Systematic changes in the incoming plate structure at the Kuril trench

Gou Fujie,1 Shuichi Kodaira,1 Mikiya Yamashita,1 Takeshi Sato,1

Tsutomu Takahashi,1 and Narumi Takahashi1

Received 24 October 2012; revised 5 December 2012; accepted 8 December 2012; published 16 January 2013.

[1] Recent seismic structural studies in trench-outer rise regionshave shown that Vp within the incoming oceanic platesystematically decreases toward the trench, probably owingto bending and fracturing of the plate. To understandthe mechanisms acting to reduce Vp, Vs is critical because theVp/Vs ratio is a sensitive indicator of lithology, porosity, and thepresence of fluid. In the outer rise region of the Kuril trench,we conducted an extensive seismic refraction and reflectionsurvey that revealed systematic changes in Vp, Vs, and Vp/Vs.Our results suggest that water content within the incomingoceanic plate increases toward the trench accompanied by thedevelopment of bending-related fractures at the top of theoceanic crust, consistent with the seawater percolation. Ourresults support the idea that plate bending and fracturing duringthe bending in the outer rise of the trench play an importantrole in the water cycle of subduction zones. Citation: Fujie, G.,S. Kodaira, M. Yamashita, T. Sato, T. Takahashi, and N. Takahashi(2012), Systematic changes in the incoming plate structure at the Kuriltrench, Geophys. Res. Lett. 40, 88–93, 10.1029/2012GL054340.

1. Introduction

[2] Dehydration processes and the expulsion of the waterfrom the subducting oceanic plate affect various subduction-zone processes, including arc volcanism and generation ofearthquakes and tremor [Hyndman and Peacock, 2003;Yamasaki and Seno, 2003; Tatsumi, 1989]. Although oceanicplates are thought to acquire water through several mechan-isms, including hydrothermal circulation near the spreadingcenters where they form, recent studies of their seismic andthermal structure suggest that most oceanic plate hydration isassociated with faulting related to plate bending in thetrench-outer rise region [Peacock, 2001; Ranero et al., 2003].[3] In the last decade, various seismic refraction and

reflection surveys have been conducted in the trench-outerrise region of the Central and South American subductionzones, where relatively young oceanic plates (about 20Ma)are subducting [Ranero et al., 2003; Ranero and Sallarès,2004; Grevemeyer et al., 2007; Ivandic et al., 2008;Contreras-Reyes et al., 2008a]. These studies showed thatbending-related faults extend from the seafloor into theoceanic upper mantle near the trench and that the compres-sional velocity (Vp) is low in the region where bending-related faults are observed.[4] The Vp reduction near the trench can be explained by

an increase of porosity owing to fracturing, the presence of

free fluids, and a rise in the degree of hydration (serpentini-zation) [e.g., Van Avendonk et al., 2011]. To better under-stand the mechanism of the Vp reduction, the shear wavevelocity (Vs) is critical because the Vp/Vs ratio, which isdirectly related to the Poisson’s ratio, is a sensitive indicatorof lithology, porosity, and the presence of fluid [Christensen,1996; Takei, 2002]. However, systematic changes in Vs

before subduction are not well constrained.[5] To investigate structural changes in an incoming plate,

focusing on its water content, we conducted an extensiveseismic refraction and reflection survey in the northwesternPacific margin, where an old part of the Pacific plate, datingfrom about 130Ma, is subducting beneath the Okhotsk plateat the Kuril trench [Müller et al. 2008]. We present modelsof Vp, Vs, and Vp/Vs derived by traveltime inversion and dis-cuss the systematic changes in the structure of the incomingplate as it approaches the subduction zone.

2. Survey and data

[6] In 2009 and 2010, we established a 500 km longseismic profile extending from 10 km south of the Kuriltrench to well seaward of the outer rise (Figure 1). Thecrustal magnetic lineations (tectonic spreading fabric) issub-parallel to the Kuril trench, and the structural variationalong the trench axis is expected to be weak here, althoughthe magnetic lineation in the southern area (where distancealong the profile x> 350 km) is obscured by pervasivevolcanism and normal faulting [Kobayashi et al., 1998].[7] We deployed 80 ocean bottom seismometers (OBSs)

along the profile at intervals of 6 km and fired a large airgun array (total volume 7200 cubic inches) from the R/VKairei at 200-m intervals for OBSs and at 50-m intervalsfor a 444-channel, 6-km-long hydrophone streamer. Wedeviated from the survey line around site 60 to avoid a groupof fishing boats. Therefore, shots between x = 352 and372 km have been excluded from the structural analysis.[8] In the time-migrated reflection data (Figure 2a), we

observed two prominent horizons interpreted as the top ofthe oceanic crust (“basement” hereafter) and the oceanicMoho discontinuity. Both horizons are smooth in the centralpart of the profile (80< x< 350 km) but rough or obscuredat both ends. The onset of horst-and-graben structure is atx� 80 km, and the fault throws gradually increase towardthe trench, as is commonly observed in this region [e.g.,Kobayashi et al., 1998; Tsuru et al., 2000].[9] Our OBSs were equipped with one vertical and two

horizontal-component geophones. On the vertical recordsections (Figures 2b–2d), we identified crustal refractions(Pg), Moho reflections (PmP), and uppermost mantle refrac-tions (Pn). The apparent velocity of the Pn refractions wasabout 8.5 km/s, which is significantly higher than the Vp oftypical oceanic mantle [White et al., 1992] but consistent

1IFREE/JAMSTEC, Yokohama, Kanagawa, Japan.

Corresponding author: Gou Fujie, IFREE/JAMSTEC, Showamachi3173-25, Kanazawa-ku, Yokohama, Kanagawa, Japan. (fujie@jamstec.go.jp)

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GEOPHYSICAL RESEARCH LETTERS, VOL. 40, 88–93, doi:10.1029/2012GL054340, 2013

with the Vp in the northwestern Pacific basin derived from aborehole study [Shinohara et al., 2008] and an active-sourceseismic structure study [Oikawa et al., 2010] in which Vp

showed strong anisotropy and was highest along the spread-ing direction.[10] To utilize the horizontal data, we calculated the radial

and transverse components by the polarization of the directwater arrivals of air gun shots. The S-wave energy should berecorded mainly on the radial component if the across-profile(along-trench) structure variation is small. Therefore, we usedonly the radial component for the Vs structural analysis.[11] As air guns in the water generally emit only P-waves,

observed shear waves were P to S (PS) converted waves.The PS converted waves are of two types, PPS and PSS.The PPS is a PS conversion of the ascending ray and hasalmost the same apparent velocity as P. The PSS is a PSconversion of the descending ray and has roughly 1=

ffiffiffi

3p

the apparent velocity of P. We observed both PPS and PSScrustal refractions with good quality (Figures 2e–2i).

3. Vp Structure

[12] To model the Vp structure along our profile, weadopted a tomographic inversion of traveltimes, whichsimultaneously determines Vp and layer boundaries [Fujieet al., 2002; Fujie et al., 2006]. At first, to clarify the lateralstructural variation objectively, we used wide-angle firstarrivals and two-way reflection times from the basementand adopted a starting model consisting of three layers thatwere almost uniform laterally (Figure 3a). The top layerwas seawater with a uniform velocity of 1.5 km/s. The

seafloor topography was derived from the multi-narrowbeam bathymetric data obtained in our survey and held fixedduring inversion. The second layer was the sediments, inwhich the velocity field was defined in terms of the veloci-ties at the top and bottom of the layer, which were distrib-uted along the profile at nodes spaced 3 km apart. The initialvalue of Vp were 1.6 km/s at the top and 2.5 km/s at thebottom, and the thickness was calculated from the two-wayreflection times from the basement. The basement was repre-sented by nodes with 2-km spacing with one degree of free-dom in the vertical direction. The third layer (below thebasement) was the oceanic crust and mantle, and its velocityfield was represented by a regular grid with nodes spaced3 km (horizontal) and 0.5 km (vertical) apart.[13] Using 62,260 wide-angle first arrivals and 1443

multichannel seismic two-way times, we simultaneouslydetermined seismic velocities and the depth of the basement(Figure 3b). During the inversion, the root mean square(RMS) of traveltime residuals was reduced from 663 to29.3 ms. To illustrate the structural changes within theincoming plate, we calculated Vp perturbations below thebasement with respect to an average Vp, which shows thatthe crustal Vp becomes lower toward the trench (Figure 3h)[14] As first arrivals alone are insufficient to resolve the

trade-off between crustal Vp and crustal thickness, we con-ducted another traveltime inversion to better constrain thelower crust and the upper most mantle by adding Mohoreflections. The second starting model was basically thesame as the initial model, but the bottom layer was dividedinto crust and mantle by incorporating the Moho (Figure 3c).Adding both 11,323 wide-angle PmP and 766 normal-incident Moho reflections, we obtained a four-layer Vp

model by traveltime inversion (Figure 3d). The RMS oftraveltime residuals was reduced from 177 to 40.2 ms.[15] To evaluate the reliability of the model, we applied a

checkerboard resolution test (CRT) to evaluate the spatialresolution of velocity perturbation, although the CRT isnot a tool for evaluating uncertainties. The checkerboardpatterns were well recovered down to depths a fewkilometers below the Moho (Figure 3e), which means thatthe lateral Vp perturbations within the oceanic crust and up-permost mantle are reliable at the scale of the checkerboardpattern. Comparing the three-layer and four-layer models,Vp perturbations within the upper crust (about 0–2 km belowthe basement) were very similar to each other, but thosewithin the deeper parts were somewhat different (Figures 3hand 3i). This is probably because Moho reflections gave ad-ditional constraints on the four-layer model, and it suggeststhat Moho reflections are indispensable for discussing thedetailed structure in the lower crust.[16] The Vp perturbations indicate that the seismic struc-

ture differs markedly between the abyssal plain (x> 350km) and the northern area (x< 350 km). The typical Vp atthe top of the oceanic crust is about 5.4 km/s in the northernarea and 5.7 km/s in the abyssal plains. The abyssal plainsformed near the spreading center by pervasive volcanismand normal faulting, and their formation affects the oceaniccrust and mantle structure. Therefore, we infer that thedifference around x= 350 km is related to the formation ofabyssal plains and not related to structural changes relatedto impending subduction. Hereafter, we confine our discus-sion to the northern area to investigate the structural changesin the incoming plate prior to subduction.

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[17] In the northern area, the upper crustal Vp begins todecrease at around the center of the outer rise bulge(x= 190 km). On the other hand, the Vp decrease for thelower crust and mantle begins at the edge of the horst-and-graben structure (x = 80 km), suggesting systematic structurechanges similar to those in the Central and South Americansubduction zones. To better understand the mechanism ofthe Vp reduction, Vs is critical.

4. Vs and Vp/Vs ratio structures

[18] Structural modeling based on marine Vs is usuallydone in three steps: (1) Vp modeling, (2) Vs modeling abovethe PS conversion interface using PPS, and (3) Vs modelingbelow the conversion interface using PSS [e.g.,Mjelde et al.,2003; Contreras-Reyes et al., 2008b]. To determine Vs by atraveltime inversion, we extended our traveltime inversionmethod by implementing a calculation method for PPS andPSS phases [Fujie et al. 2003], which is robust even incomplicated Vs models. During steps 2 and 3, Vp and layerboundaries were held fixed. In addition, in the final step,Vs above the conversion interface was also fixed.[19] We observed PPS crustal refractions and PSS crustal

refractions (Sg) of good quality at most sites, but we did

not observe mantle refractions (Sn) and Moho reflectionsat many sites. We thus focused on Vs within the upper crustand adopted a three-layer model parameterization. The typi-cal time delay between PPS and P waves was 2.0 s, and thetypical two-way time within the sediments was 0.6 s,suggesting that the Vp/Vs ratio within the sediments wasroughly 8.0. This estimation is reasonable for Vp/Vs ratioswithin sediments on oceanic plates [e.g., Shinohara et al.,2008]. The apparent velocity of crustal refractions Pg andSg suggested that the Vp/Vs ratio within the oceanic crustwas roughly 1.75. On the basis of these estimates, weconstructed a starting Vs model from the tomographic Vp

model of Figure 3b.[20] We picked 13,888 PPS crustal refractions and 16,755

PSS refractions (Sg) assuming that the conversion interfacewas the basement, as the basement was by far the mostprominent discontinuity in both the multichannel seismicsection and the Vp models. Using these picks, we obtaineda tomographic Vs model after applying inversion steps 2and 3 (Figure 3f). The third inversion step reduced theRMS of the traveltime residuals from 244 to 44.9 ms. TheCRT for the final Vs inversion step showed that the lateralVs variation within the upper crust was reliable at the scaleof the checkerboard pattern (Figure 3g).

Figure 2. Obtained seismic data. (a) Time-migrated multichannel seismic reflection section. (b,c,d) Examples of OBSvertical component data reduced by 8 km/s. (e,f,g) OBS radial component data reduced by 8 km/s. (h,i,j) OBS radialcomponent data reduced by 4.62 km/s. A 5–20Hz band-pass filter was applied to these OBS sections. See the AuxiliaryMaterial for more sections.

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[21] Similar to Vp, the upper crustal Vs, approximately 3.0km/s at the top of the oceanic crust, begins to decrease ataround x = 190 km (Figure 3j). In contrast, the Vp/Vs ratio,a sensitive indicator of water content, sharply changes atx= 140 km, implying a change in the water content withinthe upper crust (Figure 3k).

5. Discussion and Conclusion

[22] Our seismic velocity models derived by traveltimeinversion show systematic structural changes toward thetrench (Figure 3). First, Vp and Vs within the oceanic uppercrust begin to decrease roughly at the center of the outer risebulge (x= 190 km), probably owing to the increase infracture porosity caused by bending-related fracturing.[23] The Vp/Vs ratio at the top of the oceanic crust is about

1.85 and almost constant to the south of x = 140 km butbegins to increase from there. The Vp/Vs ratio dependsstrongly on water content and the aspect ratio of cracks. Asthe water content increases at the depth of the oceanic crust,the Vp/Vs ratio increases when the aspect ratio is smallerthan � 0.03 but decreases when the aspect ratio is larger[Takei, 2002]. Considering the small degree of curvature ofthe incoming plate in the outer rise, it is reasonable to

hypothesize that bending-related extensional stress createscracks with small aspect ratio (thin cracks) here [Ivandicet al., 2008]. Therefore, we propose that the water contentwithin the upper crust increases toward the trench, startingfrom x = 140 km.[24] The seismic reflection section around x = 140 km pro-

vides a clue to the mechanism of the change in the watercontent (Figure 4). The reflection from the basement issmooth and continuous to the south of x = 140 km,suggesting that the basement is little fractured, but roughand discontinuous to the north of x= 140 km, suggesting thatthe basement is pervasively fractured. We suggest that northof 140 km well-developed fractures enable water to perco-late from the bottom of the sediment into the top of theoceanic crust. South of 140 km, the small plate curvatureprobably creates the bending-related small cracks withinthe oceanic upper crust but does not cause pervasive fractur-ing at the basement. Thus, here water cannot penetrate thebasement, and the crustal water content does not changesouth of 140 km despite crustal cracking.[25] These structural changes are confined to the upper

crust until the onset of horst-and-graben structure (x< 80km), where Vp within the lower crust and uppermost mantlebecomes lower. Because the horst-and-graben structure is

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model and CRT of four-layer model parameterization. The black line in Figure 3e represents the Moho. (f,g) Vs of three-layermodel. (h,i,j) Velocity perturbations below basement. (k) Vp/Vs ratio derived from Figures 3b and 3f. Shaded areas indicateareas that are not sampled by raypaths. The contour intervals are 0.5 km/s in Vp models, 0.289 km/s in Vs models, 0.02 in theVp/Vs model, and 3% in velocity perturbations. Vertical exaggeration is 8 : 1 except in (k), where it is 15:1.

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thought to develop as a result of recurrent bending-relatednormal faulting that ruptures the entire oceanic crust andreaches deep into the oceanic mantle, the reduction of Vp

within the lower crust and uppermost mantle coincides withthe extent of the bending-related normal faulting [e.g.,Ranero et al., 2003; Ivandic et al., 2008; Contreras-Reyeset al., 2008a]. Although we could not investigate structuralchanges around the trench axis owing to the limitation ofour OBSs applicable water depth, we would expect largerstructure changes toward the trench axis.[26] The low-Vp mantle may represent serpentinization.

Mantle serpentinization would be promoted if bending-related normal faults act as conduits for water to percolatefrom the upper crust into the mantle [e.g., Ranero et al.,2003; Van Avendonk et al., 2011]. If all of the Vp reductionwithin the mantle can be attributed to serpentinization, thelargest amount of serpentinization is estimated to be roughly20% [Christensen, 2004], and this estimation is roughlyequal to those estimated in Central American subductionzone [Ivandic et al., 2010; Van Avendonk et al., 2011]. Toverify this possibility, the mantle Vs, which we could notconstrain in this study, is a key. In the trench-outer riseregion off Chile, Contreras-Reyes et al. [2008b] observedS-wave mantle refractions (Sn) of good quality and con-firmed the presence of low-Vs mantle along a trench-parallelwide-angle seismic survey line, but they could not observeSn along an across-strike seismic survey line like that ofour survey. Therefore, a trench-parallel seismic refractionsurvey in the Kuril trench might yield effective constraintson the mantle Vs and on the degree of serpentinization.[27] The most significant feature of our structure models is

the lateral variation in the Vp/Vs ratio within the upper crust,suggesting an increase in water content toward the trench.Our results support the idea that structural changes as theincoming plate approaches the subduction zone have aprofound impact on the water cycle in subduction zones.[28] The structural changes in our Vp model in the Kuril

trench, where 130Ma old lithosphere is subducting, aresimilar to those in the Central and South American subduc-tion trenches (15–50Ma) and in the Tonga trench (80Ma)[e.g., Ranero and Sallarès, 2004; Ivandic et al., 2008;Contreras-Reyes et al., 2008a; Contreras-Reyes et al.,2011]. This similarity suggests that the structural changesthat precede subduction do not depend much on plate agebut are likely controlled by bending-related faulting andfracturing. Because bending-related normal faulting isobserved at many subduction zones [e.g., Masson, 1991],

we expect that systematic structural changes before subduc-tion, including water percolation, may be shown to be com-mon features in those subduction zones.

[29] Acknowledgement. We appreciate Harm Van Avendonk and ananonymous reviewer for useful and constructive comments, which helpedimprove this manuscript.

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