Seismic anisotropy under central Alaska from SKS splitting observations

Transcription

1 Click Here for Full Article JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115,, doi: /2009jb006712, 2010 Seismic anisotropy under central Alaska from SKS splitting observations Douglas H. Christensen 1 and Geoffrey A. Abers 2 Received 21 June 2009; revised 6 November 2009; accepted 30 November 2009; published 28 April [1] Seismic anisotropy under central Alaska is studied using shear wave splitting observations of SKS waves recorded on the Broadband Experiment Across the Alaska Range (BEAAR), Splitting results can be divided into two distinct regions separated by the 70 km contour of the subducting Pacific plate. Waves that travel through the thicker mantle wedge show fast directions that are parallel to the strike of the slab. These slab parallel directions appear to indicate along strike flow in the mantle wedge, and splitting delay times increase with path length in the mantle wedge, suggesting anisotropy of 7.9% ± 0.9%. The region of along strike flow corresponds to high seismic attenuation and hence high temperatures. Along strike flow here may be driven by secular shallowing of the slab driven by subduction of buoyant Yakutat terrane crust or by toroidal flow around the east end of the Aleutian slab. Waves traveling southeast of the 70 km contour sample the Pacific plate and the nose of the mantle wedge; they show fast directions that parallel the direction of plate motion. These fast directions are most likely due to flow under the subducting Pacific plate and/or anisotropy within the subducting Pacific lithosphere. The high splitting delay times ( s) associated with these convergence parallel directions cannot be produced in the mantle wedge, which is km thick here. Thus, anisotropy shows a sharp 90 change in fabric associated with the onset of high temperature wedge flow. Citation: Christensen, D. H., and G. A. Abers (2010), Seismic anisotropy under central Alaska from SKS splitting observations, J. Geophys. Res., 115,, doi: /2009jb Introduction [2] S wave splitting observations have become a common method of inferring anisotropy in the mantle. The determination of fast direction and time delay of the split S wave is fairly straightforward and can be accomplished through a variety of common methods [e.g., Silver and Chan, 1991; Long and van der Hilst, 2005]. In most natural samples and experiments, mantle anisotropy is controlled by lattice preferred orientation of the fast, a axis of olivine, aligned parallel to flow through shear accommodated by dislocation creep [e.g., Nicolas and Christensen, 1987; Ismail and Mainprice, 1998; Zhang et al., 2000]. The fast polarization of S propagation should roughly align with flow, and in subduction zones, the expectation is that slab induced corner flow should align fast directions in the direction of convergence. However, in many if not most subduction zones, fast directions lie parallel to the strike of the arc [e.g., Fischer et al., 1998; Long and Silver, 2008]. This has led to a variety of explanations appealing to along strike or other three dimensional flow [Hall et al., 1 Geophysical Institute, University of Alaska Fairbanks, Fairbanks, Alaska, USA. 2 Lamont Doherty Earth Observatory of Columbia University, Palisades, New York, USA. Copyright 2010 by the American Geophysical Union /10/2009JB ; Kincaid and Griffiths, 2003; Kneller and van Keken, 2007], or secondary convection [Behn et al., 2007]. Alternatively, other physical mechanisms may produce fabric, such as melt mediated olivine rotation [Holtzman et al., 2003] and activation of the olivine B slip system at wet, high stress conditions [Jung and Karato, 2001]. Finally, such measurements may be complicated by variable anisotropy at different depths and complex geometries of anisotropic regions [e.g., Savage, 1999; Hartog and Schwartz, 2001; Levin et al., 2007; Abt et al., 2009], making it sometimes difficult to test structurally complicated models. [3] One of the difficulties in understanding the relationship between anisotropy and flow is the sparseness of seismic sampling in most subduction zones. Low heat flow and low seismic attenuation in the fore arc suggest that a cold nose is present in the shallow parts of the wedge, a region largely isolated from the large scale flow that advects hot material underneath arcs [Kincaid and Sacks, 1997; Kneller et al., 2005; Abers et al., 2006; Wada et al., 2008]. This region generally occurs where the slab is less than 80 km deep or at distances >20 30 km trenchward of the arc, so sampling of anisotropic fabric at length scales less than km is necessary to avoid spatial aliasing of the flow field. This is also the region in which the B type fabric is inferred to be viable, in which the olivine a (fast) axis aligns perpendicular to flow, so sharp rotations in anisotropy may be present [Kneller et al., 2005, 2007]. Thus far only a handful of sub- 1of12

2 Figure 1. Locations of seismic stations from the BEAAR experiment and shaded topography. Thick blue lines show contours to slab seismicity. Thin dark lines show active faults, and white lines show roads. Thick dashed line shows orientation of cross section in Figure 7. duction zones have sufficient density of seismographs to adequately image such flow, for example Japan [Nakajima and Hasegawa, 2004; Long and van der Hilst, 2005], Central America [Abt et al., 2009], the Marianas [Pozgay et al., 2007]. [4] In this paper we determine S wave splitting parameters using SKS waves recorded in central Alaska, from the Broadband Experiment Across the Alaska Range (BEAAR; Figure 1). From this dense array, we observe a sharp 90 rotation in fast directions from convergence parallel where the slab is shallow, to strike parallel in the deeper wedge. The abrupt boundary corresponds to the boundary in thermal structure inferred from seismic attenuation, and flow north of it likely reflects flow in the mantle wedge. However, splitting farther south is difficult to reconcile with B or any fabric within the wedge, but seems to reflect deeper anisotropy. Thus, the cold nose controls the pattern of anisotropy, but mostly by bounding the region of deeper wedge flow. 2. Alaska Regional Setting [5] The Pacific Plate subducts beneath North America at the 2800 km long Aleutian Alaska trench, as delineated by a Wadati Benioff Zone (WBZ) reaching 300 km depths in the central Aleutians and km depths beneath southcentral Alaska [Page et al., 1989; Taber et al., 1991]. The WBZ truncates abruptly east of 148 W longitude, beneath the Alaska Range, either because the plate is torn or contorts sharply here. This eastern, Denali segment of the subduction zone (148 W 151 W longitude) has been characterized as a flat slab [Gutscher and Peacock, 2003] mostly because the thrust zone dips <5 and is among the widest in the world; the WBZ deeper than 50 km dips 25 through this 2of12

3 1991]. This fast direction is nearly parallel to the strike of the subducting Pacific plate (to the south) and is also parallel to major tectonic features such as the Alaska Range and the Denali fault in the region. Convergence is highly oblique in the central Aleutians, where a seismic station at Adak shows that the anisotropic fast orientation is nearly parallel to plate motion [Bender et al., 2004], but normal to the arc at the Alaska Peninsula. Yang et al. [1995] measured splitting from local WBZ earthquakes in the Shumagin segment of the Alaska Peninsula, recorded at 4 regional stations. They found fast directions to be uniformly aligned parallel to strike for paths traversing both in the fore arc and back arc, despite the arc normal plate convergence and relatively simple slab geometry there, and an increase in splitting time with event depth indicating a mantle origin for the splitting. Thus, even in this simple setting, two dimensional flow models cannot explain the observed anisotropy. Figure 2. Distribution of earthquakes used in the study (tabulated in Data Set S1). interval and contains seismicity to 130 km depth [Ratchkovski and Hansen, 2002]. Pacific North America convergence is roughly 55 mm/yr at 20 azimuth (positive is clockwise from north), slightly oblique to the slab dip direction of [DeMets et al., 1990]. [6] The Denali segment of the subduction zone is unusual in several ways. Arc volcanism is largely absent, except for one small maar above the easternmost intermediate depth seismicity [Nye, 1999], and large active mountain ranges of the Alaska and Chugach ranges overlie it. It is also the site of one of the few terrane collisions active today; the Yakutat terrane is translating roughly with the Pacific Plate at the coast and is likely responsible for much of the active deformation around the Gulf of Alaska over the last 5 10 Ma [e.g., Bruns, 1983; Davis and Plafker, 1986; Pavlis et al., 2004]. Receiver functions from the BEAAR experiment have shown that km thick crust extends to depths of 130 km beneath the Alaska range [Ferris et al., 2003; Rossi et al., 2006; Rondenay et al., 2008], roughly the same thickness as the Yakutat terrane at the trench [Brocher et al., 1994], indicating deep subduction of buoyant crust. While it is unclear what effect the Yakutat subduction would have on the mantle flow field, it is likely that its buoyancy led to a shallowing of the slab dip over the last 5 10 Ma [Abers, 2008], perhaps displacing nominally subcontinental mantle northward. Previous studies using the BEAAR data have clearly imaged a cold nose from seismic attenuation data, which shows an abrupt transition from high attenuation over the deeper parts of the slab to very low attenuation (high Q) in the wedge where the slab is less than 80 km deep, inferred to represent a change from high to low temperature [Stachnik et al., 2004]. [7] A handful of previous shear wave splitting measurements exist for the Alaska Aleutian system. Isolated measurements at DWWSSN station COL in Fairbanks show a fast direction of 82 with a delay time of 1.55 s [Silver and Chan, 3. Data [8] Beginning in the summer of 1999 a temporary broadband seismic network was deployed across the Alaska Range as part of the BEAAR PASSCAL experiment (Figure 1). During the first year seven stations were deployed at approximately 50 km spacing in a north south line running along the Parks Highway from Nenana to Talkeetna, Alaska. During the summer of 2000 the station density was increased to include a total of 36 stations. The configuration included a closely spaced ( 10 km) north south line and a slightly greater spaced east west line which operated for the summer months. Seventeen of the sites (every other station) were then run for an additional year with the last station being removed in the fall of The broadband signals were sampled at 50 samples per second. [9] The network crossed the Alaska Range and was situated above the mantle wedge. The southern most station (TLKY) near Talkeetna was located above the 50 km contour of the subducting Pacific plate near the southern extend of the mantle wedge. The northern most station (NNA) near Nenana was located north of the deepest extent of the subducting Pacific plate as inferred from seismicity and from receiver functions. These stations were ideally located to study teleseismic arrivals which traverse the subducting plate and the mantle wedge. [10] In this paper we explore the anisotropy in the mantle (and/or crust) through SKS splitting observations. We have used only SKS waves due to the predictable polarization of the incoming wave and thus the simplicity of the analysis. Local S waves were also examined, but because intraslab earthquakes here are relatively small they only generate signals at relatively high frequencies (>1 Hz), making them sensitive to near receiver structure and difficult to interpret or compare with the simpler SKS patterns, so are not considered further. Earthquakes in the distance range between 85 and 140 and magnitudes between 5.7 and 7.9 were selected for further study. Events with poor SKS arrivals, due to radiation pattern or other source or path effects were discarded. After rejecting events with poorly resolved splitting or large errors, 56 events are included in this study (Data Set S1). 1 Unfor- 1 Auxiliary materials are available at ftp://ftp.agu.org/apend/jb/ 2009jb of12

4 Figure 3 4of12

5 Figure 4. Results from stations WON, TLKY, and HURN. (left) Back azimuth versus fast direction and (right) back azimuth versus delay time for stations WON (green squares), TLKY (blue triangles), and HURN (red circles). The fast direction scale (Figure 4, left) is extended past 90 in order to show the relationship between the green squares and to accommodate the error bars. tunately, the events are not well distributed azimuthally due to limited source regions. In fact the majority of events can be grouped into three distinct back azimuths centered around 100, 200, and 270. These events are plotted in Figure Method [11] The method that we use to determine splitting measurements is from Silver and Chan [1991], as implemented by Fischer et al. [1998]. Extracted SKS waveforms are bandpass filtered between 0.01 and 1.0 Hz. The basic technique uses a grid search to find the best fast direction and time delay which when applied to the observed SKS waves, reconstructs the original SKS wave. The best fit splitting parameters are those that reconstruct the most linear incident SKS wave, by minimizing the second eigenvalue of the crosscorrelation matrix between horizontal components. An F test on the solution provides confidence bounds. As a final quality check, results with 95% confidence bounds in fast direction greater than ±35 or delay times greater than ±0.6 s were not used in the final analysis. In some analyses, individual splitting measurements are combined in narrow back azimuth bins at each station by joint inversion, to provide more accurate path averaged parameters [Wolfe and Silver, 1998]. [12] An example of the method is shown for an event on 25 May 2001 recorded at station HURN in Figure 3. Results for stations TLKY, HURN, and WON, which are representative of most of the data, are shown in Figure 4. These plots show fast directions and delay times versus bask azimuth for the three stations. WON, which is located north of Mount McKinley over the 100 km contour of the subducting plate, shows fairly consistent fast directions (32 ± 9 ) and delay times (1.95 ± 0.63 s) at all back azimuths. This behavior is similar to many of the stations located above the thicker parts of the mantle wedge. This indicates a rather simple nearhorizontal anisotropy with a fast direction oriented parallel to the strike of the subducting slab. The southern most station TLKY also shows a fairly simple pattern with average fast directions of 23 ± 14 and delay times of 0.75 ± 0.34 s. These fast directions are perpendicular to the strike of the slab (i.e., parallel to the plate convergence direction). Station HURN shows a more complex back azimuth pattern, in which the fast directions change dramatically with back azimuth. All stations which are located about the km contour of the slab show similar behavior. Waves which arrive from the east and south have fast directions that are similar to station TLKY, while waves that arrive from the west show fast directions that are similar to station WON. This pattern is consistent with a more complicated anisotropy which could indicate either dipping anisotropy or as we will argue in section 5 a spatially varying anisotropy. 5. Results [13] The results from this study are listed in Data Set S2 and shown in Figure 5. At each station there are many SKS splitting observations which are made from events at various back azimuths and distances. In order to view the Figure 3. Example of data processing for the event on 25 May 2001 recorded at station HURN. (a) Filtered (band pass Hz) N/S and E/W components of the SKS wave. (b) Horizontal components rotated to the fast (36 ) and slow (126 ) directions. (c) Comparison of the fast and slow waveforms after the time delay (dt) has been removed. (d) Contour plot of the reconstructed fit between the two horizontal components from a grid search over possible fast directions and delay times showing the best solution (red dot) and 95% confidence region (red contour). 5of12

6 Figure 5. Compilation of SKS splitting results. Each measurement is plotted as a red line parallel to the fast direction, with length proportional to the delay time. The observations are plotted at the 100 km depth projection of the ray in order to demonstrate the back azimuthal pattern. effects of the azimuthal variation, the splitting vectors are plotted above the point where the back projected ray intersects 100 km depth; rays are projected through a simple 1D model including crust and mantle. This depth allows the azimuthal behavior to be represented on the plot by spatially separating the two regimes; only projection depths greater than 50 km do this (Figure 6) and a depth of 100 km lies generally in the region where we may expect anisotropy to exist (in the mantle wedge for the northern stations, and beneath the slab for the southern stations). Note that this depth was selected because it spatially separates the two splitting regimes and does not necessarily represent the origin of the anisotropy. [14] In general, there are three basic types of splitting behavior across the network. These behaviors can be visualized by looking at plots of fast direction versus back azimuth and delay times versus back azimuth for three typical stations in Figure 4. Stations that are above the deeper sections of the subducting slab have a fairly simple splitting distribution as seen at station WON, with all fast directions parallel to the strike of the subducting plate. Stations that are above the shallowest parts of the subducting plate resemble station TLKY, and show fast directions in the plate motion direction. The third category of stations (similar to HURN), include stations that are located above the km contour of the subducting slab. At those stations, waves that arrive from southern or eastern back azimuths show fast directions in the plate motion direction (similar to TLKY), while waves that arrive from western or northern back azimuths show fast directions parallel to the strike of the subducting plate (similar to WON). This pattern could represent a dipping anisotropy, however, the pattern is more easily explained by a spatial 6of12

7 Figure 6. SKS splitting results plotted at each station, as in Figure 5, but different depth of projection. (left) A 0 km depth projection. (right) Projected to 50 km depth. Note the clear separation between strike parallel and strike normal fast directions only when projected 50 km deep. 7of12

8 Figure 7. SKS raypaths in cross section, colored by azimuth of the fast splitting direction (scale bar) after stacking by back azimuth. Orange area shows region of high attenuation, Qs < 250 at 1 Hz [Stachnik et al., 2004]. Black curves show polynomial fits to Moho and top of slab from receiver functions [Rossi et al., 2006; Veenstra et al., 2006], triangles show stations, and circles show earthquakes. Location of cross section is shown in Figure 1. Labels denote stations used in Figure 4. Most raypaths passing through the low Qs region show strike parallel fast direction. variation of the anisotropy, which changes dramatically along the 70 km contour of the subducting slab. [15] The results show two distinctly different splitting domains. These domains are separated by the 70 km contour of the subducting Pacific plate. Rays that traverse the mantle north of this 70 km contour line have fast directions which are parallel to the strike of the subducting plate, whereas, rays that traverse the mantle south of the 70 km contour line have fast directions which are parallel to the plate convergence direction. Seismic stations located above the 70 km contour of the subducting plate clearly show both fast directions segregated by back azimuth, indicating a relatively sharp transition in anisotropy. The sharpness depends upon the depth to which rays are projected, or more importantly the actual location of the two anisotropic regions, but could be as little as km, in map view (Figures 5 and 6). 6. Discussion [16] The results reported above provide us with a fairly detailed view of anisotropy in the subduction complex in central Alaska. Any explanation will have to explain a 90 rotation of the fast direction over a very short distance Deeper Wedge [17] Several lines of evidence lead us to believe that the strike parallel fast directions are associated with along strike flow in the mantle wedge. The first indication comes from abrupt change in fast directions at the 70 km contour of the subducting plate. The two regimes are only segregated when splitting is back projected to mantle depths >50 km (Figures 5, 6, and 7), sharp boundaries in anisotropy are seen in single stations, and splitting is large (>1 s) so the source of anisotropy cannot be within the crust. [18] Second, the sharp change in direction is coincident with a known change in the properties of the mantle wedge material. Attenuation studies using the BEAAR data [Stachnik et al., 2004] show a sharp lateral boundary near the 70 or 80 km contour which separates hot highly attenuative more mobile mantle material in the thicker parts of the mantle wedge to the north, from cooler less attenuating material south of the contour in the mantle wedge corner (see Figure 7). Attenuation at mantle conditions should indicate changes in temperature to first order [Faul and Jackson, 2005] modified by the presence of any water or melt. Stachnik et al. [2004] suggest that the high attenuation region corresponds to temperatures >1200 C, lowered somewhat if water is abundant, consistent with low viscosity, flowing asthenosphere. The simplest interpretation is that the highattenuation region indicates a region of high temperatures where mantle flows readily, and the strike parallel SKS splitting fabric shows that flow. It is difficult to see why this boundary in wedge attenuation would correlate with a change in anisotropic fabric if the relevant anisotropy did not occur mostly within the wedge. [19] Finally, we see a good correlation (R 2 = 0.75) between the path averaged splitting time delays (Data Set S3) and the path length of the waves through the mantle wedge. Figure 8 shows delay times versus path lengths for SKS waves that have back azimuths around 270 (only interact with the higher attenuation/hotter portions of the mantle), and show strike parallel fast directions. Path lengths here are paths of SKS within the mantle wedge, between digitized surfaces representing the top of the slab and the upper plate 8of12

9 Figure 8. Splitting delay time versus path length within mantle wedge for paths with back azimuths near 270 and which show slab parallel fast directions. Path lengths calculated between slab surface and Moho (Figure 7). Black squares show delay times for path averages (Data Set S3), and small gray diamonds show individual measurements. Line shows linear regression on path averaged delay times. Moho from receiver functions [Rossi et al., 2006; Veenstra et al., 2006]. The delay times are clearly linearly related to path length through the mantle wedge, if just the strike parallel fabric is considered, confirming that this change in anisotropy originates within the wedge. In this comparison three outliers were omitted because they fall outside of the mantle wedge; stations NNA and AND are the farthest north and lie beyond the end of the slab, and station CZN lies east of all slab seismicity and probably past the slab s eastern termination. Ignoring these three outliers, the best fit line through the data in Figure 8 has a slope of ± s/km (1 s errors on linear regression), which if interpreted as due to constant anisotropy in a variable thickness wedge, gives anisotropy of 7.9% ± 0.9% for V s = 4.4 km/s. The y intercept is small, 0.27 ± 0.15 s, indicating the wedge anisotropy dominates the splitting. Interestingly, the strike parallel anisotropy orientation continues north of the BEAAR array to the Alaska north coast, but delay times decrease by about one half, indicating continued flow in the same direction [Litherland et al., 2007]. [20] Anisotropy of 7.9% is high compared with typical values inferred beneath continents [e.g., Savage, 1999] but not unprecedented in subduction zones. For example, detailed analysis beneath the Ryukyu arc show up to 10% anisotropy within the wedge, also with along strike fast directions [Long and van der Hilst, 2006]. Possibly the strains producing a axis alignment are unusually high near the wedge corner, or the presence of melt and fluids enhances anisotropy. Regardless, the strike parallel direction of anisotropy cannot be explained by simple olivine a axis alignment in corner flow, which should be trench normal. Fabric created by partial melt [Holtzman et al., 2003] seems less likely in Alaska, because arc volcanism is virtually absent and unusually low Poisson s ratios in the mantle [Rossi et al., 2006] are inconsistent with abundant melt. Activation of high stress slip systems such as the B fabric [Jung and Karato, 2001] also seems inappropriate for this apparently high temperature region. Similarly, the consistency of the along strike direction every northwest of where the slab is km deep seems to rule out anisotropy driven by small scale convection [Behn et al., 2007]. [21] Most likely, the anisotropy reflects along strike flow. Along strike flow may be a toroidal flow around the east edge of the Aleutian slab, as expected near along strike terminations or sharp dip changes in slabs [Kincaid and Griffiths, 2003; Kneller and van Keken, 2008]. The projected fast directions just east of the northernmost stations are rotated to more nearly E W than strike parallel, perhaps indicating some flow around the slab edge. Alternatively or additionally, a temporal shallowing of the slab dip may drive flow. Thick crust of the Yakutat terrane began subducting at the Alaska margin 6 10 Ma ago [e.g., Bruns, 1983; Pavlis et al., 2004], perhaps leading to a shallowing of slab dip as buoyant crust resists subduction [Abers, 2008]. Such changes in slab geometry would require upper plate mantle to be displaced, perhaps laterally and resulting in strike parallel stretching fabrics. 6.2 Shallow Slab Region [22] South of the 70 km contour anisotropy exhibits a sharp 90 change in fast direction to plate convergence direction. One possible explanation is that the 90 rotation is due to a change in the behavior of mantle material from normal olivine slip systems in the open mantle wedge to the B fabric within cold nose [Kneller et al., 2005]. The B fabric produces a 90 rotation in relationship between shear strain and anisotropy, and is active where water, high stresses and hence low temperatures are present. The location of our 90 rotation is located approximately where large temperature changes are inferred from attenuation, similar to such rotations in Japan and the Ryukyu arcs where this process has been invoked [Long and van der Hilst, 2005, 2006]. The major flaw in this argument is that the delay times reach 1.8 s among the southern stations (Data Set S2), or 1.7 s in back azimuth stacks (Data Set S3), far too large to be from the thin nose of the mantle wedge. In the region of interest the mantle wedge material ranges from about 30 km thick to 10 km thick at its extreme southern end. Even if this material were 10% anisotropic, the largest time delay that we could expect in 30 km of mantle wedge would be 0.7 s, and the southernmost stations would produce no more than 0.2 s splitting, compared to the s observed in the stacked measurements. It is difficult to imagine that the observed splitting could be generated in the wedge, as observed, even invoking flow driven alignment of serpentine within the cold nose [Kneller et al., 2008]. [23] Our preferred explanation is that the plate motion fast directions south of the 70 km contour represent flow from below (and/or perhaps fabric within) the subduction Pacific plate. In this scenario flow in the direction of plate motions below the Pacific plate result in the observed fast directions. In this case the mantle wedge corner still plays an important role in that this stagnant part of the wedge develops little flow driven fabric, so that the main feature in the splitting comes from below. Thus the sharp change in the observed fast direction is due to the sudden absence of mantle wedge 9of12

10 (Figure 9). In principle such an effect could be modeled by examining azimuthal patterns in anisotropy [Silver and Savage, 1994], but that requires much more continuous sampling in back azimuth than we have. Overall, shear wave splitting appears to be relatively insensitive to deeper anisotropy when a shallow anisotropic layer exists. The scale of this effect should be frequency dependent [e.g., Marson Pidgeon and Savage, 1997], and a similar effect may explain the relatively weak splitting observed from high frequency local earthquake signals. In addition, due to the perpendicular orientation of the two regions of anisotropy found in this study, nodal back azimuths in the upper layer are also nodal in the lower layer, making the identification of the deeper anisotropy through the upper layer particularly difficult. Figure 9. Shear wave splitting fast directions recovered from synthetic test, in which SKS waves propagate through two layers of differing anisotropy. Blue lines show histogram of recovered fast directions from test for incident waves arriving from all back azimuths. Anisotropy simulates strike parallel fast directions in the Alaska mantle wedge overlying the subducting plate with strike normal fast directions. Anisotropy in each layer has hexagonal symmetry with horizontal fast axis, with accumulated average delays of 1.24 s and 1.07 s in the shallow and deeper layer, respectively, oriented at 60 (red) and 30 (green) azimuth, respectively. Polarized SKS waves are propagated through these two layers, and splitting parameters are measured on the resulting seismograms in the same manner as actual data. Incoming SKS waves have sinusoidal shape and 12 s pulse duration, arriving at 13 incidence angle, calculated at 2 back azimuth intervals flow in the cold nose. It is not clear why the subplate mantle should have convergence parallel flow, this pattern differs from the along strike subslab flow inferred for a majority of subduction zones [Long and Silver, 2008], but it is also possible the pattern reflects fabric within the subducting plate. [24] It is at first puzzling that splitting to the northwest should be uniformly perpendicular to that farther southeast, if indeed the southeastern splitting is caused by anisotropy in or below the subducting plate, which should be expected to underlie the mantle wedge farther north. However, shear wave splitting does not linearly sample anisotropy uniformly along the SKS raypath, but is preferentially sensitive to anisotropy nearest the station with some azimuthal variation [e.g., Silver and Savage, 1994]. To confirm this, we made a simple calculation of splitting predicted for two anisotropic layers with horizontal fast axes orthogonal to each other, roughly mimicking our expectation for Alaska, and compared it to the prediction in which only the lower layer displayed anisotropy. This calculation sequentially splits and delays an initially polarized incident SKS wave, arriving at typical incidence angle (13 ) and a full range of back azimuths. Each final synthetic split SKS wave is then analyzed for splitting parameters in the same manner as real data. When the overall predicted delay for the upper layer is comparable to or greater than that for the lower layer, all predicted splits are within 30 of the fast direction of the upper layer 7. Conclusions [25] Shear wave splitting shows a simple pattern beneath southern Alaska. Where the slab is more than 70 km deep, fast directions uniformly parallel the slab strike. The strong correlation of splitting time and raypath length in the wedge shows that the anisotropy most likely originates from within the mantle wedge, not below it or in the overlying crust. This part of the wedge should be hot, as indicated by seismic attenuation, so olivine a axis alignment with flow seems likely. The simplest explanation is that the mantle flow is along strike, either a result of the flattening of the slab through subduction of the buoyant Yakutat block, or a simple effect of the slab end. [26] Closer to the trench, SKS splitting shows a 90 rotation over a transition no more than km wide to fast directions that are parallel to plate motion farther south. It is fairly clear that the southern domain is not generated by anisotropy in the mantle wedge, as the wedge is km thick in this region and high anisotropy (> > 10%) would be required, more than can be generated by olivine deforming by any mechanism. Most likely, this anisotropy could be from within or below the downgoing plate. Overall, these data show that flow in subduction zones is controlled by large scale plate motions, but along strike flow can dominate, explaining mantle wedge fabric without appealing to exotic mechanisms for anisotropy. [27] We see the subduction of the Pacific plate playing a major role in partitioning flow in the upper mantle. Above the Pacific plate mantle flow is parallel to the strike of the plate. Anisotropy is greatly enhanced (perhaps doubled) within the mantle wedge due to higher stresses and faster flow velocities. In central Alaska this could be particularly important due to the recent subduction of the buoyant Yakutat terrane and the subsequent shallowing of the Pacific plate and squishing of the mantle wedge. [28] Acknowledgments. The BEAAR deployment benefited from contributions by many people, including contributions from the Alaska Earthquake Information Center, the IRIS PASSCAL Instrument Center, and a large number of students who helped with the deployment and its analysis. This work is supported by National Science Foundation grant EAR References Abers, G. A. (2008), Orogenesis from subducting thick crust and evidence from Alaska, in Active Tectonics and Seismic Potential of Alaska, Geophys. 10 of 12

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