Lateral variations in SH velocity structure of the transition zone beneath Korea and adjacent regions

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117,, doi: /2011jb008900, 2012 Lateral variations in SH velocity structure of the transition zone beneath Korea and adjacent regions Ruiqing Zhang, 1,2 Qingju Wu, 1 Yonghua Li, 1 and Barbara Romanowicz 2 Received 23 September 2011; revised 11 August 2012; accepted 19 August 2012; published 29 September [1] Using seismic profiles of triplicated waveforms, we show significant lateral variations in the SH velocity (Vs) structure of the transition zone (TZ) beneath Korea and adjacent regions. Beneath Sakhalin, we detected a high Vs anomaly (2%) limited to middle regions of the TZ (mid-tz), and a large Vs jump across the 660-km discontinuity. A similar jump in Vs also occurs beneath the northern portion of the North China Craton (NCC). Beneath Korea, a high Vs anomaly (2%) in the lower TZ is inferred, accompanied by a relatively small Vs jump across the 660-km discontinuity, which is depressed by about km. The deep structure under the eastern part of northeast China (NEC) also includes a slight Vs anomaly (1%) in the lower TZ but does not exhibit significant depression of the 660-km discontinuity. Compared with previous study, our observations reveal strong regional variations of the TZ structure on a relatively short scale. These variations most likely reflect the geometrical distribution of the subducting northwest Pacific plate. Our results suggest that the subducting slab dips across the mid-tz under Sakhalin, and becomes flattened atop of the 660-km discontinuity beneath Korea, while only the tip of the slab reaches the lower TZ beneath the NEC. The TZ beneath the NCC does not show evidence of the slab stagnation. Citation: Zhang, R., Q. Wu, Y. Li, and B. Romanowicz (2012), Lateral variations in SH velocity structure of the transition zone beneath Korea and adjacent regions, J. Geophys. Res., 117,, doi: /2011jb Introduction [2] Subduction zones can be regarded as natural laboratories for studying a wide variety of large-scale geophysical processes. Subducting oceanic slabs are a major source of cold, dense material that contributes to drive mantle flow. The interaction between subducting oceanic lithosphere and upper mantle discontinuities is an important area of inquiry due to their implications for convection models of whole mantle flow versus those of convection in separate layers. [3] The northwest (NW) Pacific subduction zone, where the Pacific plate converges into northeastern Asia, is an valuable target for investigating the interactions between the subducting slab and upper mantle discontinuities beneath the continent [e.g., Tajima and Grand, 1998; Li and Yuan, 2003; Huang and Zhao, 2006]. The Pacific Plate moves in a WNW direction relative to Eurasia at a rate of about 8 cm/yr, as estimated from tectonic convergence rates along the Kuril Arc and the Izu-Bonin (or Izu-Ogasawara) Arc [Wei and Seno, 1998]. The subducting slab exhibits a relatively 1 Institute of Geophysics, China Earthquake Administration, Beijing, China. 2 Seismological Laboratory, University of California, Berkeley, California, USA. Corresponding author: R. Zhang, Institute of Geophysics, China Earthquake Administration, No. 5, Minzudaxue South Road, Haidian District, Beijing China. (zrq@cea-igp.ac.cn) American Geophysical Union. All Rights Reserved /12/2011JB complex geometry as shown in Figure 1 [Gudmundsson and Sambridge, 1998]. [4] Studies investigating the seismic velocity anomalies and the topography of the 660-km discontinuity within subduction zones can often detect the structure of the downgoing slab. Given the sensitivity of seismic velocity to mantle temperatures, the relatively cold subducting slab presents itself as a high-velocity anomaly in seismic tomography. Meanwhile, it is generally believed that there is an exothermic phase transition at the 410-km discontinuity and an endothermic phase transition from g-spinel to perovskite and magnesiowüstite at the 660-km discontinuity [e.g., Ito and Takahashi, 1989]. In the presence of a low-temperature element (i.e., the slab), the 410-km and 660-km phase transitions will be uplifted and depressed, respectively, leading to a thickened TZ [e.g., Helffrich, 2000]. An additional exothermic transformation from garnet to perovskite may occur at a depth of about 750 km [e.g., Liu, 1974; Irifune et al., 1996]. Given the low temperature, the garnet-perovskite transformation will occur uplifted, thus creating additional seismic discontinuities at around 660 km depth [e.g., Vacher et al., 1998; Weidner and Wang, 2000]. [5] Seismic studies focusing on subduction in the NW Pacific have elicited lively debate on whether the leading edge of the subducting slab is deflected when it encounters the lower boundary of the TZ, or if it instead penetrates into the lower mantle [e.g., Fukao et al., 2009]. Studies of SS precursors indicate that the TZ under the western Pacific is significantly thicker than the average global TZ thickness, implying a large-scale depression of the 660-km discontinuity 1of13

2 Figure 1. Great circle ray paths from earthquake (red stars) to stations (triangles): (a) for events 1 and 2 (gray lines), 4 and 5 (black lines), and (b) for event 3 (gray and black lines). The corresponding turning points near the 660-km discontinuity mainly sample the regions, Sakhalin (white circles), the northern portion of the NCC (purple circles), Korea (blue circles), and the NEC (yellow square). (c) Ray paths and (d) travel time curves for a source depth of 420 km calculated by the IASP91 model. The AB and CD branches turn above and below the 660-km discontinuity, respectively, while BC is reflected by the discontinuity. The timescale is shown with a velocity reduction of 6.05 km s 1, where d is epicentral distance in km. [Flanagan and Shearer, 1998; Gu and Dziewonski, 2002]. High-resolution, regional traveltime tomography studies show a 250 km thick anomaly of high P wave velocity (Vp) in the TZ beneath the NEC, extending westward to the Daxinganling Mountains [e.g., Lei and Zhao, 2005; Huang and Zhao, 2006]. This high Vp anomaly indicates that the subducting Pacific slab becomes deflected atop of the 660-km discontinuity. Receiver function analysis however has imaged only a narrow zone of 660-km depression beneath the NEC, implying that the slab may locally penetrate into the lower mantle [Ai et al., 2003; Li and Yuan, 2003]. The lateral resolution in SS precursor studies is typically km [e.g., Gu and Dziewonski, 2002], while regional tomography can provide resolutions of 100 to 150 km [Huang and Zhao, 2006]. Migrated receiver function analysis for a desirable interstation spacing of 50 km can provide resolutions on the order of tens of kilometers [Chen et al., 2005]. [6] Studies of seismic triplications related to the geometry of the subducting NW Pacific slab have yielded inconsistent results. Tajima and Grand [1998] detected high Vp anomalies in the lower TZ beneath NW Japan, west of Korea and the Bohai Sea. The anomalies are accompanied by a localized 30 km depression of the 660-km discontinuity in certain areas. Greater station coverage has facilitated assembly of seismic profiles with observations sampling various depths of the TZ. Using data collected by the Chinese national network, Y. Wang et al. [2006] inferred a depth of 730 km for the 660-km discontinuity beneath northeast Asia, but without a corresponding high Vs anomaly. Wang and Chen [2009] reported high Vp and Vs anomalies in the TZ of the 2of13

3 Table 1. Deep Seismic Events Used for Constructing Seismic Profiles Event Origin Date Origin Time Latitude ( N) Longitude ( E) Magnitude Depth (km) 1 7 Nov : Dec : Jul : /477 a /466 b 4 9 Mar : May : /520 c /519 d a Values in Y. Wang et al. [2006]. b Values in Wang and Chen [2009]. c Values in Wang and Niu [2010]. d Values in Ye et al. [2011]. subduction zone beneath Japan, which contrasted distinctively low anomalies detected beneath the Kuril Islands. Recent studies have shown a broadened 660-km discontinuity both in Vp and Vs models beneath the NEC [Wang and Niu, 2010; Ye et al., 2011], rather than the sharp velocity contrasts suggested before. Uncertainty surrounding the fate and behavior of the subducting slab may be due to complexities in plate structure [Tajima et al., 2009], or due to different methods (and resolutions associated with those methods) used by different studies. Given that some triplication studies described above were based on the same seismic events, uncertainties in their results and interpretations warrant further investigation. [7] In this report, we have assembled seismic profiles of triplication waveforms using high density data sampling to constrain the Vs structure of the TZ beneath Korea and its adjacent regions (Figure 1). The direct comparison of results from neighboring regions is preferable to comparison with globally averaged parameters [e.g., Kennett, 1993]. Previous triplication studies inferred lateral variations in the TZ structure using data sets consisting of different seismic events to sample each region [Tajima and Grand, 1998; Wang and Chen, 2009]. The lateral variations in the TZ structure described in this study were derived from the same seismic events, interpreted from records covering different azimuthal ranges (similar to methods used by Wang and Niu [2010], but using more extensive data sets). Comparing results derived from the same events reduces the effects of uncertainties in hypocenter locations. The larger data set used in this report provides consistent and robust evidence of strong lateral variations in the TZ beneath the study regions. 2. Seismic Data and Methods [8] This study utilized archival broadband seismic data from the national and numerous regional networks maintained and curated by the China Earthquake Networks Center (CENC). Additional data from a seismic array deployed in the NCC by the Institute of Geophysics, China Earthquake Administration (CEA) were also included in this study. The array consisted of 150 broadband seismic stations, and recorded numerous high signal-to-noise seismic events from 2006 to [9] Triplication studies typically use waveforms recorded at epicentral distances of These distances are optimal for recording the triplicated arrivals of reflection off the 410- (or the 660-km) discontinuity and waveforms turning above and below the discontinuity (Figures 1c and 1d). For a given waveform, triplicated arrivals have similar ray paths near the source and the receiver. The relative arrival time and amplitude among triplicates are most sensitive to velocity variations at different depths sampled in the TZ [e.g., Chen and Tseng, 2007; Wang and Niu, 2010]. [10] Deep earthquakes having magnitudes greater than 5.2 provide the best resolution for triplication sampling of the lower TZ [Brudzinski and Chen, 2003; Y. Wang et al., 2006]. For an event deeper than 410 km, the triplications from the 660-km discontinuity are free of interference from those due to the 410-km discontinuity (Figure 1d), and are thus easier to identify. Large earthquakes ensure good signal-to-noise ratios for triplication waveforms, and provide clear P and pp arrivals times at teleseismic distances for relocation purposes. [11] Resolving and matching the relative times and amplitudes between successive waveforms are significant challenges in triplication analysis. Accurate measurement depends on the quality and quantity of available data and a single record may not provide reliable result. Profiles generated by seismic arrays or collected from networks with dense station spacing provide better constraints, evident from their systematic variations in relative time and amplitude between triplicated arrivals that recorded at clustered stations. To some extent, this type of data can minimize the effects of lateral seismic heterogeneities beneath individual stations [Chen and Tseng, 2007]. Table 1 lists the events used in this study, all of which originated at focal depths greater than 350 km. The assembled seismic profiles consist of highresolution records (Figures 2 5) and provide a dense sampling of the TZ structure beneath the study regions (Figure 1). [12] The approach and procedures for processing triplications used here resemble that described in other studies [e.g., Gao et al., 2006; Zhang et al., 2008]. Tangential component seismic profiles were obtained from rotation of horizontal records and subsequently subjected to a 0.02 to 0.25 Hz band-bass filter. We use the reflectivity method [Fuchs and Muller, 1971] to calculate synthetic seismograms, with fault plane solutions taken from the Harvard CMT catalog, and source time functions estimated from teleseismic records. To minimize the uncertainty in focal depth, we visually selected the P and Pp arrival times from teleseismic data sets, and then adjusted the focal depth (as shown by Wang and Niu [2010]). [13] In forward triplication modeling, we used a modified PREM as a reference model. The modified PREM includes a TZ velocity structure similar to that of PREM [Dziewonski and Anderson, 1981] and an uppermost mantle structure derived from the IASP91 [Kennett and Engdahl, 1991]. For the rest of this paper, we will refer to it as PREM-mod. [14] Using PREM-mod, the best fitting model was obtained by adjusting parameters such as the Vs gradient of the TZ, the depth of the 660-km discontinuity and the jump in Vs across the discontinuity to minimize misfit between 3of13

4 Figure 2. Seismic profiles of events 1 and 2 sample the TZ beneath Sakhalin. Observations (black traces) are compared to synthetic waveforms (blue traces) with traveltime predictions (dotted curves) calculated using PREM-mod and the preferred model (a and c) for event 1, and (b and d) for event 2, respectively. The timescale is the same as in Figure 1. 4of13

5 Figure 3. Seismic profiles of event 3 covering azimuths (a, c, and e) larger than 240 and (b, d, and f) less than 240 sample the TZ beneath the NEC and Korea, respectively. Observations (black traces) are compared to corresponding synthetic waveforms (blue traces) with traveltime predictions (dotted curves) calculated using PREM-mod (Figures 3a and 3b) and the preferred models (Figures 3c and 3d). Comparison between observations (black traces) and synthetics (blue traces) with traveltime curves calculated by models for Korea and the NEC shown in Figures 3e and 3f, respectively. The timescale is the same as in Figure 1. 5of13

6 Figure 4. Seismic profiles of event 4 covering azimuths (a and c) larger than 240 and (b and d) less than 240 sample the TZ beneath the NCC and Korea, respectively. Observations (black traces) are compared to corresponding synthetic waveforms (blue traces) with traveltime predictions (dotted curves) calculated using PREM-mod (Figures 4a and 4b) and the preferred models (Figures 4c and 4d). The timescale is the same as in Figure 1. 6of13

7 Figure 5. Seismic profiles of event 5 covering azimuths (a and c) larger than 240 and (b and d) less than 240 sample the TZ beneath the NCC and Korea, respectively. See Figure 4 caption for further details. 7of13

8 synthetic waveforms and observations. To determine how variations in these values may affect triplication results, we conducted sensitivity tests and list the results in Appendix A. The preferred model has small misfits in both relative times and amplitudes between synthetics and observed waveforms. We accept waveform fit for each seismic profile with the correlation coefficient larger than 0.5 for time window that included the triplicated arrivals at all stations, as shown in Figures 2 5. As in the previous study [e.g., Wang and Niu, 2010], we matched relative time between successive triplicated arrivals rather than absolute arrival times. 3. Major Features in Profiles Sampling Sakhalin [15] Figure 1 shows that the hypocenters of events 1 and 2 are located closely. The high-quality tangential components corresponding to each event are shown in Figures 2a and 2b, respectively. For these two events, the turning points of rays near the 660 km mainly sample the mantle beneath Sakhalin (Figure 1). [16] Seismic profiles of events 1 and 2 clearly show three branches of the triplications within an epicentral distance range of 11 to 32. For event 1 as an example, Figure 2a shows that at distances of less than 16, the first dominant arrival is the AB branch, followed by the CD branch. The AB and CD branches intersect at a distance of 18 (crossover distance). At greater distance (after the crossover), the CD branch becomes the first arrival, followed by the AB branch. [17] Similar features appear in the seismic profiles for events 1 and 2 (Figures 2a and 2b). Compared with the corresponding PREM-mod synthetic waveforms, the observations exhibit larger separation of relative time between triplicated arrivals at all distances (12 30 ), and a far-reaching AB. For station LAY in Figure 2a, the arrivals from the CD and AB+BC branches is separated by 8.5 s larger than predicted by PREM-mod (6.0 s). Misfit in the relative time between the observations and the PREM-mod synthetics are more evident, especially in observations at shorter distances (prior to the crossover), as recorded at stations BNX, CN2 and SNY (Figures 2a and 2b). For station SNY (16 ), two dominant arrivals is separated by 6.0 s in the data, whereas they overlap and are difficult to distinguish in the PREMmod synthetics (Figure 2a). Figures 2a and 2b also illustrate that the AB branches extend over a wide range of epicentral distances. In Figure 2b, the cusp B can be clearly observed at a distance of 30, having large amplitude and following the CD branch, while the predicted AB branch terminates at a distance of 26 in PREM-mod. 4. Major Features in Profiles Sampling Beneath the NEC, NCC, and Korea [18] For events 3, 4, and 5, a remarkable feature of their records is that the time intervals between triplicated phases depend significantly on the azimuth of the ray path. At a given distance within the range of triplications, observations from stations with azimuths greater than 240 generally show longer time intervals than those from stations with azimuths less than 240. For cross-comparison, we divided seismic waveforms of each event into two different seismic profiles according to station azimuth, using the corresponding PREMmod synthetics as a guide. Seismic data from stations with azimuths close to 240 were inspected and then sorted into two separate profiles according to their waveform features. [19] The seismic profile for event 3 covering azimuths greater than 240 is compared to the PREM-mod synthetics in Figure 3a. The data shows a far-reaching AB, similar to that shown in Figures 2a and 2b, and evident in the large amplitudes of AB arrivals (80 s) observed at stations JYA, XAN and ANK (Figure 3a). Unlike the data sampling Sakhalin (Figures 2a and 2b), some observation at greater distances in Figure 3a show the time intervals between triplicate phases are in agreement with the PREM-mod predictions, for instance at stations CXT and LYN. Misfit in time intervals are also evident for stations SNY and JZO at short distance (Figure 3a), resembling patterns observed in Figures 2a and 2b. Such misfits in time intervals however are relatively small. For station SNY, the misfit in time intervals between the observed and synthetic waveforms is 2.5 s (Figure 3a). [20] The hypocenters of events 4 and 5 are also located in close proximity to each other. The seismic profiles covering azimuths larger than 240 for these two events exhibit similar features and are strongly correlated, as shown in Figures 4a and 5a. With the PREM-mod synthetics as references, the observations from these two events show longer time intervals between triplicated phases and a relatively far-reaching AB, features that are also evident in the data sampling Sakhalin (Figures 2a and 2b). Data from station HZN (Figure 4a) show the misfit up to 3.5 s in relative time between the observed and the synthetic waveforms. The similarities in seismic profiles for these two regions (Figures 2a and 2b and Figures 4a and 5a) are absent in observations at shorter distances. [21] In contrast to the profiles covering azimuths greater than 240 for events 3, 4 and 5 (Figures 3a 5a), the profiles covering azimuths smaller than 240 for these events are highly similar. These latter profiles are compared with the corresponding PREM-mod synthetics in Figures 3b 5b. The predicted time intervals and amplitude ratios between the CD and AB+BC branches in the synthetic waveforms are larger than their observed counterparts, especially for records at greater distances. The amplitudes of the AB arrivals in the PREM-mod synthetics for example, tend to decrease dramatically at distances greater than 22 (Figure 3b). This trend is less pronounced or absent in the observed data. Event 5 also exhibited discordant time intervals and amplitude ratios for the observations and PREM-mod synthetics (Figure 5b). The limited number of observations at shorter distances (Figures 4b 5b) prevents conclusive identification of the termination of cusp C. [22] The distinct differences between seismic profiles covering different azimuths for events 3, 4 and 5 offer strong evidence for lateral variation in Vs of the TZ beneath the eastern part of the NEC, the northern portion of the NCC and Korea (Figure 1). For event 3, the seismic profile covering azimuths larger than 240 samples the mantle beneath the eastern part of the NEC from about 480 to 790 km depth, while the seismic profiles of events 4 and 5 covering same azimuths (greater than 240 ) mainly sample a depth range of 470 to 785 km beneath the northern portion of the NCC. The seismic profiles of events 3, 4 and 5 covering azimuths smaller than 240 primarily sample the mantle beneath Korea 8of13

9 Figure 6. (a) The preferred SH velocity models for the study regions along with the PREM and IASP91 models. (b) Lateral variation in the TZ structure of regions sampled. Schematic cross sections showing the different geometries of the subducting Pacific slab (c) beneath A (Sakhalin) and B (NEC) and (d) beneath C (Korea) and D (NCC). at a depth range of 530 to 810 km, and thus provide the best constraint on the structure of the lower TZ. 5. Best Fitting Models for Our Study Regions [23] We matched the features displayed in seismic profiles described above to interpret the TZ structure beneath Sakhalin, the NEC, NCC and Korea (Figure 6a). We compare the data with the synthetic waveforms calculated for the corresponding preferred models in Figures 2c and 2d, 3c 5c and 3d 5d. The data are better matched by the synthetics calculated for the preferred models than for PREM-mod, both in terms of relative time and amplitude between triplicated arrivals. For event 1 (Figure 2c), the longer intervals between the AB and CD phases predicted by the preferred model are very close to the observed ones at stations BNX, CN2 and SNY. Better matching is also evident in the amplitude ratios of the CD and AB arrivals in Figures 3d 5d, especially for observations at greater distances. For the sake of brevity, we do not describe the better matching in each seismic profile in further detail. Minor discrepancies between the data and the synthetics calculated for the preferred models (e.g., DL2 in Figure 2d, and YSH, JIX in Figure 3d) may be due to the complexities in the regions sampled. [24] The best fitting Vs models beneath Sakhalin and the NCC (Figure 6a) indicate a low Vs gradient in the lower TZ (similar to the PREM, rather than the IASP91) and a large jump in Vs (8.5%) across the 660-km discontinuity. Data recorded at greater distances exhibit similarities that imply the same Vs features. The low Vs gradient can explain the far-reaching of AB observed in Figures 2c and 2d and 4c 5c. A deeper 660-km discontinuity would also extend the AB branch to a greater distance (Figure A1). This latter mechanism is unlikely because it would result in short time intervals dramatically (Figure A1), which is not consistent with our observations (Figures 2a and 2b and 4a 5a). The Vs jump (8.5%) across the 660-km discontinuity beneath Sakhalin and the NCC is abnormally large compared to that in the IASP91 (6.1%) and PREM (6.5%) models. The anomalous Vs jump can explain the longer time intervals observed at greater distances, shown in Figures 2c, 2d, 4c, 5c, and A1. [25] We infer both a low Vs gradient with a high Vs in the lower TZ, and a km depression of the 660-km discontinuity beneath Korea (Figure 6a). Seismic profiles sampling this region shows a far-reaching AB branch (Figures 3d 5d), similar to features observed in the data sampling Sakhalin and the NCC (Figures 2a and 2b and 4a and 5a). Another feature of the seismic profiles is short time intervals at greater distance relative to the PREM-mod synthetics (Figures 3b 5b). These two features both favor a deeper discontinuity. A deeper discontinuity however cannot fully explain for the observations, and such a depression would also be expected to increase time intervals at short distances (Figures 3d and A1). Figures 4d and 5d instead indicate a low Vs gradient accompanied by a high Vs anomaly in the lower TZ, which would reduce the longer time 9of13

10 intervals that resulted from a deeper discontinuity. The two factors of a deeper discontinuity and a relatively low Vs gradient combined (Figure 6a) would allow the AB branches to extend over a wide range and significantly increase its amplitude, as evident in Figures 3d 5d. Also, there is no obvious anomaly in Vs jump across the 660-km discontinuity under Korea, closer to that in the PREM. The data offers poor resolution of the mid-tz beneath Korea however, due to the limited depth range sampled ( km). [26] The TZ structure beneath the NEC exhibits a low Vs gradient in the lower TZ, accompanied by a relatively large jump in Vs (7.7%) across the 660-km discontinuity (Figure 6a). As with the interpretation of the regions beneath Sakhalin and the NCC (Figures 2c, 2d, 4c, and 5c), a depression of the 660-km discontinuity beneath the NEC is also less desirable (Figure 3c). [27] Our preferred model for the TZ beneath Sakhalin also includes a high Vs anomaly (2%) within a depth range of km in the mid-tz, when compared with the deep structure beneath the NCC. The large Vs jump across the 660-km discontinuity (8.5%) cannot fully account for abnormally longer time intervals between the AB and CD branches in observation, such as at stations BNX, CN2 and SNY (Figures 2a, 2b, and A1), indicating the presence of a high Vs anomaly in the mid-tz. A high Vs anomaly within the upper mantle on the receiver side may have contributed to increase time intervals at shorter distances, by inducing earlier arrivals of AB phases. In this case, the earlier AB arrivals would also be evident from other events recorded at same stations. Profiles from event 3 (Figure 3a) do not clearly show early AB arrivals for stations BNX, CN2 and SNY however. [28] Relative to the lower boundary of the Vs anomaly, the upper boundary is poorly constrained due to the depth sampled. For events 1 and 2, we conducted a series of sensitivity tests using different lower boundaries for this anomaly. The results indicate that the boundary is shallower than 580 km, otherwise the time intervals between the AB and CD branches cannot be matched well at shorter distances. [29] The depth of the 660-km discontinuity beneath Sakhalin is not constrained very well due to a trade-off between the depth of discontinuity and assumed depths of events in triplication study [Y. Wang et al., 2006]. For a same event, the re-determined focal depths used in this paper (Table 1) differed from those determined by other studies by as much as 8 km[y. Wang et al., 2006; Wang and Chen, 2009]. In contrast, the depth of the 660-km discontinuity beneath the NCC, NEC and Korea are relatively constrained well by using the same event records just covering different azimuths. [30] To further demonstrate the lateral variations in the Vs structure beneath the study regions, we use observations from event 3 to show that models for the TZ structure of adjacent regions can be ruled out (Figures 3e and 3f ). We compared event 3 observations from stations covering azimuths larger than 240 (sampling the NEC) with the synthetics calculated for the best fitting Vs model for Korea (Figure 3e). In Figure 3e, the predicted time intervals between AB and CD phases by the Korean model are incompatible with observations, especially for records at greater distances. Similar discrepancies between the observations (azimuths < 240 ) sampling the TZ beneath Korea and synthetics based on the Vs model for the NEC are evident in Figure 3f. 6. Discussion 6.1. Comparisons With Other Studies [31] Compared to other studies using triplication methods, our seismic profiles are based on station coverage of greater density, and cover a relatively large geographic area. These factors allow us to interpret different models for localized areas of the study regions and form an overall interpretation for regional TZ structure. [32] Using observations from event 3, mostly covering azimuths larger than 240, Wang and Chen [2009] detected a large contrast in Vs (7%) across the 660-km discontinuity. Their results were consistent with the 7.7% jump in Vs beneath the NEC reported here, but we did not detect a lower Vs anomaly beneath the 660-km discontinuity. A seismic profile constructed by Ye et al. [2011] for event 5 from stations covering azimuths larger than 250 also exhibited longer time intervals between triplicated phases, similar to the observations described here for the region beneath the NCC. This feature favors a large jump in Vs across the 660-km discontinuity as inferred from our study. [33] We detect significant lateral variations in the Vs of the TZ beneath Korea and its adjacent regions (Figure 6a). Large jumps in Vs (8.5%) across the 660-km discontinuity are inferred beneath Sakhalin and the NCC. The TZ beneath Korea exhibits only a moderate Vs jump (6.6%) accompanied by a km depression of the 660-km discontinuity. The Vs jump (7.7%) under the NEC ranges in magnitude between that found beneath the NCC (8.5%) and Korea (6.6%). The lateral variations in Vs jumps across the 660-km discontinuity are associated with Vs anomalies in the lower TZ. In contrast to the deep structure under Sakhalin (or the NCC), a 2% high Vs anomaly is found in the lower TZ beneath Korea, on the same order of that detected from the mid-tz beneath Sakhalin (2%), as well as a high anomaly detected beneath the NEC (1%). [34] The Vs anomalies (2%) identified here are consistent with the lateral Vp variations in the TZ reported by previous studies. Tajima and Grand [1998] detected high Vp anomalies of around 2% in the lower TZ of certain regions of the NW Pacific subduction zone (NW Japan and the southern Kuriles). Our results reveal significant variations at smaller scales and in different areas than those described by Tajima and Grand [1998]. [35] The Vs anomalies (2%) detected in the lower TZ between Korea differ slightly from Vp results reported by Wang and Niu [2010]. Interpreting event 5 from dense seismic networks located throughout China, Wang and Niu [2010] suggest lateral variations in Vp at around 600 km depth, spanning regions from Korea to the NEC (subregions A-C), but the anomaly in Vp is relatively small, less than 1% Implications for Models of the TZ Beneath Korea and Adjacent Regions [36] Anomalous seismic wave velocities in the mantle are typically attributed to variations in temperature rather than to chemical heterogeneity. The Vs anomaly of 2% would correspond to a thermal anomaly of about 300 K, assuming 10 of 13

11 a Vs temperature sensitivity of 0.7%/100 K from the 1300 C adiabat of the lower TZ [Cammarano et al., 2003]. Assuming a Clapeyron slope of 1.3 MPa/K (determined in recent laboratory experiments by Fei et al. [2004]) for the 660-km transformation from g-spinel to perovskite and magnesiowüstite, a 300 K temperature anomaly indicates a 12-km depression of the lower TZ. This estimation is consistent with the km depression of the 660-km discontinuity observed in the TZ beneath Korea. Beside temperature, we should note that the mantle transition zone structures are also affected by other factors, such as composition and chemical interaction between the olivine and pyroxene components. These factors have a large effect on the velocity structure of the transition zone, existence of double discontinuities, discontinuity depth and velocity gradient below the discontinuities [Y. Wang et al., 2006]. In this paper, we have ignored these factors in our interpretations of seismic results. [37] Assuming that Vs anomalies reported here are primarily due to thermal variations, the Vs anomalies (2%) can be interpreted as evidence of a dipping portion of the subucting slab within the mid-tz beneath Sakhalin and a flattened portion of the slab in the lower TZ beneath Korea (Figures 6c and 6d). Our results agree with the scenario in which the subducting slab has reached the lower the TZ, and has accumulated and flattened, thus resulting in the depression of the 660-km discontinuity detected beneath Korea. The vertical depth distribution of the Vs anomalies in the TZ indicates that the thickness of the slab may be on the order of 100 km. [38] The relatively large jumps in Vs across the 660-km discontinuity beneath Sakhalin and the NCC may indicate that no subducting slab stagnant within the lower TZ beneath these regions. For comparison, Vp and Vs increase 3% and 4% respectively, across the 660-km discontinuity beneath back arc areas of the Izu-Bonin trench, where the remnant of the Northern Philippine Sea slab rests in the lower TZ. Such anomalies of jumps are significantly smaller than those observed in surrounding regions [Tseng and Chen, 2004]. [39] The presence of water in nominally anhydrous polymorphs of olivine can also potentially cause large jump in Vs across the 660-km discontinuity. Recent studies have shown that ringwoodite may contain up to 2.3 wt. % water in its crystal structure, indicating that a relatively large amount of water can be stored in the lower TZ [e.g., J. Wang et al., 2006]. A Vs jump of 7.7% found beneath central Tibet using triplication methods was interpreted as a hydrous (1 wt. %) remnant of lithosphere resting above the 660-km discontinuity [Tseng and Chen, 2008]. Analysis of triplicate waveforms for event 5 revealed a high Vp/Vs ratio that was interpreted as the leading edge of the subducting Pacific slab trapped in a water-bearing ( wt. %) mantle [Ye et al., 2011]. [40] Taken with the Vp anomalies (of less than 1%) previously reported by Wang and Niu [2010], the 2% Vs anomaly reported here suggests a higher Vp/Vs anomaly beneath the NCC relative to that inferred beneath Korea. Recently, from the discrepancy between earthquake focal depths and the greater depths at which hydrous mineral phases break down, Green et al. [2010] suggest that the subducting slab may be dry at depth below 400 km, and thus cannot provide a pathway for significant amounts of water to enter the TZ. This may be the case for the portion of Pacific slab that has stagnated in the TZ beneath Korea, but further research is necessary to constrain assumptions concerning the water content of the slab. [41] Beneath the eastern portion of the NEC, the slightly high Vs anomaly (1%) in the lower TZ and the absence of a depression in the 660-km discontinuity indicate that only the front edge of the slab has reached the TZ. The structure beneath the NEC appears to be a transitional region between TZ areas beneath the NCC and Korea. The lateral boundaries between adjacent regions are not very sharp (Figure 6b). 7. Conclusions [42] We investigated the Vs structure of the TZ by modeling and interpreting triplications in seismic profiles from high-resolution records of deep earthquakes occurring in the NW Pacific subduction zone. We find robust evidence of lateral variations in the TZ beneath Korea and adjacent regions based on the behavior of triplications from events recorded at stations covering different azimuths. Our results reveal large jumps in Vs across the 660-km discontinuity beneath Sakhalin and the NCC. High Vs anomalies (2%) were identified both in the mid-tz beneath Sakhalin and in the lower TZ beneath Korea. For the latter region, a deeper 660-km discontinuity was also inferred, accompanied by a relative small jump in Vs across the discontinuity. The lower TZ beneath the NEC exhibits a relatively small Vs anomaly (1%) and is thus interpreted as a transitional area between the TZ beneath the NCC and Korea. [43] Lateral variations in the TZ appear to reflect the spatial geometry of the Pacific slab. We suggest that the Pacific slab has subducted across the mid-tz beneath Sakhalin, reaches the lower TZ beneath the NEC and is resting at the bottom of the TZ beneath Korea. There is no evidence for subducting slab stagnant in the lower TZ beneath the NCC. Appendix A: Forward Modeling [44] The best fitting results were obtained from the PREMmod by adjusting a) the Vs gradient of the TZ, b) the depth of the 660-km discontinuity and c) the degree of Vs jump across the discontinuity, in order to minimize differences between synthetic and observed waveforms. [45] To assess the sensitivity of triplication to these parameters, we calculated synthetic profiles using several different configurations of models and parameters. With regards to the seismic structure of the TZ, the primary differences between the IASP91 and PREM models are their respective velocity gradients. The PREM includes two velocity gradients with a change in the lower TZ, whereas the IASP91 has only one velocity gradient. For simplicity, we used the modified IASP91 models to illustrate how the modified parameters affect the triplications. [46] Figure A1 shows the traveltime predictions from four different modifications of the IASP91 models along with those predicted by the IASP91 model. The first modified model (Figure A1a) gave a relatively low Vs gradient for the lower TZ, but a fixed Vs jump across the 660-km discontinuity like the IASP91. The other two models (Figures A1b and A1c) show a large jump in Vs across the 660-km discontinuity, but a uniform Vs gradient in the TZ. The final 11 of 13

12 Figure A1. Comparisons of (top) travel times and (middle) synthetic waveforms predicted by the IASP91 model (black) and the four modified models (gray) by (a) adjusting the velocity gradient, (b and c) velocity increases across the TZ, and (d) the depth of the 660-km discontinuity. model (Figure A1d) included an increase in the depth of the 660-km discontinuity. [47] Figures A1a A1d show that the termination distance of cusp B is very sensitive to the TZ velocity gradient and the depth of the 660-km discontinuity. A relatively low Vs gradient in the lower TZ or a deeper 660-km discontinuity results in a far-reaching of AB branch. These two factors exert different effects on time intervals between triplicated phases. A deeper 660-km discontinuity dramatically increases the time intervals at shorter distances. A low Vs gradient in the lower TZ results in only a small change in relative times. The time interval between triplicated arrivals is also sensitive to the Vs jump across the 660-km discontinuity. In essence, the larger the jump in Vs, the longer time intervals at all distances (Figures A1b and A1c). These tests show that the effects of adjusting the Vs gradient, the depth of the 660-km discontinuity and the velocity jump across the discontinuity can be distinguished from one another. In many cases, modifying a single parameter does not suffice to explain the observed features, and a combination of parameter adjustments was required. Given these factors, our approach was to use the simplest set of modifications that could adequately explain the observed features. [48] Acknowledgments. Seismic data were provided by the China Earthquake Networks Center and seismic array deployed by the Institute of Geophysics, China Earthquake Administration. We are very grateful to two anonymous reviewers and to the Associate editor. Their reviews significantly improved the quality of this paper. We would also like to thank 12 of 13

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