Earth and Planetary Science Letters 311 (2011) 101–111

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Earth and Planetary Science Letters

j ourna l homepage: www.e lsev ie r .com/ locate /eps l

Subducted lithosphere beneath the Kuriles from migration of PP precursors

Nicholas Schmerr a,⁎, Christine Thomas b

a Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Rd., Washington, DC 20015 USAb Institut für Geophysik, Westfälische Wilhelms-Universität, Corrensstraße 24, 48149 Münster, Germany

⁎ Corresponding author at: NASA Goddard Space Flnamics Laboratory, Code 698, Greenbelt, MD 20771, US

E-mail addresses: nschmerr@dtm.ciw.edu (N. Schmetine@earth.uni-muenster.de (C. Thomas).

0012-821X/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.epsl.2011.09.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 11 May 2011Received in revised form 22 August 2011Accepted 2 September 2011Available online 4 October 2011

Editor: P. Shearer

Keywords:Kurile Subduction Zoneupper mantlediscontinuitiesPP precursorsP660Ptransition zone

We seismically image both thermal and chemical heterogeneity of the mantle beneath the Kurile subductionzone using P-wave energy reflected from the underside of discontinuities, arriving as precursors to the seismicphase PP. We take advantage of new broadband seismic data provided by the High Lava Plains Seismic Experi-ment and EarthScope's USArray, collecting a dataset of 31 high-quality Sumatran earthquakes sampling beneaththe Kuriles. We employ high-resolution array analysis techniques, including migration and vespagrams, toidentify precursory arrivals and study lateral variations in discontinuity depth, sharpness, and impedance ofthe mantle transition zone. We find the 410 km boundary is at 395 km near the subducting Kurile slab, thoughthe boundary is 410–425 km deep elsewhere. In regions away from subduction, we do not detect a laterallycontinuous underside reflection of P-waves from the 660 km discontinuity. However, in the vicinity of thesubducting Kurile slab, we detect robust P660P reflections from interfaces near 620–670 km depth, signifyingan increase in the impedance contrast at 660 km depth. We also detect deeper reflectors, down to 720 kmdepth, beneath the Kurile slab in a localized area. Cold, aluminum-depleted harzburgitic lithosphere residingat the base of the transition zone best explains the local enhancement of the 660 km discontinuity P-waveimpedance contrast. Our new discontinuity measurements support the hypothesis of cold, depleted lithospherestagnating at the 660 kmdiscontinuity beneath theKuriles subduction zone, and imply the 660 kmboundary canlocally impede mantle flow and produce chemical heterogeneity within the transition zone.

ight Center, Planetary Geody-A.rr),

l rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

The Kurile subduction zone of the northwestern Pacific has been akey region for investigating the nature of convection in the mantleand establishing the dynamics of subduction zones. The Kurile islandarc extends from the tip of the Kamchatka peninsula and terminatesin a complex interaction with the Japanese island arc at HokkaidoIsland. The arc is formed by the oblique convergence of relatively old(N100 Ma) (Müller et al., 1997) Pacific lithosphere subducting beneaththe Sea of Okhotsk at ~8.2 cm/yr (DeMets, 1992). Seismicity associatedwith the descending slab beneath the Kurile Islands extends into themantle transition zone (MTZ) to a depth of ~670 km, and numeroustomography and travel time studies have found evidence for highvelocitymaterials at depth (e.g., Jordan, 1976; van der Hilst et al., 1993).

Much of the early work in the Kuriles focused on determining thefate of the subducted lithosphere utilizing deep seismicity associatedwith the slab (Fedotov, 1965). Measurements of near source S, P, anddepth phase travel time anomalies indicated the presence of a highvelocity (N5%) anomaly in the vicinity of the Benioff zone (Fukao,1977). Travel time sphere residual analyses by (Jordan, 1977) found

that the Kurile slab penetrated into the lower mantle to at least900 km depth, though later studies using the same technique indicat-ed that the high velocities of the slab did not extend into the lowermantle (Fischer et al., 1988; Gaherty et al., 1991; Suetsugu, 1989).Early 3-dimensional tomography models of shear wave structurebeneath the Kuriles found evidence for high velocities located justabove the 660 km discontinuity, leading to the suggestion that theslab stalls or stagnates at the 660 km discontinuity (Fukao et al.,1992; Fukao et al., 2001; Fukao et al., 2009). A number of detailedtomography models for P and S waves (e.g., Ding and Grand, 1994;Li et al., 2008; Megnin and Romanowicz, 2000; Miller and Kennett,2006; Ritsema et al., 2011; van der Hilst et al., 1993) image a highvelocity anomaly above the 660 km discontinuity to the south nearHokkaido, stagnating on top the 660 km discontinuity in the down-dipdirection, while to the north near Kamchatka this anomaly penetratesdirectly into the lower mantle. Tomographic studies find shear wavevelocity anomalies in the slab dominate over bulk sound velocities(Gorbatov and Kennett, 2003), suggesting that the imaged velocityheterogeneity is produced by the temperature anomaly of the subduct-ing lithosphere over the compositional effects of the slab, though uncer-tainties in this relationship remain unresolved.

Another way to probe the structure beneath the Kuriles is throughseismic imaging of the depth of the upper mantle discontinuities (e.g.,Shearer, 2000). The two major upper mantle seismic discontinuitiesthat delineate the MTZ arise from solid-to-solid phase changes in

http://dx.doi.org/10.1016/j.epsl.2011.09.002mailto:nschmerr@dtm.ciw.edumailto:tine@earth.uni-muenster.dehttp://dx.doi.org/10.1016/j.epsl.2011.09.002http://www.sciencedirect.com/science/journal/0012821X

102 N. Schmerr, C. Thomas / Earth and Planetary Science Letters 311 (2011) 101–111

the mineral olivine that are sensitive to both mantle temperature andcomposition (Bina and Helffrich, 1994). The exothermic transforma-tion of olivine (ol) to wadsleyite (wd) generates the 410 km disconti-nuity (410) (Katsura and Ito, 1989), and the endothermic dissociationof ringwoodite (rw) into Mg-Perovskite (pv) and magnesiowüstite(mw) forms the 660 km discontinuity (660) (Ito and Takahashi,1989). Upper mantle discontinuity depth is perturbed by lateralchanges in mantle thermal and chemical properties, such as in thevicinity of cold subducted lithosphere in the MTZ. This owes to thedepth of each discontinuity being controlled by the Clapeyron slopeof the phase transitions, or the change in pressure of a phase transi-tion for a given change in temperature. The Clapeyron slope of the410 is positive, and is negative at the 660, resulting in a thickeningof the MTZ in the presence of a vertically continuous cold mantleanomaly, and thinning of the MTZ in the presence of vertically contin-uous hot mantle anomaly (Lebedev et al., 2003). Lateral variations inmantle composition, such as the presence of hydrogen within asubducted slab, can also alter the depth of the discontinuities andproduce topography on the boundaries (Karato, 2006).

Several other seismic discontinuities have also been detected at MTZdepths. The transformation of wadsleyite to ringwoodite gives rise to asmaller discontinuity near 520 km depth (Shearer, 1990). In some re-gions, a second discontinuity near 560 km depth related to the transfor-mation of garnet to Ca-Perovskite is observed (Deuss and Woodhouse,2001). In the vicinity of subduction zones, a low velocity zone is oftenobserved above the 410 km discontinuity, related to the presence ofpartial melt above this boundary (Song et al., 2004). The formation ofilmenite in colder regions of themantle and the completion of the garnetphase transitions at 750 km depth can produce additional discontinuitiesnear 660 depth (Weidner andWang, 1998).

Owing to the large variety of seismic sources and stations situatedaround the Kurile subduction zone, there have been numerous investi-gations utilizing reflected, refracted, and converted seismicwaves inter-acting with the MTZ discontinuities. In the southern Kuriles, triplicatedseismic waves detect a high velocity transition zone below 500 kmdepth, and a deep 690 km discontinuity, consistent with a stagnantslab in the MTZ (Tajima and Grand, 1995). The northwestern Pacific isextremely well-sampled by long-period underside reflected shearwaves, which occur as precursory energy to the seismic phase SS andare sensitive to structure half way between the source and receiver(Flanagan and Shearer, 1998; Gu et al., 1998; Shearer and Masters,1992). These seismic phases are 1–10% of the arriving SS amplitude,and require the stacking of datasets of hundreds to thousands of recordsto be observed robustly (Shearer, 1990). The SS precursory phasesdetect a large-scale depression of the 660 km discontinuity beneaththe Kuriles that is associated with the regions of higher velocity andinferred slab stagnancy in the tomographic models (Cao et al., 2010;Shearer, 1991; Shearer and Masters, 1992).

A comparable approach to studying discontinuity structure can bemade with underside reflected P wave energy arriving as a precursorto the seismic phase PP (King et al., 1975). Studies of the PP precur-sors have largely focused on the underside reflection from the 410(P410P), as the underside reflection from the 660 (P660P) is difficultto detect owing to extremely low amplitudes (Estabrook and Kind,1996; Neele and Snieder, 1992). In several recent studies, P660P hasbeen detected in different regions (Deuss et al., 2006; Thomas andBillen, 2009), suggesting that local heterogeneities can enhance theamplitude of this arrival. Past array studies of the Kuriles withshort-period PP precursors find an elevated 410 km discontinuity(Rost and Weber, 2002), but did not detect the 660 boundary.

Here we take advantage of the deployment of the broadband seis-mometers in the EarthScope USArray1 and the recent Program forArray Seismic Studies of the Continental Lithosphere (PASSCAL)

1 http://www.earthscope.org

High Lava Plains Seismic Experiment (HLP) (Carlson et al., 2005) toinvestigate detailed discontinuity structure beneath the Kuriles withPP precursors. We utilize an array migration technique that allowsus to isolate the arrivals of underside reflected P energy from thenoise and other interfering seismic phases, and focus in particularon mapping the character of the 660 km discontinuity. Our seismicsources allow us to investigate structure along a profile across thesouthern Kuriles and further test the hypothesis of a cold, stagnantslab in the lower MTZ with a method that complements recent to-mography results from this region.

2. Dataset

To study discontinuity structure, we collect a dataset of broadbandvelocity seismograms that sample beneath the Kurile subductionzone. Data were downloaded from the Incorporated Institutions forSeismology Data Management Center.2 The sources used originatedin the Sumatran subduction zone recorded between January 2005 toSeptember 2009 by the EarthScope USArray and the HLP SeismicExperiment (Fig. 1, Table 1). The HLP stations formed the backboneof our array and were supplemented by USArray stations deployedwithin 750 km from the center of the HLP. Events were required tohave a Mw≥5.8 to ensure sufficient seismic energy for our analysis.The instrument response was deconvolved from each seismogramand we studied the underside reflections of PP on the vertical compo-nent. The seismograms of each event were visually inspected to de-termine the quality of the body wave arrivals; high quality eventsdid not require any filtering to identify the PP arrivals from the back-ground noise. In addition to these high quality events, we kept earth-quakes that possessed clear PP arrivals when low-pass filtered at acorner of 10 s. The dataset initially consisted of 169 events and14,047 seismograms, after visual inspection, the dataset consisted of31 high-quality events with 2180 seismograms, with an average of116 records per event (minimum 22, maximum 231). For our analy-sis, we marked a reference time for the PP arrivals using the traveltime of the highest amplitude arrival of the first swing of the PPwaveform. We measured a signal to noise ratio for every seismogramby comparing the enveloped maximum amplitude in a 50 secondwide window centered on the pick of the PP phase to the envelopedmaximum amplitude in a 100 second window ending 50 s beforethe predicted arrival time for the Pdiff phase. To investigate frequencydependence, we generated several subsets of the data with band-passfilters with upper and lower corners at 1–10 s, 3–25 s, 6–50 s, 10–100 s, and 15–100 s.

3. Method

To study the upper mantle discontinuities beneath the Kuriles, weutilize several array methods to amplify the PP precursory arrivals outof the noise levels: velocity spectral analysis (vespagram), slownessbackazimuth analysis, and migration. To test the accuracy of ourmethodology, we also generate reflectivity synthetic seismograms(Fuchs andMüller, 1971) for each event depth and source focal mech-anism and analyze the synthetics alongside the data. We start by gen-erating 4th root vespagrams (Davies et al., 1971; Muirhead and Datt,1976; Rost and Thomas, 2002) for each event to verify that the arrivaltimes and slowness of the precursors matched predictions fromak135 (Kennett et al., 1995). We only further analyze events with asignal to noise ratio≥3.0 for both PP and P410P in the vespagrams(Fig. 2). The vespagrams allow us to determine whether the precurso-ry energy is well separated from the interfering energy of PKiKP, top-side reflections, and other non-underside reflection seismic phases.

2 http://www.iris.edu

http://www.earthscope.orghttp://www.iris.edu

a) b)

c)

15

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-5

90 100 110

45

40

235 240 245

140 150

40

50

60

d)

CMB

P410PPP

ICB

660

410

Sea of

Okhotsk

Hokkaido

Kamchatka

Kuril

es

Japan

Siberia

Fig. 1. Seismic dataset parameters and geography of the Kurile subduction zone. a) Sumatran subduction zone source regionwith events used in this study (stars). b) Station geometry of theHLP seismic array (black triangles) and location of the EarthScope Transportable Array stations used in this study (inverted triangles). c) Kurile subduction zone study region and location ofunderside reflections (circles). d) Example raypaths of PP and P410P. Underside reflections occur at the center of the seismic raypath. CMB—coremantle boundary, ICB—inner core boundary.Slab depth contours are from (Gudmundsson and Sambridge, 1998).

103N. Schmerr, C. Thomas / Earth and Planetary Science Letters 311 (2011) 101–111

When the arrivals were not separated well, the event was not consid-ered further.

We retrieve the depths of the upper mantle discontinuities using amigrationmethod adapted from (Thomas and Billen, 2009). Themigra-tion method allows the back projection of precursory energy to theunderside reflection point and lessens the size of the Fresnel zone there-by enhancing resolution (e.g., Rost and Thomas, 2009). To migrate, wegenerate a 40° by 40° grid in 1° increments centered on the reflectionpoint halfway between the average array center and source location.The grid is incremented every 5 km between 0 and 900 km depth. Wethen calculate travel times between each grid point and array station,as well as between each grid point and the source location using araytracing code through ak135. Each seismic trace is shifted by theresulting delay time and stacked. We select the maximum amplitudein a 3 second window centered on the theoretical arrival time of the

precursor, and utilize a bootstrap resampling algorithm with 300random replacement resamples to evaluate the 95% confidence ampli-tude in each stack (Efron and Tibshirani, 1986). The resulting migratedenergy recreates the X-like shape of the PP Fresnel zone, owing to theminimum–maximum travel time of underside reflected waves (Choyand Richards, 1975).

The stacked energy from the migrated section will spread alongone isochrone if only one source and receiver is used. However,using a seismic array, the spreading of energy will be less and thelargest amplitude will be focused at the reflection point, enhancingresolution. The backazimuth range for the data used in this study ison the order of 7°, which also lessens the size of the effective Fresnelzone. Past work has demonstrated array enhancement of resolution,albeit using a smaller array than the one used here (Rost and Thomas,2009). Fig. 3 includes examples of synthetics where focusing of

Table 1Event parameters and associated reflector depths.

Event Lat Lon Depth Bouncepoint Reflectors

Lat Lon d410 d660 dX1

dd/mm/yyyy hh:mm degN degE km degN degE km km km

24/07/2005 15:42 7.92 92.19 16 56.18 145.43 425 – 74025/04/2006 18:26 1.99 97.00 21 49.97 143.72 415 70027/06/2006 18:07 6.50 92.79 28 55.74 139.07 410 –17/07/2006 15:45 −9.42 108.32 21 35.21 156.05 410 –27/07/2006 11:16 1.71 97.15 20 49.84 143.78 420 –11/08/2006 20:54 2.40 96.35 22 50.93 142.75 420 630d 75001/12/2006 03:58 3.39 99.08 204 50.27 145.51 410 –07/04/2007 09:51 2.92 95.70 30 52.09 141.53 415 640d 77508/08/2007 17:04 −5.93 107.68 291 38.71 154.52 400 –13/09/2007 03:35 −2.13 99.63 22 46.10 145.99 440n –13/09/2007 16:09 −3.17 101.52 53 44.24 148.09 410n –14/09/2007 06:01 −4.07 101.17 23 43.96 147.57 405 –19/09/2007 07:27 −2.75 100.89 35 44.89 147.43 – 66020/09/2007 08:31 −2.00 100.14 30 45.87 146.59 410 68024/10/2007 02:50 −3.40 101.02 21 44.06 147.66 405 69022/01/2008 17:14 1.01 97.44 20 49.86 143.47 420 630d 75020/02/2008 08:08 2.77 95.96 26 52.03 141.80 410 71025/02/2008 08:36 −2.49 99.97 25 45.83 146.43 410 67025/02/2008 18:06 −2.33 99.89 25 46.04 146.35 400 67525/02/2008 21:02 −2.24 99.81 25 46.10 146.23 395 67029/03/2008 17:30 2.86 95.30 20 52.51 140.93 420 620d 73027/06/2008 11:40 11.01 91.82 17 59.92 137.66 420 –08/08/2008 06:37 −3.92 101.08 35 44.05 147.82 410 690 83022/11/2008 16:01 −4.35 101.26 24 45.23 147.21 420 –30/12/2008 19:49 −4.30 101.22 20 45.14 147.42 410 –15/04/2009 17:47 −3.09 100.41 6 46.95 145.79 425 –15/04/2009 20:01 −3.12 100.47 22 46.75 145.96 41513/07/2009 10:52 −9.13 119.32 67 32.73 164.66 400 –16/08/2009 07:38 −1.48 99.49 20 46.56 145.99 420 –18/08/2009 17:50 −0.91 97.95 10 47.90 144.26 410 65030/09/2009 10:16 −0.72 99.87 81 47.17 145.39 405 –

n: neg. polarity; –: reflector not observed; d: depth phase.

5

10

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wn

ess

(s/d

eg)

−300 −200 −100 0

Time (s)

120

125

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135

−300 −200 −100 0

Time (s)

Ep

icen

tral

Dis

tan

ce (

deg

) P410PPKPp410p PKPp660pP660PPP

p660pPdiffp410pPdiffPKiKPPdiff

P410PP660P PP

p660pPdiffp410pPdiff

PKiKP

Pdiff

PKPp410p PKPp660p

a)

b)

0

Fig. 2. Waveforms and vespagram from earthquake occurring on 15/04/2009-20:01 for arobust P410P and non-detection of P660P. a) Seismograms recorded at the HLP and TAarrays, aligned on themaximumamplitude of the PP arrival. The data are band-pass filteredwith corners at 10 and 100 s. Dotted lines show the predicted travel times for select seismicphases. b) 4th root vespagram of the event, positive amplitudes are black, and negativeamplitudes are white. Crosses indicate the predicted slowness and travel time for thelabeled seismic phases. In both panels, theoretical travel times and slownesses for seismicphases are from ak135.

104 N. Schmerr, C. Thomas / Earth and Planetary Science Letters 311 (2011) 101–111

stacked energy at the saddle point of the precursor reflection isobserved. A problem, however, could be a precursor phase that hastraveled out of plane. In these cases we measure the deviation fromthe theoretical reflection point using slowness-back azimuth analysis(Rost and Thomas, 2002; Rost and Weber, 2002) and if the reflectionpoint is within the Fresnel zone of a wave that would travel along thegreat circle path, we utilized the measurement (Fig. 4). Events withpoor focusing of the precursors in the migration and/or slownessbackazimuth analysis were discarded.

To select the depth of the discontinuity, we take an amplitude profileat the calculated reflection point for the source and array center (Fig. 3).This migration technique removes the effects of seismic energy at otherslownesses and enhances the arrivals from the precursors (Thomas andBillen, 2009).

4. Results

We compare the information from the vespagrams and migrationprofiles to determine the depth and amplitude of PP precursory arrivalsfrom the 410 and 660 km discontinuities. In the vespagrams, the PPprecursors arrive in a slowness window of 6.2–6.9 s/deg, though oftenthe actual arrivals are spread out over slightly larger slowness ranges(Fig. 2). Examples of well-defined precursory arrivals in our migrationsand vespagrams are shown for the events in Figs. 2–5.

We investigate discontinuity structure at several band-pass filtersof the dataset to test for frequency-dependent effects. Higher frequen-cies are contaminated by more noise, but often still contain usefulinformation about short-scale discontinuity structure. We use thebootstrap-resampling algorithm to determine whether migratedamplitudes at higher frequencies are statistically greater than the

background noise in the migration. A standard practice in the analysisof underside reflections is to use amplitudes that fall above the 2σ con-fidence interval (e.g., Flanagan and Shearer, 1998; Gu et al., 1998). Thevalidity of this approach is demonstrated in Fig. 5, where results fordifferent filters of the data are shown. The time domain corners ofthe band-pass filter are given in the top right of each panel. Thoughmore noise is present in the higher frequency results, the bootstrapalgorithm reveals two significant peaks at 400 and 675 km depth inall three band-pass filters. We obtain consistent results for data witha high frequency corner set at 0.1 Hz (10 s), though several eventsproduce clear arrivals in the 6–50 second filter. The filters with cornersat 1–10 s and 3–25 s are too contaminated by noise for further inter-pretation. The frequency analysis reveals several statistically robustpeaks and troughs; these are related to the waveform of the seismicarrivals, such as the depth phase of P410P and negative sidelobe of PP(Fig. 5). Synthetic seismograms generated for each set of event param-eters and station geometries facilitate the identification of these otherarrivals. We only further analyze events with at least a strong P410Pin the vespagram, clear focusing in the slowness backazimuth analysis,and a well-defined arrival in the migration.

The resulting 31 events are detailed in Table 1 and form a nearlylinear profile across the southern Kurile subduction zone (Fig. 1)with a kink at 40°N mirroring the bend of the Sumatran subductionzone source region. The depth of each discontinuity is determinedby finding the maximum amplitude near the predicted depth foreach boundary in the migrated sections. The accuracy of the retrieveddiscontinuity depth is controlled by the fineness of the migrated grid,though the resolution of small-scale features is limited by the migra-tion of long wavelength precursory seismic energy. We found that agrid spacing of 5 km allowed us to determine the peak amplitude

0 10 20 30 40

0

250

500

750

0 10 20 30 40 0 10 20 30 40 0 10 20 30 40

Dep

th (

km)

Distance (deg) Distance (deg)

ak135 data ak135 data

400

675

400

675

0 km

400 km

675 km

a) c)b)

Fig. 3.Migration results for the event 25/02/2008-18:06 and ak135 reflectivity synthetics, with a prominent P660P detection. a) Raypath parallel cross sections through themigrated dataand synthetic amplitudes. Positive amplitudes are blue, negative are red. The orientation of the cross sections and lateral extent of the migration grid is given by the small globe. At thecenter of each panel, we show themigration amplitudes at the predicted reflection point. The black and gray shading are respectively the positive and negative 95% confident amplitudesfrom the bootstrap resampling. b) Raypath perpendicular cross sections; the details are the same as in part (a). c) Horizontal slices through themigrated energy of the event. In all panels,the maximum amplitude is normalized to one to enhance the visibility of the migrated amplitudes.

105N. Schmerr, C. Thomas / Earth and Planetary Science Letters 311 (2011) 101–111

for each precursor and provides a reasonable tradeoff between theapproximate sensitivity of long period P-waves (Chaljub and Tarantola,1997) and efficient computation of the migrated grids.

The cross section in Fig. 6 presents migration results demonstratingthe lateral variation in the discontinuity depth beneath the Kuriles.Owing to the source and array geometry of our dataset, the majorityof our observations are densely clustered in a region spanning1000 km from 43°N to 53°N. Sampling further to the south and northof the subduction zone is sparser but still provides information aboutthe depths of the upper mantle discontinuities every 500 km. Thedepths of the upper mantle discontinuities must be corrected for varia-tions in crustal thickness and lateralmantle heterogeneity as both affectthe resulting depths. We use 1-D raytracing through CRUST 2.0 (Bassinet al., 2000) and MITP08 (Li et al., 2008) to compute correction valuesfor the entire path of PP and the precursors. The resulting correctionsare relatively small (b2 s) compared to the travel time heterogeneityobserved in the depths of the discontinuities.

In the vespagrams and migrated profiles, we observe a clear re-flection from the 410 km discontinuity in 30 of the studied events(Table 1). Beneath the Kuriles, the 410 km discontinuity occurs at adepth of 410±15 km, with the shallowest measurements in thevicinity of the subducting Pacific lithosphere (Fig. 6). A total of 15events produce reflections from the 660 km discontinuity, and with

300

360

4 8

t= -315 s Amax =0.029 Amax =0.004 Ama

4 8

t= -135 s

4

t= -125 s

Bac

kazi

mu

th (

deg

)

Slowness (s

Pdiff P660P PKiKP

Fig. 4. Slowness backazimuth analysis of event 25/02/2008-18:06 for select seismic phases,shown in the upper right of each panel. The theoretical slowness and backazimuth of eachcalculated from ak135.

the majority of 660 detections in the vicinity of the subducting Pacificlithosphere. Topography on the 660 km discontinuity is complex, andin several regions, we detect multiple reflectors in the depth range of620–830 km. We do not detect the 660 km discontinuity in eventssampling beneath the Pacific Ocean. However, from 43°N to 48°N,corresponding to the mantle lithosphere directly beneath thesubducted slab, we detect a laterally continuous reflection at 660–700 km depth (Fig. 6), and a second intermittent and weak reflectorat N800 km depth. In the MTZ region from 48°N to 53°N, associatedwith elevated P-wave velocities in MITP08 (Li et al., 2008), multiplereflectors become apparent, one at 620–650 km depth, and muchdeeper reflector ranging from 700 to 800 km depth. Further to thenorth (towards Siberia), where the slab is no longer present in theMITP08 tomography, we detect no reflection from the 660 kmdiscontinuity.

We also measure the amplitude ratios of the precursory energyrelative to the reference PP phase from the vespagrams (Fig. 7)where precursors are well separated from interfering phases such asPKiKP. The amplitude ratios provide information on the sharpnessand strength of the impedance contrast across each discontinuity(Chambers et al., 2005; Shearer and Flanagan, 1999). Amplituderatios must also be corrected for the effects of geometrical spreadingof the seismic wavefront, source radiation patterns, and anelasticity

x =0.024 Amax =0.009 Amax =0.224

8 4 8

t= -80 s

4 8

t= 0 s

0

Amax

/deg)

P410P PP

including P660P. Amplitudes are normalized to one; the reference amplitude (Amax) isseismic phase is marked with a white cross; relative travel times and slownesses are

0100200300400500600700800900

0100200300400500600700800900

0100200300400500600700800900

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Time(s)

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P410PP660PPP

15-100s

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eg)

10-100s

5

106-50s

a) b)

P400PP675P

PP

pP400P

Depth (km)

Depth (km)

Depth (km)

Fig. 5. Vespagrams andmigration amplitudes for event 25/02/2008-18:06with a prominent P660P detection. a) Vespagrams for three band-pass filters of the dataset. The band-pass filterused is shown in the upper right corner of each panel. Positive amplitudes are black, negative are white. b) Migration profiles of energy at the band-pass filters of the data in (a). Arrowsindicate the transition zone discontinuities. The profiles are located at the predicted underside reflection point for the center of the array and source location. Black and gray shading arerespectively the positive and negative 95% confident amplitudes from the bootstrap resampling.

106 N. Schmerr, C. Thomas / Earth and Planetary Science Letters 311 (2011) 101–111

effects. We correct for these effects by multiplying each measuredamplitude ratio by a correction factor computed from ak135 reflectivitysynthetics, using the PREM (Dziewonski and Anderson, 1981) values ofattenuation for the mantle. We correct our amplitude ratios to a refer-ence distance of 125°; the correction factor is then the dividendbetween the synthetic PdP/PP ratio at 125° and at the epicentraldistance of the source and array. The resulting scaling factor is used tocorrect the measured amplitude ratio from the dataset for the effectsof differential attenuation, source radiation, and geometrical spreading.We do not correct for lateral variations in attenuation structure in themantle; large lateral variations in attenuation of the PP waveform inthe upper mantle would produce correlated amplitude ratios betweenthe 410 and 660, which we do not observe. The average amplituderatio for P410P/PP is 0.0375±0.018, close to the predicted value of0.0365 for the reflection coefficient at a reference epicentral distanceof 125° in ak135. The average amplitude ratio for observations ofP660P/PP is 0.0246±0.021, nearly half that of the predicted value of0.0452 from ak135. However, in the region underlying the slab, theP660P/PP amplitude ratios become comparable to those produced byak135. The deepest reflectors (N700 km) have amplitude ratios thatare below 0.02 and the reflections are weaker to the north.

5. Discussion

Past investigations of discontinuity structure utilizing undersidereflected S-waves in the Kuriles region detect a transition zone 250–260 km thick (Flanagan and Shearer, 1998; Gu et al., 1998; Shearer,

1991; Shearer and Masters, 1992). This thickening is the result of amajor depression on the 660 km discontinuity throughout the region,consistent with the presence of cold, subducted lithosphere in theMTZ. Our results support this hypothesis and allow us to further charac-terize the properties of the subducted lithosphere in the MTZ. Mostnotable is the presence of P660P, a seismic phase absent from globalinvestigations of upper mantle structure. Long-period stacks ofunderside reflected body waves reveal well-developed P410P andS410S arrivals, however, only the S660S is routinely observed andP660P phase is missing in most analyses (Deuss, 2009). Only recentlyhave several studies revealed robust intermittent detections of P660P(Deuss, 2009; Deuss et al., 2006; Thomas and Billen, 2009). Severalpast investigations of P660P amplitude ratios explain the difficulty indetecting this seismic phase, finding that the density of velocity jumpsat 660 km depth are about 50% of their PREM values (Estabrook andKind, 1996; Shearer and Flanagan, 1999). Our results indicate a regionwith enhanced reflectivity at the 660 km discontinuity.

The reflection coefficient for an upward traveling seismic wave isdependent upon the incidence angle and impedance contrast acrossthe discontinuity. A high impedance contrast across a discontinuitywill increase the reflection coefficient. However, the reflection coeffi-cient is also dependent on the incidence angle of an incoming wave.We define incidence angle as the angle between the upgoing raypathand a vertical vector pointing towards the center of the Earth. For agiven impedance contrast and increasing epicentral distance, theincidence angle of an underside reflection increases, approaching thehorizontal for deep reflectors; for P-waves this will lower the reflection

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coefficient owing tomore energy converting to S-waves at the interface.The opposite behavior is present in S-waves, as the incidence angleincreases, the amplitude of the underside reflection will also increase.Thus deep reflectors with weak impedance contrasts are difficult todetect with PP precursors—the impedance contrast must be sufficientlyhigh to offset the effects of the increasing incidence angle.

Constraining the impedance contrast requires knowledge of thesharpness of the discontinuity, a parameter directly related to theshape of the phase loop of the associated mineralogical phase change(Stixrude, 1997). The presence of a broad gradient diffuses seismicenergy and reduces the amplitude of the underside reflections. Highfrequency investigations of the sharpness of the 660 km discontinuityaround the globe with precursors to P′P′ indicate the width of thepost-spinel phase transition is extremely sharp (b5 km) (Benz andVidale, 1993; Xu et al., 2003). The sensitivity of precursory ampli-tudes to gradient width grows at longer periods, thus large changesin the width of the gradient will produce frequency dependentchanges in the precursory amplitudes. To test the effect of gradientwidth, we generated reflectivity synthetic seismograms for a 10-

second dominant period P660P using seismic models with increasinggradient widths at 660 km depth. These tests indicate a gradientwidthN35 km is needed to reduce P660P amplitudes by more than15%, a value inconsistent with the P′P′ observations. We furtherinvestigate frequency dependence in our dataset using 5 differentband pass filter windows with upper and lower corners at 1–10 s,3–25 s, 6–50 s, 10–100 s, and 15–100 s. The amplitudes of P660Pand P410P in the 1–10 and 3–25 second windows were uncon-strained owing to large amounts of noise present in the seismograms.However, there was no systematic variation in precursory amplitudesat the longer period band pass filters. Thus, the presence of largegradients would reduce the observed P660P amplitudes makingthem more difficult to detect, a result inconsistent with the localizedenhancement of P660P amplitudes observed beneath the Kuriles, andthe observations of a globally sharp interface imaged by P′P′ studies.(Xu et al., 2003)

Several mechanisms are potentially responsible for locallyenhancing the impedance contrast across the 660 km discontinuitybeneath the Kuriles subduction zone. Seismic tomography reveals

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high velocity anomalies present in the MTZ beneath the Kurilesregion coincident with our detections of P660P (e.g., Grand, 1994; Liet al., 2008; Miller et al., 2006; van der Hilst, 1995; Widiyantoro andvan der Hilst, 1997). This high velocity anomaly extends across the660 km discontinuity into the lower mantle over a region approxi-mately 500 km wide. We show a schematic interpretation of thisstructure in Fig. 8. Seismic tomography, travel time measurements,past investigations of discontinuity structure, and our result suggestthat the slab stagnates near the Hokkaido bend. In this region wedetect a weak P660P arrival at a depth of 680–750 km. This is consis-tent with the Clapeyron slope of the post-spinel phase transition, aslowered temperatures of the slab increase the depth of the 660 kmdiscontinuity, however, the lowered temperatures alone do notexplain an increase in the impedance contrast at the phase transition,allowing us to detect P660P.

In addition to the thermal heterogeneity introduced by the down-going slab, there will be a significant change in the composition ofmaterials at 660 km depth, in particular the aluminum-content.Transformations in the non-spinel components of the mantle incor-porating Al can contribute significantly to the change in elasticityacross the 660 km discontinuity (Akaogi et al., 2002; Wang et al.,2004). The slab consists of a 10–15 km thick layer of mid-oceanridge basalt (MORB), and 80–90 km of depleted lithosphere andunderlying entrained mantle. The MORB veneer has N15 wt.% Al2O3contained within the garnet phases (Weidner and Wang, 1998). Thealuminum is extracted from the melting of the underlying mantle atthe mid-ocean ridge and concentrated into the crust, leaving an Al-depleted harzburgitic lithosphere. Changes in densities and seismicvelocities for theoretical mineral assemblages present at variousmantle temperatures and Al-content were calculated by Weidnerand Wang (1998) (hereafter referred to as WW98).

We compare our observed discontinuity structure with predictedamplitude ratios calculated using reflectivity synthetic seismogramsgenerated for seismic velocity profiles calculated in WW98 (Fig. 8). Inall cases, the amplitudes predicted by WW98 are significantly less thanak135, but similar to our observed amplitude ratios and we are able toreproduce the pattern of amplitudes ratios in Fig. 8c. It is important tostress we are not attempting to replicate the exact amplitudes ofWW98, but rather the patterns predicted by the mineral physicscalculations for changes in seismic structure. These perturbations in thetemperature and chemical content of the mantle produce observablechanges in impedance contrast and reflectivity. For ambient, higheraluminum, pyrolitic mantle, the seismic structure calculated by WW98(Fig. 8a, model for 1900 K and 5% Al), predicts relatively low P660P/PPamplitudes. A low amplitude arrival would correspond to regions awayfrom subduction where we do not detect P660P.

In the areas of lowered mantle temperatures (1700 K, and varyingaluminum content), the WW98 model predicts two separate disconti-nuities near 660 km depth, a slightly shallower boundary from thetransformation of ilmenite to Mg-perovskite, and the deeper post-spinel phase transition. Synthetic waveforms produced for thesemodels indicate that the energy from this second reflector is nearlycoincident with the P660P arrival, broadening the precursory pulsebut not necessarily detectable as a separate arrival. WW98 predicts aP660P reflector of higher amplitude in colder regions than the referencehigh aluminum pyrolitic model.

For ambient mantle temperatures (1900 K) and Al-depletion (3%),the seismic structure predicted by WW98 produces the highestsynthetic P660P/PP amplitude ratios of any of the models. Aluminumdepletion reduces the amount of garnet present in themodel, increasingthe bulkmodulus contrast across the phase transition and enhancing theimpedance contrast at the 660 km discontinuity (Hirose et al., 1999;Hirose et al., 2005).

To the south andwest of theweak, deep P660Pdetections associatedwith the slab (between 43 and 48 °), the 660 km discontinuity ispresent as a single discontinuity and not directly associated with the

high velocities of the subducting slab, even though it is still relativelydeep (660–690 km). It is in this region that we observe consistentlyhigh P660P amplitudes (Fig. 7).Wepropose this localized enhancementarises from the subduction of Al-depleted harzburgitic lithosphereacross the 660 km discontinuity. In both regions, the appearance ofP660P is consistent with oceanic lithosphere stagnating within thetransition zone.

WW98 also explored the seismic structure predicted for relativelyhighmantle temperatures. Highmantle temperatures lead to the break-down of rw to majorite garnet, perturbing the depth and sharpness ofthe 660 km discontinuity (Deuss, 2009; Deuss et al., 2006; Houser andWilliams, 2010). At temperatures N2100 K, the rw phase decomposesinto majorite garnet, switching the sign of the Clapeyron slope at the660 km discontinuity (Hirose, 2002), causing it to deepen in thepresence of high temperatures. The seismic structure predicted inWW98 for high temperature (2100 K) produces synthetic P660P ampli-tudes lower than those for the pyrolitic model, and thus difficult todetect in the Earth.

It is interesting to note that tomographic imaging of the subductingPacific plate suggests a low velocity anomaly lying directly beneath theKurile slab, leading to the suggestion that high temperature plumematerial is entrained into the transition zone (Honda et al., 2007).This region corresponds with our observations of a deep 660 beneaththe subducting slab at latitudes 43 to 48°. However, as suggested bythe synthetic modeling ofWW98, this feature is not predicted to sharp-en the 660 km discontinuity (e.g., Deuss et al., 2006). Instead, themajorite garnet to Mg-pv phase transition occurs over a much widerdepth interval than the post-spinel phase transition, creating a broadgradient and a weak P660P arrival, inconsistent with our observationof strong P660P arrivals within this region.

In the vicinity of the high velocity slab in the MTZ, we observemultiple weak P660P reflectors (Figs. 6, 7) near 620–630 km depthand near 680–750 km depth. It is likely that the shallower reflectorsare the depth phases of the PP precursors (i.e., pP660P) that stackup as coherent precursory arrivals. In each case we have to calculatethe time delay of the depth phase in relation to the main arrival andthus discriminate between cases of two reflectors and a reflectorand depth phase (see Fig. 6 for an example). If a strong pP660P isvisible, there should also be a strong pP410P phase visible as well.In several bins with multiple P660P observations we also observe anarrival after P410P suggesting the shallower reflector in these eventsis pP660P (indicated in Figs. 6 and 7). Additional arrivals near 520 kmdepth are energy from PKiKP mapped onto the migrated profile.Examination of the associated vespagram and synthetic migrationallows us to readily identify this energy, and is the motivation fornot interpreting any arrivals near 520 km depth. In addition to thevespagrams, the backazimuth slowness analysis further identifiescontaminating PKiKP arrivals. With all these complexities taken intoconsideration, there still appears to be a deep P660P reflector associ-ated with the downgoing Kurile slab.

We also consider that folding and buckling of the slab at theincreased viscosity of the lower mantle would produce alignmentand fabrics within the subducted lithosphere that may locallyenhance the impedance contrast at 660 km depth. Lattice preferredorientations of ringwoodite or shape preferred alignments of crustalmaterials would produce horizontal discontinuities within or in thevicinity of the slab. However, measurements of shear wave splittingbeneath the Kuriles finds that the majority of anisotropy observedin the slab is confined to the uppermost mantle (Fischer and Yang,1994), and it is unclear if the MTZ mineral assemblage is capable ofproducing the required anisotropy for generating underside reflec-tions (Tommasi et al., 2004).

We observe significantly less complex topography on the 410 kmdiscontinuity. Beneath the Sea of Okhotsk, tomography detects loweredmantle velocities, and we find the 410 km discontinuity near 410–425 km. Near the Kurile slab, the 410 is elevated (395 km) consistent

110 N. Schmerr, C. Thomas / Earth and Planetary Science Letters 311 (2011) 101–111

with the presence of cold subducted lithosphere entering theMTZ. In twoevents located near the subducting slab we find evidence for a polarityreversal of the 410 km discontinuity (Table 1). A negative polarityP410P would require an impedance drop with increasing depth. Onepossible explanation is the presence of a metastable wedge of olivinewithin the subducted lithosphere (e.g.,Castle and Creager, 1998), thehigher velocity anddensity ofwadsleyite overlying awedge ofmetastableolivine within the slab would produce the conditions necessary for anegative P410P reflection. However, events sampling near these observa-tions do not detect the strong amplitude and depth variations associatedwith ametastablewedge, and the Fresnel zoneof the PPprecursorswouldmake the detection of such a feature difficult to resolve (Chaljub andTarantola, 1997).

The P410P/PP amplitude ratios to the northwest of the Kurile slabare lower than the ratios observed under the Pacific (Fig. 7). Mantletransition zone mineralogy is capable of storing several weight percentof H2O (Karato, 2006), and subduction processes are an ideal mecha-nism for locally enhancing the hydrogen content of the MTZ (Ohtaniet al., 2004; Richard et al., 2006). Increased hydrogen content reducesthe pressure of the ol-wd phase transition, lowers seismic velocity,and broadens the phase transition stability field (Ohtani and Litasov,2006). The presence of hydrogen in wadsleyite will reduce the reflec-tion coefficient across the 410 and bring the discontinuity to shallowerdepths. Alternatively, buoyant hydrated material residing at the top ofthe transition zone and melting processes would locally enhance theimpedance contrast near the slab and produce deeper reflections(Bercovici and Karato, 2003; Schmerr and Garnero, 2007). We do notdetect strong variations in 410 topography or amplitude ratios, suggest-ing that lowered temperatures are capable of explaining our results,though we cannot rule out small-scale compositional heterogeneity atscale-lengths below our resolution (e.g., Zheng et al., 2007).

6. Conclusions

The detailed topography on the 410 and 660 km discontinuitiesbeneath the Kuriles subduction zone was imaged using array tech-niques applied to the PP precursors. The unprecedented dense stationspacing provided by the HLP seismic array and USArray TransportableArray enabled this approach. Our detailed array analysis showedP410P reflections from the olivine to wadsleyite phase transition at410–425 km depth, perturbed to 395 km depth in the vicinity of thesubducting Pacific lithosphere. The array analysis also revealedP660P reflections in this area from reflectors at 620–800 km depth,associated with the ilmenite to perovskite and ringwoodite to Mg-perovskite+magnesiowüstite phase transitions. This region of ro-bust, deep P660P reflections is coincident with the lowered velocitiesassociated with the subducting Kurile slab, and represents the firstdetailed analysis of P660P arrivals from beneath the Kuriles.

The detection of elevated P660P/PP amplitude ratios indicates thepresence of depleted harzburgitic lithosphere at the base of the transi-tion zone; mineral physical experiments and synthetic seismic model-ing shows depleted materials enhance the 660 km discontinuityimpedance contrast, producing P660P underside reflections. P660P isnot detected in regions situated away from subduction, further support-ing a chemical enhancement of the discontinuity impedance contrast.The observed thermal and chemical heterogeneity visible in both 410and 660 reflections is consistent with cold, depleted lithosphere in theMTZ, stagnating at the 660 km discontinuity beneath the Kuriles sub-duction zone, implying post-spinel boundary can locally impedemantleflow and produce chemical heterogeneity within the transition zone.

Acknowledgments

We are grateful for the constructive comments and suggestionsprovided by the editor Peter Shearer and two anonymous reviewersthat helped to greatly improve the quality of the manuscript. Data

were collected with the Standing Order for Data (Owens et al.,2004) software. We thank the HLP project for providing access totheir data. Data from the TA network were made freely available aspart of the EarthScope USArray facility supported by the NationalScience Foundation. The facilities of the IRIS Data Management Sys-tem, and specifically the IRIS Data Management Center, were usedfor access to waveform, metadata or products required in this study.The IRIS-DMS is funded through the National Science Foundationand specifically the GEO Directorate through the Instrumentationand Facilities Program of the National Science Foundation. Data ana-lyses were performed with the TauP (Crotwell et al., 1999), SeismicHandler (Stammler, 1993), and Seismic Analysis Code (Goldstein etal., 2003) toolkits. Figures were generated using GMT (Wessel andSmith, 1998). N.S. was supported by a Department of TerrestrialMagnetism Postdoctoral Fellowship, and C. T. was supported undergrant DFG1530/2-1.

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Subducted lithosphere beneath the Kuriles from migration of PP precursors1. Introduction2. Dataset3. Method4. Results5. Discussion6. ConclusionsAcknowledgmentsReferences