resultsby the SC method in this study (Fig. 4a). But the results by the MCmethod, as well as the previous studies using the stacking method(e.g., Chang et al., 2009; Fu et al., 2011), show dominant ESE–WNW φin South China (Fig. 5). Hence the results by the RC and SC methods onthe individual seismogram are probably biased. Because the incomingpolarizations are subparallel to the fast orientations, the energy of thetransverse component is veryweakand so thenoisemaydominate in themeasurement (Monteiller andChevrot, 2010;WüstefeldandBokelmann,2007). This possibility is confirmed by the fact that null measurementsmake up as much as 40% of the total measurements in our analysis.Fortunately, the situation is better for western China. The previousstudies using the RC and SCmethods on individual seismograms focusedon the anisotropic structure in southern Tibet and the Tienshan orogen.The φ in those regions (NE–SW) is neither parallel nor perpendicular tothe incoming polarizations, so the results obtained from the individual

seismograms in those regions are reliable (e.g., Huang et al., 2000;McNamara et al., 1994).

4. Discussion

The shear-wave splitting signals extracted from the XKS phases andthe S phases from the teleseismic deep earthquakes are generallyconsidered to reflect the anisotropy in the upper mantle, either in thelithosphere or asthenosphere (e.g., Long and Silver, 2009; Savage, 1999;Silver, 1996). In regions where the lithosphere lacks significantdeformation and the lithospheric anisotropy is weak, the observationsmainly reflect the anisotropy in the asthenosphere (e.g., Conrad et al.,2007). As a result of simple shear induced by the APM, the LPO in theasthenosphere is rotated toward the infinite strain axis. Thus the fast axesof anisotropy (φ) are parallel with either the mantle flow or the sheardirection (i.e., the APMdirections) (Karato et al., 2008; Zhang andKarato,1995). In contrast, in regions with extensive lithospheric deformation,anisotropy in the lithosphere may be another source for shear-wavesplittingobservations. Threemajor categoriesof lithosphericdeformationthatmay be encountered are transcurrent, uniaxis compression (actuallytranspression in most cases), and extensional regimes (Silver, 1996). Foreither transcurrent or transpression deformation, the fast orientations ofanisotropy (φ) are generally parallelwith themajor surface features, suchas the strikes of orogens and faults (e.g., Nicolas, 1993; Savage, 1999;Silver, 1996). In the case of an extensional regime, φ would be parallelwith the extension direction (Savage, 1999; Silver, 1996).

We compared the φ by the MC method with the APM directionsderived from theHS3-NUVEL1Amodel (Gripp andGordon, 2002) at the90 stations where the MC method yielded reliable measurements(Fig. 6a; Table S2). As a whole, φ is consistent with the APM directionwith anangular difference smaller than 25° (Fig. 6b). The results suggest

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that the observed shear-wave splitting is closely related to APM-drivenanisotropy in the asthenosphere due to the mineral LPO there. Thecorrelation between φ and the APM direction is relatively low for thestations in western China; some of the angular differences reach 35°(Fig. 6b). In western China φ appears to be better related to the surfacefeatures and lithospheric structures, e.g., φ is generally parallel with thestrikes of orogens and boundary-faults (Fig. 10) while perpendicular tothemaximumhorizontal stressσH (Heidbach et al., 2010) (Fig. 6e and f).Therefore, anisotropy in the lithosphere may significantly contribute tothe observed shear-wave splitting in western China. We also compareour measurements with the GPS results relative to the ITRF97(International Terrestrial Reference Frame, epoch 1997.0) (Wang et al.,2001) (Fig. 6c). In eastern China, theGPS results are consistentwith bothφ and the APMdirection, suggesting that the crust and uppermantle arehighly coupled and APM-driven anisotropy exists in the asthenosphere.In western China, φ seems to be consistent with the GPS results(especially in south Tibet), but not correlated with the APM direction(Figs. 6c and 10). This result also argues for lithospheric anisotropy inwestern China rather than asthenospheric anisotropy.

In summary, to first order, the observed shear-wave splitting isclosely related to asthenospheric anisotropy resulting frommineral LPOcaused by the APM. In western China, lithospheric anisotropy as a resultof lithospheric deformation may significantly contribute to the obser-

Fig. 6. Comparisons of the fast polarization direction φ (red bars) (a) with the direction ofthe absolute plate motion (APM) (Gripp and Gordon, 2002), (c) with the GPS results(purple arrows) (Wang et al., 2001), and (e)with the orientations ofmaximumhorizontalstress σH (black bars) (Heidbach et al., 2010). The GPS results in (c) and σH in (e) are theaverage values within 2° around each seismic station (Audoine et al., 2004). The rightpanels (b, d and f) show the histograms of the angular differences betweenφ andAPM (b),between φ and GPS results (d), and between φ and σH (f). The orange columns show theresults for all the stations inChina,while the blue columns show the results for the stationsin western China.

vations. In the following we discuss themechanisms that may affect theobservations in different parts of China.

4.1. Northeast China

Northeast (NE) China is characterized by widespread intraplatevolcanism in the Great Xing'an Ranges and Changbai Mountain and theaccompanying deformation caused by multi-episode extension since theLate Mesozoic (Ren et al., 2002; Tian et al., 1992; Wang et al., 2006).Global and regional seismic tomography shows that the subductingPacific slab from the Japan Trench becomes stagnant in the mantletransition zone under eastern China, and a bigmantle wedge has formedin the upper mantle above the Pacific slab and the stagnant slab underEast Asia (Huang and Zhao, 2006; Zhao, 2004). Mantle convection in thebig mantle wedge and deep slab dehydration may induce hot and wetupwellingflows, leading to the formationof active intraplate volcanism inNE China, such as the Changbai andWudalianchi volcanoes (Duan et al.,2009; Lei and Zhao, 2005; Zhao et al., 2004, 2009).

Iidaka and Niu (2001) studied anisotropy in the crust using Moho-converted phases and suggested that the crust in eastern China is almostisotropic. Li and Niu (2010) proposed that fossil anisotropy frozen in thelithosphere due to extension is the source for their observed shear-wavesplittingwithNW–SEφ inNEChina. In contrast, Liuet al. (2008) suggestedthat the measurements could be explained by the LPO of metastableolivine within the Pacific slab (e.g., Jiang et al., 2008) and by back-arcasthenospheric flow in the mantle wedge above the slab. Thesemechanisms, however, cannot explain the observations that uniformNW–SE oriented anisotropy exists in not only NE China but also broadareas in East Asia (Fig. 5). Two-layer anisotropy was revealed at HIA andULN, suggesting that the NW–SE oriented anisotropy (being parallel withAPM and the subduction direction) spreads across the Great Xing'anRange and continues to Central Mongolia thousands of kilometerswestward (Barruol et al., 2008; Gao et al., 1994) (Fig. 5). Thus it isimproper to attribute the large-scale splitting observations to any local-scale structures. Note that the frontier of the subducted Pacific slab hasreached to the eastern margin of the Great Xing'an Range, not furtherwestward (e.g., Huang and Zhao, 2006). The anisotropy in the lithosphereand that related to the subducted Pacific slab may contribute to or affectour observations, but can hardly be the dominant factor in such broadregions. For example, the observed δt is smaller in the southeastern part(i.e., the Changbai Mountain) than that in northwestern part of NE China(Fig. 7). As mentioned above, ongoing upwelling flow is expected in theuppermantle under the Changbai volcano (Zhao et al., 2009), and it musthave canceled someof theAPM-drivenmineral LPO. Thewidespreadnullsat station MDJ (Fig. 4b) also suggest weak anisotropy in this area.

The NNE–SSW φ (at GNH, MOH and also in the upper layer underHIA)measured in theGreatXing'anRange is subparallelwith the strike ofthe range and faults (Fig. 7). It may reflect anisotropy in the lithospherethatwrapped around the Siberian craton (Barruol et al., 2008; Li andNiu,2010; Liu et al., 2008). A similar measurement is obtained at station CN2near the Tanlu fault (Fig. 7); the fault-parallel φ could be caused bytranscurrent deformation in the lithosphere (Ren et al., 2002; Silver,1996; Zinke and Zoback, 2000).

4.2. North China

North China, usually named the North China Craton, is in a quitesimilar situation to that of NE China. Most of the above arguments aboutthe anisotropic structure under NE China remain suitable for NorthChina. However, North China may have suffered more specific tectonicevolution. It is considered to be anArchean cratonwhich consists of threesub-blocks: theWestern Block, Eastern Block and the Trans-North ChinaOrogen in themiddle (Fig. 8) (Zhaoet al., 2001). Theorogen in themiddleis a suture zone formed during the Precambrian when the Western andEastern blocks amalgamated (Zhao et al., 2001). The Western Block, ortheOrdos Block, is a stable block characterized by a thickmantle root and

Fig. 7.Distribution of the shear-wave splittingmeasurements (red bars) in Northeast China. The splitting parameters (φ, δt) are indicated by the orientation and length of the bars. Atstation HIA where multi-layer anisotropy has been revealed, the fast axes in the upper and lower layers are shown as open and solid black bars, respectively. The bold blue arrowsand thin purple arrows indicate the APM directions and the GPS results with the scales shown in the upper-right inset. The dark red bars denote the orientations of σH. The orangelines denote the active faults, and the yellow rose diagram shows the statistics of the fault strikes. The two solid triangles denote theWudalianchi and Changbai volcanoes. The otherlabeling is the same as Fig. 1.

Fig. 8. Distribution of the shear-wave splitting measurements in North China. The labeling is the same as Fig. 7.

112 Z. Huang et al. / Earth and Planetary Science Letters 306 (2011) 105–117

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the lack of internal deformation since thePrecambrian (HuangandZhao,2006; Tian et al., 2009; Zhao et al., 2001). The Eastern Block, in contrast,has been strongly reactivated since the Late Mesozoic and Cenozoic; thelithosphere has been thinned from ~200 km in the Archean to thepresent ~80 km (e.g., An and Shi, 2006; Chen et al., 2008; Griffin et al.,1998; Menzies et al., 2007).

The anisotropic features in the three sub-blocks are different. In theEastern Block, φ aligns WNW–ESE, which is consistent with thedirections of APM and the Pacific plate subduction (Fig. 8) (Bai et al.,2010; Chang et al., 2009; Liu et al., 2008; Zhao and Xue, 2010). Theobservations thus reflect asthenospheric anisotropy due to the mineralLPO in the mantle flow. However, the role of Pacific plate subduction isnot clear. Seismic tomographyhas revealed that the stagnant Pacific slabhas reached to the middle orogen and the upper mantle under theEastern Block is characterized by extensive low-velocity anomalies(Huang and Zhao, 2006; Tian et al., 2009; Xu and Zhao, 2009). Both thehigh heat flow and thin lithosphere suggest a hot mantle beneath theEastern Block (An and Shi, 2006; Chen et al., 2008; Hu et al., 2000). Notethat high temperatures can reduce viscosity, and so the development ofLPO becomes much easier under the shear induced by the APM (Karatoet al., 2008). Therefore the Pacific plate subduction has played a veryimportant role in the formation of anisotropy under the Eastern Block.

In the middle orogen, orogen- and fault-parallel φ (NNE–SSW) ismeasured (Fig. 8, SHZ and upper layer at BJT and TAY) (Bai et al., 2010;Zhao et al., 2008; Zhao and Xue, 2010). The orogen is the transition zonefrom thick lithosphere in the Western Block (N120 km) to thin lithos-phere (~80 km) in the Eastern Block (An and Shi, 2006). Zhao and Xue(2010) suggested that there is a sublithopheric corridor under theorogen; the mantle material in the asthenosphere may flow along thecorridor andgenerate the LPOwith theNNE–SSWorientation. Thismodelis supported by the tomographic images, which revealed a low-velocityzone in theuppermostmantle right beneath the orogen (Tian et al., 2009;Xu and Zhao, 2009; Zhao and Xue, 2010). Bai et al. (2010), however,carefully analyzed the shear-wave splittingat thepermanent stations andargued for a two-layer model under the orogen. The φ in the lower layeralignsWNW–ESE, which is consistentwith the APMdirection, whileφ inthe upper layer is subparallel with the orogen and fault strikes, reflectinganisotropy in the lithosphere (Bai et al., 2010; Huang et al., 2008).

There are fewmeasurements in theWestern Block (Ordos) because ofthe lack of seismograph stations there. Station YCH is located just in thewestern margin of the Ordos Block. The events recorded by YCH mainlyoccurred in the Tonga subduction zone andmost of the rays pass throughthe upper mantle under the Ordos Block. Thus themeasurements at YCHcanbeused to constrain theanisotropywithin the stableblock. The resultsshow a NW–SE φ under the Ordos Block, which is consistent with theresults at the portable stations in the northern Ordos (Fig. 8) (Zhao et al.,2008). TheNW–SE, evenNNW–SSE (HHC in Fig. 8)φ is different from theAPM direction and so cannot be explained by the APM-driven LPO in theasthenosphere. TheOrdos Blockmaybe very cold anddeep-rooted and sothe development of LPO is not easy (Hu et al., 2000; Huang and Zhao,2006; Tian et al., 2009; Zang et al., 2005). This cannot be the result of thepresent lithospheric deformation either, because there is no significantinternal deformationwithin the Ordos Block since the Precambrian (Zhaoet al., 2001). Thus, themost plausible explanation for the anisotropy in theOrdos Block is that it is fossil anisotropy frozen in the lithosphere whenthe block formed (Zhao et al., 2008).

4.3. South China

SouthChinawas formedby the collision of theYangtzeandSoutheast(SE) China blocks in the Precambrian (e.g., Charvet et al., 1996), and latersuperimposed by broad thin-skinned fold-and-thrust belts (trendingNE–SW) formed by the intracontinental shortening in the Mesozoic(Fig. 9) (Charvet et al., 1996; Li and Li, 2007). While the dominantWNW–ESE φ (APM-parallel, Fig. 9) suggests that the anisotropy existsmainly in the asthenosphere, frozen anisotropy in the lithosphere may

also contribute to the shear-wave splitting signals in areas with stronglithospheric deformation. At station ENH in the Sichuan Block, theobservations reveal a complex anisotropic structure (Figs. S5 and S7d),which may indicate multilayer anisotropy. The measurements may alsoreflect lateral variationsof the anisotropy under ENH (Fig. 9) because themulti-layer anisotropy itself can be revealed as anaccumulative result bythe MC method (Silver and Long, 2011).

SE China is located near the boundary between the Eurasian andPhilippine Sea plates. Seismic tomography has revealed significant low-velocity anomalies,whichmay reflect strongmantleupwellingoccurringin this region in the LateMesozoic andCenozoic (Huang and Zhao, 2006;Huang et al., 2010; Zhou et al., 2006). Thus the APM-driven LPO cannotbewell developed in the uppermantle under SE China. As the PhilippineSea plate ismoving toward the northwest at a rate of ~82 mm/yr (Fig. 9)(Seno et al., 1993), SE China is under extensive NW–SE contraction(Sibuet andHsu, 2004). TheNE–SWφ in this area is parallelwith the faultstrikes (Fig. 9), and so it should reflect the anisotropy in the lithosphereunder NW–SE compression (Savage, 1999; Silver, 1996; Zinke andZoback, 2000).

4.4. Western China

The western China region is characterized by extensive lithosphericdeformation resulting fromthe India–Asia collision(e.g., Tapponnier et al.,2001; Yin, 2010 and references therein). The Himalayan orogen, TibetanPlateau, Tienshan orogen, and even the Baikal rift zone are all consideredto be in the tectonic domain of the India–Asia collision (Molnar andTapponnier, 1975; Tapponnier et al., 1982, 2001; Tapponnier andMolner,1977; Yin, 2010). The φ orientation in western China is generally parallelwith the strikesof theorogensandactive faultswhileperpendicular to theorientations of the maximum horizontal stress σH (Figs. 6e, f and 10)(Heidbach et al., 2010), as revealed bymany previous studies (Chen et al.,2006, 2010; Dricker et al., 2002; Flesch et al., 2005; Herquel andTapponnier, 2005; Huang et al., 2000; Li and Chen, 2006; Li et al., 2010;McNamara et al., 1994; Silver, 1996; Sol et al., 2007; Vinnik et al., 2007;Wang et al., 2008). Thus the observed shear-wave splitting is mostprobably causedbyanisotropy in the lithosphere and its spatial variationsmainly reflect the large-scale pattern of lithospheric deformation (e.g.,Flesch et al., 2005;Wang et al., 2008). Many studies have suggested thatthe crust and the underlying mantle are completely decoupled beneathYunnan in the southeastern margin of the plateau where φ suddenlyrotates from N–S to nearly E–W(Fig. 10) (e.g., Flesch et al., 2005; Huanget al., 2007; Lev et al., 2006; Royden et al., 1997, 2008; Sol et al., 2007;Tapponnier et al., 2001). ButWang et al. (2008) argued for crust–mantlemechanical coupling during deformation; the observed dramatic spatialrotation of φ reflects the deformation style from simple shear in theTibetan Plateau transitioning to pure shear (E–W extension) in Yunnan.In general, φ in Yunnan agrees well with the APM direction and the GPSresults (relative to the ITRF97) (Fig. 10) (Bai et al., 2009; Huang et al.,2007; Lev et al., 2006), supporting the APM-driven anisotropy contrib-uting to the observations. Our measurements at KMI can be fitted wellwith two-layer anisotropy and φ in the lower layer is parallel with theAPM direction (Fig. 10), which confirms the existence of APM-drivenanisotropy under Yunnan.

Whether APM-driven anisotropy in the asthenosphere exists widelyunder western China is actually a critical issue, because it is concernedwith the effect of the India–Asia collision on the evolution in thesublithospheric mantle (e.g., Liu et al., 2004). As mentioned above, thesmall angulardifferencesbetweenφ andtheAPMdirection (b35°; Fig. 6)indicate that the observed shear-wave splitting is closely related to theAPM. As increasing data have become available, the complex anisotropicstructure, e.g., the multi-layer anisotropy, has been clarified. Two-layeranisotropy has been revealed at the stations in SE Tibet (KMI) and NETibet (Li et al., 2011), near the Tienshan orogen that is over 1000 kmaway from the frontier of the India–Asia collision (WUS, WMQ; Li et al.,2010), and even inMongolia (ULN,Ulaanbaatar; Barruol et al., 2008) and

Fig. 9. Distribution of the shear-wave splitting measurements in South China. The measurements with the SCmethod at ENH are shown as black bars, and the blue circle denotes theincidence angle of 10°. The other labeling is the same as Fig. 7.

114 Z. Huang et al. / Earth and Planetary Science Letters 306 (2011) 105–117

theBaikal rift zone. The fast axes of the lower-layer anisotropy at all thesestations show good consistency with the APM direction. Many previousstudies showed that the fast axes of one-layer anisotropy are also parallelwith the APM direction (e.g., Barruol et al., 2008; Dricker et al., 2002;Gao et al., 1994; Huang et al., 2007; Lev et al., 2006; Li et al., 2011). Allthese results suggest that APM-driven anisotropy exists widely in theasthenosphere under western China, i.e., the Tibetan Plateau, Tienshanorogen and even Mongolia and the Baikal rift zone (Figs. 5 and 10).Although the lithosphere in this region may have suffered strongerdeformation than any other region in the world as a result of the India–Asia collision (Tapponnier et al., 2001; Yin, 2010), from the viewpoint ofseismic anisotropy, the asthenosphere may have evolved in its ownwayand been less affected by the mountain buildings in the overlyinglithosphere.

Special attention, however, should be paid on the interpretation ofsplitting in south Tibet. As only four permanent stations (GZE, LIZ, LSAand SQH) are located in south Tibet, our results could not provide acomprehensive view on anisotropy there. Fortunately, the resultsaccumulated in the past decades provide important information onthis issue (e.g., Chen et al., 2010; Gao and Liu, 2009; Huang et al., 2000).Because south Tibet is located in the frontier of the India–Asia collisionwhere the Indian plate has subducted into the upper mantle and evenreached the mantle transition zone (e.g., Li et al., 2008; Zhao et al.,2011b), consistentmineral LPO in the asthenosphere is unlikely. Thus theAPM-drivenanisotropymaynot exist in this region. Themeasurements insouth Tibet show strong lateral variations and tend to be well correlatedwith the strikes of active faults andGPS results (Figs. 5 and 10) (e.g., Chenet al., 2010; Huang et al., 2000), which also argue against APM-drivenanisotropy while favoring anisotropy in the lithosphere. The δt splittingtimes may reach more than 2 s (GZE; Fig. 4; Table S1) (Chen et al., 2010;Huang et al., 2000), suggesting that the thickness of the anisotropic layershould be at least 230 km (for anisotropy of 4%; Silver and Chan, 1988).Because the depth of the lithosphere–asthenosphere boundary is only

~200 km (Zhao et al., 2011b) and the crustal thickness is ~70–80 km(Yuan et al., 1997), the thickness of the lithospheric mantle beneathTibet is no more than 130 km. Hence the crustal anisotropy mustcontribute to the observed splitting. Two origins of crustal anisotropyshould be considered. One is the LPO of middle-lower crustal mineralssuch as amphibole developed in deep crustal flow, which may causeshear-wave splitting with fast axes (φ) consistent with the GPS results(Gaoand Liu, 2009; Roydenet al., 2008). Theother is fault-parallelφnearactive faults, whichmay result from the crustal fault-fabric accompany-ing the fault activities (Wang et al., 2008; Zinke and Zoback, 2000). Gaoand Liu (2009) analyzed the shear-wave splitting measurements at LSAand confirmed the multi-layer anisotropy (Fig. 10), i.e., upper-layeranisotropy in the lower-middle crust and lower-layer anisotropy in thelithospheric mantle.

5. Conclusions

We have made a detailed study of the shear-wave splitting at 138permanent seismograph stations in China. Our results have providednew insights into the anisotropic structure and mantle dynamics underthe Chinese continent. Three different methods (the RC, SC and MC)were used simultaneously to better estimate the splitting parametersandavoid potential bias.Most of the events used in our analysis occurredin the Tonga subduction zone with back-azimuths of 110°–130°, whichare subparallel to the fast orientations of the anisotropy (i.e., φ for theMC method) in eastern China. Thus the splitting parameters in easternChina determined by a routine (RC or SC) method on the individualseismograms may be seriously biased.

The φ fast directions in eastern China, to first-order, are consistentwith the APMdirections andGPS results, suggesting that the shear-wavesplitting reflects asthenospheric anisotropydue tomineral LPOdrivenbyAPM. In NE China and North China, the subduction of the Pacific plateplays an important role in the LPO development. Under the orogens in

Fig. 10. Distribution of the shear-wave splitting measurements in western China. The labeling is the same as Fig. 7.

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the western part of eastern China, anisotropy in the lithosphere contri-butes significantly to the observations, while the observations in theOrdosBlock reflect fossil anisotropy frozen in the stableArcheanblock. InSE China, the NE–SWφ is caused by anisotropy in the lithosphere due tothe strong NW–SE contraction between the Eurasian and Philippine Seaplates.

The most significant feature in western China is that φ is perpen-dicular to the orientations of the maximum horizontal stress σH whileparallel with the strikes of the orogens and faults. The observed spatialvariations in anisotropy reflect the large-scale pattern of lithosphericdeformation, accompanyinga transition fromsimple shear in the TibetanPlateau to pure shear in the surrounding regions. In south Tibet, crustalanisotropy, either due to deep crustalflowor the fault-fabric, contributesto the observed splitting. In addition, φ also exhibits good correlationwith theAPMdirections andGPS results, andmanystudies have revealedthat APM-driven anisotropy exists widely in the asthenosphere underwestern China. These results suggest that the mountain building hascaused significant deformation in the lithosphere, but has less affectedthe underlying asthenosphere, from theviewpoint of seismic anisotropy.

Acknowledgments

Data used in the study were archived and managed by the ChinaSeismic Network Data Center and IRIS DMC. We thank Prof. S. Chevrotand Dr. V. Monteiller for providing us the code formultichannel splittinganalysis. This work was supported partially by the National NaturalScience Foundation of China (Grant No. 40634021), a grant (Kiban-A17204037) toD. Zhao fromthe JapanSociety for thePromotionof Scienceand the Scientific Research Foundation of Graduate School of Nanjing

University. D. Zhao and Z. Huangwere also supported by the Global-COEprogramof TohokuUniversity. Prof. P. Shearer (editor),M. Savage and ananonymous reviewer provided constructive comments which improvedthe manuscript. Most of figures were made by using GMT (Wessel andSmith, 1998).

Appendix A. Supplementary data

Supplementarydata to this article canbe foundonline atdoi:10.1016/j.epsl.2011.03.038.

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