A complex 660 km discontinuity beneath northeast China

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1 R Available online at Earth and Planetary Science Letters 212 (2003) 63^71 A complex 660 km discontinuity beneath northeast China Yinshuang Ai, Tianyu Zheng, Weiwei Xu, Yumei He, Dan Dong Institute of Geology and Geophysics, Chinese Academy of Sciences, Deshengmenwai, Qijiahuozi, Chaoyang District, Beijing , PR China Received 14 November 2002; received in revised form 6 May 2003; accepted 15 May 2003 Abstract We present a detailed seismic study of the 660 km discontinuity beneath northeast China using the receiver function technique. We use seismic data collected from 24 broadband stations in northeast China. Analysis of these data shows that the 660 km discontinuity is locally depressed in the region from longitude E to E and latitude 40.0 N to 44.0 N, and then it splits into multiple discontinuities in the surrounding regions. The complexity of the 660 km discontinuity beneath the subduction zones is probably attributable to the interaction between the upper mantle and subducted slab. We speculate that the flattening of the subducted lithosphere near the bottom of the upper mantle causes the multiple discontinuity structure, and that the slab penetrating the 660 km discontinuity into the lower mantle causes the narrow 660 km depression area. ß 2003 Elsevier Science B.V. All rights reserved. Keywords: the 660 km discontinuity; receiver functions; slab; phase change; northeast China 1. Introduction The 660 km discontinuity (hereafter called the 660) is very important for studying the dynamics of the Earth and mantle mineralogy, especially in a region where mantle plumes or subducted plates are located. The Japan trench, one of the oldest active subduction zones in the world, is an ideal location to study the interaction between subducting slab and upper mantle discontinuity. Compared with other subduction zones, the Japan subduction zone has a lower slab dip (30 ), with * Corresponding author. Tel.: ; Fax: address: ysai@mail.igcas.ac.cn (Y. Ai). relative velocity of about 90 mm/yr at the trench. Based on magnetic anomalies and the magnetic time scale, the Japan subduction zone has an age of about 130 Myr since lithosphere began entering the trench. Starting 100 Myr ago, the Paci c plate has increased in size with continued spreading along its margins and has moved farther north so that the western end of the Kula^ Paci c ridge was about to be subducted beneath the Asian margin in the vicinity of Japan [1]. The Sea of Japan probably started to open during the Late Cretaceous or Early Tertiary as a result of the Kula^Paci c ridge subduction [2]. On the basis of thermodynamic studies of the phase change at 660 km depth and by xing the mantle temperature, Deal and Nolet [3] obtained a thickness of the Japan slab of about 85 km, with potential mantle temperature of 1180 C X / 03 / $ ^ see front matter ß 2003 Elsevier Science B.V. All rights reserved. doi: /s x(03)

2 64 Y. Ai et al. / Earth and Planetary Science Letters 212 (2003) 63^71 Beneath northeast China, the Paci c plate is being subducted towards the northwest along the Japan Trench, and anomalies at the 660 may be present due to plate and mantle interaction. Experimental results from mineral physics studies suggest that, instead of a single discontinuity, multiple discontinuities may exist around 660 km depth in the subduction zones [4]. In this case, multiple discontinuities represent three phase changes, garnet to ilmenite, the breakdown of Q-spinel, and ilmenite to perovskite. This subduction zone mineralogy is also intimately related to temperature in the subduction zone. The e ect of the subducting Paci c slabs beneath northeast China has been debated for a long time. Previous studies using a variety of methods have found very complicated structures at the bottom of the upper mantle transition zone there, but the results about the 660 are still controversial. Recent tomographic studies by Fukao et al. [5] suggest that subducted slabs entering into the transition region tend to be stagnant rst and then spread subhorizontally at the bottom of the transition zone beneath northeast China. These tomographic results are consistent with the observations of SS precursors [6]. If these results are correct, a broad depression of the endothermic 660 phase boundary would be expected [7], and this depression might be observed with short and long period seismic data [8]. This broad 660 depression is also supported by a study using ScS reverberations [9]. However, previous SS studies were limited in resolution due to the large Fresnel zone of the SS precursors, and a several hundred kilometer de ection of the 660 discontinuity within a subducted slab might not be well imaged by SS precursors. On the other hand, tomographic results from Bijwaard et al. [10] show that the high velocity anomalies beneath Japan continue all the way down to the core^mantle boundary. They interpret this as evidence that subducted slabs probably penetrate the 660 beneath northeast China. If slabs penetrate the 660, a narrow band depression area would be found instead of a broad depression of the 660 phase boundary. Niu and Kawakatsu [11,12] used P-to-S converted phases to determine the absolute depth of the upper mantle discontinuities beneath northeast China, based on seven events from station MDJ. They report a multiple discontinuity structure rather than a single depressed 660 km discontinuity. Moreover, Li [8] studied the upper mantle structure beneath stations MDJ and HIA using the receiver function migration technique [13]. She found a depression area on the bottom of the 660 discontinuity but could not constrain it due to lack of more useful data. The transition region beneath northeast China was not well constrained in the past due to the lack of seismic stations there. However, coverage has improved rapidly in recent years with the installation of seismic stations by the NE China Seismic Experiment-Changbai and the Chinese National Digital Seismic Network (CNDSN). Here we use these newly available seismic data and perform a detailed study of the 660 km discontinuity beneath northeast China. We employ the receiver function technique, which uses converted waves at the upper mantle discontinuities, to study the 660. The density of the seismic data allows us to stack many receiver functions that have the same piercing points at the 660 and to identify with certainty the converted energy from the upper mantle discontinuities. The seismic coverage also allows us to study in detail local variations of the 660 in the presence of a subducted slab. Our stacking results indicate that the 660 is locally depressed, splitting into multiple seismic discontinuities beneath northeast China. We discuss data and method, seismic results, and interpretations in the following sections. 2. Data and method 2.1. Data selection The seismic data used are from one GSN permanent station (MDJ), 19 NE China Seismic Experiment-Changbai stations and four CNDSN broadband stations (Fig. 1). MDJ started recording in More than 1000 teleseismic events with magnitude greater than 5.8 have been recorded by that station. In the NE China Seismic Experiment, 19 stations with Guralp 30T sensors and six-channel Passcal recorders began working

3 Y. Ai et al. / Earth and Planetary Science Letters 212 (2003) 63^71 65 Fig. 1. Studied region in northeast China, along with two latitudinal stacking lines AAP, BBP (AAP along N, BBP along N), and longitudinal stacking lines CCP, DDP (CCP along E, DDP along E). The results of stacking receiver functions along the lines AAP, BBP, CCP, DDP are shown in Fig. 3. Symbols represent seismic stations (triangles) and the piercing points of converted S wave at the 410 km depth (circles) and at the 660 km depth (crosses). Contours of the subducting slab depth (km) are from [14]. between late June and mid-september, Ten of them were in place until April The four broadband stations from the CNDSN are DL2 (Dalian), SNY (Shenyang), CN2 (Changchun) and BNX (Binxian), whose sensors and recording system were made in China. Those four stations have been continuously recording since September We selected seismic observations in the distant range between 30 and 90 for earthquakes with magnitude greater than 5.8. For the four CNDSN stations, we were able to use recordings of earthquakes with magnitude greater than 5.5 thanks to the good conditions of the station site. Recordings at distances closer than 30 are complicated by upper mantle triplications and interference of PP phases, and P waves start to di ract along the core^mantle boundary after 90. We selected records with clear P waves and high signal-to-noise ratios. The selected events constitute a reasonable azimuthal coverage for the studied region (Fig. 2) Receiver function A receiver function, calculated by the deconvolution of the vertical component from the radial component for a three-component teleseismic P waveform, can be simpli ed as [15,16]: Hðf Þ¼Yðf Þ=Xðf Þ ð1þ where X(f) is the spectrum of vertical components, Y(f) is the spectrum of radial components, and H(f) is the receiver function. Straightforward spectrum division is sometimes unstable due to the trough spectrum of X(f). Wiener deconvolution (i.e., time-domain deconvolution) is thus introduced to solve the problem

4 66 Y. Ai et al. / Earth and Planetary Science Letters 212 (2003) 63^71 Fig. 2. Distribution of all events used. Triangle indicates approximate location of the seismic stations. maximum entropy deconvolution method [18]. Seismic observations are windowed from 20 s before to 100 s after the P wave arrivals. A 2.5 Hz Gaussian parameter is used to extract receiver functions from three-component broadband seismograms. After deconvolution, a second-order zero phase Butterworth bandpass lter with corner frequencies 0.03 Hz to 0.3 Hz is applied to all of the receiver functions. Table 1 lists the number of receiver functions and station information. Due to the weak converted phases present in the receiver functions, we stacked many receiver functions in order to produce an image of the upper mantle discontinuities. We used a common conversion point stacking method, similar to that used by Dueker et al. [19,20]. The delay time of the converted phase relative to the direct P wave, however, depends on both the epicentral distance and the depth of the discontinuity. A move-out correction is required to account for this travel [17]. The basic principle of Wiener deconvolution is to minimize the L2 errors between actual and expected output for the Wiener lter. The Wiener lter coe cients can thus be solved by Toeplitz equation: ½RŠ½HŠ T ¼½CŠ T ð2þ where [R] is the autocorrelation matrix of input x(t), [H] T is the vector of Wiener lter coe cients (i.e., receiver function in our paper), and [C] T is the cross-correlation vector between input x(t) and output y(t). The receiver function obtained by Wiener deconvolution depends on both autocorrelation and cross-correlation coe cients. Both are calculated in a limited time window under the assumption that the data outside the window are zero. The spectrum resolution of the receiver function is limited by this assumption. Thus, the maximum entropy process is introduced to calculate both autocorrelation and cross-correlation coe cients in order to improve the resolution and precision of the spectrum [17]. From the above dataset, a total of 987 high quality receiver functions are obtained by the Table 1 Station information and number of receiver functions (RF) used Station Latitude Longitude Number Network of RF ( ) ( ) BACH PASSCAL CANY PASSCAL CBAI PASSCAL CH2A PASSCAL DAND PASSCAL DHAA PASSCAL DRAG PASSCAL FROG PASSCAL JANA PASSCAL JILA PASSCAL JIYU PASSCAL LIAO PASSCAL LING PASSCAL MANG PASSCAL PSIA PASSCAL THAA PASSCAL TILL PASSCAL WUSU PASSCAL YABA PASSCAL BNX CNDSN CN CNDSN DL CNDSN SNY CNDSN MDJ GSN

5 Y. Ai et al. / Earth and Planetary Science Letters 212 (2003) 63^71 67 time dependence on the epicentral distance so that the depth of the discontinuity before receiver functions can be stacked. We stacked receiver functions after correcting for this distance and depth dependence for each receiver function. Ray tracing was used to calculate the delay time of converted phase relative to the direct P wave for each receiver function. We chose two assumed depths for mantle discontinuities: 410 km and 660 km. We also calculated the piercing points location for each receiver function at two assumed depths. In the calculation, the IASP91 velocity model [21] was used. The delay time of the converted S phase relative to the direct P wave in a attened Earth is [19]: T Pds ðpþ ¼ Z 0 3D hpffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffii V s ðzþ 32 3p 2 3 ðzþ3p2 dz ð3þ V 32 p where p is the P wave ray parameter in s/km and D is the depth of the discontinuity. The move-out corrections were carried out by adjusting the delay time predicted by Eq. 3 of each individual receiver function to the delay time predicted at the average epicentral distance of all the receiver functions used for stacking. The delay time at the average epicentral distance for all receiver functions is: T Pds ðp ave Þ¼ Z 0 hpffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffii V s ðzþ 32 3p 2 ave3 V 32 p ðzþ3p2 ave dz 3D ð4þ where p ave is the P wave ray parameter in s/km at the average epicentral distance and D is the depth of the discontinuity, so that the Pds move-out is de ned as: NT Pds ðpþ ¼T Pds ðpþ3t Pds ðp ave Þ ð5þ After these move-out corrections, we converted each receiver function from the time domain to the depth domain based on ray tracing. We binned the receiver functions according to their converted S ray piercing points at the two assumed depths, 410 km and 660 km, and stacked the receiver functions in each bin. The distributions of converted S wave ray piercing points at 410 km and 660 km are illustrated in Fig. 1. For this study, we used circle bins with 100 km radius, 20 km bin spacing. In this case, adjacent bins overlap by 180 km along the stacking line. Data from at least four stations and a minimum of 20 traces in each bin can be stacked. 3. Results We have selected four lines (Fig. 1) to stack receiver functions for imaging the 660 km discontinuity. The converted energy at the 660 can be traced well along line AAP (Fig. 3). From west to east along this line, the 660 exhibits three di erent characteristics. It changes depths from around 670^680 km between and in the rst section to about 690 km between and in the second section. The second section is located exactly at the tip of the subducting slab beneath northeast China, where it splits into two separate discontinuities, at depths of 665 km and 715 km respectively, east of longitude In addition, there is also a continuous phase converted at depths between 550 km and 600 km from longitude 127 to the east, which may be related to conversion in the slab. The 660 also exhibits a complex feature along line BBP (Fig. 3). It is split into two discontinuities between longitudes and 127.5, with an upper one around 660^670 km and a lower one around 680^690 km disappearing around longitude The discontinuities become a single discontinuity depressed to 695 km between longitudes and It then splits again into two discontinuities east of longitude 130.3, at depths of about 665 km and 715 km respectively. The energy converted from the top discontinuity is strong. We also nd a continuous phase converted at depths between 550 km and 600 km, which may be related to the conversion in the slab. The same complex feature of the 660 is also observed along two longitudinal lines CCP and DDP (Fig. 3). Along line CCP, between latitudes 40.5 and 42.0, the 660 is split into two discontinuities with depths about 670 km and 690 km, respectively. The lower one has weak converted phases. These two discontinuities merge to one

6 68 Y. Ai et al. / Earth and Planetary Science Letters 212 (2003) 63^71 Fig. 3. The stacking receiver functions for line AAP to line DDP. Green dots are earthquakes that occurred within 35^45 N (M s 4.0) from 1973 to present with depths between 500 km and 700 km. Positive polarity energy is plotted in red. At depths between 310 and 540 km and 541 and 800 km, we stack all the receiver functions whose piercing points at 410 km and at 660 km are within the same bin, respectively. Character sp represents a dissociation of Q-spinel, gt-il represents the reaction from garnet to ilmenite, and il-pv represents the reaction from ilmenite to perovskite. around latitude The second section is located at latitude 42.0 to 43.8, where there is only one obvious discontinuity located about 680 km, with stronger converted energy. The third section in this line is located north of latitude Three separate discontinuities are visible at depths around 660 km, 710 km and 760 km, respectively. Similarly to line CCP, the 660 km along line DDP also exhibits complex behaviors. Only one discontinuity is present at a depth of about 680 km between latitudes 40.2 and It becomes depressed to a depth about 695 km from 40.9 to 43.6 and then it splits into two visible discontinuities around 665 km and 690 km from 43.6 to We constructed a detailed map of the characteristics of the 660 beneath northeast China by stacking the receiver functions within bins with a radius of 100 km. Only the bins with more than 20 receiver functions are used in the analysis.

7 Y. Ai et al. / Earth and Planetary Science Letters 212 (2003) 63^71 69 Fig. 4. Illustration of the 660 km discontinuity structures beneath northeast China. The red area represents the depression zone with one discontinuity greater than 680 km, the green area represents the multiple seismic discontinuities zone, and the blue area represents only one discontinuity at less than 680 km depth. The white area is an uncertainty region due to very few stacking receiver functions in each bin or with more random noise in the stacked receiver functions. The green line is a political boundary. Contours of the subducting slab depth (km) are from [14]. A simple illustration of stacked results for the discontinuities around the 660 is shown in Fig. 4. The results suggest that the 660 is depressed in a narrow zone, located at the tip of the subducting slab (with depths around 690^700 km). We also nd that there are multiple discontinuities at the bottom of a transition zone surrounding the depression region. In the west about 200 km away from the depression zone, only one discontinuity at depth about 665 km is found. We deduce that the subducting slab has little e ect on this region. The receiver function technique has frequently been used for studying upper mantle discontinuities in recent years, and this technique has advantages in that the results are insensitive to velocity heterogeneities in the source region. In the common conversion point stacking algorithm, moveout corrections are calculated by ray tracing the S-P arrival time between the upper mantle discontinuities and the stations. Therefore, velocity heterogeneity beneath the studied region can cause some errors in stacking results. In the above stacks, we did not consider the e ects of velocity heterogeneity and used the one-dimensional IASP91 velocity model to calculate move-out corrections. To explore image robustness further, we used a bootstrap re-sampling method [22,23] to assess the robustness of the image. Moreover, to test systematic heterogeneity e ects, we performed some experiments. For example, in constructing cross-section AAP of Fig. 3 we picked up the maximum peak of stacked receiver functions between 620 and 730 km depth in each common converted point bin with 500 times bootstrap re-sampling and stacked receiver functions, which allowed us to assess the mean and standard deviation for each bin. In most cases, the mean is consistent with the result from cross-section AAP, and the standard deviation ranges from 3 to 6 km for each column of bins. To test for systematic small-scale (200^400 km) heterogeneity e ects, we did an experiment on the e ects of using a velocity model with larger perturbation than the model used. In the new model, there is a 300 km lateral and 100 km vertical anomaly layer above the 660. Within this layer, there is a 6% peak to peak V p and V s variation compared with the standard IASP91 model. Also, using cross-section AAP of Fig. 3 for an example, we picked up the maximum peak of stacked receiver functions between 620 and 730 km depth in each common converted point bin with two di erent velocity models. In most cases, the stacking structures have little sensitivity to the velocity model, since the depths of the discontinuities only vary by about 1%. In our region of study, the 660 has about 30 km peak-to-peak variations. We are able to conclude that small-scale heterogeneity can produce some shifts in the computed discontinuity depths. However, this in uence does not greatly a ect the nal results. We also did some tests on the e ect of bandpass ltering of the seismograms. With a bandpass lter with corner frequencies 0.03^0.4 Hz, the converted energy from the 660 is traced well. 4. Discussion Stagnant slabs exist in the transition zone region beneath northeast China. The e ect of stag-

8 70 Y. Ai et al. / Earth and Planetary Science Letters 212 (2003) 63^71 nant slabs on the discontinuities in the transition zone region can be well constrained by studying the variation of the 660. The newly available seismic data signi cantly improve coverage and allow us to map out the lateral variations of the 660 in this region. Overall, we nd a very complicated structure. At the tip of the subducting slab beneath northeast China, only one depressed 660 km discontinuity is found, and a multiple discontinuity structure is found in the surrounding area of the depressed zone. This complex structure is di erent from that of other regions [24,25]. Computation of seismic pro les from mineral physics suggests that there are three reactions occurring between 600 and 750 km depth in subduction zones [4]. The computed pro les along the 1000 K adiabat are consistent with our receiver function stacking results [4]. We can infer that the structure has a much lower temperature at the bottom of transition zone beneath northeast China. The multiple discontinuity structure is a possible result of slab piling up on the discontinuity and the narrow 660 depression zone could imply a penetration of slabs through the discontinuity. In mineral physics, non-olivine components play an important role in explaining the 660 in the subducting zone [4]. From above Fig. 3, we can deduce that the breakdown of Q-spinel to magnesium-perovskite and magnesiowu«stite, the phase change from garnet to ilmenite and the phase change from ilmenite to perovskite all exist in this region. In Fig. 3 we use the symbols sp and gt-il, as well as il-pv, representing the three reactions cited above, respectively. As shown in this gure, the narrow depression area in the study area corresponds to the breakdown of the Q-spinel to magnesium-perovskite and magnesiowu«stite. Northeast of the subducting region, the discontinuity at depths around 650^670 km may represent the phase change from garnet to ilmenite, and the discontinuity at depths about 710^730 km may represent the phase change from ilmenite to perovskite. North of line CCP (Fig. 3), there are three discontinuities with depths about 660 km, 710 km and 760 km. We interpret the above two discontinuities as representing gt-il and il-pv phase changes, respectively. This structure is similar to the result obtained by Niu and Kawakatsu [11]. 5. Conclusions The results of stacking receiver functions demonstrate that the de ection or attening of the subducted lithosphere near the bottom of the upper mantle, as proposed in previous studies in the region, cause a multiple discontinuity structure. They also indicate that the slabs penetrating the 660 km discontinuity into the lower mantle cause the narrower 660 km depression area. The base of the transition zone is not simply a dissociation of Q-spinel, and reactions of non-olivine components such as a reaction from garnet to ilmenite or a reaction from ilmenite to perovskite also occur. We have also found slab images between the depths of 550 and 600 km east of 127.0, although the slab pictures are not very remarkable. Unfortunately, there is a lack of teleseismic data in this region. This condition will be remedied with the installation of national seismic network stations and portable broadband seismic stations in northeast China in the next few years. Acknowledgements This work was supported by the National Science Foundation of China (Grant ). We thank L.X. Wen for helpful discussions and review, Elliot Klein and Francis Wu for proofreading the manuscript. Constructive reviews by Ken Dueker and Eric Sandvol greatly improved the manuscript. We also thank the IRIS Data Management Center for the data, the Chinese National Digital Seismic Network Center in the Institute of Geophysics, Seismological Bureau of China, for the data used from four broadband stations. The GMT software package distributed by Wessel and Smith [26] was used in plotting gures.[rv] References [1] T.W.C. Hilde, S. Uyeda, L. Kroenke, Evolution of the

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