The development of seismic anisotropy below south-central ...

Geophys J Int (2021) 225 548ndash554 doi 101093gjiggaa603Advance Access publication 2020 December 21GJI Seismology

The development of seismic anisotropy below south-central Alaskaevidence from local earthquake shear wave splitting

E Karlowska 12 I D Bastow1 S Rondenay3 R Martin-Short2 and R M Allen2

1Department of Earth Science and Engineering Imperial College London London SW 7 2BP UK E-mail ibastowimperialacuk2Seismological Laboratory University of California Berkeley CA 94720 USA3Department of Earth Science University of Bergen N-5020 Bergen Norway

Accepted 2020 December 11 Received 2020 December 8 in original form 2020 September 11

S U M M A R YThe Transportable Array in south-central Alaska spans several subduction zone featuresbackarc forearc and volcanic arc making it an ideal tool to study subduction zone anisotropyShear wave splitting analysis of 157 local earthquakes of mb ge 30 that occurred between2014 and 2019 yields 210 high-quality measurements at 23 stations Splitting delay times (δt)are generally small (δt asymp 03 s) increasing with distance from the trench Arc-parallel fastdirections φ are only seen in the forearc but rotate to arc-perpendicular φ in the backarcObserved φ values generally do not parallel teleseismic SKS splitting results implying that thelatter is sensitive primarily to subslab mantle flow not mantle wedge dynamics The forearclocal-earthquake signal likely originates from anisotropic serpentinite in fractures atop thesubducting Pacific Plate with possible additional signal coming from fractures in the NorthAmerican crust Mantle wedge corner flow potentially with additional arc-perpendicularanisotropy in the subducting slab explains backarc anisotropy

Key words North America Seismic anisotropy Subduction zone processes Volcanic arcprocesses

1 I N T RO D U C T I O N

The processes operating along the gt50 000 km length of Earthrsquossubduction zone system are debated In the mantle 3-D along-arcflow 2-D corner flow and complex toroidal flow patterns at slabedges have variously been hypothesized in different settings (egLong amp Silver 2008 Abt et al 2009) Fracture systems in the down-going and overriding plate with or without a thin serpentinite layeratop the downgoing plate (Abers et al 2017) may also characterizesome subduction zones (see Long 2013 for a review) Key to resolv-ing these tectonic and geodynamic subduction zone characteristicsis the measurement of seismic anisotropy the directional depen-dence of seismic wave speed Much subduction zone anisotropy isexpected to result from the development of lattice preferred orien-tation (LPO) fabrics in crust and mantle minerals such as olivineHowever b-type as opposed to the more common a-type c-type ore-type olivine LPO (Karato et al 2008) can develop when mantlewedge conditions are suitably high differential stress and low tem-perature (eg Zhang amp Karato 1995) This changes the relationshipbetween strain crystal alignment and the resulting anisotropy theflow is perpendicular to the anisotropic fast direction not parallelto it (eg Nakajima amp Hasegawa 2004 Kneller et al 2005)

Shear wave splitting utilizes the observation that when a shearwave encounters an anisotropic medium it splits into two orthogonal

shear waves from local earthquakes one travelling faster than theother (eg Silver amp Chan 1991) The splitting is quantified by thetime delay (δt) between the two shear waves and the orientation (φ)of the fast shear wave Key to resolving different sources of seismicanisotropy at subduction zones is analysis of shear wave splitting ina variety of waveforms including both teleseismic (eg SKS) andS wave from local earthquakes the former are path averages of theentire upper mantle below a station the latter afford resolution ofshallower anisotropic fabrics

Local S-wave splitting studies often reveal an arc-parallel to arc-perpendicular transition in φ from the forearc to the backarc (egTonga Smith et al 2001 Middle America Abt et al 2009) Somestudies however report only arc-parallel directions [eg Long ampSilver 2008 (various locations) the Caribbean Pinero-Feliciangeliamp Kendall 2008 the Aleutians Yang et al 1995] or only arc-perpendicular directions (eg Scotia Muller 2001) elsewheremore complex patterns are observed (eg Kamchatka Levin et al2004) Measurements of δt from local S waves also vary betweensubduction zones worldwide δt asymp 03 plusmn 04 s at the Hikurangi sub-duction zone (Morley et al 2006) δt asymp 08 plusmn 05 s at Ryukyu (Longamp van der Hilst 2006) and δt asymp 15 plusmn 04 s at the Aleutians (Long ampSilver 2008) Smaller delay times are sometimes cited as evidencethat a strong mantle wedge flow field is lacking and vice versa (egLong 2013)

548Ccopy The Author(s) 2020 Published by Oxford University Press on behalf of The Royal Astronomical Society All rights reserved Forpermissions please e-mail journalspermissionsoupcom

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ealth Library user on 04 May 2021

Seismic anisotropy below south-central Alaska 549

South-central Alaska where 50 mm yrminus1 (Sauber et al 1998)northward-verging subduction of the Pacific Plate is ongoing be-neath North America is an ideal study locale for subduction zonedynamics because recent deployment of the Transportable Array(TA) network spans both forearc and backarc settings (Fig 1) Pre-vious SKS splitting studies in this part of Alaska have suggestedtoroidal mantle flow as the dominant cause of the observations(Christensen amp Abers 2010 Hanna amp Long 2012 Venereau et al2019) Most recently the SKS splitting study of McPherson et al(2020) corroborates this view except below the Kenai Peninsulawhere they suggest that there is likely little-to-no mantle above theplate interface However source-side splitting analysis (Walpoleet al 2017) suggests that the SKS signal originates from belownot above the subducting slab To address this debate and to betterconstrain sources of anisotropy beneath the region we perform ashear wave splitting study of local earthquakes in Alaska

2 L O C A L E A RT H Q UA K E DATA S E T A N DS H E A R WAV E S P L I T T I N GM E T H O D O L O G Y

Seismograms of magnitude mb ge 30 and depth 10ndash212 km earth-quakes in the region 55ndash665N 142ndash166W occurring betweenJanuary 2014 and April 2019 were obtained from the IRIS DataManagement Center for 23 TA broad-band stations From thisinitial data set a total of 814 earthquakendashstation pairs were ex-amined for which the S-wave incident angle is within the shearwave window (SWW) The SWW is the vertical cone bound byic = sinminus1 (VsVp) where S-wave particle motions are not dis-turbed by P head wave and SndashP conversions at the free surface(Booth amp Crampin 1985) A zero-phase Butterworth bandpass filterwith corner frequencies of 01ndash10 Hz was applied to all seismo-grams This frequency range is similar to that adopted in analogouslocal earthquake shear wave splitting studies (eg Long amp van derHilst 2006)

Splitting analysis was carried out using the method of Teanbyet al (2004) which is based on the traditional Silver amp Chan (1991)method with errors estimated using the method of Walsh et al(2013) Horizontal components are rotated and time-shifted to min-imize the second eigenvalue of the covariance matrix for particlemotion within a time window around the shear wave arrival Thisprocess is similar to linearizing the particle motion and minimizingtangential component shear wave energy The traditional Silver ampChan (1991) approach takes a single manually picked shear waveanalysis window In the cluster analysis approach of Teanby et al(2004) the splitting analysis is performed for a range of windowlengths and cluster analysis is utilized to find measurements that arestable over many different windows All splitting parameters weredetermined after analysis of 100 different windows each windowencapsulates at least a full cycle of S-wave energy with the rangeof window start and end times spanning approximately half to afull wavelengthmdashsufficient to resolve splitting that results in ellip-tical or cruciform particle motion (the latter can result when δt iscomparable to the wavelength of the S-wave energy eg Booth ampCrampin 1985) The result chosen by the cluster algorithm is theone from the most stable cluster with the lowest error (calculated viaan F-test to obtain the 95 per cent confidence interval) An exam-ple high-quality splitting result is shown in Supporting InformationFig S1

Some studies (eg Saltzer et al 2000 Wirth amp Long 2010) havedemonstrated a bias towards near-surface layers in high-frequency

splitting results The filter bands used in our local splitting analy-sis overlap with the SKS studies [eg Venereau et al (2019) used004ndash03 Hz compared to our 01ndash1 Hz) so we expect frequency-dependent effects to be minimal in our study Nevertheless weattempted splitting analysis of both local and SKS waveforms forstations M22K O19K and O20K using filter corner frequenciesof 01ndash05 Hz This reduced the high-frequency content of the localearthquake data set and pushed the SKS analysis to higher frequen-cies For almost all local earthquakes evidence for coherent shearwave energy from which acceptable quality splitting measurementscould be made was lacking Three exceptions to this rule yielded lo-cal earthquake splitting results that showed no clear change in φ buta slight increase in δt and associated errors (eg from 038 plusmn 003 to055 plusmn 005 s at O20K) notably still much lower delay times than inthe published SKS data sets from the region For the SKS data setsplitting analysis in the frequency range 01ndash05 Hz was attemptedfor the same stationndashearthquake pairs as Venereau et al (2019) forstations M22K O19K and O20K (Supporting Information TableS2) For four of the original seven earthquakes where an acceptablequality measurement could be obtained we found near-identicalsplitting parameters as for the 004ndash03 Hz range

3 R E S U LT S

We obtained 210 splitting measurements at 23 TA stations from 157earthquakes of depth 18ndash204 km Arc-parallel φ results are gener-ally only observed in the forearc arc-perpendicular φ directionsdominate the backarc (Fig 1) Observations of δt range from 010to 096 s 96 per cent are le05 s with an average 032 plusmn 003 s (Sup-porting Information Table S1) Our results have errors σφ le1375

and σ δt le017 s As expected δt generally increases with path length[calculated by ray tracing through the ak135 velocity model of Ken-nett et al (1995) Fig 2] so we calculate per cent anisotropy bydividing δt with ak135 predicted S-wave traveltimes Anisotropy isalmost exclusively lt3 per cent across the network with an averageof 104 per cent Stations O19K and O20K account for sim25 of allmeasurements Increased seismicity there is attributable to the cur-vature of a subducting slab (eg Ratchkovski amp Hansen 2002) Mostof the earthquakes within the SWW were located in south-centralAlaska A few events in the SWW north of 53N were unfortu-nately of too low signal-to-noise ratio to conduct a reliable shearwave splitting analysis Maps showing per cent anisotropy ratherthan δt (Fig 1) are shown in Supporting Information Fig S2

Fabric type can have a big impact on splitting parameters fornon-vertical S-wave phases (eg Savage 1999) Approximately 13of our S waves have incidence angles (θ ) lt20 95 per cent arelt35 Examining splitting delay times as a function of θ we findthat δtθlt20 = 0306 s ranging from 012 to 053 s δtθgt20 = 0322 sranging from 01 to 096 s Performing a KolmogorovndashSmirnov teston the two ic families of data reveals that they are near-identicalwith a high P-value of 58 per cent indicating that we cannot precludethe hypothesis that the two data sets sample the same distribution

4 D I S C U S S I O N

41 Causes of seismic anisotropy and comparison toprevious studies

A striking observation in Fig 1 is the lack of correlation betweenour local earthquake-derived φ directions and SKS splitting resultsCorroborating earlier studies (Christensen amp Abers 2010 Hanna

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550 E Karlowska et al

(a) (b)

(c) (d)

Figure 1 (a) Tectonic setting of south-central Alaska with major fault lines (Colpron et al 2007) TA seismograph stations and earthquakes (circles) thatproduced high-quality splitting measurements Solid arrows absolute plate motion (APM) in the hot spot (HS) and no-net-rotation (NNR) reference frames(Gripp amp Gordon 2002) Slab20 contours are after Hayes et al (2018) (b) Local S-wave splitting measurements in south-central Alaska plotted at the station(c) As per (b) but plotted at the ray-path midpoint (d) Close-up of stations O19K and O20K plotted at the ray-path midpoint

amp Long 2012) Venereau et al (2019) suggested that mantle flowabove the subducting Pacific Plate was a primary cause of SKSanisotropy in the region the later SKS study of McPherson et al(2020) generally supports this view but adopted a subslab mantleflow hypothesis to explain observations in the Kenai Peninsula area

where they point out that there is likely little-to-no mantle above theplate interface The anticorrelation of our φ observations with theSKS studies may indicate that the SKS data set is sensitive primarilyto intraslab or subslab anisotropy Corroborating this hypothesis therecent source-side splitting global study of Walpole et al (2017)

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Figure 2 Average splitting delay times (δt) for 25 km bins of path lengthL Path length is calculated assuming ray paths through the ak135 mode ofKennett et al (1995) Error bars show the standard deviation of the delaytimes in each bin with the contribution of each weighted by its signal-to-noise ratio Numbers below each data point are the number of measurementscontributing to each bin The dashed line is a linear least-squares fit showingan increasing trend in average delay time with path length

which is inherently biased towards subslab anisotropy in subduc-tion zones presents φ and δt observations for Alaska akin to theSKS studies at least in the area where our local and SKS splittingcomparisons are being made On the other hand a similar studyof source-side splitting by Lynner amp Long (2014) found somewhatscattered φ observations below Alaska albeit with a weak trendin φ that aligns approximately with the subducting plate motionThey attributed much of the scatter to the significant distances raypaths travel through the slab below Alaska in contrast to other re-gions of source-side study such as Central America where signalcontamination from the slab is not so apparent

Comparing values of δt for local earthquakes and SKS arrivalshas to be done cautiously because there can be a demonstrablebias towards near-surface structure in local earthquake analysesSpecifically δt has in some areas been shown to decrease athigher frequencies (eg Marson-Pidgeon amp Savage 1997 Wirthamp Long 2010) with near-surface anisotropic regimes contribut-ing more at higher frequencies (eg Saltzer et al 2000) With thiscaveat in mind it is nevertheless interesting to note that we con-strain markedly smaller δt values than the SKS studies [δt asymp 032 shere eg Venereau et al (2019) found δtSKS asymp 119 s) Our localearthquake δt observations can therefore be used cautiously to cor-roborate the hypothesis that the SKS and local earthquake data setsare dominated by different anisotropic layers

High aspect ratio melt inclusions generally result in higher percent anisotropy than we observe so we do not favour that hypoth-esis for Alaska (eg Keir et al 2005 Bastow et al 2010) Our δtobservations also contrast with larger ones at some other subductionzones where hypotheses of mantle wedge flow have been favouredFor example Smith et al (2001) interpreted δt asymp 13 plusmn 03 s inTonga as along-arc mantle flow Long amp van der Hilst (2006) citedδt asymp 08 plusmn 05 s as evidence for a 2-D wedge corner flow belowRyukyu Large δt times are not globally ubiquitous however δtobservations akin to ours have been noted in the Caribbean (Pinero-Feliciangeli amp Kendall 2008) and South America (Polet et al 2000)with these studies generally arguing against mantle wedge flowYang et al (1995) suggest that the crust contributes δt asymp 01 s tothe 01ndash035 s total δt observed beneath the Aleutians HoweverAlaskan forearc and backarc structural trends do not mirror theabrupt change in φ (Fig 1) so a continental crustal origin for the

anisotropy is not an obvious candidate to explain the results Fewstations in our study are perfectly suited to an isolated study ofanisotropy in the 50 km thick (eg Martin-Short et al 2018) upperplate However M22K within the Yakutat terrane where there islittle-to-no mantle wedge is well placed for such analysis δt rangesfrom 011s for the shallowest earthquakes to 032 s for those ex-ceeding 100 km depth φ parallels surface geological trends Witha mean of δt = 021 s at M22K it is clear that the North Ameri-can upper plate contributes some signal to our observations butalso that it generally does not dominate them particularly where φ

shows no parallelism with geological trends Intriguingly δt showsa gentle increase of sim033 s with path length over sim300 km (Fig 2)In central America Abt et al (2009) constrained similar δt patternsto those observed here (sim03 s over sim200 km) and attributed it tomantle wedge anisotropy

42 Forearc anisotropy

We see some evidence for arc-perpendicular and subperpendicularanisotropy in the Alaskan forearc (Fig 1) perhaps the result of fab-rics in the subducting slab itself Tian amp Zhao (2012) for examplefound evidence for arc-perpendicular anisotropy in the subductingPacific Plate which they interpreted to originate from mid-oceanridge formation However there is also evidence for arc-parallelanisotropy in the Alaskan forearc (Figs 1b and c) with station O20Kparticularly well located for analysis of this signature (Fig 1d) Insome cases φ parallels geological trends so we cannot precludethe possibility that some splitting (sim 02 s) is accrued in the NorthAmerican crust (Fig 2) Indeed in their anisotropic P-wave tomog-raphy study Gou et al (2019) found some evidence for arc-parallelφ in our study area However the aforementioned abrupt transitionto arc-perpendicular anisotropy northwest of O20K despite the lackof change in structural trends argues for a deeper contribution tothe observations

Abers et al (2017) suggests that the O20K region lies above acold mantle wedge lsquonosersquo that is decoupled from the core of mantlewedge (Fig 3) The arc-parallel anisotropy we observe is thereforeunlikely the result of flow in the mantle wedge When reviewed inlight of the P- and S-wave study of Tian amp Zhao (2012) which foundno evidence for anisotropy in Alaskarsquos cold mantle wedge nose weconclude that our O20K results are unlikely to be influenced bymantle flow which likely only dominates further away from the arc(Gou et al 2019)

Arc-parallel anisotropy can result from arc-parallel faults in aserpentinite layer on the top of subducting slabs (Faccenda et al2008) for short (10 km long) faults such a layer can produce δtasymp 026ndash035 s similar to the mean δt asymp 032 plusmn 003 s we observeA serpentinite layer is expected to form if the forearc mantle issufficiently hydrated by water released by the downgoing slab Cor-roborating the Abers et al (2017) view that the Alaskan mantlewedge is only moderately hydrated using receiver function con-straints on VpVs ratios Rossi et al (2006) estimated the extent ofserpentinization in the Alaskan mantle wedge to be lt30 per centfor slab depths lt80 km Further constraints on mantle wedge ser-pentinization come from recent tomographic imaging studies in theregion (eg Martin-Short et al 2016 2018 Berg et al 2020) thatimage low wave speeds atop the subducting Pacific Plate slab Berget al (2020) however suggest low serpentinization and moderatemantle wedge hydration below Alaska The low wave speeds maytherefore instead be the result of other subduction zone processessuch as inclusion of crustal velocity material in the wedge The

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(b)(a)

Figure 3 (a) Earthquakes producing splits at stations O19K (triangles) and O20K (circles) projected onto line AndashB in Fig 1(d) Subducting upper slab extentis after Hayes et al (2018) Moho depths are after Martin-Short et al (2018) The cold nose extent is after Abers et al (2017) The projected ray paths arecalculated according to the ak135 velocity model of Kennett et al (1995) (b) Splitting delay times projected onto line AndashB showing a sim 03 s increase in δtaway from the trench

serpentine in faults hypothesis for the O20K splitting observationsis thus not unambiguous Additionally we cannot completely ruleout the possibility of b-type olivine fabrics here (eg Kneller et al2005) However the P-wave study of Gou et al (2019) presentedample evidence for anisotropy in the crust and subducting slab be-low this region but little-to-none in the mantle wedge Thereforewe propose that anisotropy in the forearc region is likely producedby a combination of anisotropy in the downgoing plate a thin layerof serpentinite on top of the slab andor the North American crustwith minimal influence from the mantle wedge itself

43 Backarc anisotropy and implications for subslabmantle flow

Station O19K is well placed to examine backarc anisotropy belowAlaska (Figs 1 and 3) In general φ is perpendicular to geologicaltrends so we rule out continental crustal anisotropy as the domi-nant explanation for our results Alaskarsquos cold mantle wedge noseends lt100 km from where the North American crust meets thenose tip (Abers et al 2017 Fig 3a) Backarc results at O19K andelsewhere are therefore too distant from the trench to be samplingb-type olivine fabrics which are only thought to develop in the coldlsquonosersquo of the mantle wedge (eg Song amp Kawakatsu 2013) Assum-ing a-type (or c-type or e-type) olivine LPO our arc-perpendicularobservations may therefore be illuminating a 2-D corner flow aspreviously proposed by Long amp Silver (2008) Supporting this hy-pothesis δt increases away from the trench (Fig 3b) It is notablethat our δt observations are smaller than those in the backarc re-gions of some other subduction zones For example Long amp vander Hilst (2006) found δt asymp 08 s in their study of Ryukyu Withthe caveat that frequency-dependent effects may be exerting strongcontrol on δt in our study our smaller delay times are perhaps theresult of a thinner corner flow anisotropic layer Alaska has a thickerupper plate [sim50 km (Martin-Short et al 2018) compared to 35ndash40 km in Ryukyu (Taira 2001)] and generally shallower earthquakes[sim114 km compared to sim145 km in Long amp van der Hilst (2006)]in the less steep Alaskan slab However we acknowledge the obliq-uity of subduction in the Cook Inlet area so the 2-D corner flow

assumption may be overly simplistic here Indeed Kneller amp VanKeken (2007 2008) model complex flow patterns above obliquecurved and varying-dip slabs and highlight the potential for complexresulting anisotropic patterns along-strike variations in slab geom-etry lead to trench-parallel pressure gradients and are thus a possiblemechanism for 3-D flow (eg Kneller amp Van Keken 2008) Severalof our splitting observations show evidence for arc obliquity (Figs 1and 3) implying some departure from simple 2-D flow models islikely Future studies of Alaskan anisotropy could usefully tacklethe challenge of carrying out 3-D modelling to further pinpoint thelikely variations in wedge and subslab flow patterns Specificallythe influence of Alaskarsquos variable slab dip slab curvature obliquesubduction and slab edge would all need to be considered carefully

As indicated earlier our local earthquake splitting observationssuggest a substantial subslab contribution to the SKS data sets ofVenereau et al (2019) and McPherson et al (2020) McPhersonet al (2020) already support this hypothesis for the Kenai Peninsulawhere there is likely little-to-no mantle above the plate interfaceOur observations are also not easily explained by the mantle flowhypothesis of Jadamec amp Billen (2010) who consistent with SKSmeasurements more broadly in Alaska predict strong mantle wedgeflow due to the proximity of the slab edge in Alaska and its influencein generating a toroidal mantle flow field

Studies of subslab anisotropy do exist For example Faccendaamp Capitanio (2013) modelled the development of LPO in olivinendashenstatite aggregates in a 3-D slab rollback flow field They proposedtwo zones of subslab anisotropy At shallow depths simple shearbeneath the slab generates a ge100 km thick layer of arc-normalφ At greater depths the fast axes remain arc normal but plungeparallel to the subducting slab This layer systematically overlies adeeper layer or lsquocorersquo of arc-parallel φ generated by pure shear inthe slab retreat direction This anisotropic core is strongest near slabedges as per our study area where the divergence of the horizontalsubslab flow is greatest Such a geodynamic scenario which hasbeen suggested as appropriate for the Aegean for example (Oliveet al 2014) seems inappropriate for Alaska given the largely arc-parallel fast directions in the SKS data set SW of the slab edge(Fig 1b) Instead subduction of the oceanic asthenosphere may bethe dominant source of subslab anisotropy in our study area as has

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been suggested by Song amp Kawakatsu (2013) for central AlaskaEither way further modelling of the SKS data set would be neededto confirm or refute any subslab hypothesis which our data set isinherently incapable of resolving

A combination of shallower 2-D corner flow and deeper 3-Dtoroidal flow has been suggested in Cocos subduction zone (Sotoet al 2009) To this end the observed discrepancy between the SKSand local S-wave data sets in south-central Alaska could simply bedue to the data sets being sensitive to different parts of mantle wedgeOur local S-wave splitting results may be sensitive to a relativelysmall-scale shallow 2-D corner flow while the SKS data sets aresensitive to deeper 3-D flow (Jadamec amp Billen 2010 Venereau et al2019) additional arc-perpendicular anisotropy in the subductingslab may contribute to both However the Cook inlet segment isquite some distance from the slab edge and geodynamic modellingsuggests that toroidal flow may not dominate this far south (egJadamec amp Billen 2010)

5 C O N C LU S I O N S

We report weak shear wave splitting from earthquakes in south-central Alaska with δt = 010ndash096 s with δt generally increasingaway from the trench In φ we observe an arc-parallel to arc-perpendicular forearc to backarc transition With some local excep-tions (eg M22K and O20K) continental crustal structural trendscannot generally explain the observations suggesting a deeperanisotropic source Forearc anisotropy region is likely producedby a combination of the downgoing plate a thin layer of serpenti-nite on top of the slab andor North American crust with relativelylittle influence from the mantle wedge Backarc results indicatea 2-D corner flow in the presence of a-type olivine LPO (poten-tially with additional arc-perpendicular anisotropy in the subductingslab) akin to the Long amp Silver (2008) hypothesis for mantle wedgeflow in Alaska If correct our interpretations imply that SKS split-ting results for the Kenai Peninsula region of south-central Alaskaare explained best by subslab mantle flow not flow in the mantlewedge

A C K N OW L E D G E M E N T S

Two anonymous reviewers provided helpful feedback that promptedus to think more deeply about our results Seismograms come fromthe Incorporated Research Institutions for Seismology Data Man-agement Center which is funded through the Seismological Facil-ities for the Advancement of Geoscience and EarthScope (SAGE)Proposal of the National Science Foundation under CooperativeAgreement EAR-126168 TA network data were made freely avail-able as part of the EarthScope USArray facility operated by IRISand supported by the National Science Foundation under Coop-erative Agreements EAR-1261681 (Busby amp Aderhold 2020) IBacknowledges support from Natural Environment Research CouncilGrant NES0141361

R E F E R E N C E SAbers G Van Keken P amp Hacker B 2017 The cold and relatively dry

nature of mantle forearcs in subduction zones Nat Geosci 10(5) 333ndash337

Abt DL Fischer KM Abers GA Strauch W Protti JM amp GonzalezV 2009 Shear wave anisotropy beneath Nicaragua and Costa Rica im-plications for flow in the mantle wedge Geochem Geophys Geosyst10(5) doi1010292009GC002375

Bastow I Pilidou S Kendall J-M amp Stuart G 2010 Melt-induced seismic anisotropy and magma assisted rifting in Ethiopiaevidence from surface waves Geochem Geophys Geosyst 11doi1010292010GC003036

Berg EM Lin F-C Allam A Schulte-Pelkum V Ward KM ampShen W 2020 Shear velocity model of Alaska via joint inver-sion of Rayleigh wave ellipticity phase velocities and receiver func-tions across the Alaska Transportable Array J geophys Res 125(2)doi1010292019JB018582

Booth D amp Crampin S 1985 Shear-wave polarizations on a curved wave-front at an isotropic free surface Geophys J Int 83(1) 31ndash45

Busby RW amp Aderhold K 2020 The Alaska Transportable Array asbuilt Seismol Res Lett 91 3017ndash3027

Christensen D amp Abers G 2010 Seismic anisotropy under centralAlaska from SKS splitting observations J geophys Res 115(B4)doi1010292009JB006712

Colpron M Nelson J amp Murphy D 2007 Northern Cordilleran terranesand their interactions through time GSA Today 17(45) 4ndash10

Faccenda M amp Capitanio F 2013 Seismic anisotropy around subductionzones insights from three-dimensional modeling of upper mantle de-formation and SKS splitting calculations Geochem Geophys Geosyst14(1) 243ndash262

Faccenda M Burlini L Gerya TV amp Mainprice D 2008 Fault-inducedseismic anisotropy by hydration in subducting oceanic plates Nature455(7216) 1097ndash1100

Gou T Zhao D Huang Z amp Wang L 2019 Aseismic deep slab andmantle flow beneath Alaska insight from anisotropic tomography Jgeophys Res 124(2) 1700ndash1724

Gripp AE amp Gordon RG 2002 Young tracks of hotspots and currentplate velocities Geophys J Int 150(2) 321ndash361

Hanna J amp Long MD 2012 SKS splitting beneath Alaska regionalvariability and implications for subduction processes at a slab edgeTectonophysics 530 272ndash285 doi101016jtecto201201003

Hayes GP Moore GL Portner DE Hearne M Flamme H FurtneyM amp Smoczyk GM 2018 Slab2 a comprehensive subduction zonegeometry model Science 362(6410) 58ndash61

Jadamec MA amp Billen MI 2010 Reconciling surface plate motionswith rapid three-dimensional mantle flow around a slab edge Nature465(7296) 338ndash341

Karato S-I Jung H Katayama I amp Skemer P 2008 Geodynamic sig-nificance of seismic anisotropy of the upper mantle new insights fromlaboratory studies Annu Rev Earth Planet Sci 36(1) 59ndash95

Keir D Kendall J-M Ebinger C amp Stuart G 2005 Variations in latesyn-rift melt alignment inferred from shear-wave splitting in crustalearthquakes beneath the Ethiopian rift Geophys Res Lett 32(23)doi1010292005GL024150

Kennett BLN Engdahl ER amp Buland R 1995 Constraints on seismicvelocities in the Earth from traveltimes Geophys J Int 122(1) 108ndash124

Kneller E amp Van Keken P 2007 Trench-parallel flow and seismicanisotropy in the Mariana and Andean subduction systems Nature450(7173) 1222ndash1225

Kneller E amp Van Keken P 2008 Effect of three-dimensionalslab geometry on deformation in the mantle wedge implica-tions for shear wave anisotropy Geochem Geophys Geosyst 9(1)doi1010292007GC001677

Kneller E Van Keken PE Karato S-I amp Park J 2005 B-type olivinefabric in the mantle wedge insights from high-resolution non-Newtoniansubduction zone models Earth planet Sci Lett 237(3ndash4) 781ndash797

Levin V Droznin D Park J amp Gordeev E 2004 Detailed mapping ofseismic anisotropy with local shear waves in southeastern KamchatkaGeophys J Int 158(3) 1009ndash1023

Long MD 2013 Constraints on subduction geodynamics from seismicanisotropy Rev Geophys 51(1) 76ndash112

Long MD amp Silver PG 2008 The subduction zone flow field fromseismic anisotropy a global view Science 319(5861) 315ndash318

Long MD amp van der Hilst RD 2006 Shear wave splitting from localevents beneath the Ryukyu arc trench-parallel anisotropy in the mantlewedge Phys Earth planet Inter 155(3) 300ndash312

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Lynner C amp Long M 2014 Sub-slab anisotropy beneath the Sumatra andcircum-Pacific subduction zones from source-side shear wave splittingobservations Geochem Geophys Geosyst 15(6) 2262ndash2281

Marson-Pidgeon K amp Savage M 1997 Frequency-dependent anisotropyin Wellington New Zealand Geophys Res Lett 24(24) 3297ndash3300

Martin-Short R Allen RM amp Bastow ID 2016 Subduction geometrybeneath south central Alaska and its relationship to volcanism GeophysRes Lett 43(18) 9509ndash9517

Martin-Short R Allen R Bastow ID Porritt RW amp Miller MS 2018Seismic imaging of the Alaska subduction zone implications for slabgeometry and volcanism Geochem Geophys Geosyst 19(11) 4541ndash4560

McPherson AM Christensen DH Abers GA amp Tape C 2020 Shearwave splitting and mantle flow beneath Alaska J geophys Res 125(4)doi1010292019JB018329

Morley AM Stuart GW Kendall J-M amp Reyners M 2006 Mantlewedge anisotropy in the Hikurangi subduction zone central North IslandNew Zealand Geophys Res Lett 33(5) doi1010292005gl024569

Muller C 2001 Upper mantle seismic anisotropy beneath Antarctica andthe Scotia Sea region Geophys J Int 147(1) 105ndash122

Nakajima J amp Hasegawa A 2004 Shear-wave polarization anisotropyand subduction-induced flow in the mantle wedge of northeastern JapanEarth planet Sci Lett 225(3ndash4) 365ndash377

Olive J-A Pearce F Rondenay S amp Behn MD 2014 Pronouncedzonation of seismic anisotropy in the Western Hellenic subduction zoneand its geodynamic significance Earth planet Sci Lett 391 100ndash109

Pinero-Feliciangeli L amp Kendall J-M 2008 Sub-slab mantle flow par-allel to the Caribbean plate boundaries inferences from SKS splittingTectonophysics 462(1ndash4) 22ndash34

Polet J Silver P Beck S Wallace T Zandt G Ruppert S Kind Ramp Rudloff A 2000 Shear wave anisotropy beneath the Andes fromthe BANJO SEDA and PISCO experiments J geophys Res 105(B3)6287ndash6304

Ratchkovski NA amp Hansen RA 2002 New evidence for segmentationof the Alaska subduction zone Bull seism Soc Am 92(5) 1754ndash1765

Rossi G Abers G Rondenay S amp Christensen D 2006 Unusual mantlePoissonrsquos ratio subduction and crustal structure in central Alaska Jgeophys Res 111(B9) doi1010292005JB003956

Saltzer RL Gaherty JB amp Jordan TH 2000 How are vertical shearwave splitting measurements affected by variations in the orientation ofazimuthal anisotropy with depth Geophys J Int 141(2) 374ndash390

Sauber J Mcclusky S amp King R 1998 Correction to lsquoRelation of ongoingdeformation rates to the subduction zone process in southern AlaskarsquoGeophys Res Lett 25(2) 215ndash215

Savage M 1999 Seismic anisotropy and mantle deformation what havewe learned from shear wave splitting Rev Geophys 37(1) 65ndash106

Silver PG amp Chan WW 1991 Shear wave splitting and subcontinentalmantle deformation J geophys Res 96(B10) 16 429ndash16 454

Smith GP Wiens DA Fischer KM Dorman LM Webb SC ampHildebrand JA 2001 A complex pattern of mantle flow in the Laubackarc Science 292(5517) 713ndash716

Song T amp Kawakatsu H 2013 Subduction of oceanic asthenospherea critical appraisal in central Alaska Earth planet Sci Lett 36782ndash94

Soto GL Ni JF Grand SP Sandvol E Valenzuela RW SpezialeMG Gonzalez JMG amp Reyes TD 2009 Mantle flow in the RiverandashCocos subduction zone Geophys J Int 179(2) 1004ndash1012

Taira A 2001 Tectonic evolution of the Japanese island arc system AnnuRev Earth Planet Sci 29(1) 109ndash134

Teanby N Kendall J-M amp Van der Baan M 2004 Automation of shear-wave splitting measurements using cluster analysis Bull seism Soc Am94(2) 453ndash463

Tian Y amp Zhao D 2012 Seismic anisotropy and heterogeneity in theAlaska subduction zone Geophys J Int 190(1) 629ndash649

Venereau CMA Martin-Short R Bastow ID Allen RM ampKounoudis R 2019 The role of variable slab dip in driving mantleflow at the eastern edge of the Alaskan subduction margin insights fromshear-wave splitting Geochem Geophys Geosyst 20(5) 2433ndash2448doi1010292018GC008170

Walpole J Wookey J Kendall J-M amp Masters T-G 2017 Seismicanisotropy and mantle flow below subducting slabs Earth planet SciLett 465 155ndash167

Walsh E Arnold R amp Savage M 2013 Silver and Chan revisited Jgeophys Res 118(10) 5500ndash5515

Wirth E amp Long M 2010 Frequency-dependent shear wave splittingbeneath the Japan and IzundashBonin subduction zones Phys Earth planetInter 181(3ndash4) 141ndash154

Yang X Fischer KM amp Abers GA 1995 Seismic anisotropy beneaththe Shumagin Islands segment of the AleutianndashAlaska subduction zoneJ geophys Res 100(B9) 18 165ndash18 177

Zhang S amp Karato S 1995 Lattice preferred orientation of olivine aggre-gates deformed in simple shear Nature 375(6534) 774ndash777

S U P P O RT I N G I N F O R M AT I O N

Supplementary data are available at GJI online

Figure S1 High-quality splitting measurement example for stationO19K (a) Recorded seismograms filtered 01ndash1 Hz (b) Two hor-izontal components rotated into radial and tangential directionsmdashtop two are originals and bottom two are corrected with splittingalgorithm In this example the long axis of the anisotropy ellipsehappens to align with the backazimuth (c) Top closer look atwaveforms in the window bottom particle motion before and aftercorrection (LndashR) (d) Contour map with lines corresponding to onestandard deviation Thick line shows the 95 per cent confidencelevel for stability of a given measurement (e and f) Variations insplitting measurements over different windows and during clusteranalysis respectivelyFigure S2 (a) Tectonic setting of south-central Alaska with majorfault lines (Colpron et al 2007) TA broad-band seismograph sta-tions and earthquakes (circles) that produced high-quality splittingmeasurements Solid arrows absolute plate motion (APM) in thehot spot (HS) and no-net-rotation (NNR) reference frames (Gripp ampGordon 2002) Slab20 contours are after Hayes et al (2018) majorfault lines are after Colpron et al (2007) (b) Local S-wave splittingmeasurements in south-central Alaska plotted at the station (c) Asper (b) but plotted at the ray-path midpoint (d) Close-up of stationsO19K and O20K plotted at the ray-path midpointFigure S3 (a) Incidence angle (θ ) for all splitting measurements(b) Comparison of δt for θ lt 20 and θ ge 20

Please note Oxford University Press is not responsible for the con-tent or functionality of any supporting materials supplied by theauthors Any queries (other than missing material) should be di-rected to the corresponding author for the paper

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South-central Alaska where 50 mm yrminus1 (Sauber et al 1998)northward-verging subduction of the Pacific Plate is ongoing be-neath North America is an ideal study locale for subduction zonedynamics because recent deployment of the Transportable Array(TA) network spans both forearc and backarc settings (Fig 1) Pre-vious SKS splitting studies in this part of Alaska have suggestedtoroidal mantle flow as the dominant cause of the observations(Christensen amp Abers 2010 Hanna amp Long 2012 Venereau et al2019) Most recently the SKS splitting study of McPherson et al(2020) corroborates this view except below the Kenai Peninsulawhere they suggest that there is likely little-to-no mantle above theplate interface However source-side splitting analysis (Walpoleet al 2017) suggests that the SKS signal originates from belownot above the subducting slab To address this debate and to betterconstrain sources of anisotropy beneath the region we perform ashear wave splitting study of local earthquakes in Alaska

2 L O C A L E A RT H Q UA K E DATA S E T A N DS H E A R WAV E S P L I T T I N GM E T H O D O L O G Y

Seismograms of magnitude mb ge 30 and depth 10ndash212 km earth-quakes in the region 55ndash665N 142ndash166W occurring betweenJanuary 2014 and April 2019 were obtained from the IRIS DataManagement Center for 23 TA broad-band stations From thisinitial data set a total of 814 earthquakendashstation pairs were ex-amined for which the S-wave incident angle is within the shearwave window (SWW) The SWW is the vertical cone bound byic = sinminus1 (VsVp) where S-wave particle motions are not dis-turbed by P head wave and SndashP conversions at the free surface(Booth amp Crampin 1985) A zero-phase Butterworth bandpass filterwith corner frequencies of 01ndash10 Hz was applied to all seismo-grams This frequency range is similar to that adopted in analogouslocal earthquake shear wave splitting studies (eg Long amp van derHilst 2006)

Splitting analysis was carried out using the method of Teanbyet al (2004) which is based on the traditional Silver amp Chan (1991)method with errors estimated using the method of Walsh et al(2013) Horizontal components are rotated and time-shifted to min-imize the second eigenvalue of the covariance matrix for particlemotion within a time window around the shear wave arrival Thisprocess is similar to linearizing the particle motion and minimizingtangential component shear wave energy The traditional Silver ampChan (1991) approach takes a single manually picked shear waveanalysis window In the cluster analysis approach of Teanby et al(2004) the splitting analysis is performed for a range of windowlengths and cluster analysis is utilized to find measurements that arestable over many different windows All splitting parameters weredetermined after analysis of 100 different windows each windowencapsulates at least a full cycle of S-wave energy with the rangeof window start and end times spanning approximately half to afull wavelengthmdashsufficient to resolve splitting that results in ellip-tical or cruciform particle motion (the latter can result when δt iscomparable to the wavelength of the S-wave energy eg Booth ampCrampin 1985) The result chosen by the cluster algorithm is theone from the most stable cluster with the lowest error (calculated viaan F-test to obtain the 95 per cent confidence interval) An exam-ple high-quality splitting result is shown in Supporting InformationFig S1

Some studies (eg Saltzer et al 2000 Wirth amp Long 2010) havedemonstrated a bias towards near-surface layers in high-frequency

splitting results The filter bands used in our local splitting analy-sis overlap with the SKS studies [eg Venereau et al (2019) used004ndash03 Hz compared to our 01ndash1 Hz) so we expect frequency-dependent effects to be minimal in our study Nevertheless weattempted splitting analysis of both local and SKS waveforms forstations M22K O19K and O20K using filter corner frequenciesof 01ndash05 Hz This reduced the high-frequency content of the localearthquake data set and pushed the SKS analysis to higher frequen-cies For almost all local earthquakes evidence for coherent shearwave energy from which acceptable quality splitting measurementscould be made was lacking Three exceptions to this rule yielded lo-cal earthquake splitting results that showed no clear change in φ buta slight increase in δt and associated errors (eg from 038 plusmn 003 to055 plusmn 005 s at O20K) notably still much lower delay times than inthe published SKS data sets from the region For the SKS data setsplitting analysis in the frequency range 01ndash05 Hz was attemptedfor the same stationndashearthquake pairs as Venereau et al (2019) forstations M22K O19K and O20K (Supporting Information TableS2) For four of the original seven earthquakes where an acceptablequality measurement could be obtained we found near-identicalsplitting parameters as for the 004ndash03 Hz range

3 R E S U LT S

We obtained 210 splitting measurements at 23 TA stations from 157earthquakes of depth 18ndash204 km Arc-parallel φ results are gener-ally only observed in the forearc arc-perpendicular φ directionsdominate the backarc (Fig 1) Observations of δt range from 010to 096 s 96 per cent are le05 s with an average 032 plusmn 003 s (Sup-porting Information Table S1) Our results have errors σφ le1375

and σ δt le017 s As expected δt generally increases with path length[calculated by ray tracing through the ak135 velocity model of Ken-nett et al (1995) Fig 2] so we calculate per cent anisotropy bydividing δt with ak135 predicted S-wave traveltimes Anisotropy isalmost exclusively lt3 per cent across the network with an averageof 104 per cent Stations O19K and O20K account for sim25 of allmeasurements Increased seismicity there is attributable to the cur-vature of a subducting slab (eg Ratchkovski amp Hansen 2002) Mostof the earthquakes within the SWW were located in south-centralAlaska A few events in the SWW north of 53N were unfortu-nately of too low signal-to-noise ratio to conduct a reliable shearwave splitting analysis Maps showing per cent anisotropy ratherthan δt (Fig 1) are shown in Supporting Information Fig S2

Fabric type can have a big impact on splitting parameters fornon-vertical S-wave phases (eg Savage 1999) Approximately 13of our S waves have incidence angles (θ ) lt20 95 per cent arelt35 Examining splitting delay times as a function of θ we findthat δtθlt20 = 0306 s ranging from 012 to 053 s δtθgt20 = 0322 sranging from 01 to 096 s Performing a KolmogorovndashSmirnov teston the two ic families of data reveals that they are near-identicalwith a high P-value of 58 per cent indicating that we cannot precludethe hypothesis that the two data sets sample the same distribution

4 D I S C U S S I O N

41 Causes of seismic anisotropy and comparison toprevious studies

A striking observation in Fig 1 is the lack of correlation betweenour local earthquake-derived φ directions and SKS splitting resultsCorroborating earlier studies (Christensen amp Abers 2010 Hanna

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The development of seismic anisotropy below south-central ...

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